Department of Physiology: CNS Physiology-Review & Illustrations

Anatomical Review

Brain figures and animations (Figure 1), (figure 2), (animation 1), (animation 2). For animations, download the animation file and download and install this program to run the animation.

Spinal cord (figure 1)

Brainstem (figure 1)

Cerebral cortex (figure 1)

Neuronal pool (circuits)

The nucleus (center) within CNS is a collection of intercommunicated neurons. Each center has its own special characteristics of organization (circuits or pools) which cause it to process signals in its own special way.

Inhibition within the CNS

the inhibitory mechanisms within the CNS are of two types: (1) Presynaptic inhibitory mechanism: In which the inhibition occur at the presynaptic neuron before the signal reaches the synapse itself. Presynaptic inhibition can be achieved by two different mechanisms. (2) Postsynaptic inhibitory mechanism: This type of inhibition can be due to the generation of IPSP at the postsynaptic membrane or the the synaptic fatigue or the presence of refractory period at the postsynaptic neuron.

According to the anatomicall arrangement, inhibition can be of two types.

Somatosensory functions of the CNS

According to the type of energy or stimulus that stimulates receptors, there are five different types of sensory receptors:

[1] Mechanoreceptors, which detect mechanical deformation of the receptor or of cells adjacent to the receptor which include

Tactile sensations (touch, pressure, vibration, tickles, itch),
Hearing,
Equilibrium,
Position sense (Joint receptors, Stretch receptors in muscle),
Baroreceptors in carotid sinus

[2] Thermoreceptors,

[3] Pain receptors (nociceptors),

[4] Electromagnetic receptors (photoreceptors),

[5] Chemoreceptors,

Olfactory receptors
Taste receptors
Osmoreceptors
Carotid body O₂ receptors

In general, clinically, senses can be classified into three types:

[A] Somatic senses

1. Tactile sensations (carried by type A & C nerve fiber).

Touch,
Pressure,
Tickling,
Itch, the
Vibratory (vibration detection is between 60 up to 700 cycles/sec), Vibratory and proprioceptive sensations are closely related, when one is depressed, so is the other
Stereognosis

2. Position (or proprioceptive), it is carried by type A nerve fiber. Proprioceptive sensation is either conscious and is carried by leminiscal pathway or subconscious and is carried by spinocerebral tract (as will described later). It can be divided into two subtypes:

Static proprioceptive sensation, which means conscious recognition of the orientation of the different parts of the body with respect to each other and,
Dynamic proprioceptive sensation (Kinesthesia), which means conscious recognition of movements and the rates of movement of the different parts of the body.

3. Pain sensation (carried by Aδ & C nerve fibers).

4. Thermal sensation (cold sensation is carried by the Aδ and C, whereas, the warm is carried by only C fibers).

[B] Special senses:

Special sensations are the complex sensations for which the body has some specialized sense organs.

Vision,
Smell,
Taste,
Hearing, and
Equilibrium sensations (rotational and linear acceleration).

[C] Visceral sensations: Which are those concerned with perception of the internal environment such as

Baroreceptors
Osmoreceptors
Chemoreceptors

General properties of receptors:

The sensitivity of receptors (The stimulus or the energy for which a sensory receptor is most sensitive (i.e. has lowest threshold for detection) is called the adequate stimulus)
The specificity of the nerve fiber attached to the receptor (Each nerve fiber is specialized to transmit only one modality of sensation)
The ability to generate a receptor potential (generator potential). The relationship between the action potential of the nerve fiber attached to the receptors and stimulus intensity is shown in figure. The brain can recognize the intensity of the stimulus that is transmitted to it by: [A] Variation in the frequency of the action potential generated by the activity in a given receptor (called temporal summation, or frequency coding) and [B] By variation in the number of receptor activated (called spatial summation or population coding).
Adaptation or desensitization of receptors (Tonic receptors, & Phasic receptors).

Assessment of tactile Sensitivities: Regional sensitivity to light touch can be checked by gentle contact with a fingertip or a slender wisp of cotton. The two-point discrimination test (see figure) provides a more detailed sensory map of tactile receptors. Two fine points of a bent paper clip or another object are applied to the skin surface simultaneously. The person then describes the contact. When the points fall within a single receptive field, the person will report only one point of contact. A normal person loses two-point discrimination at 1 mm (0.04 in.) on the surface of the tongue, at 2–3 mm (0.08–0.12 in.) on the lips, at 3–5 mm (0.12–0.20 in.) on the backs of the hands and feet, and at 4–7 cm (1.6–2.75 in.) over the general body surface.

Applying the base of a tuning fork to the skin tests vibration receptors. Damage to an individual spinal nerve produces insensitivity to vibration along the paths of the related sensory nerves. If the sensory loss results from spinal cord damage, the injury site can typically be located by walking the tuning fork down the spinal column, resting its base on the vertebral spinous processes.

Notes: Various descriptive terms indicate the degree of sensation sensitivity in an area. Anesthesia implies a total loss of sensation. The person cannot perceive touch, pressure, pain, or temperature sensations in that area. Hypesthesia (hypoesthesia) is a reduction in sensitivity. Paresthesia is the presence of abnormal sensations such as the pricking sensation when an arm or leg “falls asleep” as a result of pressure on a peripheral nerve.

Pain

Types of pain: There are two types of pain; acute pain and chronic pain (see table).

Types of pain receptors: The pain receptors (nociceptors) are all free nerve endings and are of tonic type. Pain receptors can be classified into 3 types according to the type of stimulus that excite them and these are:

Mechanosensitive pain receptors.
Thermosensitive pain receptors.
Chemosensitive pain receptors.

Referred pain: That is the pain felt in a part of the body considerably remote from the tissues causing the pain.
The mechanism of the referred pain: The visceral pain fibers enter the spinal cord and synapse with second order neuron that also receives pain fiber from the skin. When the visceral pain fibers are stimulated, pain signals from the viscera are then conducted through the same spinal neurons that conduct pain signals from the skin, and person has the feeling that the sensations actually originate in the skin itself.

The rules that determine the areas to which the pain is referred are:

Dermatomal rule
Brain interpretation rule
Facilitation effects rule

Visceral pain: It is the pain from different viscera of the abdomen and chest. The visceral pain is transmitted through type C nerve fibers that run in the sympathetic or parasympathetic nerves. The viscera have somatic receptors for pain sensation only.

Central inhibition of pain: Pain can be controlled by:

Notes: Allodynia, painful perception produced by normally innocuous stimuli, is one manifestation of sensitization. An example of this phenomenon is the temporary sensitization of the skin to light touch following sunburn. Hyperalgesia refers to the increased response to a painful stimulus resulting from sensitization.

Thermal Sensations

Types of receptors:

[A] The cold receptors which respond maximally to temperature slightly below body temperature. The fall in body temperature below 30^o C stimulates only the cold receptors and the warm receptors remain inactive. Temperature below 10^o C will stimulate the nociceptors and give pain sensation.
[B] The warmth receptors which respond maximally to temperature slightly above body temperature. This will be seen until the temperature reaches 45^o C. Temperature beyond this will not stimulate the thermoreceptors, but stimulates the nociceptors which give pain sensation.

Types of nerve fibers: The cold sensation is carried by the Aδ and C, whereas, the warm is carried by only C fibers.
In most areas of the body there are three to ten times as many cold receptors as warmth receptors.
they are tonic and at the same time phasic type of receptors.

Somatic sensory pathways within CNS

The main ascending somatic sensory pathways within the spinal cord (see figure) and through the brain (see figure) are

The dorsal column pathway (leminiscal system). The dorsal column carries the following sensations:

Fine touch and pressure (including weight, shape, Size, texture, stereognosis),
Vibration, and
Conscious proprioception

The anterolateral pathways (spinothalamic pathway). It carries the following sensations:

Crude touch and pressure,
Pain,
Thermal,
Tickle,
Itch, and
Sexual sensations.

The spinocerebellar pathways carries unconscious proprioceptive sensations.

The spinotectal pathways (not shown in figures) originate from dorsal horn and ascend through lateral column of the spinal cord after crossing the spinal cord and terminate into superior colliculus and is concerned with spinovisual reﬂex. It carries pain, thermal , and tactile information to superior colliculus.

Lesions of the spinal pathways of somatic sensations.

Higher interpretation of sensory signals

This is achieved by the following areas of cerebral cortex (see figure and figure)
In the primary somatic sensory area

The spatial orientation of the different parts of the opposite side of body were represented
The size of the area of representation is directly proportional to the number of specialized sensory receptors in each respective peripheral area of the body
Lesions in the primary somatic sensory area

Somatic sensations do not abolish indicating that perception may occur at subcortical level and it is possible in the absence of the cortex. If the lesion is associated with thalamic lesion, then there is a loss of sensations in the contralateral side of the body.
The person is unable to localize discretely the different sensations in the different parts of the body.
He is unable to judge exactly the degrees of pressure against his body, unable to judge exactly the weights of objects, unable to judge shapes or forms of objects, unable to judge texture of materials.

In the sensory association areas
The general function of the sensory association areas is to provide a higher level of interpretation of the sensory signals by giving the brain the simplest meaning and characteristic of the sensory signal. Damage to the sensory association area is associated with specific deficits known as agnosias, in which there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss.^[

Damage can affect the ability to recognize objects even though the objects can be felt (tactile agnosia). Loses the ability to recognize complex objects and complex forms by the process of feeling them is called astereognosis, even though there is no specific sensory deficit

In Wernicke’s area

Located in the posterior part of the temporal lobe
It is highly developed in the dominant side of the brain (left side)
Plays the greatest role in interpretation of the complicated meanings of different sensory experiences.

Damage to the general interpretive area affects your abilityto interpret what you see or hear, even though you understandthe words as individual entities. For example, if your general interpretive area were damaged, you might still understand the meaning of the spoken words sit and here, because word recognition takes place in the auditory association areas. But you would be totally bewildered by the request sit here. Damage to another portion of the general interpretive area might leave you able to see a chair clearly, and to know that you recognize it, but you would be unable to name it because the connection to your visual association area has been disrupted.

Brain lesions: The symptoms resulting from injuries to key areas of the brain have been a primary source of information about the role those areas play. Following are some examples of symptoms that may occur following trauma or stroke to specific brain regions.

Parietal lobe lesion: Dysfunction in this part of the brain causes people to ignore objects on the opposite side of the body—even their own arm and leg. Patients may dress only half their body and even deny that the opposite arm or leg belongs to them.

Temporal lobe lesion: An injury here can impair the ability to identify familiar objects. Some may not even recognize their own face. In other instances, the person may lose the ability to differentiate between sounds, causing him to lose any appreciation of music.

Frontal lobe lesion: A lesion or injury here can result in severe personality disorders and cause socially inappropriate behavior

Somatomotor functions of the CNS

Descending spinal pathways involved in motor control

1. The lateral pathways are concerned with voluntary movement of the distal muscles (e.g., muscles of the arm and hand).

The lateral corticospinal tract,
The rubrospinal tract
The lateral medullary reticulospinal tracts,
The lateral vestibulospinal tract,

2. The ventromedial pathways innervate the proximal and axial muscles to help maintain head position and posture, muscle tone and gross movements of the neck, trunk, and proximal limb muscles. Sensory information about the body position and balance is derived from the visual and vestibular systems and is conveyed via three major tracts:

The anterior corticospinal tract,
The tectospinal tract,
The medial pontine reticulospinal tracts,
The medial longitudinal fasciculus

NOTE: The medial longitudinal fasciculus links the three main nerves which control eye movements, i.e. the oculomotor, trochlear and the abducent nerves, as well as the vestibulocochlear nerve. The purpose of the medial longitudinal fasciculus is to integrate movement of the eyes and head movements.

The motor functions of the CNS can be divided into:

Movement, There are three classes of movements:

[a] Reflexes which are involuntary, rapid, stereotyped movement such as eye-blink, coughing, knee jerk and graded control by eliciting stimulus.

[b] Voluntary movement which is complex actions such as reading, writing, playing piano. They are purposeful, goal-oriented and learned type of activity which can be improved with practice.

[c] Mixed pattern which combine voluntary & reflexive acts such as chewing, walking, running. It is initiated and terminated voluntarily, but once initiated it become repeatitive and reflexive.

Posture and balance,
Communication.

Each segment of the spinal cord has several millions neurons in its gray matter and these are:

The posterior sensory neurons
The anterior motor neurons

The cells of the anterior horn of spinal cord (anterior motor neurons) or motor cranial nuclei and their efferent fibers that run to motor units are also called the lower motor neurons to distinguish them from the upper motor neurons of the higher motor control centers. The lower motor neuron is the final common path (see figure) for all efferent impulses directed at the muscle, i.e. descending spinal pathways that involved in motor control.

The anterior motor neurons are of three types:

The alpha motor neurons: Which give off large nerve fibers (type A alpha nerve fiber) that innervate the large skeletal contractile muscle fibers (extrafusal muscle fibers) forming the motor units.
The gamma motor neurons or gamma efferent neurons: Which give off nerves fibers (type A gamma nerve fiber) that innervate very small specialized skeletal muscle fibers called intrafusal fibers which are part of the muscle spindle (act as receptors).
The interneurons: These are small neurons that have many interconnections one with the other. Most of the incoming sensory signals from the spinal nerves are transmitted first through interneurons where they are appropriately processed and then terminate on the anterior motor neurons. They are either excitatory or inhibitory interneurons (such as Renshaw cells).

Renshaw inhibitory cells serve two functions:

lateral inhibition is to focus or sharpen the signals, i.e. to allow transmission of the primary signal while suppressing the tendency for signals to spread to adjacent neurons.
Recurrent inhibition is important to allow only the initial impulses arriving at a motoneuron to pass through easily while the late impulses will find the anterior horn cells partially inhibited and will therefore produce a smaller motor discharge than the initial excitatory impulses

Reflexes: The basic unit of reflexes is the reflex arc. The arc consists of:

Sense organ (receptor),
Afferent neuron (sensory),
Interneurone, one or more synapses in a central integrating station or sympathetic ganglion,
Efferent neuron (motor), and
Effector.

The activity in the reflex arc is modified by multiple inputs converging on them from higher motor control centers through descending spinal pathways that involved in motor control. Sensory signals enter the cord through the sensory roots. After entering the cord, every sensory signal travels to two separate destinations:

The sensory nerve or its collateral terminate in the gray matter of the cord and elicit local segmental motor responses.
The signals travel to higher and lower segmental levels of the cord itself or to the brain stem or even to the cerebral cortex.

Proper control of muscle requires not only excitation of the muscle by the anterior motor neurons but also continuous feedback of information from each muscle to the nervous system which is achieved by two special types of sensory receptors and these are:
1. Muscle spindles:

They are distributed throughout the belly of muscle
Send information to the NS about the muscle length and the rate of change of its length.
Excited upon increase in length (stretch) of the muscles

2. Golgi tendon organs:

They are located among the fascicles of a tendon between it and the muscle itself
Send information about tension or rate of change of tension.
Excited upon increase in muscle tension.

Normally, there is a continuous sustained muscle contraction even the muscle is at rest or inactivity and it is called muscle tone. The causes of muscle tone are:

Anatomical arrangement of muscle, i.e. muscle usually in a state of slight stretch due to the muscle-bone origin-insertion arrangement which slightly stretches the muscle and in turn stretches the muscle spindle and activate muscle spindle stretch reflex.
Alpha and gamma motor neurons normally receive a basal level of excitatory stimulation from the brain through the following descending tracts: (see figure)

Corticospinal tract, to stimulate voluntary muscle contration
Lateral vestibulospinal tracts to stimulate extensor (antigravity) muscles, and inhibit flexors.
Pontine reticulospinal tract, to stimulate extensor (antigravity) muscles .
Rubrospinal tract (excites flexors and inhibit extensors).

These excitatory pathways are held in check directly or indirectly by inhibitory descending pathways on gamma motor neurons by:

Medullary reticulospinal tract
Rubrospinal tract (excites flexors and inhibit extensor).

Through descending spinal tracts of the upper motor neurons (mainly the pyramidal tracts, vestibulospinal tract, and reticulospinal tracts) and the activities of the basal ganglia and cerebellum, all influence directly or indirectly:

[1]. The cells of the anterior horn of spinal cord or motor cranial nuclei from which the lower motor neuron runs to motor unit. Therefore, the lower motor neuron is the final common path for all efferent impulses directed at the muscle. The inputs converging on the motor neurons bring about voluntary activity, adjust body posture to provide a stable background for movement, and coordinate the action of various muscle to make movements smooth and precise.

[2]. the transmission of neuronal impulses through spinal reflex arcs. There are two mechanisms by which descending projections may control transmission through segmental reflex arcs:

By their excitatory or inhibitory action on the neurons involved in the spinal reflex arc.
By their inhibitory action on the terminal part of afferent sensory fibers before their synapse with the next neuron (presynaptic inhibition).

Muscle spindle stretch reflex: It is the only monosynaptic reflex in the body

Responsible for establishment of muscle tone
Stabilization of body position during tense motor action
Damping or smoothing function of the stretch reflex against unsmoothed motor signals
Enhancement of extrafusal muscle fiber contraction (gamma loop servo system)

The Golgi tendon reflex (inverse stretch reflex)

Prevents the development of too much tension on the muscle
Equalize the contractile forces of the separate muscle fibers. That is, those fibers that exert excess tension become inhibited by the reflex, whereas those that exert too little tension become more excited because of the absence of reflex inhibition.

The withdrawal (flexor) reflex

Polysynaptic reflex
Includes the following circuits, Diverging circuits, reciprocal inhibition circuits, crossed extensor reflex circuit, after-discharge circuit.

The higher motor control systems:

See figure of the hierarchy of higher motor control

The higher motor control systems involve the structures that control all motor activities executed at the brainstem level and spinal cord and these are:

The pyramidal system.

Motor cortex
Pyramidal (corticospinal and corticobulbar) tracts

The extrapyramidal system.

Basal ganglia,

Reticular formation, and Reticulospinal tracts

Vestibular nuclei, and Vestibulospinal tract

Red nuclei, and Rubrospinal tract

Tectum (superior colliculi), and Tectospinal tract, and is involved in the control contralateral neck muscles),

Olivary nucleus, and olivospinal tract

Cerebellum.

The motor cortex

It is located directly in front of the central sulcus and occupying approximately the posterior one half of the frontal lobes
Has extensive connections with other areas of cerebral cortex and with subcortical structures
The motor cortex divided into three separate divisions

The primary motor cortex
The supplementary motor cortex
The premotor cortex

The primary motor cortex:

Located directly in front of the central sulcus in the precentral gyrus
From this area motor neurons originated and send their fibers directly or indirectly to the anterior motor neurons of the spinal cord through the corticospinal tract and to the brain stem through corticobulbar tract.
The spatial orientation of different muscles of the opposite side of the body is represented in this area (except for the lower two thirds of the face which is represented bilaterally) (see figure).
The surface area of representation of each muscle is proportional to the skill with which the part is used in fine voluntary movement. The areas involved in speech and hand movements are especially large in the cortex.
More than one half of the entire primary motor cortex is concerned with controlling of conscious voluntary fine, precise, discrete (separate) skilled movements of the hands and the muscles of speech which are highly developed in human being.
This area increases muscle tone by facilitation of stretch reflex.
Damage to this area causes lack of patient's fingers coordination and patient cannot precisely contract just one digit or a particular group of digits.

The supplementary motor cortex (see figure):

Located on medial surface of the frontal lobe slightly anterior to the primary motor cortex.
It is responsible for generating mental planning of complex sequence of events of a motor act that need bimanual coordination (for example tying shoe laces) and sends these instructions to the premotor Area.
Damage to this area leads to the following: Patient cannot tie shoe laces because of impaired selection of a particular movement sequence.

The premotor cortex:

Located anterior to primary motor area and below the supplementary motor area on the lateral side of the hemisphere.
It assembles the details of the mental planning of a motor act received from the supplementary motor area. In another words, this area instructs the primary motor area of how to do the motor act by constructing the details of the muscles involved in each event of a motor act and arrange the muscle contractions in order.
This area is also active during “mental rehearsal” for a movement. With repetition, the proper pattern of stimulation becomes stored in your premotor cortex as a ready-made program (also called engram) such as tying shoe laces.
Damage to premotor cortex area leads to the following:
1. Patient cannot initiate the movement the patient wishes to make.
2. Patient exhibits motor apraxia (defect in motor performance without paralysis) because the selection of a particular movement is impaired.
3. Reappearance of grasping reflex. E.g. when you have a patient in coma and you try to put a thing in his hand, he grasps it. Grasp reflex: happens when you put something the baby's hand, he will just grasp it, and this is before the age of 2 years, normally after 2 years, this area is well developed and this reflex will be inhibited.
4. Loss of bimanual coordination.

Within the premotor cortex the following areas are present (see figure):

Broca’s area for speech: This is the ward formation area. In most people (97%), both Broca's area and Wernicke's area are found in only the left hemisphere of the brain. Damage to it (Broca's aphasia or non-fluent aphasia) does not prevent a person from vocalizing, but it does make it impossible for the person to speak whole words rather than uncoordinate utterances or an occasional simple words such as “no” or “yes”.
The voluntary eye movement area for controlling eye and eyelid movements such as blinking. It is part of the frontal cortex in the human brain. Situated just anterior to the premotor cortex, it includes the frontal eye fields (so-named because they are believed to play an important role in the control of eye movements). Damage to this area prevents a person from voluntarily moving the eyes toward different objects. Instead, the eyes tend to lock on specific objects, an effect controlled by signals from the occipital region. Damage to this area, by stroke, trauma or infection, causes tonic deviation of the eyes towards the side of the injury. This finding occurs during the first few hours of an acute event such as cerebrovascular infarct (stroke) or hemorrhage (bleeding).This area also controls eyelid movements such as blinking.
Head rotation area: Stimulation this area will elicit head rotation. This area is closely associated with the eye movement field and presumably related to directing the head toward different objects.
Area for hand skills: Damage to this area causes the hand movements become incoordinate and nonpurposeful, a condition called motor apraxia.

Pyramidal tracts

This tract originates about 40% from the primary motor cortex, few of them from giant pyramidal cells, also called Betz cells, 40% from premotor cortex, and 20% from the somatic sensory areas at the parietal lobe.
These fibers are terminate either in the brainstem (corticobulbar) or spinal cord (corticospinal) and are involved in control of motor functions of the body.
Regardless of the location of their cell bodies, pyramidal tract fibers begin their descent from the cortex as a corona radiata (radiating crown) before forming the internal capsule. At the level of medulla, 80% of fibers (originated mainly from primary motor area) then crosses to the opposite side and descends in the lateral corticospinal tracts of the cord. Finally, these fibers terminate on excitatory (for the agonist muscles) or inhibitory (for antagonist muscles) interneurons that in turn synapse with anterior motor neurons. These fibers are concerned with contralateral distal limb muscles and hence with skilled movements especially of the hands and fingers. The pyramidal tract is a major controller of muscle activity for finger movements for performance of voluntary, purposeful activity such as writing, typing, tying knots, fastening buttons and playing musical instruments. However, some of the fibers terminate directly on the anterior motor neurons. A few of the fibers (20%, originated mainly from premotor area) do not cross to the opposite side in medulla but pass ipsilaterally down the cord through the reticular formation as ventral (anterior) corticospinal tracts, but these fibers terminate bilaterally mainly at the level of synapse with motor neurons. These fibers are concerned with axial and proximal limb (girdle) muscle contraction.
The neurotransmitter of the pyramidal system is glutamate and/or aspartate.
The cerebral cortex itself and subcortical structures (basal ganglia, brainstem reticular formation, Olive, Red nucleus, and cerebellum) all receive simultaneously strong signals from the pyramidal tract every time a signal is transmitted down the spinal cord to cause a motor activity.
A highly selective damage to lateral corticospinal tract is associated with deficit with fine skilled movements of hand and fingers. On the other hand, lesion of ventral corticospinal tract causes difficulty with balance, walking, and climbing.

The extrapyramidal system:

Which includes all those portions of the brain and brain stem and their fibers that contribute to motor control but that are not part of the pyramidal system. This system is concerned mainly with:

Postural control and stability,
Inhibits unwanted muscular activity,
Maintains muscle tone,
It is responsible for facial expression such as sadness, irony and happiness and swallowing.

Extrapyramidal system includes:

Basal ganglia,
Reticular formation,
Vestibular nuclei,
Red nuclei,
Tectum (superior colliculi),
Olivary nucleus,
Cerebellum.

The descending spinal extrapyramidal tracts are crossed lateral pathway except vestibular nucleus which has in addition crossed and uncrossed ventromedial pathways to the cervical segments of the spinal cord (see figure).

The main spinal extrapyramidal tracts include:

Tectospinal tract (from the superior colliculus of the tectum and is involved in the control contralateral neck muscles),
Vestibulospinal tract (from vestibular nuclei),
Reticulospinal tracts (from pontine and medullary reticular formation),
Rubrospinal tract (from the red nucleus).

The red nucleus:

Pink in fresh specimens because of an iron-containing pigment in many of the cells. Centrally placed in the upper mesencephalic reticular formation of the brain stem.
The red nucleus has a somatotopical representation of all the muscle of the body similar to the motor cortex but far less developed fineness of representation.
This nucleus gives rise the rubrospinal tract that crosses to the opposite side in the lower brain stem and follows a course parallel to the lateral corticospinal tract and terminate directly or indirectly (through interneuron) on the anterior motor neurons. The corticorubral pathway serves as an accessory route for the transmission of discrete signals from the motor cortex to the spinal cord.
The rubrospinal tract is involved in large movements of proximal musculature of the limbs.

The reticular formation:

Reticular formation is a diffused mass of neurons and nerve fibers, which form an ill-defined meshwork of reticulum in central portion of the brainstem.
The neurons of the reticular formation receive collateral nerve ending from the spinal cord, eyes, ears, cortex, and many subcortical structures.
Efferent fibers from it pass both upward and downward in the axis of the NS that play important roles in the adjustment of endocrine secretion, regulation of sensory input and consciousness.

Based on functions, reticular formation along with its connections is divided into two systems:

Descending reticular system.
Ascending reticular activating system.

Descending reticular system: Its motor functions, can be divided into pontine (facilitatory) and medullary (inhibitory areas).

The pontine reticular formation has a general stimulatory effect on both extensors and flexors, with the predominant effect on extensors (antigravity muscles) (through pontine reticulospinal tract).
The medullary reticular formation has a general inhibitory effect on both extensors and flexors, with the predominant effect on extensors. The inhibitory area tends to inhibit the extensors (through medullary reticulospinal tract).

The ascending projections of the reticular formation are involved in 4 different types of functions:

The regulation of posture
The control of muscle tone
The modulation of pain sensation
The coordination of autonomic functions. This is because centers controlling inspiration, expiration, the normal rhythm of breathing, heart rate and blood pressure, gastrointestinal activities, and ocular (pupillary) reflexes have been identified in the medulla and pontine reticular formation.

Ascending Reticular Activating System (ARAS) : The mesencephalic and upper pontine portions (facilitatory) of the reticular formation is giving an ascending monoaminergic (NE, 5-HT) and cholinergic projections to the cerebral cortex, subcortical structures and thalamus to increase wakefulness as well as the responsiveness to sensory stimuli, a state known as arousal and provide intrinsic activation of large areas of the brain. Tumor or lesion in ARAS leads to sleeping sickness or coma. The impact of head injury on ARAS also causes coma.

RAS is subject to excitation and therefore subject to increased levels of activation by two stimuli:

Sensory stimuli: All the sensory pathways send collaterals to ARAS. These sensory signals on entering the reticular formation may activate the RAS and awaken the subject immediately. This is called arousal reaction.
Retrograde stimuli from almost all parts of the cerebrum especially from motor regions to the reticular formation via direct fiber pathways. This may explain the importance of moving around when one wishes to remain awake.

RAS is subject to inhibition and therefore subject to decrease levels of activation, which can lead to sleep. This can be achieved by the areas of reticular formation in the brain stem below the midlevel of the pons and throughout the medulla oblongata which is inhibitory to the spinal cord segmental activities through inhibitory reticulospinal tracts and can be inhibitory to RAS that may lead to sleep. Damage to this area causes the cerebrum to become active and to remain active indefinitely as if it remains continuously awake.

Tectum

Located in the dorsal region of the mesencephalon (mid brain).
It consists of four nuclei that form four mounds on the dorsal surface, collectively called Corpor Quadrigemina, two superior colliculi and two inferior colliculi.
The tectum is responsible for auditory (inferior colliculi) and visual (superior colliculi) reflexes mediated by tectospinal tract to the contralateral cervical regions of the spinal cordr for postural movements of the head in response to visual and auditory stimuli.

Tegmentum

Located in the ventral region of the mesencephalon (mid brain).
It contains the reticular formation, the periaqueductal gray, the substantia nigra, and the red nucleus.
It is rich in dopamine and serotonin neurons.
The tegmentum is considered to be part of the pleasure system, or reward circuit, one of the major sources of incentive and behavioural motivation. It is also shown to process various types of emotion and security motivation, where it may also play a role in avoidance and fear-conditioning.

Vestibular Nuclei

Located at the border of the pons and medulla.
The functions of these nuclei are:

A. Maintain the body, neck and the head in an upright balanced position: This is achieved by excitation of antigravity muscles especially related to movements of the head through:

Uncrossed descending pathways known as the lateral vestibulospinal tracts,
Crossed medial vestibulospinal tracts and
Two-ways traffic with the cerebellum (reflex of equilibrium).

Lesions involving the vestibular nerve, nuclei, and descending pathways will result in problems such as stumbling or falling towards the side of the lesion.

B. Maintain the retinal image while the head is moving: The vestibular nucleus in association with cranial nerves III (oculomotor N), IV (troclear N.), VI (abducent N.), and XI (accessary N.), controlling the head, neck, and eye muscles forming vestibulo-ocular reflex. The vestibule-ocular reflex helps maintain fixation of the eyes (fovea of the retina) on an object of interest with movement of the head. The vestibule-ocular tract also controls saccadic eye movements (voluntarily turning both of our eyes horizontally to the left or right in order to see a new object of interest).

C. Conscious awareness of balance: The vestibule-cortical tract through the thalamus to the inferior parietal gyrus of cerebral cortex provides awareness of balance or not (dizziness).

The basal ganglia:

The Basal ganglia are a group of functionally related nuclei located bilaterally in the inferior cerebrum, diencephalon and midbrain.
The basal ganglia do not make any direct sensory or motor connections with the spinal cord, their contribution to the control of movement is made indirectly through the sensory and motor cortex (see figure) (see figure).

Physiologically, the basal ganglia composed of the following nuclei:

1. Caudate nucleus
2. Globus pallidus (Globus pallidus interna and Globus pallidus externa) (GP, GPi, GPe)
3. Putamen
4. Substantial nigra (pars reticulata and pars compacta) (SN, SNr, SNc)
5. Subthalamic nuclei (STN) (body of Luys)

These structures have high oxygen consumption. They suffer rapidly from hypoxia or ischemia. The copper content of the substantia nigra and the nearby locus ceruleus is particularly high.
There are two neuronal circuit in basal ganglia:

Direct Excitatory pathway (Stimulates motor cortex by decreasing the inhibition on the thalamus, thus Promotes Movement
Indirect Inhibitory pathway (Inhibits motor cortex) by increasing the inhibition on the thalamus, thus Inhibits Movement

Basal ganglia neurotransmitters are:

Dopamine: It selectively excites the direct pathway and inhibits the indirect pathway, thus promotes movement.
Acetylcholine: It inhibits the direct pathway and excites the indirect pathway, thus inhibits movement
GABA, is inhibitory in the neuronal circuit
Glutamate, is excitatory in the neuronal circuit

NOTES: Overactivity in the indirect pathway is a major factor in Parkinsonian signs, while underactivity in the indirect pathway is a major factor in hyperkinetic disorders such as hemiballism (violent involuntary throwing movements of the limb after a lesion in the contralateral subthalamic nucleus), Huntington disease (which is an autosomal dominant hyperkinetic disorder characterized by chorea, dementia and behavioral disturbance).

The main functions of basal ganglia are:

[1] Shared in control the learned complex pattern motor activity: Basal ganglia in association with the motor and sensory cortex and cerebellum responsible for planning, programming and timing of the learned complex pattern of motor activity such as writing a letters of alphabet, cutting paper with scissors, hammering nails, shooting basketball through a hoop, passing a football, throwing a baseball, the movements of shoveling dirt, when dressing, and virtually any other of our skilled movements. The basal ganglia in association with the cerebellum nuclei modify movement on a minute-to-minute basis.

[2] Control the instinctive cognition of the sequences of motor response: Some of the basic instinctive cognitions and their motor responses need to be executed without thinking for too long time, to respond quickly and appropriately. The basal ganglia plays major role in this instinctive cognitive control of the sequence of motor response. A good example of this would be for a person to see a lion approach and then respond instantaneously and automatically by [a] turning away from the lion, [b] beginning to run, and [c] even attempting to climb a tree. Without the cognitive functions, the person might not have the instinctive knowledge, without thinking for too long time, to respond quickly and appropriately.

[3] Control the speed of movement and the scale of movements: For instance, one may write a small “a” on a piece of paper or a large letter “a” on a chalkboard. Regardless of his choices, the proportional characteristics of the letter remain the same. Patients with severe lesions of the basal ganglia, these speed and scaling functions are poor or even not existent.

[4] Control the posture: The basal ganglia in association with other structures help to control the axial and girdle movements of the body (i.e. control of posture) to provide the background positioning of the body and proximal limbs so that the more discrete motor functions of the hands and feet can then be performed.

[5] Inhibit muscle tone throughout body: The feedback loops from the cortex through the basal ganglia and then back to the cortex make virtually all these loops negative feedback loops. This effect results inhibitory signals transmitted from the basal ganglia to cerebral cortex. Therefore, widespread destruction of the basal ganglia causes muscle rigidity throughout the body.

The cerebellum

It is especially vital to the control of very rapid muscular activities such as running, typing, playing the piano, and talking. Loss of this area of the brain can cause almost total incoordination of these activities even though its loss causes no paralysis of any muscle.

Cerebellum has wide interconnections with various parts of the NS structures and with the peripheral sensory receptors.

The main functions of cerebellum

1. Cerebellum is planning, programming and timing of sequential pattern of the motor activities and for the next movement at the same time that the present movement is occurring. This is achieved in association with motor, sensory cortex, and basal ganglia. In addition. In cerebellar dysfunction, this capability is seriously disturbed especially for rapid movements, which can lead to extreme incoordination and failure of progression of the purposeful movements of the hands, fingers, and feet, a condition called dysdiadochokinesia. In which jumbled movements occur instead of the normal coordinate movements. In addition, speech is affected, a condition called dysarthria.
2. Cerebellum monitors (assesses) the strength of the motor signals, compares it with the actual strength that reaches the anterior motor neuron and the rate of movement, and makes corrective adjustments in the motor activities elicited by other parts of the brain and stops the movement precisely at the intended point, thereby preventing the overshoot. Prevention of overshooting by the cerebellum is called the damping function of the cerebellum. In cerebellar dysfunction, overshooting does occur (the effect is called dysmetria or past pointing), the conscious centers of cerebral cortex recognize this and initiate a movement in the opposite direction to bring the arm to its intended position. But again the arm, because of its momentum, overshoots, and appropriate corrective signals must again be instituted by the cerebral cortex. Thus they are oscillates back and forth past its intended point for several cycles before it finally fixes on its mark. This effect is called an action or intention tremor.
3. Predictive function of cerebellum: The cerebellum functions with the spinal cord and brain stem to control postural and equilibrium during movements. This is achieved by predictive function of cerebellum. Cerebellum is especially important in controlling the balance between agonist and antagonist muscle contractions during rapid changes in body positions as dictated by the vestibular apparatuses. This is achieved by the predictive function (efference copy) of the cerebellum who analyzes the information dictated from peripheral sensory receptors (especially from the muscles, joints, and skin surface) and vestibular nuclei about the rate and direction of movement of each part of the body and compute these information to predict the position of these parts of the body within the next 15-20 msec and therefore, provide almost instantaneous correction of postural motor signals as necessary for maintaining equilibrium even during extremely rapid motion, including rapidly changing directions of motion. Besides movements of the body, cerebellum also plays a role in predicting other events. Cerebellar dysfunction causes extreme disturbance of equilibrium during performance of rapid motions than during stasis.
4. Cerebellar control of the muscle spindles: Cerebellum receives extreme amount of information from the muscle spindles via the dorsal spinocerebellar tracts. In turn, from cerebellum, signals are transmitted into the brain stem (pontine and medullary reticular formation) and motor cortex to stimulate the gamma efferent fibers that innervate the muscle spindles themselves. This pattern of arrangement forms a cerebellar stretch reflex or negative stretch reflex. When the muscle is already contracted, any sudden release of the load on the muscle that allows it to shorten will elicit reflex muscle inhibition rather than reflex excitation to oppose the shortening of the muscle in the same way that the positive stretch reflex opposes lengthening of the muscle.
Loss of the cerebellar component of the stretch reflex will result to an effect called rebound in which, if a person with cerebellar disease is asked to pull upward strongly on an arm while the physician holds it back at first and then lets go, the arm will fly back until it strikes the face instead of being automatically stopped. In normal state, the cerebellum instantaneously and powerfully sensitizes this stretch reflex mechanism whenever a portion of the body begins to move unexpectedly in an unwilled direction. In addition, hypotonia is occurred in cerebellar dysfunction which results from loss of facilitation of the motor cortex and brain stem nuclei by the tonic discharge of the deep cerebellar nuclei.
5. Control of ballistic movements: Many rapid movements of the body, such as the movements of the fingers in typing, the movements of the eyes when reading or when looking at successive points along a road when a person is moving in a car, where the eyes jump from one position to the next.

The prefrontal cortex (frontal association cortex):

These areas are located in the frontal lobe anterior to the motor regions. The functions of these areas are:

1. To control the types of behavior that should be followed for each social or physical situation.
2. These areas prevent distractibility (inability to concentrate) from a sequence of thoughts.
3. Elaboration of thought, i.e. an increase in depth and abstractness of the different thoughts. Elaboration of thought is important to:
A. Prognosticates.
B. Plan for the future.
C. Delay action in response to incoming sensory signals so that the sensory information can be weighed until the best course of response is decided.
D. Consider the consequences of motor action even before these are performed.
E. Solve complicated mathematical, legal, or philosophical problems.
F. Correlate all avenues of information in diagnosing rare disease.
G. Control one’s activities in accord with moral laws.

The thalamus

Ovoid mass of gray matter, situated bilaterally in diencephalon.
Thalamus receives afferents from cortex (thalamic radiation), subcortical structures, spinal cord. It sends efferents to whole cerebral cortex (thalamic radiation), hypothalamus, basal ganglia, and limbic system. Thalamic radiation contains both thalamocortical and corticothalamic fibers. All these fibers between thalamus and cerebral cortex pass through internal capsule
Functions of the thalamus:

1. Anatomical and functional gateway for cerebral cortex: Impulses of almost all the sensations (with the exception of the olfactory system) reach the thalamic nuclei. After being processed in the thalamus, i.e. i.e. integrated and modified, the impulses are carried to specific areas of cerebral cortex through thalamocortical fibers.
2. Center for determining quality of sensations: Thalamus is also the center for determining the quality of sensations, i.e. to recognize the type, location and other details of sensations, and also to determine whether a sensation is pleasant or unpleasant and agreeable or disagreeable. This is done in collaboration with the cerebral cortex, the limbic system and hypothalamus.
3. Role in arousal and alertness reactions: Because of its connections with nuclei of reticular formation, thalamus plays an important role in arousal and alertness reactions. It facilitates the cerebral cortex by raising its excitability up to the level necessary to do all cerebral functions. Without the thalamus, the cerebral cortical functions are markedly depressed.
4. Center for reflex activity: Since the sensory fibers relay here, thalamus forms the center for many reflex activities.
5. Center for integration of motor activity: Through the connections with cerebellum and basal ganglia, thalamus serves a gateway for these structures to the cerebral cortex.

The Limbic System (see figure) (see figure)

It is the entire basal system of the brain that mainly controls the person’s emotional behavior and drive (instinctual behavior) but also many higher mental functions, such as learning and formation of memories. .
The primary structures within the limbic system include the amygdala, hippocampus, thalamus, basal ganglia, and cingulate gyrus.
The hypothalamus is a major output pathway of the limbic system and has communicating pathways with all levels of this system.
Functions of the limbic system include:

1. The limbic system in humans is a center for basic emotional drives. There are few synaptic connections between the cerebral cortex and the structures of the limbic system especially amygdala (which is the emotion center of the brain), that perhaps helps to explain why we have so little conscious control over our emotions. The cingulate gyrus coordinates smells, sights, and pleasant memories, and induces an emotional reaction to pain, and helps regulate aggressive behavior. There is a closed circuit of information flow between the limbic system and the thalamus and hypothalamus called the Papez circuit. In the Papez circuit, a fiber tract, the fornix, connects the hippocampus to the mammillary bodies of the hypothalamus, which in turn project to the anterior nuclei of the thalamus. The thalamic nuclei, in turn, send fibers to the cingulate gyrus, which then completes the circuit by sending fibers to the hippocampus. Through these interconnections, the limbic system and the hypothalamus appear to cooperate in the neural basis of emotional states.
2. Linking the conscious intellectual functions of the cerebral cortex with the unconscious autonomic functions of the brain stem.
3. Facilitating memory storage and retrieval. The hippocampus plays an essential role in the formation of new memories and recall the old one.
4. Motivation: The sensory cortex, motor cortex, and association areas of the cerebral cortex enable you to perform complex tasks, but it is largely the limbic system that makes you want to do them. For this reason, the limbic system is also known as the motivational system.

Hypothalamus

The hypothalamus may be visualized as an integrating centre where several neural and endocrine influences originating throughout the brain converge. After due processing, the neural and endocrine outputs of the hypothalamus diverge to all parts of the body (see figure).

The hypothalamus is the part of the diencephalon which forms the floor and part of the lateral wall of the third ventricle. It is the size of an almond.
It is connected to the pituitary gland by the pituitary stalk (hypophysial stalk).
The hypothalamus is a major central component of the limbic system.
The main hypothalamic centers are:

Sympathetic center
Endocrinal center
Food intake center
Thermoregulation center
Water balance center
Salt appetite center
Biological clock center
Reward and punishment center
Fear, Rage, Placidity centers
Sexual behavior

[I] Autonomic function: The hypothalamus is considered as a "sympathetic center" which responds to emotions by generalized sympathetic activation (the alarm response).
[II] Endocrinal function: The hypothalamus controls the secretion of hormones from the anterior and the posterior pituitary glands. (see animation A & B), for animations, download the animation file and download and install this program to run the animation..
[III] Control of food intake: The hypothalamus contains a “food intake controlling system” which controls the appetite for food. Food intake controlling system is under the influence of limbic system especially amygdala. Food intake controlling system consists of two centers; an "appetite or feeding center" (tonically active) which stimulates the appetite, and a "satiety center" which inhibits the appetite center. The hypothalamic food intake controlling system is adjusted to a "set point" to maintain a specific body weight for each individual.The drive to eat is inﬂuenced by both short-term factors related to the daily pattern of meals and by long-term factors related to the energy stores in adipose tissue (see figure).

In the short term, hunger is induced by hypoglycemia and by the gastrointestinal peptide hormone ghrelin. After a meal is consumed, the sensation of satiety is mediated via the vagus nerve, due to distention of the stomach and by the release of the gastrointestinal hormone, cholecystokinin.
The major long-term regulator of eating is leptin, a polypeptide hormone released by adipocytes, which stimulates the satiety center and inhibit hunger center, thereby inhibiting eating. Plasma leptin concentration reﬂects the size of the total body fat store; there is a feedback loop in which high levels of body fat should increase leptin levels and decrease feeding. Patients who are obese are poorly responsive to leptin, which may contribute to the development and maintenance of overeating.

[IV] Thermoregulation: The hypothalamic thermostat consists of a "heat loss center" in the anterior "hypothalamus and a "heat gain center" in the posterior hypothalamus. Both centers are interconnected and work in a reciprocal manner.

Stimulation of the heat loss center leads to cutaneous vasodilation and profuse sweating. The heat loss center is sensitive to any change in blood temperature (± 0.02°C) which is an indicator of the body core temperature (through central thermoreceptors located in hypothalamus, spinal cord, abdominal organs and other internal location). It responds mainly to any increase in core temperature to prevent hyperthermia.

Stimulation of the heat gain center leads to cutaneous vasoconstriction, increased muscle tone, shivering and stimulation of catecholamine secretion. The heat gain center responds to input signals coming from cutaneous thermoreceptors which monitor the surface temperature (through skin peripheral thermoreceptors). It responds mainly to cooling of the skin to prevent hypothermia.

[V] Control of water balance: Hypothalamus regulates body water by creating the sensation of thirst through thirst center and conserve water. Thirst center is found in the lateral hypothalamus. It receives stimulatory input signals from different sources.
[VI] Control of salt appetite: There is a "salt appetite center" in the anterior hypothalamus very close to the osmoreceptors. Its cells are sensitive to changes in plasma osmolality, as well as the level of sodium in the plasma (osmosodium receptors). They are stimulated by hyponatremia, hypotonicity or hypovolemia to increase the appetite for salt (craving for salt).
[VII] Control of cyclical phenomena (biological clock) by suprachiasmatic nucleus: The pacemaker of the circadian rhythm is found in the suprachiasmatic nucleus of hypothalamus which plays an important role in setting the biological clock by its connection with retina via retino-hypothalamic fibers. Through the efferent fibers, it sends circadian signals to different hypothalamic nuclei, the pineal gland, and the reticular formation to maintain the circadian rhythm of sleep, hormonal secretion, thirst, hunger, appetite, etc.
[VIII] Role in learning and memory: The hypothalamus contains a reward center. When stimulated, it gives a sense of reward; i.e. relaxation, pleasure and satisfaction. In addition, there is a punishment center, when stimulated it gives a sense of punishment; displeasure fear and terror. Less potent reward and punishment centers are found in the amygdala, the hippocampus and other areas of the brain.
[IX] Control of motor responses to emotions: The motor responses to emotions are controlled by complex mechanisms that involve the association areas of the limbic system.

Fear is an unpleasant emotional state which involves a sense of insecurity because of impending danger or evil. It is produced by stimulation of the fear center in the hypothalamus. Amygdaloid nuclei in the temporal lobes activate the fear center in the hypothalamus. Reactions to fear include cowering, avoidance, and sweating, pupillary dilation, turning the head from side to side to seek escape and flee.

Rage is violent anger. It is produced by stimulation of a certain area of the lateral hypothalamus (rage area). This area is tonically inhibited by the placidity area of the hypothalamus, and the limbic system.

Placidity is produced by stimulation of the placidity area of the hypothalamus.

The amygdaloid nuclei facilitate the rage area and inhibit the placidity area (see figure).

[X] Sexual behavior: Libido and sexual activity is mainly under the control of the cerebral cortex and limbic system which are sensitive to sex hormones. In humans, the sexual functions have become extensively encephalized and conditioned by social and psychic factors. However, hormones play small role in the sexual behavior in humans (for example testosterone and estrogen increase libido, i.e. sexual interest and drive, in males). This is because of the greater degree of encephalization of sexual functions in humans.

The memory

It is the capacity of the brain to store what is learned, and subsequently recalls information.
memories are caused by the formation of memory traces. Memory traces are new pathways or facilitated pathways (i.e. synaptic plasticity).
There are four distinct types of memory (see figure):

Sensory memory (immediate memory). Retain sensory signals in the sensory areas of the brain for a very short interval of time (a sec or less) and it is due to presynaptic potentiation.
Short-term memory, (STM or, primary memory) (or working memory): It is the memory of facts, words, numbers, letters, or other information for a few sec to a few minutes at a time but lasting only as long as the person continues to think about the numbers or facts. For instance, one can memorize the digits of a telephone number for a short period of time after looking up the number in the telephone directory. The possible mechanisms for primary memory are:

Reverberating circuit theory.
Presynaptic facilitation or inhibition.

3. Intermediate-term memory. It lasts for many minutes or even weeks. They will eventually be lost unless the memory traces are activated enough to become more permanent. The possible mechanism for such memory is chemical changes in the presynaptic terminal or the postsynaptic membrane causing the memory paths to become facilitated for days or weeks thereafter.

4. Long-term memory, that lasts for years. The secondary long-term memory is the storage in the brain of highly overlearned information as one's own name and address. The possible mechanism for such memory is anatomical or physical changes in the synapses.
The information transformation from the intermediate-term memory to long-term memory is enhanced by the following factors:

Excitation, stimulation of the reward or punishment systems.
Repeated practice or rehearsal of the experience
Association of old and new data.

This memory is difficult to disrupt, and it is seldom affected in retrograde amnesia.

This is because information in the long-term memory occupies large areas of the brain and more than one "copy" of the information is stored in different regions of the brain. The access to the long-term memory is very rapid, e.g. one immediately remembers his name if he is asked about it.

Stages in the formation of memory (see figure):

Encoding: The sensory information, visual (picture), acoustic (sound), and semantic (meaning) are changed (encoded) into different formats to be able to store.
Codification: Save the information into a direct association with other memories of the same type of information
Storage or consolidation: Creation of a permanent record of the encoded information.

Language and Speech

It is important to note that the left hemisphere is usually dominant with respect to language, even in left-handed people (see figure).
Perception of spoken words is the function of the primary auditory sensory area in the temporal lobe at the floor of the lateral sulcus. Signals are then conveyed to the adjacent auditory sensory association area. This area understands the meaning of the heard words. It feeds the message of the heard words to the Wernicke’s area which correlates them with other related items stored in memory in the process of thinking.
Written words are perceived by the primary visual sensory area in the occipital lobe. The signals are then conveyed to the visual sensory association area which understands the meaning of words. The message is then fed to the Wernicke’s area.
Wernicke's area is found in the posterior part of the superior temporal gyrus. It receives input signals from the sensory association areas. It is the memory store tor language. It decides what words are suitable and in what sequence to express a certain idea.
It is connected with Broca's area (word formation center) in the premotor cortex via the "arcuate fasciculus.
Broca’s area is in the prefrontal association cortex. It stores the motor programs for different words. When it is activated, it stimulates the motor cortex at certain pattern to produce words by coordinated contractions of the respiratory, laryngeal, pharyngeal, lingual and labial muscles.

Clinical evidence indicates that Wernicke’s area is essential for the comprehension, recognition, and construction of words and language, whereas Broca’s area is essential for the mechanical production of speech. Patients with a defect in Broca’s area show evidence of comprehending a spoken or written word but they are not able to say the word. In contrast, patients with damage in Wernicke’s area can produce speech, but the words they put together have little meaning.

Brain waves (see figure)

1. Alpha waves (α): They occur at frequency between 8-13/sec and their voltage usually is about 50 microvolts and are found in EEGs of almost all normal adult persons when they are awake in a quiet resting state of cerebration with closed eyes and occur most intensely in the occipital region but also be recorded at times from the parietal and frontal regions of the scalp. It is assumed that the alpha waves result from spontaneous activity of the thalamocortical system and possibly including the RAS pathways.
2. Beta waves (β): They occur at frequency of more than 14-25 cycles/sec and rarely 50 cycles/sec. These are most frequently recorded from the parietal and frontal regions of the scalp. Most beta waves appear during alert wakefulness or open the eyes in bright light.
3. Theta waves (τ): They have frequency of between 4-7 cycles/sec. These occur mainly in the parietal and temporal regions in children, but they also occur during emotional stress in some adults, particularly during disappointment and frustration. These same waves also occur in many brain disorders.
4. Delta waves (δ): They include all the waves below 3.5 cycles/sec. These occur in infancy, in deep sleep (slow wave sleep), and in serious organic brain disease. These waves can occur strictly in cortex independent of activities in lower regions of the brain.

Sleep

During each night a person goes through stages of two different types of sleep (see figure) that alternate with each other. These are called:

Slow wave sleep
Rapid eye movement sleep (REM, desynchronized sleep or paradoxical sleep)

Persons who are deprived of sleep become irritable, fatigued, disoriented and unable to concentrate. Personality disorders such a paranoid thoughts, auditory and visual illusions or hallucinations may be encountered in persons who are deprived of sleep. Deprivation of REM sleep may lead to anxiety disorders. Prolonged wakefulness or complete deprivation from either REM or slow-wave sleep is often associated with weight loss in spite of increased caloric intake and progressive malfunction of the mind and also causes abnormal behavioral activities of the NS and eventual death. Therefore, sleep in some way not clear yet restores both normal sensitivities of and normal balance among the different parts of the CNS.
During infancy and childhood, the reduction in sleep time from 16 hours to 10 hours occurs almost entirely by a reduction of the amount of time spent in REM sleep.
In adulthood, the sleepimg time is about 7 hours, the reduction in sleep time is caused by a reduction in the time spent in the sleep stages of slow-wave sleep.
Elderly individual spends less than 6 hours of each day sleeping.Phase 4 sleep declines gradually, and may disappear in the elderly causing their sleep to be light and interrupted. This may force such individuals to take afternoon naps to compensate for lost sleep.

Mechanism of sleep
1. The metabolic theory: Sleep is due to accumulation of sleep-inducing factor (factor-S) and Serotonin.

3. The passive theory (sleep centers): Due to inactivation of ascending reticular activating system.

2. The active theory: There are specific centers which induce slow wave sleep, others which induce REM sleep.

Cerebrospinal fluid (CSF) system

It has a volume of about 150 ml and found in the ventricles of the brain, in the cisterns around the brain, and in the subarachnoid space around both the brain and the spinal cord.
CSF is formed at a rate of about 500 ml / day. One thirds of this fluid originates as a ultrafiltration of blood plasma through choroidal capillaries in the four ventricles, mainly in the two lateral ventricles. Two third or more is secreted by choroid plexus and ependymal cells which actively transport sodium, chloride and bicarbonate ions into the ventricles and water follows the resulting osmotic gradient. CSF circulation is shown in figure.
CSF eventually drains back into the venous system at specialized areas of arachnoid membrane called arachnoid granulations. CSF is drained into arachnoid villi from where it enters subdural sinuses. The reabsorption shows passive diffusion caused by hydrostatic pressure difference. The villi function like valves that allow the fluid and its contents to flow readily into the blood of the venous sinuses while not allowing blood to flow backward in the opposite direction. Normally, this valve action of the villi allows CSF to begin to flow into the blood when its pressure is about 1.5 mm Hg greater than pressure of the blood in the venous sinuses. Then as the CSF pressure rises still higher, the valves open widely, so that under normal conditions, the pressure almost never rises more than a few mm of Hg higher than pressure in the venous sinuses. The rate of production of CSF by the choroid plexus must be the same as the rate of CSF absorption at arachnoid granulations.
CSF pressure when measured when one is lying in a horizontal position from lumbar spinal segments (between L3 and L4), ranges from 70 to 150 mm of CSF.
CSF is an isotonic fluid. CSF is similar to brain interstitial fluid. This is because the free communication between the brain interstitial fluid and CSF. Has very little protein, greater Na, Cl, and Mg ion concentrations than in plasma, lower K, Ca ions, HCO3-and glucose (30% less) concentrations than in plasma.

Blood-brain barrier (BBB)

The blood-brain barrier protects interstitial ﬂuid surrounding neurons from changes in the plasma composition. It also prevents the escape of neurotransmitters from their functional sites in the CNS into the general circulation.
There are two important factors which facilitate development of blood brain barrier. They are: [1] Presence of tight junctions between the endothelial cells of brain capillaries. [2] The end feet of astrocytes ending on the brain capillaries also contribute to the barrier (see figure).
Cerebral capillaries at birth lack blood brain barrier. In adults, the blood brain barrier is fully developed and hence only lipid soluble molecules and respiratory gases (CO₂, O₂) can pass through brain capillaries.Glucose is an important exception and is a crucial substrate for neuronal tissue. Glucose enters the brain by facilitated diffusion via carriers present in capillary endothelial cell membranes.
Large molecules do not pass through the BBB easily such as plasma proteins, cholesterol.
Non lipid soluble molecules do not penetrate into the brain.
Molecules that have a high electrical charge to them are slowed.

The BBB can be broken down by:

Hypertension (high blood pressure)
Development: the BBB is not fully formed at birth.
Hyperosmolarity
Microwaves
Radiation
Infection
Trauma, Ischemia, Inflammation, Pressure

Cerebral blood flow (CBF)

In an adult, CBF is typically 750 millilitres per minute or 15% of the cardiac output. This equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute.
As in the coronary circulation, CBF is autoregulated, meaning that it remains constant between a blood pressure of 50-150 mm Hg. Within the physiological range, autoregulation protects the brain against hypoxic damage with a reduction in cerebral perfusion pressure, and against hyperaemia, capillary damage and cerebral edema with increased perfusion pressure.
Arterial O₂ and CO₂ concentrations help in determining regional CBF. Unlike the coronary circulation, cerebral resistance vessels are more sensitive to PCO₂ than PO₂. Therefore, even slight increase in PCO₂ can cause a large increase in CBF. The response to arterial carbon dioxide tension is mediated by the hydrogen ion concentration in the extracellular fluid of the vascular smooth muscle.