Special Senses Physiology-Review & Illustrations
Anatomical Review (figure)
Intraocular Fluids (IOF) (figure)
- Aqueous humor is produced by the ciliary body & flows from the posterior chamber through the pupil to fill the anterior chamber of the eye.
- It is normally reabsorbed into the canal of Schlemm, a venous channel at the junction of the cornea & iris.
- The normal average intraocular pressure (IOP) is about 16 mm Hg with a range of 12-20 mm Hg.
The Eye & Light Refraction
- The light is refracted at the anterior surface of cornea, & at the anterior & posterior surfaces of the lens.
- Most of the RP of the eye is provided by the anterior surface of the cornea.
- The importance of the lens is its ability to increase its RP by increasing its curvature.
Accommodation
- The process by which the curvature of the lens is changed to enable the eye to maintain a focus on the retina as the distance from the eye to the object changes.
- The curvature of the lens is regulated by tension of the lens ligaments which is regulated by the ciliary body. When an object is 6 meters or more from the eye, the ciliary muscle is relaxed & tension is greatest in the lens ligaments & because the lens capsule has a considerable elasticity & the lens substance is malleable, the lens is pulled into a flattened shape (figure A). As the object moves closer to the eye, the ciliary muscle contracts & causes relaxation of lens ligaments (figure B). When the lens ligaments are relaxed the lens becomes more convex in shape & its RP increases.
- The ciliary muscle is controlled almost entirely by parasympathetic nerve signals transmitted to the eye from the third cranial nerve nucleus (oculomotor nerve) in the brain stem. Stimulation of parasympathetic nerves contracts the ciliary muscle which relaxes the lens ligaments & increases the RP of the lens & the eye will be capable of focusing on nearer objects.
The near point of vision
- The nearest point to the eye at which an object can be brought into clear focus by accommodation.
- The near point recedes (moves away from the eye) throughout life (from approximately 9 cm at age 10 to approximately 83 cm at age 60).
- At the age of 40-45, the loss of accommodation is usually sufficient to make reading & close work difficult. This condition is known as presbyopia & can be corrected by convex lens glasses.
Errors
of Refraction (see figure)
Hyperopia or farsightedness: This is usually due to short anteroposterior diameter of an eyeball and & parallel rays of light are brought to a focus behind the retina. To overcome this abnormality the ciliary muscle must contract to increase the strength of the lens. The prolonged muscular effort caused by sustained accommodation is tiring & may cause headaches & blurring of vision. This defect can be corrected by convex lens glasses.
Myopia or nearsightedness: In myopia, when the ciliary muscle is completely relaxed, the parallel rays of light from distant objects are focused in front of the retina. It is usually due to an eyeball with too long anteroposterior diameter. No mechanism exists by which the eye can decrease the RP of its lens (strength of the lens) to less than that which exists when the ciliary muscle is completely relaxed. So myopics have no mechanism by which they can ever focus distant objects sharply on the retina. However, as an object comes nearer to the eye it finally comes near enough that its image will focus on the retina. Then when the object comes still nearer to the eye, the person can use his mechanism of accommodation to keep the image focused clearly. Myopia is said to be genetic in origin. In young adults, the extensive close work involved in activities such as studying accelerates the development of myopia. Myopia can be corrected by concave lens glasses.
Astigmatism: This is a condition in which the curvature of the cornea is not uniform i.e. the degree of curvature in one of its planes is different. So the light rays passing through the different planes do not all come to a common focal point. Astigmatism may also be produced if the curvature of the lens is not uniform, but this condition is rare. It is corrected by cylindrical lenses.
Retina (figure)
- The retina contains visual receptors & 4 types of neurons: bipolar cells, ganglion cells, horizontal cells & amacrine cells.
- The visual receptors or photoreceptors → The rods & the cones → They synapse with bipolar cells & horizontal cells
- The bipolar cells transmit signals from rods & cones & horizontal cells to ganglion cells & amacrine cells
- The horizontal cells transmit signals horizontally in between the receptor cells & between the receptor cells & the bipolar cells
- The amacrine cells transmit signals in 2 directions; either directly from bipolar cells to ganglion cells, or horizontally in between the bipolar cells, the ganglion cells, &/or other amacrine cells
- light must pass first through the ganglion cells & the other layers of the retina to reach the layer of rods & cones located on the outer side of the retina, this affects the visual acuity.
- The pigment layer of retina absorbs light rays, preventing the reflection of light rays back through the retina.
- There are no visual receptors overlying the optic disc so this spot is blind (the blind spot).
- At the posterior pole of the eye there is a minute area in the retina, the macula lutea, which occupies a total area of less than 1 mm2. The central portion of the macula is the fovea centralis, a depression in the retina of 0.4 mm in diameter. The fovea centralis is the point where visual acuity is greatest. When attention is fixed on an object the eyes are normally moved so that light rays coming from the object fall on the fovea
Rods & Cones
- Fovea contains only cones which are densely packed
- No convergence of cones on ganglion cells in the fovea
- The fovea has a big cortical representation
- In the fovea the blood vessels & the layers of neurons are pushed aside
Photochemistry of Vision
- The photosensitive compounds (pigments) located in outer segments of cones and rods.
- In rods the photosensitive compounds is called Rod pigment (rhodopsin → scotopsin + retinal) → has peak sensitivity to light at a wavelength of 505 nanometers (green-senstive pigment).
- In the cones the photosensitive compounds are called: cone pigments (photopsins + retinal) → has peak sensitivity to light at a wavelength of 445 nanometers (blue-sensitive pigment), 535 nanometers (green-sensitive pigment), and 570 nanometers (red-sensitive pigment).
Excitation of Rods & cones:
- In resting dark conditions (figure) (figure, left image)
- Bleaching: A process by which the entire rhodopsin molecule (light-sensitive pigment in the outer segment of photoreceptor, = 11-cis retinal + Opsin) is broken down into all-trans retinal and opsin due to absorption of photons (see figure).
- Metarhodopsin II (activated rhodopsin)
activates transducin (a type of G protein).
- Transducin activates phosphodiesterase (PDE).
- Phosphodiesterase → breaks down cGMP → cGMP levels decline (figure, right image) → gated sodium channels close → the rate of Na+ entry into the cytoplasm then decreases → the membrane potential drops from -40 mV toward -70 mV (hyperpolarization) → ↓inhibitory neurotransmitter (glutamate) release from photoreceptor → less generation of IPSP at the bipolar cell surface → more excitation of bipolar cells → more generation of EPSP at the surface of ganglion cells → propagation action potentials along the optic nerve as an .
The all-trans retinal is converted to its original shape (11-cis isoform). This conversion requires energy in the form of ATP. Once the retinal has been converted, it can recombine with scotopsin. The rhodopsin molecule is now ready to repeat the cycle.
The mechanism of dark & light adaptation
- Dark adaptation: Progressive increase in the sensitivity of retina to
light (decrease in light threshold) after spending a considerable period of
time in brightly lighted surroundings and then moves to a dimly lighted
environment. It is nearly maximal in about 20 minutes. The increase in sensitivity after spending time in darkness environment
is because the retinal & opsins in rods & cones are converted back into
the light-sensitive pigments, & vitamin A is reconverted back into retinal
to give still additional light-sensitive pigments.
- Light adaptation: Progressive decrease in the sensitivity of retina to light (increase in light threshold) after spending a considerable period of time in dimly lighted surroundings and then moves to a brightly lighted environment. It is nearly maximal in about 5 minutes. The decrease in sensitivity after spending time in brightly lighted environment is because large amounts of the photochemical in rods & cones have been reduced to retinal & opsins & much of retinal has been converted into vitamin A. So the concentrations of the photochemical are considerably reduced & consequently the sensitivity of the eye to light is reduced (high visual threshold).
- Pupillary light reflex: Is a change in pupillary size. This can
cause adaptation of about 30-fold within a fraction of a second, because
of changes in the amount of light allowed through the pupillary opening.
Visual Pathways & Visual defects (figure)
Pupillary light reflexes
- Central nervous system (direct & indirect) arrangement (figure).
- The function of the light reflex is to help the eye adapt extremely rapidly to changing light conditions, as was explained earlier.
- The afferent and the efferent limbs of the direct and indirect pupillary reflex as follow:
The efferent limb of the reflex is (oculomotor nerve): Pretectal nucleus → parasympathetic nucleus of the oculomotor nerve (Edinger-Westphal nucleus of the oculomotor) ipsilaterally (DIRECT) and contralaterally (INDIRECT) → preganglionic fibers of oculomotor N. → ciliary ganglion → postganglionic fibers in ciliary nerve → constrictor pupillae.
Accommodation Reflex (The Near Response)
- When looking at a near object or when the object is moving towards the eyes; three reflex responses occur: Pupillary constriction, Accommodation, Convergence
- The neuronal pathways (see figure). The afferent limb of the reflex is as follow: Optic nerve → optic tract → LGB → optic radiation → visual occipital cortex with final integration in the visual association areas. The efferent limb of the reflex is as follow: Occipital cortex → superior colliculus → the oculomotor nerve nucleus & the Edinger-Westphal nucleus of the oculomotor nerve supplying the sphincter muscle of the iris, the medial rectus muscle & the ciliary muscle.
Hearing
Anatomy of the external ear, middle ear, and inner ear (figure)- The external ear funnels sound waves to the external auditory meatus.
- From the meatus, the external auditory canal passes inward to the tympanic membrane (eardrum).
- The middle ear is an air-filled cavity, in the temporal bone, that opens via the auditory (eustachian) tube into the nasopharynx and through the nasopharynx to the exterior.
- The auditory tube is usually closed, but during swallowing, chewing, and yawning it opens, ventilating the middle ear and keeping the air pressure on the two sides of the eardrum equalized.
- In the middle ear, there are three auditory ossicles; the malleus, incus, and stapes.
- The malleus is attached to the center of the tympanic membrane, and at its other end, the malleus is bound to the incus by ligaments.
- On the other end, the incus in turn articulates with the head of the stapes. The foot plate of the stapes is attached by a ligament to the walls of the oval window.
- Two small skeletal muscles, the tensor tympani, and the stapedius are also located in the middle ear (figure).
- In order to open the auditory tube, the
tensor tympani muscle (innervated by mandibular branch of trigeminal N),
attaching to the auditory tube and the malleus, must contract. This occurs
during swallowing, yawning, and sneezing.
- Stapedius muscle (innervated by facial N) is protective; this protection is achieved by the action of the stapedius muscle, which attaches to the neck of the stapes. When sound becomes too loud, the stapedius muscle contracts and dampens the movements of the stapes against the oval window. This action helps to prevent nerve damage within the cochlea.
- Inner ear is made up of two parts, one within the other. The bony labyrinth is a series of channels in the petrous part of the temporal bone. Inside these channels is a fluid called perilymph (is almost identical with cerebrospinal fluid). The membranous labyrinth, is more or less duplicates the shape of the bony channels and is filled with a fluid, called endolymph (contains a high concentration of potassium and a low concentration of sodium, which is exactly opposite to the contents of perilymph) . There is no communication between the spaces filled with endolymph and those filled with perilymph.
- Labyrinth is composed of cochlea, which is concerned with hearing, and the vestibular apparatus, which is concerned with equilibrium.
- Cross-section of the spiral canal of cochlea & Organ of Corti (figure)
- The cochlear portion of the labyrinth is a coiled tube divided throughout its length by the Reissner’s membrane and the basilar membrane into three chambers (scalae). The upper, scala vestibuli, and the lower, scala tympani (both contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema, and communicate directly with the subarachnoid space around the brain), and in the middle is the scala media (does not communicate with the other two scalae and it contains endolymph).
- Transmission of Sound Waves in the Cochlea (figure): The sound waves → the tympanic membrane sets into motion → auditory ossicles set into motion→ movements of the foot plate of stapes at the oval window → set up traveling waves in the perilymph of the scala vestibule (figure) → movement of Reissner’s membrane → displacement of the basilar membrane → bending of the hairs cell of organ of corti (figure) → depolarization of the hair cells (figure)→ synaptic transmitter is released that depolarizes the afferent neurons in contact with the hair cells. Conversely displacement of the hairs in the opposite direction hyperpolarizes the cells, and less transmitter is released.
- The
Central Auditory Pathways (see figure). Many collateral fibers from the auditory tracts pass directly into the reticular activating system
of the brain stem. This system projects diffusely upward into the
cerebral cortex and downward into the spinal cord and activates the
entire nervous system in response to a loud sound. Other collaterals go
to the vermis of the cerebellum, which is also activated in the event of a sudden noise
- Cochlear tuning: It is tuning our cochlea to receive some frequencies better than others. The is achieved by the outer hair cells (OHCs) and by inner hair cells (IHCs). The two mechanisms are as follow: [A] Sound → stimulates OHCs → nerve signals to the superior olivary nucleus by way of the sensory neurons → the superior olivary nucleus sends signals immediately to the OHCs by way of the motor neurons → the outer hair cells contract by about 10% to 15% → reduces the basilar membrane’s freedom to vibrate → some regions of the organ of Corti sending fewer signals to the brain than neighboring regions → the brain can better distinguish between the more active and less active hair cells and sound frequencies. [B] The other mechanism of cochlear tuning involves superior olivary nucleus which sends efferent fibers to the cochlea that synapse with the sensory nerve fibers near the base of the IHCs. The efferent fibers can inhibit the sensory fibers from firing in some areas of the cochlea, and thus enhance the contrast between signals from the more responsive and less responsive regions.
- Determination of Sound Frequency: Low frequency sounds cause maximal activation of the basilar membrane near the apex of cochlea, sounds of high frequency activate the basilar membrane near the base of the cochlea, and intermediate frequencies activate the membrane at intermediate distances between these two extremes (figure). Cochlear nerve fibers from each respective area of the basilar membrane terminate in a corresponding area of the cochlear nuclei in brain stem and this organization continues all the way up to the cerebral cortex. Therefore, the major method used by the nervous system to detect different sound frequencies is to determine the position along the basilar membrane that is most stimulated. This is called the “place principle” for determination of sound frequency.
- Determination of sound Loudness: Loudness is determined by the auditory system in the following ways: [A] By the frequency of action potentials in single auditory nerve fibers: As the sound becomes louder, the amplitude of vibration of the basilar membrane and hair cells also increases so that the hair cells excite the nerve endings at more rapid rates. [B] By the total number of receptors (hair cells) stimulated: As the amplitude of vibration of the basilar membrane increases, it causes more of the hair cells to become stimulated, so that the impulses are transmitted through many nerve fibers.
- Sound Localization: Determination of the direction from which a sound comes in the horizontal plane depends upon: [A] Detecting the difference in time between the arrival of the sound stimulus in the two ears. [B] It also depends upon the fact that the sound is louder on the side closest to the source. The two mechanisms above cannot tell whether the sound is coming from in front of the person or behind or from above or below. This discrimination is achieved mainly by the pinnae of the two ears. Sounds coming from directly in front of the individual differ in quality from those coming from behind, because each pinna is turned slightly forward. In addition, reflections of the sound waves from the pinnal surface change as sounds move up or down. Thus, the pinna plays an important role in locating sounds in the vertical plane.
- Vestibular system: Give information about the position and movement of the head in space, maintains equilibrium, coordinate eye, head, and body movements, and permits the eyes to remain fixed on a point in space as the head moves . This is achieved by mechanoreceptors within the vestibular Apparatus (figure). These mechanoreceptors are located within three structures: utricle, saccule (located on the inside surface of each utricle and saccule is a small sensory area called a macula), three semicircular canals (contain the sensory areas called crista located within the three ampullae of the three semicircular canals). Over the surface of the sensory areas there are receptors cells called hair cells. The maculas in the utricle and saccule detect linear acceleration and the static position of the head with respect to gravity. The semicircular canals detect rotational acceleration. The basic principal of stimulation of hair cells in all these receptors is bending of the cilia of hair cell due to movements of head.
- Responses to Linear Acceleration: When the head is accelerated or tilted linearly in any direction → the otoliths are displaced → bending of hairs and generating activity in the nerve fibers. In each macula, the different hair cells are oriented in different directions so that at different positions of the head, different hair cells become stimulated, i.e. some of them are stimulated when the head bends forward, some when it bends backward, others when it bends to one side, and so forth. The patterns of stimulation of the different hair cells inform the nervous system about the position of the head with respect to the pull of gravity. The maculas also discharge tonically in the absence of head movement, because of the pull of gravity on the otoliths.
- Responses to Rotational Acceleration: When the head begins to rotate in any direction → the endolymph in the semicircular canals remains stationary, because of its inertia, while the semicircular canals turn → The endolymph is displaced in a direction opposite to the direction of rotation → The fluid pushes the cupula, deforming it and thereby bending the processes of the hair cells and changing the impulse rates → With continued rotation, the inertia of the endolymph is overcome, and the endolymph moves with the semicircular canal at the same rate → The cupula returns to its resting position, and impulse rates return to the normal tonic level → When rotation stops, changes that are opposite to the initial ones occur → The endolymph, due to its momentum, continues to move in the direction of rotation, after the canal has come to rest → This bends the cupula and therefore the hairs in the opposite direction to that at the beginning of movement → The impulse rate is appropriately altered → Finally the endolymph stops moving and the cupulae regain their resting position and the sense of movement ceases.
- Central vestibular pathway (figure)
SMELL (olfaction)
- The olfactory receptors, which are chemoreceptors, are located in a specialized portion of the nasal mucosa, the olfactory mucous membrane (or olfactory epithelium).
- The olfactory receptors (located on the cilia of the olfactory receptor cells) respond only to substances that are in contact with the olfactory epithelium and are dissolved in the thin layer of mucus that covers it. Binding of an odor producing molecule to the receptor causes activation of G proteins, which in turn activate adenylate cyclase. There is an increase in intracellular cAMP that opens Na+ channels in the olfactory receptor membrane and produces a depolarizing receptor potential.
- Volatile substances that at least slightly water soluble, and at least slightly lipid soluble can be smelled.
- Discrimination of differences in the intensity of any given odor is poor.
- When one is exposed continuously to an odor, the perception of the odor decreases and eventually ceases due to the fairly rapid adaptation. It is specific for the particular odor being smelled, and the threshold for other odors is unchanged. It has been postulated that most of the adaptation occurs in the central nervous system.
- Central
olfactory pathways (figure)
TASTE
- Taste buds (figure) are the sense organs for
taste. They contain taste cells (figure), “the gustatory receptors”.
- The taste buds are located in the mucosa of the epiglottis, palate, and pharynx and on three types of papillae of the tongue; the circumvallate papillae, which form a V line on the posterior surface of the tongue, the fungiform papillae over the flat anterior surface of the tongue, and the foliate papillae located on the lateral and posterior surfaces of the tongue (figure).
- Central taste Pathways (figure). The mechanism by which most stimulating substances react with the taste villi to initiate the receptor potential is by binding of the taste chemicals to protein receptor molecules that protrude through the villus membrane. This in turn opens ion channels, which allow sodium ions to enter and depolarize the cell. Then the taste chemical is gradually washed away from the taste villus by the saliva, which removes the stimulus. The type of receptor protein in each taste villus determines the type of taste that will elicit responses.
- The ability of humans to discriminate differences in the intensity of tastes, like intensity discrimination in olfaction is relatively crude.
- Taste sensations adapt rapidly, it is almost certain that most of the adaptation occurs in the central nervous system.