Department of Physiology: ANS Physiology-Review & Illustrations

Comparison of autonomic and somatic NS.

Anatomical organization of SNS (figure) & (figure)

The preganglionic neurons have their cell bodies in the intermediolateral gray column of spinal cord segments (T1-L2) and their axons pass through the anterior nerve roots into the corresponding spinal nerves.

The course of the preganglionic fibers can be one of the following three:

They can synapse with postganglionic neurons in the paravertebral sympathetic ganglion that they enter.
They can pass up or down the chain to end in paravertebral ganglia at higher or lower levels than the point of entrance.
They can pass through the sympathetic chain without synapsing, then through one of the sympathetic nerves radiating outward from the chain and finally terminating in one of the prevertebral (collateral) ganglia such as the celiac ganglion.

Postganglionic neurons originate either in one of the sympathetic chain ganglia or in a prevertebral ganglion travel to the various organs. Some of the postganglionic fibers pass back from sympathetic chain through gray rami communicantes into the spinal nerves at all levels of spinal cord

Anatomic organization of PNS

PNS has a cranial outflow and a sacral one (figure)
Cranial outflow the preganglionic fibers leave the CNS through cranial nerves III, VII, IX, and X.
Sacral outflow the preganglionic neurons originate in the intermediate gray column of spinal cord segments S2 and S3 and occasionally S1 and S4.
The preganglionic fibers in both outflows end on short postganglionic neurons in peripheral ganglia located near or on the visceral structures.

Chemical transmission at autonomic junctions (figure)

Synthesis of ACh and catecholamines (figure)

ACh Receptors (Cholinergic Receptors):

ACh activates two types of receptors:

Muscarinic receptors: found in all effector cells stimulated by the postganglionic neurons of the PNS and in those stimulated by the postganglionic cholinergic neurons of the SNS (sweat dland).
Nicotinic receptors: found in the synapses between the preganglionic and postganglionic neurons of both the SNS and PNS. These receptors are also present at many non-autonomic nerve endings, for instance, in the membranes of skeletal muscle fibers at the neuromuscular junction.

Both muscarinic and nicotinic ACh receptors are also found in large numbers in the brain.

Adrenergic Receptors:

There are two major types of adrenergic receptors:

Alpha receptors; divided into α₁ and α₂
Beta receptors; divided into β₁, β₂ and β₃

NE → excites mainly α receptors, but excites the β receptors to a less extent as well.

E → excites both types of receptors approximately equally.

The distribution and the effects of adrenergic receptors is shown in figure. To make it easily remember the following figure is a simplified version of alpha and beta receptor effects.

Adrenaline:

It increases systolic pressure, by increasing the force of contraction of heart and cardiac output via β1 Receptors.
It decreases diastolic pressure by reducing the total peripheral resistancevia β2 Receptors stimulation on the skeletal blood vessels.
It causes constriction of blood vessels.

Noradrenaline:

It increases diastolic pressure due to its general vasoconstrictor effect via alpha receptor.
It also increases the systolic pressure slightly.
It has strong effect on blood vessels than on the heart

SNS versus PNS effects on the effectors (figure)

Mass sympathetic discharge (figure):

In emergency or stressful situation whether “emotional stress” or “physical stress” (fear or fright, severe pain, exercise, bleeding, etc.), the hypothalamus is stimulated by signals passing via afferents from sense organs and cerebral cortex (paths unknown), and signals are transmitted from hypothalamus downward through the reticular formation of brainstem and into the spinal cord to cause massive sympathetic discharge, i.e. the SNS discharges as one unit (mass discharge), to prepare the individual to cope with the emergency. These changes increase the physical fitness to prepare the individual for “fight” or “flight”.