ORGANIZATION OF AUTONOMIC NERVOUS SYSTEM - PowerPoint Presentation

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ORGANIZATION OF

AUTONOMIC NERVOUS SYSTEM

THE AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system (ANS) is a motor or efferent system which connects the central nervous system with all smooth muscle, cardiac muscle and gland cells of the body.

This system helps to control visceral functions such as arterial blood pressure, gastrointestinal motility and secretion, urinary bladder emptying, sweating, body temperature, heart rate and several other activities.

Some of these functions are controlled almost entirely by the ANS and some only partially.

AUTONOMIC NERVOUS SYSTEM CONT’D

It is a two-neuron pathway, so that there is a ganglion between the CNS and the effector.

The ratio of preganglionic to postganglionic neurons is not one to one.

There are more postganglionic than preganglionic neurons.

Preganglionic neurons are

myelinated

whereas, postganglionic ones are

unmyelinated

.

The ANS is controlled by various parts of the brain, mainly the hypothalamus, but also the prefrontal cortex, the limbic lobe, the cerebellum and the reticular formation. The ANS is subdivided into two divisions, the parasympathetic (

craniosacral

outflow) and the sympathetic (thoracolumbar outflow).

Functional anatomy of the parasympathetic nervous system

The

craniosacral

outflow of the autonomic nervous system is associated with 4 of the 12 cranial nerves and with three segments of the sacral portion of the spinal cord region.

Cranial nerves transmitting preganglionic parasympathetic fibres are cranial nerves III, VII, IX, and X.

The nuclei ( collections of preganglionic nerve cell bodies within the central nervous system) associated with these cranial nerves are the

Edinger-Westphal

nucleus

for the third cranial nerve;

superior salivary nucleus

for the seventh;

inferior salivary nucleus

for the ninth and the

dorsal motor nucleus

of the

vagus

for the tenth.

The only points of emergence of parasympathetic nerves from the spinal cord are in the pelvic splanchnic nerves arising from segments S2, S3, and S4.

The cell bodies of the preganglionic neurons are found in the lateral columns of the grey matter in this part of the spinal cord.

Parasympathetic ganglia between the CNS and the effector (

i.e

cell bodies of the postganglionic neurons) are generally located near to or lie within innervated organ.

In the cranial region of the system, there are four ganglia, the

ciliary

,

pterygopalatine

, submandibular and

otic

, forming pathways to the eyes,, lacrimal glands, salivary glands and the oral and nasal mucosae.

The

vagus

nerves have a wide distribution including the heart, respiratory tract and gastrointestinal tract as far as the proximal colon.

T

he ganglia are located within the organs themselves e.g. the intramural plexuses in the gastrointestinal tract. The distal colon, rectum, kidney, bladder and sexual organs are supplied by the pelvic splanchnic nerves.

Because of the position of the ganglia, it can be seen that the parasympathetic preganglionic neurons are relatively long compared with the postganglionic neurons.

Functional anatomy of the sympathetic nervous system

Preganglionic cell bodies of the sympathetic or thoracolumbar outflow of the autonomic nervous system are located only in 14 spinal cord segments between T1 and L2.

In these spinal cord segments, the preganglionic cell bodies are present midway between the dorsal and ventral horn cells in a column of cells called the

intermediolateral

cell column.

Sympathetic ganglia are arranged as two interconnected chains immediately outside the spinal cord.

Each chain or trunk consists of 22 or 23 ganglia extending alongside the whole length of the spinal cord.

Another group, the collateral or

prevertebral

ganglia, are found at various points along the abdominal aorta.

Their names reflect their positions, so that the coeliac ganglion is at the junction of the aorta and coeliac artery, while the superior mesenteric and inferior mesenteric ganglia lie adjacent to their respective arteries.

The preganglionic sympathetic neurons have their cell bodies in the lateral columns of the grey matter in the thoracolumbar region of the spinal cord.

The axons leave in the ventral roots of the spinal nerves and then enter the sympathetic chains in the white rami

communicantes

.

The latter are so-named because they contain

myelinated

axons.

NEUROTRANSMITTERS OF THE AUTONOMIC NERVOUS SYSTEM

Neurotransmitters are endogenous chemicals that enable neurotransmission.

It is a type of chemical messenger which transmits signals across a chemical synapse, such as a neuromuscular junction, from one neuron (nerve cell) to another ‘‘target’’ neuron, muscle cell, or gland cell.

They include, acetylcholine (used by neurons in the PNS and CNS ), norepinephrine, serotonin, dopamine, GABA, glutamate, and endorphin

The neurotransmitter released by the preganglionic

fibers

in both parasympathetic and sympathetic nervous system is acetylcholine

The transmitter-receptor interaction is excitatory; so that autonomic ganglia are collections of excitatory synapses.

At the

neuro

-effector junctions,

the

neurotransmitters

differ in the two divisions. In parasympathetic, the postganglionic fibres release acetylcholine (cholinergic neurons), whereas in the sympathetic , noradrenaline is released (noradrenergic neurons).

The sympathetic and parasympathetic system exert opposing actions on their target organs.

The fact that the two divisions release different chemicals into the tissue largely explains the different responses.

Also the nature of the tissue receptors influence whether individual responses are excitatory or inhibitory.

The acetylcholine receptors

Acetylcholine activates two different types of receptors, muscarinic and nicotinic receptors.

Acetycholine

activates both receptors.

The muscarinic receptors are found in all effector cells stimulated by the postganglionic neurons of the parasympathetic nervous system as well as those cells stimulated by the postganglionic cholinergic neurons of sympathetic system.

The nicotinic receptors are found in the synapses between pre and postganglionic neurons of both the sympathetic and parasympathetic systems and also in the membranes of skeletal muscle fibres at neuromuscular junction.

The adrenergic receptors

There are two major types of adrenergic receptors, alpha receptors and beta receptors. Both alpha and beta receptors can be subdivided into alpha1and alpha 2 receptors; and beta1 and beta 2 receptors.