Autonomic Nervous System I


Autonomic Nervous System I

 

(Recommended Reading: Chapter 10; Berne and Levy)

Autonomic Nervous System

 

 

The autonomic nervous system (ANS) is considered to be an efferent system that controls smooth and cardiac muscle contraction, exocrine secretion, endocrine secretion, and metabolism. ANS helps to maintain a proper internal environment by coordinating the visceral activities of the body. The homeostatic mechanisms provided by ANS occur below the level of consciousness and therefore are involuntary. This contrasts with the somatic nervous system that controls the skeletal muscle of the body and is voluntary. ANS is organized into three divisions: sympathetic, parasympathetic, and enteric nervous systems. For sympathetic and parasympathetic nervous systems, each effector cell of an organ is influenced by a sequence of two neurons. A preganglionic neuron whose cell body arises from the central nervous system and forms a synapse with a postganglionic neuron at a location outside of the spinal cord.

 

Sympathetic:

 

Cell bodies of preganglionic neuron of this division are located in the intermediolateral horn of the spinal cord. The axons of the myelinated preganglionic sympathetic neurons leave the spinal cord via the ventral roots of the spinal cord at the level of T1-L2 (thoracolumbar). The preganglionic neurons then make synapses with postganglionic neurons in the paravertebral sympathetic chain ganglia (the upper chain ganglia merge into superior, middle and inferior cervical ganglia) or in the prevertebral collateral ganglia (celiac, superior and inferior mesenteric ganglia). The nonmyelinated axons of the postganglionic neurons travel from these chain and collateral ganglia to the tissues that they innervate (see figures).

 

Parasympathetic:

 

Like the sympathetic nervous system, each effector cell is influenced by two neurons in series. The cell bodies of preganglionic neurons of this division are located in the cranial portion of the brainstem and in the sacral segments of the spinal cord (craniosacral). Unlike the sympathetic system, the preganglionic neuron is long and terminates close to the effector organ, making synapses with the postganglionic neuron that is present close to or within the organ. The cranial part of this system is supplied by the four cranial nerves (cranial nerve #3=oculomotor, 7=facial, 9=glossopharyngeal, and 10=vagus). These nerves innervate the head, thoracic viscera, and most of the abdominal viscera. The neurons from the sacral (S2-S4) part of the parasympathetic nervous system innervate the lower abdominal and pelvic viscera.

 

Cranial nerves:

 

 

Some ANS functions are served by:

 

 

Autonomic nervous system.

Figure 1. Autonomic nervous system.

 

Axons of the sympathetic preganglionic neurons leave the spinal cord via the ventral roots and end in the sympathetic ganglion chain or in the collateral ganglia where cell bodies of postganglionic neuron are located. Axons of the parasympathetic preganglionic neurons leave the brain stem (cranial outflow) and end near the visceral structures in the head, thorax and upper abdomen. Another group of axons of the parasympathetic preganglionic neuron leave the S2-S4 of the spinal cord and end near the viscera in the pelvic region.

 

 

Sympathetic Division

 

Parasympathetic Division

 

Neurotransmitters released by neurons of the ANS

 

The major neurotransmitter released by the preganglionic neurons of the sympathetic and parasympathetic system is acetylcholine (ACh). ACh acts on the nicotinic cholinergic receptors on postganglionic neurons and causes depolarization and firing of the postganglionic neuron. The action potentials that arrive at the nerve terminal of the postganglionic neuron then cause release of synaptic vesicles containing norepinephrine or ACh by the process of exocytosis. The neurotransmitter released by the sympathetic postganglionic neuron is norepinephrine (NE) whereas the neurotransmitter released by the parasympathetic postganglionic neuron is ACh.

Biosynthesis of ACh, NE and E

 

The postganglionic sympathetic fibers that innervate blood vessels of skeletal muscles and sweat glands in the skin are cholinergic, i.e., ACh is the neurotransmitter. They are referred to as the “cholinergic sympathetic” pathway. The adrenal medulla is innervated by the preganglionic cholinergic fiber. Adrenal gland releases both epinephrine (80-90%) and norepinephrine (10-20%).

 

The effects of the sympathetic nervous system are more widespread, massive, diffuse, and generalized than those of the parasympathetic system. This is because of the divergent nature of the sympathetic nervous system. Each preganglionic neuron synapses with many postganglionic neurons each with different neuro-effector junctions. Furthermore, stimulation of the adrenal gland results in release of both epinephrine and norepinephrine (catecholamines) into the blood stream. Also, the catecholamines are deactivated more slowly than ACh.

 

In general, sympathetic nervous system stimulates those activities that are expressed during emergency and stress situations, also called the “fight, fright and flight activities”. There is expenditure of energy due to acceleration of the rate and force of heartbeat, increase in blood pressure and sugar.

 

The effects of the parasympathetic nervous system are localized and discrete, and ACh is inactivated quickly by the enzyme cholinesterase. The parasympathetic system stimulates those activities that bring about conservation and restoration of energy stores. The two systems act in concert to maintain the internal environment of the organism for a given physiological state of the individual.