Nervous System, Autonomic Health Article

Definition

The autonomic nervous system is a network of nerves that regulate involuntary control of cardiac muscle, organ smooth muscle, and glands such that basic biological processes such as digestion and breathing can occur without conscious thought.

Description

The peripheral nervous system consists of nerves that must travel outside of the brain and spinal cord in order to contact organs, glands, and muscles. Under the umbrella of the peripheral nervous system are the somatic and autonomic nervous systems. The somatic nervous system is responsible for controlling voluntary movements during activities such as walking while the autonomic nervous system regulates involuntary tasks such as food digestion. More specifically, the somatic division mediates voluntary or reflexive control of skeletal

muscles while the autonomic nervous system is responsible for the involuntary and reflexive control of glands, organ smooth muscle, and cardiac muscle.

The autonomic nervous system has three components:

• sympathetic nervous system
• parasympathetic nervous system
• enteric nervous system

The enteric nervous system is the less common of the three and is responsible for coordinating the digestive functions of the gastrointestinal tract, pancreas, and gall bladder. The two other subdivisions of the autonomic nervous system, parasympathetic and sympathetic, work in concert to subconsciously control other bodily functions, such as heart rate, blood pressure, digestion, metabolism, reproduction, breathing, excretion, sweating, and temperature.

The parasympathetic and sympathetic divisions have similar organizations but are distinguishable at the anatomical, biochemical, and functional levels. Both systems are organized into a two-neuron chain. The first neuron in this chain is referred to as a preganglionic neuron and the second as a postganglionic neuron. The nucleus containing cell bodies of preganglionic neurons are found in the brain and spinal cord of the central nervous system. The preganglionic neuron extends a fiber process, known as an axon, outside of the central nervous system to make contact with the cell body of the postganglionic neuron. The place where the axon of the preganglionic neuron meets the cell body of the postganglionic neuron is called a synapse. The synapses of the autonomic nervous system are outside of the brain and spinal cord of the central nervous system in specialized structures known as autonomic ganglia.

The preganglionic neurons of the parasympathetic nervous system originate in the brainstem and sacral spinal cord. These preganglionic neurons communicate with postganglionic neurons by extending very long axons that release the neurotransmitter, acetylcholine. The synapses of the parasympathetic ganglia are usually in or near the targeted organ. The postganglionic neuron expresses protein receptors on the surface that are capable of responding to acetylcholine. The postganglionic neurons have very short axons that release acetylcholine onto the targeted organ to modulate the intrinsic activity of that particular organ. These organs include the eye, lacrimal gland, salivary gland, heart, bronchi and lungs, small intestine, stomach, gallbladder, liver, pancreas, large intestine, rectum, genitalia, blood vessels, and bladder. Each of these targeted organs expresses acetylcholine receptors to respond to the parasympathetic nervous system.

The preganglionic neurons of the sympathetic nervous system originate in the thoracic and upper lumbar regions of the spinal cord. These preganglionic neurons send very short axons to synapse in the paravertebral or in the prevertebral ganglia. The paravertebral ganglia lie in close proximity to the spinal cord. The postganglionic neurons of the paravertebral ganglia send axons to the head, trunk, and limb regions. The other organs in the body receive inputs from the prevertebral ganglia which is further away from the spinal cord and closer to the target organ. An exception to organization is the adrenal gland which is directly contacted by preganglionic neurons of sympathetic nervous system. Identical to the parasympathetic nervous system, the preganglionic neurons of the sympathetic nervous system communicate by releasing the neurotransmitter acetylcholine. However, the postganglionic neurons of the sympathetic nervous system differ in that they release norepinephrine onto the targeted organ. An exception to this is in the sweat glands where sympathetic postganglionic neurons release acetylcholine instead of norepinephrine. The target organs of the sympathetic nervous system include many of the same ones as the parasympathetic nervous system.