Autonomic Nervous System

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Autonomic Nervous System...

Autonomic Nervous System:

The autonomic nervous system serves as a relay station to the visceral organs of the body. It is an efferent system, meaning it sends out motor impulses to the organs. It regulates many body functions to maintain homeostasis.

Most motor pathways usually have one motor neuron from the brain or spinal cord. In the autonomic nervous system, the message is transferred at the synapse from one neuron to a second one that carries it to the muscle or gland cell.

The autonomic nervous system is actually part of the peripheral nervous system in that it consists of motor portions of some cranial and spinal nerves. Because it's functioning is so specialized, however, the ANS is usually discussed as a separate entity.

The autonomic nervous system consists of certain motor neurons that conduct impulses from the spinal cord or brain stem to cardiac muscle tissue, smooth muscle tissue, and glandular tissue.

The ANS consists of the parts of the nervous system that regulate involuntary functions such as the heartbeat, contractions of the stomach and intestines, and secretions by glands.

The ANS consists of two divisions:

the sympathetic nervous system

the parasympathetic nervous system

Often, they function in opposition to each other.

Autonomic neurons are the motor neurons that make up the ANS. These autonomic neurons are called visceral motor neurons. These neurons will lead to smooth muscle, cardiac muscle and glands. These structures are called visceral effectors and they will stimulate muscle contraction or relaxation and the glands will either increase or decrease their secretions.

The autonomic nerve pathway from the CNS to a visceral effector consists of two motor neurons that synapse into a ganglion (a group of neural cell bodies) outside of the CNS. The first of the two motor neurons is called the preganglionic neuron, which will conduct the impulses between the CNS and the ganglion. The second neuron or the postganglionic neuron will conduct impulses from the ganglion to the visceral effector.

Sympathetic division

The sympathetic division is dominant in stressful situations, which include anger, fear, or anxiety, as well as exercise. The fibers arising from the sympathetic division are mainly from the lumbar regions of the spinal cord. They primarily release norepinephrine (adrenalin) and stimulate the muscle or gland. Effects of norepinephrine include:

increase in rate and force of contraction of the heart

dilation of the pupils

increase in blood pressure

Parasympathetic division

The parasympathetic division brings the body back to a normal state after the stress response and is most active under ordinary, relaxed conditions. It acts as a balance for the sympathetic system. This division is sometimes called the “rest and repose” system and continuously secretes short acting acetylcholine.

Effects include:

decrease in blood pressure

decrease in blood flow to skeletal muscles

increase in digestive tract activity

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