Understanding the Autonomic Nervous System: Sympathetic and Parasympathetic Systems, Lecture notes of Neuroscience

Autonomic Nervous System

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The autonomic nervous system (ANS) controls the non-voluntary motor function in

your body. Remember that in the somatic nervous system (the system that controls

voluntary muscle movement) you have a neuron in the ventral horn of the spinal cord (

or  motor neurons) that send out axons that synapse directly on skeletal muscle. In the

ANS, spinal cord neurons send out axons that synapse in ganglia in the body. It is the

muscle tissue of organs such as stomach, GI tract and lungs. It also involves smooth

muscles around arteries for blood flow control and smooth muscles of the eye such as the

iris and the smooth muscle that controls the eye lens for focus. These postganglionic

axons can also synapse onto glands and cardiac muscle. The highest control center for the

ANS is the hypothalamus.

Note that axons that are travelling to a ganglion are called preganglionic fibers and

axons leaving the ganglion are called postganglionic fibers.

There are two parts of the ANS:

A major group of neurons that are involved in the sympathetic nervous system is in the

gray matter of the spinal cord. It is called the intermediolateral cell column (IML),

found at spinal cord levels T1-L2. These neurons send axons (preganglionic fibers) out

of the spinal cord via the ventral root and synapse onto neurons in the sympathetic chain

ganglion. Some will bypass the sympathetic chain ganglia and synapse on other ganglia

in the body, e.g., superior mesenteric ganglia, etc. The neurotransmitter used in the

synapse in the sympathetic chain ganglion (or other ganglia) is ACh. The neurons in the

sympathetic chain ganglion send out axons (postganglionic fibers), which travel in nerves

(PNS) and synapse on the effector organ, e.g., smooth muscle, glands etc. The

neurotransmitters used at those synapses are norepinephrine, ACh and dopamine. Not