Differences between the somatic & autonomic systems

Written by keri gardner
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Differences between the somatic & autonomic systems
The somatic nervous system includes sensory nerves for touch. (Jupiterimages/Pixland/Getty Images)

Sensory receptors, sensory nerves and motor nerves make up your somatic and autonomic nervous systems. Together they are termed the peripheral nervous system. Your central nervous system contains the brain and spinal cord and your peripheral nervous system contains all the other nerves in your body and connects them to the central nervous system. The somatic system functions under voluntary control, while the autonomic system controls involuntary muscles, organs and glands.

Peripheral Nervous System

A change in your internal or external environment stimulates your sensory receptors. The receptors consist of cell membranes, whole nerve cells or groups of cells. Classification of receptors may be according to the type of stimulus: mechanical, thermal, light, chemical or smell. The afferent fibres carry sensory information to the spinal cord, while the efferent fibres carry messages from the spinal cord to your muscle, gland or organ.


The somatic nervous system involves only one effector neuron, while the autonomic nervous system uses two effector neurons and ganglia. A neuron is a nerve cell that conducts nerve impulses and a ganglion is a group of nerve cells. A response to a stimuli moves from the central nervous system, through a neuron and into a muscle using the somatic system. In an autonomic response, the signal moves from the central nervous system, through a neuron, into a ganglion and into a second neuron, where it branches into muscles, glands or organs.

Effects of Neurotransmitters

The effect of the somatic system on a neuron is always excitation. The chemical that transmits the impulse to the reactive muscle is acetylcholine. The autonomic system regulates its response by excitation or inhibition, depending on the tissue receiving the signal. It uses one chemical to excite, acetylcholine and another chemical to inhibit, norepinephrine. The autonomic nervous system is responsible for the "fight or flight" response when your body reacts to danger.

Division of System

The autonomic nervous system is divided into two divisions called the parasympathetic and sympathetic. The divisions are classified according to anatomy and function. Most organs have nerve fibres from both parasympathetic and sympathetic divisions. For example, the parasympathetic system reduces heart rate and blood pressure, while the sympathetic system causes the opposite effect. The somatic nervous system does not have separate divisions.

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