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13.1: Introduction to the Somatic Nervous System

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  • Chapter Learning Objectives

    After studying this chapter, you will be able to:

    • Describe the components of the somatic nervous system
    • Describe the structures of general and special senses
    • Describe regions of the central nervous system that contribute to somatic functions
    • Explain the stimulus-response motor pathway
    • Describe the components of a basic reflex arc and distinguish between different types of reflexes

    The somatic nervous system includes all the afferent, sensory receptors from the skin, skeletal muscles, joints and special senses which are responsible for our conscious perception of the environment. It also includes the efferent, motor division which reaches the skeletal muscles and is responsible for our voluntary motor response. In contrast, the autonomic nervous system, which is discussed in the next chapter, includes afferent sensory receptors that are from visceral organs and are responsible for unconscious perception as well as an efferent motor division that reaches cardiac muscle, smooth muscle or glands for involuntary motor responses.

    Our somatic senses make us aware of our surroundings or state of our body through peripheral sensory receptors. These receptors send sensory information through cranial and spinal nerves to the central nervous system. The neurons of the central nervous system need to process the information, a step called integration. Once processed, if a motor response is required, the neurons of the central nervous system will send a signal to skeletal muscles to move. Let's take the example of touching a hot stove (Figure \(\PageIndex{1}\)). Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. This triggers an action potential (a change in electrical properties of a neuron), which travels along the sensory fiber from the skin through the dorsal spinal root to the spinal cord, and directly activates a ventral horn motor neuron. That neuron sends a signal along its axon to excite the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. The basic withdrawal reflex explained above includes afferent somatosensory input (the painful stimulus), central processing (the synapse in the spinal cord), and efferent motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii).

    In this chapter, we will examine all the components of the somatic nervous system. For the sensory input, we will look at sensory receptors of general and special senses and how the sensory information is sent to the CNS. Once the information is received in the appropriate region of the CNS, central processing takes place to integrate the various information. Lastly, the CNS will send a motor response to the periphery.

    Pot on blue flame of stove top
    Figure \(\PageIndex{1}\): Too Hot to Touch. When high temperature is sensed in the skin, a withdrawal is initiated by the muscles of the arm. Sensory neurons are activated by a stimulus, which is sent to the central nervous system, and a motor response is sent out to the skeletal muscles that control this movement. (Image Credit: "Flame" by Steve Buissinne, Pixabay is in the Public Domain)

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    OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at