Somatic Nervous System | Definition, Function & Example

By | September 22, 2019

Somatic Nervous System

Somatic Nervous System

Somatic Nervous System Introduction:

The Somatic Nervous System (SoNS) is part of the Peripheral Nervous System (PNS). This part of the peripheral nervous system is also known as the ‘Voluntary Nervous System’.

The somatic nervous system (SoNS) contains the neurons which are associated with the striated muscle fibers or skeletal and influence the voluntary movement of the body. The Peripheral Nervous System (PNS) is composed of all the neurons which exist outside the brain and the spinal cord. It works as an intermediary medium between the Central Nervous System (CNS) and the muscles, skin, and sensory organs.

The nerves of the Peripheral Nervous System (PNS) send the electrochemical signals back and forth between the Central Nervous System (CNS), and the rest of all body occurs. A large part of the Peripheral Nervous System (PNS) is made up of 12 pairs of the cranial nerves and 31 pairs of the spinal cord nerves.

In these nerves, some the neurons have a sensory function and the other neurons have a motor function. The neurons with motor function are those which innervate striated muscles from the somatic nervous system.

Somatic Nervous System

Parts of the Somatic Nervous System

The term “somatic nervous system” is drawn from the Greek word soma, which suggests “body,” that is suitable for considering it’s the only technique that transmits the information or data to and from the Central Nervous System (CNS) to the other parts of the body.

The somatic nervous system contains two major forms of neurons:

Sensory Neurons

Sensory Neurons, additionally called afferent neurons, are accountable for carrying the information or data from the nerves to the Central Nervous System (CNS).

Motor Neurons

Motor Neurons, additionally called efferent neurons, are accountable for carrying the information or data from the brain and spinal cord to muscle fibers throughout the body.

The neurons which made the somatic nervous system develop superficially from the structure and connect to the muscles of the body, and carry signals from muscles and sensory organs back to the central nervous system or CNS.

The Nerve cell body is present within the system, and also the nerve fiber then comes and terminates within the skin, sense organs, or muscles.

Functions of the Somatic Nervous System

  • The Somatic Nervous System (SNS) consists of both afferent nerves which travel toward the Central Nervous System (CNS) and the efferent nerves are responsible for sending the electrochemical signals toward the rest of the body.
  • The spinal cord and brain process the input from the variety of sources and integrate them before a response devising. This response refers to the location and strength of the muscle contraction toward the different parts of the body.
  • So, the primary function of the Somatic Nervous System (SoNS) is to create a connection between the Central Nervous System (CNS) and striated muscles to enable complex movement and complex behavior.
  • Reflex Arcs and the Somatic Nervous System: In addition, the Somatic Nervous System also mediates the subset of involuntary muscle responses, known as ‘reflex arcs’.
  • The reflex arcs occur in the result of an extremely quick response in muscle contraction to a stimulus, with the minimal intervention from the brain.
  • And for the most voluntary muscle contraction, the impulse originates in the brain or brainstem, and reflex action brought about with only single sensory and motor neuron which synapse in the body part of the spinal cord.
  • The motor neuron response is “hard-wired” for the specific stimulus practically.

The example of reflex action is the ‘knee-jerk’ response to the stimulus of the patellar ligament in the knee.

Somatic Nervous System

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Examples of the Somatic Nervous System Response:

The Somatic Nervous System (SoNS) is the link between Central Nervous System (CNS) with the sensory neurons and motor neurons of Somatic Nervous System which communicate with the brain and the spinal cord. The striated skeletal muscles receive the signals for contraction on the basis of stimuli relayed to the Central Nervous System under the voluntary control.

For instance, a dancer is integrating her or his memory of the music on the stage and choreography in the Central Nervous System (CNS) to the movement of her or his muscles directly through the Somatic Nervous System (SoNS). Before the music start and till the last bow and smile from the still readiness of her or his body, the neurons of the Somatic Nervous System (SoNS) signals to every large and small striated muscle group in the body which basis on the direction of the Central Nervous System (CNS).

Motor Neurons:

The motor neurons are the neural pathway which is the result of skeletal muscle contraction. The skeletal muscle contraction can be divided into two main kinds of neurons; in the central nervous system, the upper motor neurons and the lower motor neuron of Somatic Nervous System (SoNS). The lower motor neurons may be a part of spinal or cranial nerves which innervate the muscle fibers and cause their contraction directly. The upper motor neurons consist of their cell bodies in the precentral gyrus of the brain. This area locates towards the posterior end of the frontal lobe in the cerebral cortex which associates with the primary motor cortex. The axons of upper motor neurons associated with the voluntary muscles movement which travels toward the Central Nervous System (CNS) in the two pathways as the Corticospinal and the Corticobulbar tracts. The neurons with axons which travel along with Corticobulbar tract synapse with lower motor neurons in the brain. From the cranial nerves, the axons of lower motor neurons like the oculomotor, trochlear or the trigeminal nerves which involve with the skeletal muscle contraction in the neck, face, jaw and the tongue.

Neuromuscular Junction:

An alpha motor neuron creates a neuromuscular junction of the axon terminus with striated muscle fibers, from that point acetylcholine released as a neurotransmitter. When potential action reaches the axon terminus of an alpha motor neuron, the voltage-gated ion channel permits to enter calcium ions into the neuron. These of ion induces the fusion of the synaptic vesicle with a membrane of plasma which is the result of releasing of acetylcholine into the neuromuscular junction. Then, the acetylcholine binds the nicotinic receptors on the muscle cells. These all receptors are the ion channels which open upon the ligand binding that leads to ions cascade in the fiber of muscles lead to contraction of muscles.

Sensory Neurons:

The Somatic Nervous System (SoNS) has afferent sensory neurons gives the information to the Central Nervous System about the muscle length, muscle tension, joint angle, and the presence of noxious stimuli.


The body of a muscle also consists of muscle spindles in addition to typical extra refusal muscle fibers. These are small sensory organs which consist of specialized muscle fibers having a central non-contractile segment.

The afferent neurons have sensory dendrites in this region. These dendrites consist of ion channels which open in response to mechanical force on cell. The opening of ion channels produces an action potential in the sensory neurons when the stretch of muscle spindle occurs. The presence of mechanically gated ion channels permits the neurons to take detailed information about the condition of muscle and its contractile activity.


Nociceptors are the pain receptors which found along the body, and it is an essential part of injury prevention, most especially in the muscle fibers. These neurons are activated in the response of damaging of the stimuli, including heat, cold, or extreme forces. When nociceptors are present, it prevents us from the hyperextending joints, overstretching of the muscles, and our protection from a wide range of injuries.