What Is the Somatic Nervous System?

The somatic nervous system carries motor and sensory signals to and from the central nervous system (CNS). Because of this bodily system, we are able to control our physical movements and process four of the five senses—smell, sound, taste, and touch.

Learn more about the somatic nervous system, including its location, function, and parts. We also provide a few examples of how the somatic nervous system works, as well as how it is different from the autonomic nervous system.

Somatic Nervous System Location

The somatic nervous system includes all of the nerves that extend from the brain and spinal cord. It does not include the brain and spinal column themselves, both of which are part of the central nervous system. There are two types of somatic nerves: cranial and spinal.

Cranial Nerves

The somatic nerves that extend from the brain are known as cranial nerves and are located on the back of the head and neck. There are 12 pairs of cranial nerves in total, each of which splits to carry signals between both sides of the brain and body. These nerves are often involved in neuromuscular disorders.

Spinal Nerves

The somatic nerves that extend from the spinal column are known as spinal nerves. There are 31 pairs of spinal nerves. Eight come from the cervical portion of the spine, 12 are in the thoracic region, both the lumbar and sacral regions have five spinal nerves, and one is near the tailbone. A disease or injury in any of these areas can result in a loss of sensation and function below that area.

Somatic Nervous System Function

If you're wondering what the somatic system does, it has two basic functions:

• Movement control: The somatic nervous system plays a vital role in initiating and controlling the movements of your body. This system is responsible for nearly all voluntary muscle movements.
• Sensory input: The somatic system is also responsible for processing sensory information that arrives via external stimuli. It processes the senses of hearing, smell, taste, and touch.

How exactly does this complex system work? Answering this question requires a closer look at its key parts.

Parts of the Somatic Nervous System

The term somatic is drawn from the Greek word soma, which means "body." This is appropriate considering that it is this system that transmits information back and forth between the CNS and the rest of the body.

The somatic nervous system contains two main types of neurons (nerve cells):

• Sensory neurons, also known as afferent neurons, are responsible for carrying information from the body to the CNS.
• Motor neurons, also known as efferent neurons, are responsible for carrying information from the brain and spinal cord to muscle fibers throughout the body.

The body of the neuron is located in the CNS and the axon (a portion of the neuron that carries nerve impulses away from the cell body) projects and terminates in the skin, sensory organs, or muscles.

Other components of the somatic nervous system include:

• Ganglia, which are large groups of nerve cells that are closely related
• Glial cells, which don't actively transmit signals but, instead, support nervous system cell functions
• Nuclei, or nerve cell clusters that share the same function or connections
  

Reflex Arcs

In addition to controlling voluntary muscle movements, the somatic nervous system is also associated with involuntary movements known as reflexes (or reflex actions). These reflexes are controlled by a neural pathway known as a reflex arc.

Reflex arcs include sensory nerves that carry signals to the spinal cord, often connect with interneurons there, then immediately transmit signals down the motor neurons to the muscles that triggered the reflex.

During a reflex, muscles move involuntarily without input from the brain; you don’t have to think about doing these things. This occurs when a nerve pathway connects directly to the spinal cord. Examples of reflex actions include:

• Jerking your hand back after accidentally touching a hot pan
• Involuntary jerking when your doctor taps on your knee  
  

Somatic vs. Autonomic Nervous Systems

The somatic and autonomic nervous systems are both parts of the peripheral nervous system, which allows the brain and spinal cord to receive and send information to other areas of the body. However, they have different functions.

The somatic nervous system connects the central nervous system with the body's muscles and skin. Its primary function is to control voluntary movements and reflex arcs, while also helping us process the senses of touch, sound, taste, and smell. 

An example of a somatic system function is if you are out for a jog in the park one brisk winter morning and as you run, you step on a patch of slick ice. Once your foot starts to slip, your somatic nervous system carries a message to the muscles in your legs, enabling you to catch yourself and avoid a fall.

In comparison, the autonomic nervous system connects the CNS with visceral organs (heart, stomach, etc.) This system regulates a variety of involuntary body processes, some of which include heartbeat, blood flow, breathing, body temperature, and emotion response.

Diseases of the Somatic Nervous System

Somatic nervous system diseases are those that impact the peripheral nerves outside of the brain and spinal cord. Diseases that impact the peripheral nerve fibers of the somatic nervous system can cause what is known as peripheral neuropathy.

Peripheral neuropathy leads to nerve damage, resulting in numbness, weakness, and pain, often in the hands and feet. This damage can be caused by physical injury or trauma, diabetes, blood or vein issues, autoimmune diseases, and more.

Other somatic nervous system diseases include:

• Brachial plexus neuropathy, or damage to nerves in the upper shoulder, resulting in pain in the shoulders or arms
• Guillain-Barre syndrome, which is when the immune system attacks the nerves
• Myasthenia gravis, an autoimmune disorder that leads to muscle weakness and fatigue
• Nerve compression syndromes involve pain, weakness, numbness, or tingling due to a pinched nerve
• Trigeminal neuralgia, a neuropathic condition causing shock-like pain or burning in the face

Several additional factors can ultimately lead to damage to the somatic nervous system, thus impacting its function. Among them are certain medications, exposure to toxins, and infections such as shingles, Lyme disease, and HIV.

Signs of Somatic Nervous System Problems

The symptoms experienced with a somatic nervous system issue can vary depending on whether the damage is to the motor nerves (which control movement) or sensory nerves (which affect the senses).  

Signs of motor nerve damage include:

• Loss of movement control
• Muscle cramps or spasms
• Tremors or twitching
• Wasting of muscles (muscle shrinkage)

If there is damage to the sensory system, the following symptoms may exist:

• Inability to feel things you touch
• Numbness or tingling
• Sharp or burning pain in the damaged area

Treatments for Somatic Nervous System Conditions

Treatments used for somatic nervous system issues range from taking medication or doing physical therapy to needing nerve ablation or surgery. Other options include acupuncture, transcutaneous electrical nerve stimulation (TENS), and using devices or equipment to assist with the physical movement process.

Healthcare professionals who specialize in treating somatic nervous system issues include:

• Neurologist - a physician trained in the treatment of nervous system disorders
• Neurosurgeon - a surgeon trained to conduct brain and spine surgeries
• Neuropsychologist - a mental health professional trained to provide services related to cognitive function

Preventing Peripheral Neuropathy

While diseases that impact the somatic nervous system are not always preventable, there are lifestyle changes you can make that may help prevent peripheral neuropathy.

Some strategies that may help include:

• Avoiding alcohol
• Correcting vitamin deficiencies
• Eating a healthy diet
• Getting regular exercise
• Maintaining a healthy weight
• Quitting smoking

It is also essential to treat chronic health conditions such as diabetes, which may play a role in the onset of peripheral neuropathy.