You settle down on your couch and decide to read a book lying on your coffee table. In response to this thought, your brain sends a signal to your arm and causes skeletal muscle contraction, which enables you to pick up the book. Or, you touch a hot pan and immediately withdraw your hand with a skeletal muscle movement that is an involuntary reflex. Guess what? In both instances, you’ve used your somatic nervous system. Please continue reading to learn more about this vital part of the human body.

What is the somatic and autonomic nervous system?

The human nervous system consists of two main parts. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system consists of all the nerves that connect the central nervous system and various parts of the body, such as the organs, glands, and muscles. 

The peripheral nervous system is further divided into the somatic nervous system and the autonomic nervous system. Electrical signals in the autonomic nervous systems (sympathetic and parasympathetic) control involuntary body processes, such as breathing, heart rate, blood pressure, digestion, sexual arousal, and the fight or flight response.  

The somatic nervous system is the voluntary nervous system. Meaning you choose when to activate electrical impulses in this part of the nervous system. This part of the neural pathway controls mainly voluntary muscle movements (skeletal muscles) but also some involuntary muscle responses (reflexes).

The somatic nervous system and autonomic nervous system work in tandem. For example, when you urinate, the autonomic nervous system causes smooth muscle contraction to enable urine to leave the bladder, but some somatic (voluntary) movement is needed to urinate.

What does the somatic nervous system do?

The somatic nervous system works in two main ways - to control voluntary movement and to process sensory input.

What are the 3 parts of the somatic nervous system?

The somatic nervous system includes sensory and motor nerves that work together to enable sensory and motor control. Meaning, they allow us to control physical movement through somatic motor neurons and to process information collected by our senses (sight, sound, smell, taste, and touch) through sensory neurons. Somatic nervous system issues can arise in various parts of this complex system.

Afferent and efferent neurons

• Afferent (sensory) neurons and afferent nerves carry signals from the rest of the body to the brain and spinal cord.
• Efferent (motor) neurons and efferent nerves carry signals from the brain and spinal cord to the rest of the body.
• The reflex arcs relay information from sensory organs to skeletal muscle fibers.

Cranial and spinal nerves

The somatic nervous system consists of two types of somatic nerves, which branch out and end in nerve endings under the skin's surface.

• 12 pairs of cranial nerves originate from the brain (specifically the back of the brain called the brain stem). Motor nerves control facial muscles and glands. Sensory nerves help us see, hear, taste, smell, and feel touch. Some cranial nerves have both sensory and motor neurons. Examples of neurological disorders involving the cranial nerves include trigeminal neuralgia and Bell’s palsy.
• 31 pairs of spinal nerves extend out from various levels of the spinal cord. These nerve fibers relay both motor and sensory information through efferent and afferent neurons. Any injury or disease of the spinal nerves can result in a loss of function or loss of sensation below that level. 
• Peripheral nerves are present outside the brain and spinal cord. They relay information between the brain and the rest of the body. 

Upper and lower motor neurons

The somatic nervous system has two types of neurons:

• Upper motor neurons originate in the front part of the brain called the primary motor cortex. These nerve cells carry signals from the brain to the spinal cord and control voluntary movements, speed, and coordination. They also control involuntary muscle responses such as posture and muscle memory.
• Lower motor neurons carry signals from the spinal cord to the muscle fibers in the body. They control motor function through muscle contraction and also include the somatic reflex arc. Lower motor neurons further consist of alpha motor neurons, beta motor neurons, and gamma motor neurons. 

What is an example of a somatic response?

Involuntary movements like pulling your hand away from a hot pan are somatic responses mediated through the somatic reflex arc. Other examples of somatic nervous system functions include voluntary movements like walking, lifting weights, swallowing, blinking, etc., which are mediated through motor fibers. Feeling sensations like touch, vibration, muscle tension, etc., are examples of somatic sensory neuron function.

What are some examples of somatic nervous system problems?

Injuries, infections, and medical conditions like diabetes can damage peripheral nerve fibers, resulting in a condition called peripheral neuropathy. This condition is associated with symptoms like pain, numbness, tingling, and muscle weakness, commonly in the hands and feet. 

Nerve compression syndromes like carpal tunnel syndrome and sciatica are a result of pinched peripheral nerves and can lead to numbness, tingling, pain, and weakness. 

Myasthenia gravis is an autoimmune disorder in which there is a breakdown of communication between nerves and muscles at the neuromuscular junction.

Amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease, is an example of motor neuron disease involving lower and upper motor neurons.

References:

1. https://www.ncbi.nlm.nih.gov/books/NBK556027/
2. https://my.clevelandclinic.org/health/body/23291-somatic-nervous-system