The Somatic Nervous System

Welcome to this video about the somatic nervous system, which, along with the autonomic nervous system, composes the peripheral nervous system.

Let’s take a moment and review the nervous system briefly.

Nervous System Review

The central nervous system includes the brain and spinal cord.

The peripheral nervous system, which includes all of the neurons outside of the central nervous system, has two divisions:

• The autonomic nervous system controls involuntary processes (such as the heartbeat) and includes the sympathetic, parasympathetic, and enteric nervous systems.
• The somatic nervous system includes the sensory (afferent) and the motor (efferent) nervous systems and controls voluntary processes, such as standing and walking. Reflex arcs, which involve involuntary reflex movements, are also associated with the somatic nervous system.

Afferent (sensory) nerves take sensory input, such as a smell or pain response, from the body and send it to the central nervous system for processing.

Efferent (motor) nerves send back an impulse or a motor output response to muscles, glands, and organs from the central nervous system. Most nerves of the somatic nervous system are mixed and carry signals in both directions.

With reflex responses, such as the patellar reflex, the response is mediated within the spinal cord. An afferent or sensory nerve takes an impulse to the spinal cord, and an interneuron in the spinal cord transmits the impulse to an efferent or motor nerve that immediately sends back a response.

In most other cases, the impulses travel to the spinal cord and up tracts in the spinal cord to the brain, which sends back a response.

Nerves in the Somatic Nervous System

The nerves that comprise the somatic nervous system include the spinal nerves and the cranial nerves.

Cranial Nerves

First, let’s look at the cranial nerves.

There are 12 pairs of cranial nerves. Cranial nerves I and II emerge from the cerebellum and cranial nerves III through XII from the brain stem:

• Cranial nerves I, the olfactory nerves, are sensory nerves associated with the sense of smell.
• Cranial nerves II, the optic nerves, are sensory nerves associated with vision.
• Cranial nerves III, the oculomotor nerves, are motor nerves associated with extraocular movements, pupil constriction, opening of the eyelids, and the shape of the lens.
• Cranial nerves IV, the trochlear nerves, are motor nerves associated with downward and inward movement of the eyes.
• Cranial nerves V, the trigeminal nerves, are mixed nerves that control the muscles of mastication, lateral eye movement, and sensations in the face, scalp, and mucous membranes in the mouth and nose.
• Cranial nerves VI, the abducens nerves, are motor nerves that control movement of the eyes laterally.
• Cranial nerves VII, the facial nerves, are mixed nerves that control facial movements, mouth closing, eye closing, sense of taste on anterior two-thirds of tongue, saliva, and tear production.
• Cranial nerves VIII, the vestibulocochlear nerves, are sensory nerves associated with the inner ear, including hearing and equilibrium.
• Cranial nerves IX, the glossopharyngeal nerves, are motor nerves that affect the pharynx for phonation and swallowing and the parotid glands and sense of taste on the posterior one-third of the tongue as well as saliva and tear production.
• Cranial nerves X, the vagus nerves, are mixed nerves associated with the larynx and pharynx—talking and swallowing—and sensations in the larynx, pharynx, and viscera (including the heart, respiratory muscles, stomach and gallbladder).
• Cranial nerves XI, the spinal accessory nerves, are motor nerves that control the trapezius and sternocleidomastoid muscles in the neck.
• Cranial nerves XII, the hypoglossal nerves, are motor nerves that control the movement of the tongue.

Spinal Nerves

Now, let’s look at the spinal nerves, the other component of the somatic nervous system. There are 31 pairs of spinal nerves, and all of these nerves are mixed, both sensory and motor.

The spine has 31 segments, with each segment containing a pair of spinal nerves that branch out to other parts of the body. The spinal nerves are grouped and numbered according to the level at which they arise from the spinal cord. The 31 pairs of nerves and nerve roots in the spinal cord include:

• 8 cervical pairs that innervate the face and head
• 12 thoracic pairs that innervate the upper body
• 5 lumbar pairs that innervate the lower body
• 5 sacral pairs that innervate the lower back and pelvis
• 1 coccygeal pair that innervates the buttocks area

The nerves emerging from the superior part of the spinal cord do so almost horizontally, but those emerging from the posterior part tend to do so in a downward direction.

The reason for this is that the spinal cord extends the length of the vertebral column at birth, but then the vertebral column grows more quickly than the cord, so the lengthened vertebral column causes the spinal nerves near the posterior end to descend beyond the cord end, resulting in the formation of the cauda equina.

Each spinal nerve is formed from two roots. The dorsal or posterior root is the sensory portion. It is characterized by the dorsal root ganglion, an enlarged area that contains the cell bodies of sensory neurons whose axons carry sensory information into the spinal cord where they connect with other neurons.

The ventral or anterior root is the motor portion. It is formed by the axons of motor neurons whose cell bodies are in the gray matter of the spinal cord, so these axons carry impulses out into the body.

The dorsal and ventral roots combine to form mixed (sensory and motor) spinal nerves that emerge from the vertebral canal through an intervertebral foramen.

After each spinal nerve exits the intervertebral foramen, it divides into the dorsal ramus, which innervates the muscles and skin of the back, and the ventral ramus, which innervates the muscles and skin of the front and sides of the trunk and the limbs.

The white ramus and gray ramus convey autonomic signals for the sympathetic nervous system.

PLEXUSES
Except in the thoracic area, anterior spinal nerves from different segments combine and intertwine to form nerve networks called plexuses instead of going directly to a specific part of the body. This results in nerve fibers from different spinal nerves associated with a particular part of the body coming together to form new nerves. Important plexuses include:

• The cervical plexus in both sides of the neck combines fibers from cranial nerves I through IV to supply nerves to the muscles and skin of the neck and fibers from cranial nerves III through V into the phrenic nerves, which send motor impulses to the muscles of the diaphragm.
• The brachial plexus in the shoulders combines fibers from cranial nerves IV through VIII and spinal nerve T1. The nerves that emerge include the musculocutaneous, ulnar, median, radial, and axillary nerves, among others.
• The lumbar plexus lies on the posterior abdominal wall and combines fibers from spinal nerves T12 and L1 through L4. The lumbar plexus gives rise to the iliohypogastric, ilioinguinal, genitofemoral, lateral cutaneous, femoral, and obturator nerves.
• The sacral plexus lies on the posterior pelvic wall and combines fibers from spinal nerves L4, L5 and S1 through S4. The nerves that emerge include the superior gluteal nerve, inferior gluteal nerve, tibial nerve, and sciatic nerve.

Each spinal nerve relates to a specific dermatome, or area of skin that receives sensations from the spinal nerve. Dermatomes are evenly spaced in the trunk and are relatively horizontal, encircling the body, but they run longitudinally in the limbs.

Somatic Nervous System Disorders

There are a number of disorders associated with the somatic nervous system:

• Amyotrophic lateral sclerosis, commonly known as ALS, is a motor neuron disease that causes weakness and muscle atrophy.
• Brachial plexus neuropathies result from damage of the nerves, causing severe shoulder pain and weakness. Brachial plexus injuries may result from birth trauma, such as when the baby’s neck is stretched too far to one side.
• Cauda equina syndrome caused by trauma to the nerve roots in the cauda equina results in numbness, pain, and weakness in the saddle area of the body and the legs.
• Guillain-Barré syndrome is a progressive form of polyneuritis that causes inflammation and segmental demyelination of the peripheral nerves.
• Herpes zoster, also called “shingles,” is an infection of the dorsal root ganglia caused by herpes virus varicella-zoster, the virus that causes chicken pox. The infection causes vesicular skin lesions and pain along a dermatome.
• Myasthenia gravis is an autoimmune disorder that results in impaired transmission of nerve impulses, producing progressive weakness of skeletal muscles.
• Nerve compression syndromes occur when irritation or pressure damages the nerve, such as may occur with repetitive injuries, resulting in numbness, weakness, and pain. Carpal tunnel syndrome is a common nerve compression syndrome.
• Trigeminal neuralgia is a disorder of Cranial nerve V, the trigeminal nerve, which causes severe paroxysmal attacks of pain in the face.

The somatic or voluntary nervous system is vitally important to the ability to initiate and control movement because it controls almost all voluntary muscle activity in addition to processing sensory input. Thus, the somatic nervous system allows us to perceive through sight, smell, vision, and touch what is in our environment and to respond through voluntary movement.

Review

Before we go, let’s go over a few questions:

1. All of the following are managed by the somatic nervous system EXCEPT:

1. The sympathetic stress response
2. Voluntary motor responses
3. Involuntary reflexes
4. Sensory responses

 
2. The inability for an individual to smile symmetrically indicates possible dysfunction of which of the following cranial nerves?

1. IV (Trochlear)
2. V (Trigeminal)
3. VI (Abducens)
4. VII (Facial)

 
3. Which root of the spinal nerve is responsible for sensory functions?

1. Ventral root
2. Anterior root
3. Dorsal root
4. Lateral root

 
That’s all, thanks for watching and happy studying!