Which division of the peripheral nervous system activates skeletal muscles?

The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.[1][2]

The somatic nervous system consists of sensory nerves carrying afferent nerve fibers, which relay sensation from the body to the central nervous system (CNS), and motor nerves carrying efferent nerve fibers, which relay motor commands from the CNS to stimulate muscle contraction.[3]

The a- of afferent and the e- of efferent correspond to the prefixes ad- (to, toward) and ex- (out of).

Structure[edit]

There are 43 segments of nerves in the human body.[4] With each segment, there is a pair of sensory and motor nerves. In the body, 31 segments of nerves are in the spinal cord and 12 are in the brain stem.[4] Besides these, thousands of association nerves are also present in the body.[citation needed]

Thus the somatic nervous system consists of two parts:

• Spinal nerves: They are mixed nerves that carry sensory information into and motor commands out of the spinal cord.[5]
• Cranial nerves: They are the nerve fibers that carry information into and out of the brain stem.[4] They include smell, eye muscles, mouth, taste, ear, neck, shoulders, and tongue.[5]

Function[edit]

The somatic nervous system controls all voluntary muscular systems within the body, and the process of voluntary reflex arcs.

The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). Stimuli from the precentral gyrus are transmitted from upper motor neurons, down the corticospinal tract, to lower motor neurons (alpha motor neurons) in the brainstem and ventral horn of the spinal cord: upper motor neurons release a neurotransmitter called glutamate from their axon terminal knobs, which is received by glutamate receptors on the lower motor neurons: from there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

Reflex arcs[edit]

A reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a two-element chain, of which in the human body there is only one, also called a monosynaptic reflex (there is only one synapse between the two neurones taking part in the arc: sensory and motor). The singular example of a monosynaptic reflex is the patellar reflex. The next simplest reflex arc is a three-element chain, beginning with sensory neurons, which activate interneurons inside of the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex ("knee jerk") activated by tapping the patellar tendon, contribute to ordinary behavior.

Other animals[edit]

In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For vertebrates, however, the response of a skeletal striated muscle fiber to a neurotransmitter – always acetylcholine (ACh) – can only be excitatory.

The peripheral nervous system includes the nerves and ganglia outside the brain and spinal cord.  The PNS includes the peripheral nerves, neuromuscular junctions, cranial nerves, and spinal nerves. This system also carries information to and from the central nervous system.

The peripheral nervous system (PNS) is made up of two divisions: the somatic nervous system and the autonomic system. Each part of this system plays a vital role in how information is communicated throughout the body.

This article discusses what the peripheral nervous system is, how it works, and the influence that it has on how the body functions, including what happens when there is a problem affecting the PNS.

What Is the Peripheral Nervous System?

The nervous system is divided into two parts: the central nervous system and the peripheral nervous system. The central nervous system (CNS) includes the brain and spinal cord, while the peripheral nervous system includes all of the nerves that branch out from the brain and spinal cord and extend to other parts of the body, including muscles and organs.

The primary role of the PNS is to connect the CNS to the organs, limbs, and skin. The nerves of the PNS extend from the central nervous system to the outermost areas of the body. The peripheral system allows the brain and spinal cord to receive and send information to other areas of the body, which allows us to react to stimuli in our environment.

The four primary functions of the PNS are to:

• Control autonomic body functions
• Control motor movements
• Digestion
• Relay sensory information to the central nervous system

Examples of processes controlled by the peripheral nervous system include dilating or constricting the pupils in response to light, stimulating digestion, activating the sweat response, controlling blood flow, and regulating heart rate.

The nerves that make up the peripheral nervous system are actually the axons or bundles of axons from nerve cells or neurons. In some cases, these nerves are very small, but some nerve bundles are so large that they can be seen by the human eye.

Structures of the Peripheral Nervous System

The peripheral nervous system itself is divided into two parts: the somatic nervous system and the autonomic nervous system. Each of these components plays a critical role in how the peripheral nervous system operates.

The Somatic Nervous System

The somatic system is the part of the peripheral nervous system responsible for carrying sensory and motor information to and from the central nervous system. The somatic nervous system derives its name from the Greek word soma, which means "body."

The somatic system is responsible for transmitting sensory information as well as for controlling voluntary movement. This system contains two major types of neurons:

• Motor neurons: Also called efferent neurons, motor neurons carry information from the brain and spinal cord to muscle fibers throughout the body. These motor neurons allow us to take physical action in response to stimuli in the environment.
• Sensory neurons: Also called afferent neurons, sensory neurons carry information from the nerves to the central nervous system. The sensory neurons allow us to take in sensory information and send it to the brain and spinal cord.

The Autonomic Nervous System

The autonomic system is the part of the peripheral nervous system that's responsible for regulating involuntary body functions, such as blood flow, heartbeat, digestion, and breathing.

In other words, it is the autonomic system that controls aspects of the body that are usually not under voluntary control. This system allows these functions to take place without needing to consciously think about them happening. The autonomic system is further divided into two branches:

• Sympathetic system: By regulating the flight-or-fight response, the sympathetic system prepares the body to expend energy to respond to environmental threats. When action is needed, the sympathetic system triggers a response by accelerating heart rate, increasing breathing rate, boosting blood flow to muscles, activating sweat secretion, and dilating the pupils.
• Parasympathetic system: This helps maintain normal body functions and conserve physical resources. Once a threat has passed, this system will slow the heart rate, slow breathing, reduce blood flow to muscles, and constrict the pupils. This allows the body to return to a normal resting state.

Recap

The PNS is made up of two divisions: the somatic nervous system and the autonomic nervous system. The somatic system contains sensory and motor neurons. It sends and receives sensory information and motor signals. The autonomic system is responsible for regulating involuntary body functions.

Nerves in the Peripheral Nervous System

The PNS is composed of nerves that are responsible for carrying signals between the central nervous system and the parts of the body that lie outside the CNS. This includes information from the senses, organs, and muscles.

The axons of these nerve cells are bundled together and can be found throughout the body. Information is received by the dendrites of these cells, the information travels down the axon to the cell body. The message can then be communicated to other cells.

The nerves that make up the peripheral nervous system connect with either the spinal cord or brain in order to transmit information to the CNS.

PNS Nerves

The main nerves that make up the peripheral nervous system include:

• Brachial plexus
• Femoral nerve
• Lateral femoral cutaneous nerve
• Peroneal nerve
• Sciatic nerve
• Spinal accessory nerve
• Tibial nerve

Spinal Nerves

Spinal nerves are responsible for transmitting information from the muscles, organs, and glands to the spinal cord. There are 31 spinal nerves that branch out to different areas of the body from the spinal cord. 

Cranial Nerves

The cranial nerves are responsible for the receptors found in the head and neck area. Instead of connecting with the spinal cord, these nerves travel directly to the brain. There are 12 pairs of cranial nerves that transmit motor and sensory information from areas including the face, mouth, eyes, nose, and ears.

Recap

The PNS is made up of cranial nerves that connect directly with the brain and spinal nerves that connect with the spinal cord.

Diseases of the Peripheral Nervous System

There are a number of diseases and disorders that can affect the peripheral nervous system. Problems with the sensory and motor neurons in the PNS can result in changes in sensation, muscle weakness, or muscle paralysis.

Some of the conditions that can affect the PNS include:

• Amyotrophic lateral sclerosis (ALS)
• Carpal tunnel syndrome
• Diabetes
• Epstein-Barr virus
• Guillain-Barre syndrome
• Lupus
  
• Peripheral neuropathy
• Peripheral nerve injuries
• Rheumatoid arthritis
• Thoracic outlet syndrome
• Tumors
• Vitamin deficiency
  

Symptoms of PNS diseases and disorders can include numbness, pain, tingling, burning, sensitivity to touch, and muscle weakness. 

Recap

Disorders, injuries, toxins, and viruses can cause problems with the peripheral nervous system. Such conditions can lead to symptoms related to sensation, muscle strength, and muscle control.

Treatment for PNS Conditions

Treatment for peripheral nerve problems depends on the cause and focuses on treating the underlying disorder and offering supportive care. For example, some PNS conditions may be caused by injury while others may be related to underlying health conditions, viruses, toxins, or genetic disorders. 

In addition to treating conditions that may be causing dysfunction or damage to PNS nerves, treatment may also involve surgery, physical therapy, occupational therapy, speech/language therapy, and respiratory support.

A Word From Verywell

The peripheral nervous system plays a critical role in motor and sensory function. If you are experiencing symptoms such as muscle weakness, numbness, loss of sensation, or sensitivity, talk to a healthcare provider for further evaluation.

Frequently Asked Questions

• What does the peripheral nervous system do?

  The nerves of the peripheral nervous system are responsible for relaying information between the body and the brain. This includes involuntary body functions such as breathing, blood flow, and heartbeat as well as sensory information and control of voluntary movement.

• What is the peripheral nervous system made up of?

  The peripheral nervous system is made up of the nerves and ganglia that lie outside of the brain and spinal cord. It begins with the spinal nerves that branch off from the spinal cord and the cranial nerves that connect directly to the brain.

• What are the two divisions of the peripheral nervous system?

  The two divisions of the PNS are the somatic nervous system and the autonomic nervous system. The somatic system includes the nerves that transmit motor and sensory information to and from the CNS. The autonomic system regulates automatic body functions including digestion and blood pressure. 

• How would an injury to the central nervous system affect the peripheral nervous system?

  Injuries to the CNS may affect the ability of the brain and spinal cord to send signals to the peripheral nervous system. For example, damage to the spinal cord may affect the ability to transmit motor and sensory information to the rest of the body.

  Some research suggests that traumatic brain injury may be associated with an increased risk for disorders that can affect the peripheral nervous system, such as ALS.

  What nervous system activates the skeletal muscles?

  The somatic nervous system provides control of skeletal muscle movement.

  Which division of the peripheral nervous system activates skeletal muscles quizlet?

  The peripheral nervous system is divided into the somatic and autonomic nervous systems, and the autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems. The somatic nervous system controls voluntary skeletal muscles.

  Which division of the peripheral nervous system activates?

  Answer and Explanation: The division of the peripheral nervous system that activates skeletal muscle is called the somatic nervous system. The peripheral nervous system consists of sensory and motor neurons.

  Is skeletal muscle controlled by sympathetic or parasympathetic?

  The sympathetic nervous system regulates skeletal muscle motor innervation and acetylcholine receptor stability - PMC.