Peripheral Nervous System
The peripheral nervous system (PNS) is made up of both nerves and ganglia, and its primary role is to connect the central nervous system (CNS) to the organs, limbs, and skin. The nerves of the PNS extend from the CNS to these outermost areas of the body. Axons that originate from neurons located in the CNS make up the PNS and are responsible for delivering signals to the most distant parts of the body. The PNS, unlike the CNS, is not protected by bone or the blood—brain barrier. This makes it vulnerable to toxins and mechanical injury.
Anatomy and Physiology
The PNS consists of 31 pairs of spinal nerves that originate in the spinal cord and 12 pairs of cranial nerves that originate in the brain. There are two important types of cells within the PNS: glial cells and neurons. Glial cells of the PNS include Schwann cells that surround nerve fibers and perineuronal satellite cells that surround the cell body. Both types of cells produce myelin sheaths around axons and some cell bodies of ganglia. Myelin is essential for fast communication between neurons. Schwann cells also have the ability to become phagocytes in response to nerve injury and inflammation. Neurons specialize in rapid nerve impulse conduction, allowing for the exchange of signals with other neurons. The body of the neuron, or soma, is located in the CNS while the axon projects and terminates in the skin, organs, or muscles, allowing for rapid communication between the CNS and PNS.
Somatic and Autonomic Nervous Systems
The PNS is divided into two parts: the somatic and autonomic nervous systems. The somatic nervous system conveys and processes conscious and unconscious sensory information including vision, pain, and touch. Additionally, this system is responsible for motor control of voluntary muscles, allowing for coordinated muscle activity that can be adjusted based on an animal’s environment. This system contains two major types of neurons: sensory neurons that carry information from the nerves to the CNS and motor neurons that carry information away from the CNS toward muscle fibers throughout the body.
In addition to controlling voluntary movements, the somatic nervous system is associated with involuntary movements. These involuntary movements are known as reflex arcs, during which muscles move involuntarily without input from the brain and a nerve pathway connects directly to the spinal cord. Placing one’s hand on a hot stove and pulling it away quickly is an example of a reflex arc. A sensory receptor responds to an environmental stimulus (hot stove) and afferent fibers (A delta and C pain fibers) convey this signal through peripheral nerves to the gray matter of the spinal cord. The afferent root enters the spinal cord and synapses with an interneuron, which synapses with alpha motoneurons. Alpha-motoneurons transmit an impulse to voluntary muscles, allowing the person to pull his or her hand away.
The autonomic nervous system conveys and processes sensory input from visceral organs in addition to motor control of the involuntary and cardiac musculature. It is responsible for control of involuntary or visceral bodily functions including cardiovascular, respiratory, digestive, urinary, and reproductive systems. Examples of these body functions include heartbeat, digestion, and breathing. Additionally, it plays a key role in the way the body handles stress and recovers from stressful situations.
Sympathetic and Parasympathetic Nervous Systems
The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system regulates the “fight-or-flight” response, allowing the body to function under stress. A sympathetic response dilates pupils, inhibits salivation, relaxes the bronchi (to ease breathing), decreases digestive activity, and stimulates secretion of epinephrine and norepinephrine from the kidney. Additionally, stimulation increases blood flow to skeletal muscles, increases chronotropic and inotropic effects of the heart, and releases glucose stores from the liver. All of these reactions enable a person to respond immediately to an emergent, stressful situation such as avoiding a car accident while driving.
Based on the organization of the sympathetic nervous system, it is also referred to as the thoracolumbar or adrenergic system. All preganglionic fibers within the sympathetic nervous system emerge from the thoracic and upper two lumbar levels of the spinal cord. Norepinephrine and epinephrine are the primary neurotransmitters released by postganglionic fibers within this system and are responsible for many effects seen in the fight-or-flight response. The sympathetic system is designed to exert its effects over widespread body regions for a sustained period of time.
The parasympathetic nervous system regulates the “rest and digest” response, returning the body to normal function in order to conserve physical resources. A parasympathetic response constricts pupils, stimulates salivation, reduces heart rate, constricts bronchi, stimulates digestive activity, and contracts the bladder. All of these responses allow a person to conserve energy during times of relaxation, preparing the person for more stressful situations. The vagus nerve (the 10th pair of the 12 cranial nerves) is the main regulator of automatic functions within the parasympathetic nervous system.
The parasympathetic nervous system is also referred to as the craniosacral or cholinergic system. Preganglionic fibers within the parasympathetic nervous system emerge with several cranial nerve pairs (III—oculomotor, VII—facial, IX—glossopharyngeal, and X—vagus) and at the sacral division (S2—S4) of the spinal cord. Acetylcholine is the neurotransmitter released by postganglionic fibers within the parasympathetic nervous system and is responsible for many effects of rest and digest. The parasympathetic nervous system is organized to respond transiently to a stimulus in a localized region of the body.
See also: Afferent Tracts; Autonomic Nervous System; Central Nervous System; Cranial Nerves; Nociception
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