11.6: Peripheral Nervous System - Biology

11.6: Peripheral Nervous System - Biology

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One Piano, Four Hands

Did you ever see two people play the same piano? How do they coordinate all the movements of their own fingers, let alone synchronize them with those of their partner? The peripheral nervous system plays an important part in this challenge.

What Is the Peripheral Nervous System?

The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside of the central nervous system (CNS). The main function of the PNS is to connect the CNS to the rest of the organism. It serves as a communication relay, going back and forth between the CNS and muscles, organs, and glands throughout the body.

Tissues of the Peripheral Nervous System

The tissues that make up the PNS are nerves and ganglia. Ganglia are nervous tissues that act as relay points for messages transmitted through nerves of the PNS. Nerves are cable-like bundles of axons that make up the majority of PNS tissues. Nerves are generally classified on the basis of the direction in which they carry nerve impulses as sensory, motor, or mixed nerves. See examples of sensory and motor never in Figure (PageIndex{3}).

  • Sensory nerves transmit information from sensory receptors in the body to the CNS. Sensory nerves are also called afferent nerves.
  • Motor nerves transmit information from the CNS to muscles, organs, and glands. Motor nerves are also called efferent nerves.
  • Mixed nerves contain both sensory and motor neurons, so they can transmit information in both directions. They have both afferent and efferent functions.

Divisions of the Peripheral Nervous System

The PNS is divided into two major systems, called the autonomic nervous system and the somatic (or sensory-somatic) nervous system. Both systems of the PNS interact with the CNS and include sensory and motor neurons, but they use different circuits of nerves and ganglia.

Somatic Nervous System

The somatic nervous system primarily senses the external environment and controls voluntary activities in which decisions and commands come from the cerebral cortex of the brain. For example, when you feel too warm, decide to turn on the air conditioner, and walk across the room to the thermostat, you are using your somatic nervous system. In general, the somatic nervous system is responsible for all of your conscious perceptions of the outside world and all of the voluntary motor activities you perform in response. Whether it’s playing piano, driving a car, or playing basketball, you can thank your somatic nervous system for making it possible.

Structurally, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Figure (PageIndex{2})). Cranial nerves are in the head and neck and connect directly to the brain. Sensory cranial nerves sense smells, tastes, light, sounds, and body position. Motor cranial nerves control muscles of the face, tongue, eyeballs, throat, head, and shoulders. The motor nerves also control the salivary glands and swallowing. Four of the 12 cranial nerves participate in both sensory and motor functions as mixed nerves, having both sensory and motor neurons.

Spinal nerves of the somatic nervous system emanate from the spinal column between vertebrae. All of the spinal nerves are mixed nerves, containing both sensory and motor neurons. Spinal nerves also include motor nerves that stimulate skeletal muscle contraction, allowing for voluntary body movements.

Autonomic Nervous System

The autonomic nervous system primarily senses the internal environment and controls involuntary activities. It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. In general, the autonomic nervous system is responsible for all the activities that go on inside your body without your conscious awareness or voluntary participation.

Structurally, the autonomic nervous system consists of sensory and motor nerves that run between the CNS (especially the hypothalamus in the brain) and internal organs (such as the heart, lungs, and digestive organs) and glands (such as the pancreas and sweat glands). Sensory neurons in the autonomic system detect internal body conditions and send messages to the brain. Motor nerves in the autonomic system function by controlling the contractions of smooth or cardiac muscle or glandular tissue. For example, when sensory nerves of the autonomic system detect a rise in body temperature, motor nerves signal smooth muscles in blood vessels near the body surface to undergo vasodilation, and the sweat glands in the skin secrete more sweat to cool the body.

The autonomic nervous system, in turn, has two subdivisions: the sympathetic division and parasympathetic division. The two subdivisions of the autonomic system are summarized in Figure (PageIndex{4}). Both affect the same organs and glands, but they generally do so in opposite ways.

  • The sympathetic division controls the fight-or-flight response. Changes occur in organs and glands throughout the body that prepare the body to fight or flee in response to a perceived danger. For example, the heart rate speeds up, air passages in the lungs become wider, more blood flows to the skeletal muscles, and the digestive system temporarily shuts down.
  • The parasympathetic division returns the body to normal after the fight-or-flight response has occurred. For example, it slows down the heart rate, narrows air passages in the lungs, reduces blood flow to the skeletal muscles, and stimulates the digestive system to start working again. The parasympathetic division also maintains the internal homeostasis of the body at other times.

Disorders of the Peripheral Nervous System

Unlike the CNS, which is protected by bones, meninges, and cerebrospinal fluid, the PNS has no such protections. The PNS also has no blood-brain barrier to protect it from toxins and pathogens in the blood. Therefore, the PNS is more subject to injury and disease than is the CNS. Causes of nerve injury include diabetes, infectious diseases such as shingles, and poisoning by toxins such as heavy metals. Disorders of the PNS often have symptoms such as loss of feeling, tingling, burning sensations, or muscle weakness. If a traumatic injury results in a nerve being transacted (cut all the way through), it may regenerate, but this is a very slow process and may take many months.


  1. Describe the general structure of the peripheral nervous system, and state its primary function.
  2. What are ganglia?
  3. Identify three types of nerves based on the direction in which they carry nerve impulses.
  4. Outline all of the divisions of the peripheral nervous system.
  5. Compare and contrast the somatic and autonomic nervous systems.
  6. When and how does the sympathetic division of the autonomic nervous system affect the body?
  7. What is the function of the parasympathetic division of the autonomic nervous system? What specific effects does it have on the body?
  8. Name and describe two disorders of the peripheral nervous system.
  9. Give one example of how the CNS interacts with the PNS to control a function in the body.
  10. For each of the following types of information, identify whether the neuron carrying it is sensory or motor and whether it is most likely in the somatic or autonomic nervous system.
    1. Visual information
    2. Blood pressure information
    3. Information that causes muscle contraction in digestive organs after eating
    4. Information that causes muscle contraction in skeletal muscles based on the person’s decision to make a movement
  11. The cranial nerves:
    1. Carry sensory information
    2. Carry motor information
    3. Are part of the somatic nervous system
    4. All of the above
  12. True or False. All of the spinal nerves carry both sensory and motor information.
  13. True or False. The sympathetic nervous system enhances digestion to provide more energy for the body.

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Mindfulness techniques have been shown to reduce symptoms of depression as well as those of anxiety and stress. They have also been shown to be useful for pain management and performance enhancement. Specific mindfulness programs include Mindfulness-Based Stress Reduction (MBSR) and Mindfulness Mind-Fitness Training (MMFT). You can learn more about MBSR by watching the video below.

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11 Mammalian Tissues

In biology, tissue is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. The English word is derived from the French tissu, meaning something that is woven, from the verb tisser, “to weave”.

The study of human and animal tissues is known as histology or, in connection with disease, histopathology. For plants, the discipline is called plant anatomy. The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.

Introduction to the Adrenal Glands

Figure 9.6.2 Each of the two adrenal glands is found above a kidney.

The adrenal glands are endocrine glands that produce a variety of hormones. Adrenal hormones include the fight-or-flight hormone adrenaline and the steroid hormone cortisol. The two adrenal glands are located on both sides of the body, just above the kidneys, as shown in Figure 9.6.2. The right adrenal gland (on the left in the figure) is smaller and has a pyramidal shape. The left adrenal gland (on the right in the figure) is larger and has a half-moon shape.

Each adrenal gland has two distinct parts, and each part has a different function, although both parts produce hormones. There is an outer layer, called the adrenal cortex, which produces steroid hormones including cortisol. There is also an inner layer, called the adrenal medulla, which produces non-steroid hormones including adrenaline.

Neurobiology of substance P and the NK1 receptor

Substance P belongs to a group of neurokinins (NKs), small peptides that are broadly distributed in the central nervous system (CNS) and peripheral nervous system (PNS). The biological effects of substance P in the CNS, namely regulation of affective behavior and emesis in the brain and nociception in the spinal cord, are mediated by its binding to the NK1 receptor. The substance P-NK1 (SP-NK1) receptor system is the most extensively studied NK pathway, and in contrast to receptors for other neurotransmitters, such as glutamate, which have high expression throughout the CNS, only a minority of neurons (5% to 7%) in certain CNS areas express the NK1 receptor. The NK1 receptor is distributed in the plasma membrane of cell bodies and dendrites of unstimulated neurons, but upon substance P binding, the NK1 receptor undergoes rapid internalization, followed by rapid recycling to the plasma membrane. Release of substance P is induced by stressful stimuli, and the magnitude of its release is proportional to the intensity and frequency of stimulation. More potent and more frequent stimuli allow diffusion of substance P farther from the site of release, allowing activation of an approximately 3- to 5-times greater number of NK1 receptor-expressing neurons. Recent studies employing pharmacologic or genetic inactivation of NK1 receptors demonstrate the important role of the SP-NK1 receptor system in the regulation of affective behavior and suggest that inhibition of this pathway may be a useful approach to treatment of depression and associated anxiety.

11.6 Infectious Polyneuropathy: Guillain-Barre Syndrome (GBS)/Acute Inflammatory Demyelinating Neuropathy

Guillain-Barre Syndrome is a polyneuropathy that is caused by a reaction to a viral infection, surgery or vaccination. Sometimes, the cause is unknown. It is believed that the immune system “sees” the proteins coating myelin surrounding axons as very similar to the protein markers on virus or bacterial pathogens and continues to destroy myelin after the infectious agents have been removed from the body. Sensory losses often precede motor losses, but both afferent and efferent nerves are affected. This condition generally begins in the feet and hands and progresses proximally over the course of a few hours, days, or weeks. The symptoms include pain, loss of sensation, and muscle weakness in the most distal parts. Deep tendon reflexes can be absent. In some cases, the condition spreads rapidly, involving muscles of respiration, in which case it can be life-threatening. Also, sometimes the autonomic nerves are affected, causing dangerous changes in heart rate and blood pressure.

For most patients, spontaneous full recovery occurs, but may take weeks, months or years. Some patients experience lasting neurological deficits, and less than 10% succumb to the disease. Plasmapheresis and intravenous immunoglobulins (IVIG) are the two main immunotherapy treatments for GBS. Plasmapheresis attempts to reduce the body’s attack on the nervous system by filtering antibodies out of the bloodstream. Similarly, administration of IVIG neutralizes harmful antibodies and inflammation. Pain-reducing medications are also used in treating GBS.

Physical therapy is a very important component of the treatment of GBS. Muscle strengthening, endurance training, and gait training are the main goals of PT interventions. Splints, orthotics, and assistive devices are sometimes helpful in returning patients to functional activities.

The prognosis of GBS is considered to be generally positive, with most people regaining muscle strength, sensation, and pre-disease activity levels. Some, however, continue to have pain, weakness, and abnormal sensations throughout life. Older patients and those with longer lasting acute phases of GBS are more likely to have poorer outcomes.

Guillain–Barré syndrome (GBS) is a rapid-onset muscle weakness as a result of damage to the peripheral nervous system. 11.6 Resource 01 – . “File:Guillain-barré syndrome – Nerve Damage.gif” by Doctor Jana is licensed under CC BY 4.0 International

Watch the video: Νευρικό Σύστημα. Μέρος B: Περιφερικό Νευρικό Σύστημα (November 2022).