Which cells carry information by sending nerve impulses throughout the body?

The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord.

Thus, the peripheral nervous system includes

• The nerves that connect the head, face, eyes, nose, muscles, and ears to the brain (cranial nerves Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more )

• The nerves that connect the spinal cord to the rest of the body, including the 31 pairs of spinal nerves

• More than 100 billion nerve cells that run throughout the body

Using the Brain to Move a Muscle

Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate in the brain, as when a person consciously decides to move a muscle—for example, to pick up a book.

Or the impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to sense pain or a change in temperature. This sensory information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways:

• The sensory nerve pathway to the brain

• The motor nerve pathway to the muscle

• If sensory receptors in the skin detect pain or a change in temperature, they transmit an impulse (signal), which ultimately reaches the brain.

• The impulse travels along a sensory nerve to the spinal cord.

• The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord.

• The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord.

• The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information, located deep in the brain.

• The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors).

• The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse.

• The nerve carrying the impulse crosses to the opposite side at the base of the brain.

• The impulse is sent down the spinal cord.

• The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord.

• The impulse travels out of the spinal cord along the length of the motor nerve.

• At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where the impulse stimulates the muscle to move.

If the sensation occurs suddenly and is severe (as when stepping on a sharp rock or picking up a cup of very hot coffee), the impulse may travel to the spinal cord and directly back to the motor nerve, bypassing the brain. The result is a quick response of a muscle—by immediately withdrawing from whatever is causing the pain. This response is called a spinal reflex.

Dysfunction of peripheral nerves may result from damage to any part of the nerve:

• Axon (the part that sends messages)

• Body of the nerve cell

• Myelin sheath (the membranes that surround the axon and that function much like insulation around electrical wires, enabling nerve impulses to travel quickly)

Damage to the myelin sheath is called demyelination Overview of Demyelinating Disorders Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation... read more , as occurs in Guillain-Barré syndrome Guillain-Barré Syndrome (GBS) Guillain-Barré syndrome is a form of polyneuropathy causing muscle weakness, which usually worsens over a few days to weeks, then slowly improves or returns to normal on its own. With treatment... read more .

Typical Structure of a Nerve Cell

A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. The impulses from the axon cross a synapse (the junction between two nerve cells) to the dendrite of another cell.

Each large axon is surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one.

Insulating a Nerve Fiber

Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation around an electrical wire, the myelin sheath enables nerve signals (electrical impulses) to be conducted along the nerve fiber with speed and accuracy. When the myelin sheath is damaged (called demyelination), nerves do not conduct electrical impulses normally.

Peripheral nerve disorders can affect

• One nerve (mononeuropathy Mononeuropathy Mononeuropathy is damage to a single peripheral nerve. Pressure on a nerve for a long time is the most common cause of mononeuropathy. The affected area may tingle, feel prickly, or be numb... read more

  )

• Two or more peripheral nerves in separate areas of the body (multiple mononeuropathy Multiple Mononeuropathy Multiple mononeuropathy is the simultaneous malfunction of two or more peripheral nerves in separate areas of the body. It causes abnormal sensations and weakness. (See also Overview of the... read more )

• Many nerves throughout the body but usually in about the same areas on both sides of the body (polyneuropathy Polyneuropathy Polyneuropathy is the simultaneous malfunction of many peripheral nerves throughout the body. Infections, toxins, drugs, cancers, nutritional deficiencies, diabetes, autoimmune disorders, and... read more )

• A (the part of the spinal nerve connected to the spinal cord)

• A plexus Plexus Disorders Plexuses (networks of interwoven nerve fibers from different spinal nerves) may be damaged by injury, tumors, pockets of blood (hematomas), or autoimmune reactions. Pain, weakness, and loss... read more (a network of nerve fibers, where fibers from different spinal nerves are sorted and recombined to serve a particular area of the body)

• The neuromuscular junction Overview of Neuromuscular Junction Disorders Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors... read more (where the nerve and muscle connect)

If motor nerves (which control muscle movement) are damaged, muscles may weaken or become paralyzed. If sensory nerves (which carry sensory information—about such things as pain, temperature, and vibration) are damaged, abnormal sensations may be felt or sensation may be lost.

Synapse

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Nerve Cells and Fibers

Causes

Peripheral nerve disorders can be hereditary or acquired (caused by exposure to toxins, injury, infections, or metabolic or inflammatory disorders).

Table

Disorders that may resemble peripheral nerve disorders

Certain disorders cause progressive deterioration of the nerve cells in the spinal cord and brain that control muscle movement (motor neuron diseases Amyotrophic Lateral Sclerosis (ALS) and Other Motor Neuron Diseases (MNDs) Motor neuron diseases are characterized by progressive deterioration of the nerve cells that initiate muscle movement. As a result, the muscles stimulated by these nerves deteriorate, become... read more ) as well as in the peripheral nerves. Motor neuron diseases can resemble peripheral nerve disorders, which affect nerve cells outside the brain and spinal cord rather than those in the spinal cord or brain. Motor neuron diseases may be caused by viruses (such as the polio Polio Polio is a highly contagious, sometimes fatal enterovirus infection that affects nerves and can cause permanent muscle weakness, paralysis, and other symptoms. Polio is caused by a virus and... read more virus), be inherited, or have no clear known cause (such as amyotrophic lateral sclerosis Amyotrophic Lateral Sclerosis (ALS) and Other Motor Neuron Diseases (MNDs) Motor neuron diseases are characterized by progressive deterioration of the nerve cells that initiate muscle movement. As a result, the muscles stimulated by these nerves deteriorate, become... read more ).

Neuromuscular junction disorders Overview of Neuromuscular Junction Disorders Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors... read more are distinct from peripheral nerve disorders, although they may have similar consequences such as muscle weakness. The neuromuscular junction is where the ends of peripheral nerve fibers connect to special sites on a muscle’s membrane. The nerve fibers release a chemical messenger (neurotransmitter) that sends a nerve impulse across the neuromuscular junction and signals a muscle to contract. Neuromuscular junction disorders include

• Infant botulism Infant Botulism Infant botulism is a potentially life-threatening infection that causes muscle weakness. It develops in infants who eat food containing spores of Clostridium botulinum bacteria. (See... read more

• Eaton-Lambert syndrome Eaton-Lambert Syndrome Eaton-Lambert syndrome is an autoimmune disorder that impairs communication between nerves and muscles, causing weakness. Eaton-Lambert syndrome usually precedes, occurs with, or develops after... read more

• Myasthenia gravis Myasthenia Gravis Myasthenia gravis is an autoimmune disorder that impairs communication between nerves and muscles, resulting in episodes of muscle weakness. Myasthenia gravis results from malfunction of the... read more

• Dysfunction caused by certain insecticides Insecticide Poisoning Insecticides are chemicals used to kill insects. Some insecticides are also dangerous to humans. Many insecticides can cause poisoning after being swallowed, inhaled, or absorbed through the... read more (organophosphate pesticides) or chemical warfare agents Nerve Chemical-Warfare Agents There are many types of chemical-warfare agents that affect different parts of the body. Nerve agents (sometimes incorrectly called "nerve gas") affect how nerves transmit signals to muscles... read more (such as sarin gas and Novichok) or by use of certain drugs (such as curare)

Novichok was developed in Russia and has been used in assassination attempts. Curare has been used to help relax muscles during surgery and to paralyze and kill when placed on the tip of poison darts.

Disorders that affect muscle rather than nerves (as peripheral nerve disorders do) also cause muscle weakness. Muscle disorders may be categorized as

• Hereditary, such as Duchenne muscular dystrophy, Becker muscular dystrophy Duchenne Muscular Dystrophy and Becker Muscular Dystrophy Muscular dystrophies are a group of inherited muscle disorders in which one or more genes needed for normal muscle structure and function are defective, leading to muscle weakness of varying... read more , familial periodic paralysis Familial Periodic Paralysis Familial periodic paralysis is a rare inherited disorder that causes sudden attacks of weakness and paralysis. There are four different forms, which involve abnormalities in how electrolytes... read more , limb-girdle muscular dystrophy Limb-Girdle Dystrophy Limb-girdle dystrophy encompasses a group of muscular dystrophies that can be inherited in various ways. The muscles of the shoulder or pelvis are affected. Muscular dystrophies are a group... read more , myotonia congenita Myotonia Congenita Myotonia congenita is an inherited disorder that causes muscle stiffness and muscle enlargement. It affects what most people think of as muscle, the type that can be contracted to move the various... read more (Thomsen disease), and myotonic dystrophy Myotonic Dystrophy Myotonic dystrophy is a rare muscular dystrophy. This disorder affects the ability to relax the muscles at will. Myotonia refers to delayed relaxation after muscle contraction, which can cause... read more (Steinert disease)

• Endocrine, such as acromegaly (excessive growth due to overproduction of growth hormone), Cushing syndrome Cushing Syndrome In Cushing syndrome, the level of corticosteroids is excessive, usually due to taking corticosteroid drugs or overproduction by the adrenal glands. Cushing syndrome usually results from taking... read more

  , diabetes mellitus, hyperthyroidism Hyperthyroidism Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more
  (an overactive thyroid gland), and hypothyroidism Hypothyroidism Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions. Facial expressions become dull, the voice... read more
  (an underactive thyroid gland)

• Inflammatory, such as infections (usually viral) and polymyositis and dermatomyositis Autoimmune Myositis Autoimmune myositis causes inflammation and weakness in the muscles (polymyositis) or in the skin and muscles (dermatomyositis). Muscle damage may cause muscle pain and muscle weakness may cause... read more

• Metabolic, such as lipid Other Rare Hereditary Disorders of Lipid Metabolism Lipid metabolism disorders are hereditary metabolic disorders. Hereditary disorders occur when parents pass the defective genes that cause these disorders on to their children. There are different... read more

  and glycogen storage diseases Glycogen Storage Diseases Glycogen storage diseases are carbohydrate metabolism disorders that occur when there is a defect in the enzymes that are involved in the metabolism of glycogen, often resulting in growth abnormalities... read more , alcoholism, and hypokalemia Hypokalemia (Low Level of Potassium in the Blood) In hypokalemia, the level of potassium in blood is too low. A low potassium level has many causes but usually results from vomiting, diarrhea, adrenal gland disorders, or use of diuretics. A... read more (low potassium levels)

Doctors do tests to determine whether the cause of weakness is a muscle, a neuromuscular junction, or a nerve disorder.