What is the nervous system?

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The major control system of the body, which synchronizes physiologic activity by interpreting incoming stimuli and which is responsible for memory and reasoning; it is composed of the central nervous system (the brain and spinal cord) and the peripheral nervous system (nerve processes, sensory receptors, and ganglia).
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Structure and Functions

The nervous system serves as the major control system of the human body. It is responsible for the synchronization of body parts, the integration of physiologic activity, the interpretation of incoming stimuli, and all intellectual activity, including memory and abstract reasoning. The nervous system regulates these activities by communication between various nerve cells; by controlling the actions of skeletal, smooth, and cardiac muscle; and by stimulating the secretion of products from various glands of the body.

Anatomically, the nervous system is divided into the central nervous system, which is composed of the brain and the spinal cord, and the peripheral nervous system, which includes all nervous structures outside the central nervous system—primarily nerve processes, sensory receptors, and a limited number of cells of the nervous system that are located in special structures known as ganglia. Ganglia are found at various locations throughout the body. They are the only locations of neurons outside the central nervous system. Information from incoming cells can be transmitted to the ganglion cells, which in turn can transmit that information to other locations.

Although the brain and the spinal cord contain several different types of cells that are morphologically unique, there is only one type of functional cell present, which by convention is always referred to as the neuron. The neuron is one of the few cells in the body that cannot reproduce; a fixed number of these cells develop in infancy, and the number never increases, though it can decrease in the event of injury or disease.

The neuron consists of a cell body that is similar to that of the typical animal cell familiar to most people. In addition, the neuron has extensions called processes. In the typical neuron, there are two types of processes: dendrites and axons.

Usually a neuron has many dendrites. Dendrites are very short; they receive information from nearby cells and relay that information to the cell body. Each cell has only a single axon, which may be very long, extending up and down the spinal cord or from the spinal cord to the ends of the fingers or toes. The axons conduct information from the cell bodies to the effectors—that is, the muscles and glands—or to other neurons.

Functionally, the nervous system is divided into two areas: the somatic nervous system and the autonomic nervous system. The somatic system controls posture and locomotion by stimulating the skeletal muscles. It is responsible for knowing where the body is in space and for ensuring that there is sufficient muscle contraction (tone) to maintain posture. Responses of the somatic system occur through the motor neurons.

The autonomic nervous system regulates internal activities through the innervation, or nerve stimulation, of the smooth muscles or the glands. It is anatomically different from the somatic nervous system in that the stimulation of body parts always involves two neurons. The cell body of the second neuron in the sequence is located in a ganglion outside the central nervous system.

The autonomic nervous system is broken down further into two divisions: the sympathetic and the parasympathetic. The sympathetic system is also known as the “fight or flight” reaction, since it evolved from the mechanism in lower animals by which an animal would prepare to fight a predator or run from it. More commonly, it is referred to in humans as the adrenaline response, which is active during stressful situations, strenuous physical activity, public performance, or competition.

The parasympathetic system, which is responsible for the digestive functions of the body, controls stimulation of salivary gland secretions, increased blood flow to digestive organs, and movement of material through the digestive system. The sympathetic and parasympathetic systems usually function in balance; the parasympathetic system predominates after meals, and the sympathetic system predominates during periods of stress or physical activity.

Neurons communicate with other neurons or effectors through the release of chemical messengers known as neurotransmitters. At the termination of the axon, there is a widened area known as the synaptic knob, which produces and stores neurotransmitters. The effects of neurotransmitters are always localized and of short duration. There are many types of neurotransmitters, some of which are well known, such as acetylcholine and norepinephrine.

Neurotransmitters are released in response to an electrical impulse that is conducted along the axon. Once released, a neurotransmitter binds to cells that have appropriate receptors on their dendrites. Neurotransmitters may either stimulate or inhibit the activity of the second cell. If there is significant stimulation of the second cell, it will conduct the information along its axon and release a neurotransmitter from the axon terminal, which will in turn stimulate or inhibit the next neuron or effector. There must be a mechanism for the immediate removal of neurotransmitters from the synaptic cleft if the stimulation of the second neuron is to cease and if other impulses are to be conducted.

Neurotransmitters can influence only those cells that have the appropriate receptors on their surfaces. It is through the neurotransmitter-receptor complex that neurotransmitters are able to influence cells, and any alteration of the number or type of receptors on a cell membrane will lead to an alteration of cellular functioning.

The axons of some neurons are covered with multiple layers of a cell membrane known as myelin. The myelin is produced by specialized cells in the brain known as oligodendrocytes and by cells in the peripheral axons known as Schwann cells. Myelin serves as an insulator for axons and is effective in speeding up the conduction of nerve impulses. It is essential for the normal functioning of the nervous system.

The brain and spinal cord are enclosed by three membranes of dense connective tissue called the meninges, which separate the nervous system from other tissue and from the skull and spinal cord. From the outside inward, they are the dura mater, the arachnoid mater, and the pia mater. Many of the blood vessels of the brain travel through the meninges; therefore, the surface of the brain is very vascular and is subject to bleeding or clotting after trauma.

Disorders and Diseases

Diseases of the nervous system can be arranged into several general categories: infections, congenital diseases, seizure disorders, circulatory diseases, traumatic injury, demyelinating diseases, degenerative diseases, mental diseases, and neoplasms.

Infections of the nervous system are described according to the tissues infected. If the meninges are infected, the disease is known as meningitis; if the brain tissue is infected, the disease is referred to as encephalitis. The development of abscesses in the nervous tissue can also occur. The conditions described can be caused by viruses, bacteria, protozoa, or other parasites.

In most cases, the organism that causes meningitis is spread via the bloodstream. It is also possible for infections to be spread via an infected middle ear or paranasal sinus, a skull fracture, brain surgery, or a lumbar puncture. The infectious agent can usually be determined by analyzing the spinal fluid. Bacterial infections are treated with antibiotics, while viral infections receive only supportive treatment.

An abscess of nervous tissue is usually a complication resulting from an infection at some other anatomical site, particularly from middle-ear infections or sinus infections. Abscesses may also occur following penetrating injuries. The abscess can create pressure inside the skull and, if left untreated, may rupture and lead to death.

Viral encephalitis is an acute disease that is often spread to humans by arthropods from animal hosts. After a carrier insect bites a human, the virus is spread to the brain of the human via the bloodstream. The specific causative agent often goes undiagnosed. Some well-known forms of encephalitis are herpes simplex encephalitis, poliomyelitis, rabies, and cytomegalovirus encephalitis. In addition, some forms of encephalitis fall into the category of slow virus infections, which have latent periods as long as several years between the time of infection and the development of encephalitis.

Other serious infections include neurosyphilis, which occurs in the late stages of untreated syphilis infections; toxoplasmosis, a protozoan infection that is extremely dangerous to fetuses but rarely causes serious problems in adults; cerebral malaria; and African trypanosomiasis, which is also known as sleeping sickness.

Congenital diseases of the brain vary in the degree of malfunction they produce. Spina bifida is a general term for a group of disorders in which the vertebrae do not develop as they should. As a result, the spinal cord may protrude from the lower back. In some cases, the effects may be so minimal as to produce no symptoms; in other cases, however, these malformations may lead to major neurologic impairment.

Hydrocephalus is another congenital malformation, one that may lead to an increase in the size of the ventricles of the brain. It may be caused by blockage of the flow of spinal fluid in the fetus. In some cases, the spinal fluid produced by the nervous system fills the ventricles and limits the space available for the growing brain and nervous tissue. The result under these conditions is the presence of larger-than-normal ventricles and a smaller-than-normal amount of nervous tissue.

A seizure disorder is any sudden burst of excess electrical activity in the neurons of the brain. Epilepsy is a general term for seizure disorders. The condition may be mild and have only minimal effects, or it may be severe, leading to convulsions. The cause is often unknown, but epilepsy may result from infection, trauma, or neoplasms.

Cerebrovascular accident (CVA) is the term used to describe a variety of malfunctions of blood circulation in the nervous system that are not a result of trauma. More commonly, the term stroke is used to describe the condition. Strokes have many causes that generally fall into two categories: ischemic and hemorrhagic.

Ischemic strokes are those in which the nervous tissue is deprived of oxygen as a result of an impairment of blood flow to the area. An ischemic stroke is most commonly the result of a blood clot that blocks the blood vessels leading to the brain or the blood vessels in the brain itself. Since the cells can live for only a few minutes without oxygen, an ischemic stroke can result in neurological impairment or even death.

In hemorrhagic strokes, there is bleeding in the brain itself. It may be caused by hypertension or by the rupture of a weakened blood vessel, which is known as an aneurysm. Both ischemic and hemorrhagic strokes lead to the death of neurons in the affected area. The degree of damage to the brain is determined by the number of cells destroyed by the oxygen deprivation.

Traumatic injury to the brain can generally be classified as penetrating or nonpenetrating. Penetrating injuries produce a risk of infection as well as bleeding at the site of the wound. Since many large blood vessels are located in the meninges, even injuries that penetrate only into the meninges may be sufficient to cause serious injury. Nonpenetrating injuries may also cause bleeding of the meninges, which can limit blood flow to the nervous tissue or put excessive pressure on the tissue.

Injury to the spinal cord may result in severing the spinal cord from the brain. If this should occur, communications between the brain and any structures below the area of the injury are lost, as is all sensory and motor function in those areas. Since neurons are unable to regenerate and axon repair is limited, there is little hope for reversal of this condition, although extensive research is being conducted in this area.

Demyelinating diseases are those that result in changes in the myelin sheaths of neurons. The most common example is multiple sclerosis, which affects myelin in the central nervous system but not in the peripheral nervous system. Although there are varying degrees of severity, the condition causes limb weakness, impaired perception, and optic neuritis, among other things. Some cases present only mild symptoms, while others are degenerative and can lead to death, sometimes within months. Many patients, however, survive for more than twenty years. The cause of these diseases is not yet clear, although viral infections have been associated with some demyelinating diseases.

Degenerative diseases are those in which there is a gradual decline in nervous function. The disease may be hereditary, as in the case of Huntington’s disease, or may occur without any apparent genetic basis, as in the case of Parkinson’s disease. Parkinson’s disease involves the death of certain neurons in the brain and a decreased concentration of neurotransmitters. As the disease progresses, there is a gradual loss of motor ability and, ultimately, a complete loss of motor function. Not much is known about neurotransmitter replacement or mechanisms to stop degenerative diseases.

Little is known about mental diseases such as schizophrenia and manic depression. They appear to involve abnormal levels of neurotransmitters or errors in the membrane receptors associated with those neurotransmitters. Success in localizing the causes of these diseases has been slow in coming; there has been much more success in the development of medications to treat them.

Cancer of the brain can be primary or metastatic. Metastatic tumors, the more common variety, can arise from any source. Of the primary neoplasms, the most common are those derived from glial cells, which are responsible for more than 65 percent of all primary neoplasms. The second most common are neoplasms resulting from transformation of cells of the meninges. Since neurons cannot divide, neuron tumors are almost nonexistent except in children.

Perspective and Prospects

When the control system of the body experiences a malfunction, the effects are wide ranging. Since the nervous system is responsible for regulating so many diverse activities, nervous-system injury or disease must be treated immediately if the patient is to survive. This problem is further complicated by the fact that the brain is a difficult organ to study, because of its location within the skull and because its cells are vital and can be studied only after they have died.

Disease or injury of the cells of the nervous system, especially the brain, creates problems that are unique to that organ for several reasons, including the fact that those cells cannot repair themselves and cannot divide. In addition, the cells of the brain are restricted to a limited area. The cells of the nervous system are unique in that they are so highly specialized that they are not capable of cell division. As a result, humans have the greatest number of neurons during early childhood. Any neural injury or disease that kills cells results in a decreased number of neurons. Furthermore, the space in the skull is tightly packed with cells and cerebrospinal fluid, leaving no room for blood that might result from an injury or fluid accumulation due to tissue infection or tumors. Any of these conditions will increase the pressure within the skull and will also increase the extent of the injury to the nervous tissue.

Although there is no mechanism for replacing cells that have died, the prognosis is not totally bleak. There are cells in the brain that can, in the event of disease or injury, assume the responsibilities of the dead cells. For example, a person who has lost the capacity to speak following a stroke may be retaught to speak using cells that previously did not perform that function.

Among the problems with which the nervous system must cope, there are many things that can go wrong at the synapse of a neuron. The cell may produce too little or too much neurotransmitter. It is possible that the neurotransmitter may not be released on cue or that, if it is released, the postsynaptic cells will not have the appropriate receptors. There also may be no mechanism for removal of the neurotransmitter from the synaptic cleft. These are only a few of the problems that can interfere with communication between different neurons or between neurons and other effectors. As science learns more about the communication system of neurons, efforts to correct these problems will intensify. Already there are many drugs available that can alter activity at the synapse. Correcting these errors can lead to methods for the treatment of mental diseases.

Someday it may be possible to transplant healthy neurons from one person to another. This procedure may permit physicians to prevent total paralysis in a person who has suffered a broken neck or total loss of motor function in an individual who suffers from Parkinson’s disease. In 1990, normal neurons were grown in tissue culture for the first time. Such scientific breakthroughs will lead to more and better treatments for individuals who suffer from diseases of the nervous system.

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