Nervous System
NEURAL NETWORK
1.The neural network of the human nervous system consists of billions of nerve cells known as neurons. They relay information to and from the central nervous system (the brain and spinal cord) in the form of electrical impulses. Neurons can sense changes inside and outside the body. They also control thinking and feeling, and muscles and glands.
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| NEURON STRUCTURE The neuron consists of a central portion called the cell body and extensions called dendrites and axons. Dendrites receive stimuli in the form of electrical impulses. Impulses travel from the cell body along the axons to stimulate other cells in the body--for example, another neuron, a gland cell, or a muscle. A substance called myelin surrounds the axon. Gaps in the sheath are known as nodes of Ranvier. At the tip of the axon is a junction called a synapse. |
TRANSMISSION OF NERVE IMPULSE
When the neuron is at rest, the axon maintains a chemical balance by keeping more potassium ions inside the cell and more sodium ions outside. When a signal is transmitted, the myelin sheath is stimulated at the nodes of Ranvier, allowing the ions to leak through. Potassium and sodium ions change places, creating an electric signal that travels along the axon.
SYNAPSE
The space between two neurons is called a synapse. Neurons communicate through the synapse, usually by means of chemicals known as neurotransmitters. When the impulse reaches the synapse, chemical transmitters ferry the impulse from the presynaptic side to receptors on the postsynaptic side, and the signal reaches the next neuron. The first neuron returns to a resting state, and the impulse travels on.
| I | INTRODUCTION |
Nervous
System, those elements within the animal organism that are concerned with
the reception of stimuli, the transmission of nerve impulses, or the activation
of muscle mechanisms.
| II | ANATOMY AND FUNCTION |
The reception of stimuli is the function of
special sensory cells. The conducting elements of the nervous system are cells
called neurons; these may be capable of only slow and generalized activity, or
they may be highly efficient and rapidly conducting units. The specific response
of the neuron—the nerve impulse—and the capacity of the cell to be stimulated
make this cell a receiving and transmitting unit capable of transferring
information from one part of the body to another.
| A | Nerve Cell |
Each nerve cell consists of a central
portion containing the nucleus, known as the cell body, and one or more
structures referred to as axons and dendrites. The dendrites are rather short
extensions of the cell body and are involved in the reception of stimuli. The
axon, by contrast, is usually a single elongated extension; it is especially
important in the transmission of nerve impulses from the region of the cell body
to other cells. See Neurophysiology.
| B | Simple Systems |
Although all many-celled animals have some
kind of nervous system, the complexity of its organization varies considerably
among different animal types. In simple animals such as jellyfish, the nerve
cells form a network capable of mediating only a relatively stereotyped
response. In more complex animals, such as shellfish, insects, and spiders, the
nervous system is more complicated. The cell bodies of neurons are organized in
clusters called ganglia. These clusters are interconnected by the neuronal
processes to form a ganglionated chain. Such chains are found in all
vertebrates, in which they represent a special part of the nervous system,
related especially to the regulation of the activities of the heart, the glands,
and the involuntary muscles.
| C | Vertebrate Systems |
Vertebrate animals have a bony spine and
skull in which the central part of the nervous system is housed; the peripheral
part extends throughout the remainder of the body. That part of the nervous
system located in the skull is referred to as the brain; that found in the spine
is called the spinal cord. The brain and the spinal cord are continuous through
an opening in the base of the skull; both are also in contact with other parts
of the body through the nerves. The distinction made between the central nervous
system and the peripheral nervous system is based on the different locations of
the two intimately related parts of a single system. Some of the processes of
the cell bodies conduct sense impressions and others conduct muscle responses,
called reflexes, such as those caused by pain (see Reflex).
In the skin are cells of several types
called receptors; each is especially sensitive to particular stimuli. Free nerve
endings are sensitive to pain and are directly activated. The neurons so
activated send impulses into the central nervous system and have junctions with
other cells that have axons extending back into the periphery. Impulses are
carried from processes of these cells to motor endings within the muscles
(see Muscle). These neuromuscular endings excite the muscles, resulting
in muscular contraction and appropriate movement. The pathway taken by the nerve
impulse in mediating this simple response is in the form of a two-neuron arc
that begins and ends in the periphery. Many of the actions of the nervous system
can be explained on the basis of such reflex arcs, which are chains of
interconnected nerve cells, stimulated at one end and capable of bringing about
movement or glandular secretion at the other.
| D | The Nerve Network |
The cranial nerves connect to the brain by
passing through openings in the skull, or cranium. Nerves associated with the
spinal cord pass through openings in the vertebral column and are called spinal
nerves. Both cranial and spinal nerves consist of large numbers of processes
that convey impulses to the central nervous system and also carry messages
outward; the former processes are called afferent, the latter are called
efferent. Afferent impulses are referred to as sensory; efferent impulses are
referred to as either somatic or visceral motor, according to what part of the
body they reach. Most nerves are mixed nerves made up of both sensory and motor
elements.
The cranial and spinal nerves are paired;
the number in humans are 12 and 31, respectively. Cranial nerves are distributed
to the head and neck regions of the body, with one conspicuous exception: the
tenth cranial nerve, called the vagus. In addition to supplying structures in
the neck, the vagus is distributed to structures located in the chest and
abdomen. Vision, auditory and vestibular sensation, and taste are mediated by
the second, eighth, and seventh cranial nerves, respectively. Cranial nerves
also mediate motor functions of the head, the eyes, the face, the tongue, and
the larynx, as well as the muscles that function in chewing and swallowing.
Spinal nerves, after they exit from the vertebrae, are distributed in a bandlike
fashion to regions of the trunk and to the limbs. They interconnect extensively,
thereby forming the brachial plexus, which runs to the upper extremities; and
the lumbar plexus, which passes to the lower limbs.
| E | Autonomic Nervous System |
Among the motor fibers may be found groups
that carry impulses to viscera. These fibers are designated by the special name
of autonomic nervous system. That system consists of two divisions, more or less
antagonistic in function, that emerge from the central nervous system at
different points of origin. One division, the sympathetic, arises from the
middle portion of the spinal cord, joins the sympathetic ganglionated chain,
courses through the spinal nerves, and is widely distributed throughout the
body. The other division, the parasympathetic, arises both above and below the
sympathetic, that is, from the brain and from the lower part of the spinal cord.
These two divisions control the functions of the respiratory, circulatory,
digestive, and urogenital systems.
| III | DISORDERS OF THE NERVOUS SYSTEM |
Consideration of disorders of the nervous
system is the province of neurology; psychiatry deals with behavioral
disturbances of a functional nature. The division between these two medical
specialties cannot be sharply defined, because neurological disorders often
manifest both organic and mental symptoms. For a discussion of functional mental
illness, Mental Illness.
Diseases of the nervous system include
genetic malformations, poisonings, metabolic defects, vascular disorders,
inflammations, degeneration, and tumors, and they involve either nerve cells or
their supporting elements. Vascular disorders, such as cerebral hemorrhage or
other forms of stroke, are among the most common causes of paralysis and other
neurologic complications. Some diseases exhibit peculiar geographic and age
distribution. In temperate zones, multiple sclerosis is a common degenerative
disease of the nervous system, but it is rare in the Tropics.
The nervous system is subject to infection
by a great variety of bacteria, parasites, and viruses. For example, meningitis,
or infection of the meninges investing the brain and spinal cord, can be caused
by many different agents. On the other hand, one specific virus causes rabies.
Some viruses causing neurological ills affect only certain parts of the nervous
system. For example, the virus causing poliomyelitis commonly affects the spinal
cord; viruses causing encephalitis attack the brain.
Inflammations of the nervous system are
named according to the part affected. Myelitis is an inflammation of the spinal
cord; neuritis is an inflammation of a nerve. It may be caused not only by
infection but also by poisoning, alcoholism, or injury. Tumors originating in
the nervous system usually are composed of meningeal tissue or neuroglia
(supporting tissue) cells, depending on the specific part of the nervous system
affected, but other types of tumor may metastasize to or invade the nervous
system (see Cancer). In certain disorders of the nervous system, such as
neuralgia, migraine, and epilepsy, no evidence may exist of organic damage.
Another disorder, cerebral palsy, is associated with birth defects.
Receptors
Movement may occur also in direct response to an outside stimulus; thus, a tap
on the knee causes a jerk, and a light shone into the eye makes the pupil
contract. These involuntary responses are called reflexes. Various nerve
terminals called receptors constantly send impulses into the central nervous
system. These are of three classes: exteroceptors, which are sensitive to pain,
temperature, touch, and pressure; interoceptors, which react to changes in the
internal environment; and proprioceptors, which respond to variations in
movement, position, and tension. These impulses terminate in special areas of
the brain, as do those of special receptors concerned with sight, hearing,
smell, and taste.
Reflex action
Reflex action
