Nervous System
What's in a nerve?
Planet Science investigates nerve cells
What are nerves?
Nerves are cells called neurons, which make up our nervous system. Nerves are specialized cells - they carry
messages from one part of the body to another, as tiny electrical signals. These messages are also known as
nerve impulses. Do you want to run? Your nerves will tell your leg muscles to move.
Nerve cells carry messages from one part of the body to another
There are three different types of neurons, each with a slightly different function.
1. sensory neuron picks up stimuli from the internal or external environment and converts each stimulus
into a nerve impulse. The impulse travels along the sensory neuron until it reaches an interneuron,
usually in the brain or spinal cord.
2. An interneuron is a neuron that carries nerve impulses from one neuron to another. Some interneurons
pass impulses from sensory neurons to motor neurons.
3. A motor neuron sends an impulse to a muscle or gland, and the muscle or gland reacts in response.
Did you know?
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Nerves are the oldest and longest cells in the body!
The neuron from your spinal cord to your big toe can be up to 1 meter long!
Sketch and Label:
Discovery Education Nerve Article:
Description
The nervous system is a network of specialized cells within the animal organism that is concerned with the
reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms; in other
words, the nervous system coordinates, analyzes, and responds to the information coming from within the
organism, as well as from the environment.
Citations
Citation (MLA)
Nervous System Funk & Wagnalls, 2005 . Encyclopedia Article.
Discovery Education. Web. 11 November 2014. <http://www.discoveryeducation.com/>.
Citation (APA)
Funk & Wagnalls, (2005). Nervous System. [Encyclopedia Article]. Available from
http://www.discoveryeducation.com/
NERVOUS SYSTEM,
network of specialized cells within the animal organism that is concerned with the reception of stimuli, the
transmission of nerve impulses, or the activation of muscle mechanisms; in other words, the nervous system
coordinates, analyzes, and responds to the information coming from within the organism, as well as from the
environment.
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.
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.
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.
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 and elsewhere are cells of several types called receptors; each is
especially sensitive to particular stimuli. Free nerve endings sensitive to pain are directly activated. The neurons
so activated typically 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. 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.
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 various
regions of the trunk and to the limbs. They interconnect extensively, thereby forming the brachial plexus, which
proceeds to the upper extremities; and the lumbar plexus, which proceeds to the lower limbs.
Autonomic Nervous System.
This system controls the functions of the respiratory, circulatory, digestive, and urogenital systems. It has
traditionally been regarded as consisting 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. Toward the end of the
20th century many researchers began to recognize the existence of a third division of the autonomic nervous
system: the enteric, comprising the complex network of nerves regulating the activity of the digestive system.
Although governed by the central nervous system, notably via the vagus nerve, it is capable of acting
autonomously and is sometimes called the “second brain.“
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, see Mental Disorders. 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 of
organic damage may be readily apparent. Another disorder, cerebral palsy, is associated with birth defects.
F.A.M., FREDERICK A. METTLER, M.A., Ph.D., M.D. For further information on this topic, see the
Bibliography, sections 493. Anatomy and physiology- 494. Body systems, 497. Brain.
Resources:
http://www.planet-science.com/categories/over-11s/human-body/2011/05/what's-in-a-nerve.aspx