General Functions of the Nervous System

The nervous system is composed predominantly of neural tissue, but also includes some blood vessels and connective tissue. Neural tissue consists of two cell types: nerve cells, or neurons (nu'ronz), and neuroglia (nu-rog'le-ah) or glial cells. Neurons are specialized to react to physical and chemical changes in their surroundings. Small cellular processes called dendrites (den'dritz) receive the input, and a longer process called an axon (ak'son) or nerve fiber carries the information away from the cell in the form of bioelectric signals called nerve impulses (fig. 10.1). Nerves are bundles of axons. Neuroglia were once thought only to fill spaces and surround or support neurons. Today we know that they have many other functions, including nourishing neurons and perhaps even sending and receiving messages.

An important part of the nervous system at the cellular level is not a cell at all, but the small spaces between neurons, called synapses (sin'aps-ez). Much of the effort of the nervous system centers on sending and receiving electrochemical messages from neuron to neuron at synapses. The actual carriers of this information are biological messenger molecules called neurotransmitters (nu"ro-trans-mit'erz).

The organs of the nervous system can be divided into two groups. One group, consisting of the brain and spinal cord, forms the central nervous system (sen'tral ner'vus sis'tem) or CNS, and the other, composed of the nerves (cranial and spinal nerves) that connect the central nervous system to other body parts, is called the peripheral nervous system (pe-rif'er-al ner'vus sis'tem) or

Spinal Nerve Neuron

Dendrites

Cell body

Axon

Figure

Neurons are the structural and functional units of the nervous system (600x). Neuroglial cells surround the neuron, appearing as dark dots. Note the location of the neuron processes (dendrites and a single axon).

Dendrites

Cell body

Axon ure 10.1

Figure

Neurons are the structural and functional units of the nervous system (600x). Neuroglial cells surround the neuron, appearing as dark dots. Note the location of the neuron processes (dendrites and a single axon).

Central And Peripheral Nervous System

The nervous system includes the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial nerves and spinal nerves).

Figure

The nervous system includes the central nervous system (brain and spinal cord) and the peripheral nervous system (cranial nerves and spinal nerves).

PNS (fig. 10.2). Together these systems provide three general functions—sensory, integrative, and motor.

Structures called sensory receptors at the ends of peripheral neurons provide the sensory function of the nervous system (see chapter 11, p. 402). These receptors gather information by detecting changes inside and outside the body. They monitor external environmental factors such as light and sound intensities as well as the temperature, oxygen concentration, and other conditions of the body's internal environment.

Sensory receptors convert their information into nerve impulses, which are then transmitted over peripheral nerves to the central nervous system. There the signals are integrated—that is, they are brought together, creating sensations, adding to memory, or helping produce thoughts. Following integration, conscious or subconscious decisions are made and then acted upon by means of motor functions.

The motor functions of the nervous system employ neurons that carry impulses from the central nervous system to responsive structures called effectors. These effectors are outside the nervous system and include muscles that contract in response to nerve impulse stimulation, and glands that secrete when stimulated. The motor portion of the peripheral nervous system can be subdivided into the somatic and the autonomic nervous systems. Generally the somatic nervous system is in volved in conscious activities, such as skeletal muscle contraction. The autonomic nervous system controls viscera, such as the heart and various glands, and thus controls unconscious actions.

The nervous system can detect changes in the body, make decisions on the basis of the information received, and stimulate muscles or glands to respond. Typically, these responses counteract the effects of the changes, and in this way, the nervous system helps maintain homeostasis. Clinical Application 10.1 discusses migraine headaches, a common medical problem attributed to the nervous system that may involve its blood supply as well as neurons.

Neurons vary considerably in size and shape, but they share certain features. For example, every neuron has a cell body, dendrites, and an axon. Figure 10.3 shows some of the other structures common to neurons.

A neuron's cell body (soma or perikaryon) contains granular cytoplasm, mitochondria, lysosomes, a Golgi apparatus, and many microtubules. A network of fine threads called neurofibrils extends into the axons and supports them. Scattered throughout the cytoplasm are many membranous packets of chromatophilic substance (Nissl bodies), which consist of rough endoplasmic retic-ulum. Cytoplasmic inclusions in neurons contain glyco-gen, lipids, or pigments such as melanin.

Near the center of the neuron cell body is a large, spherical nucleus with a conspicuous nucleolus. Mature neurons generally do not divide; neural stem cells do.

Dendrites are usually highly branched, providing receptive surfaces to which processes from other neurons communicate. (In some kinds of neurons, the cell body itself provides such a receptive surface.) Often the den-drites have tiny, thornlike spines (dendritic spines) on their surfaces, which are contact points for other neurons.

A neuron may have many dendrites, but usually only one axon. The axon, which often arises from a slight elevation of the cell body (axonal hillock), is a slender, cylindrical process with a nearly smooth surface and uniform diameter. It is specialized to conduct nerve impulses away from the cell body. The cytoplasm of the axon includes many mitochondria, microtubules, and neurofibrils (ribosomes are found only in the cell body). The axon may give off branches, called collaterals. Near its end, an axon may have many fine extensions, each with a specialized ending called an axon terminal. This ends as a synaptic knob very close to the receptive surface of another cell, separated only by a space called the synaptic cleft.

In addition to conducting nerve impulses, an axon conveys biochemicals that are produced in the neuron cell body, which can be quite a task in these very long cells. This process, called axonal transport, involves vesicles, mitochondria, ions, nutrients, and neurotransmitters that move from the cell body to the ends of the axon.

Migraine

The signs of a migraine are unmistakable—a pounding head, waves of nausea, sometimes shimmering images in the peripheral visual field, and extreme sensitivity to light or sound. Inherited susceptibilities and environmental factors probably cause migraines. Environmental triggers include sudden exposure to bright light, eating a particular food (chocolate, red wine, nuts, and processed meats top the list), lack of sleep, stress, high altitude, stormy weather, and excessive caffeine or alcohol intake. Because 70% of the millions of people who suffer from migraine worldwide are women, hormonal influences may also be involved.

Although it is considered a headache, a migraine attack is actually a response to changes in the diameters of blood vessels in the face, head, and neck. Constriction followed by dilation of these vessels causes head pain (usually on one side), nausea and perhaps vomiting, and sensitivity to light.

Migraine Types

The two major variants of migraine are called "classic" and "common." Ten to 15% of sufferers experience classic migraine, which lasts four to six hours and begins with an "aura" of light in the peripheral vision. Common migraine usually lacks an aura and may last for three to four days. A third, very rare type, familial hemi-plegic migraine, may lead neurolo gists to finally understand precisely how all migraines occur.

Familial hemiplegic migraine runs in families. In addition to severe head pain, it paralyzes one side of the body for a few hours to a few days and may cause loss of consciousness. This form of migraine results from a single amino acid change in a neuronal protein calcium channel in three brain regions (cerebellum, brain stem, and hippocampus). Interestingly, two other types of mutations in the responsible gene cause two different nervous system disorders. A shortened protein causes episodic ataxia, a movement disorder that makes a person periodically walk as if intoxicated. Extra copies of a particular amino acid in the protein cause spinocerebellar ataxia type 6, which causes chronic lack of coordination.

Treatments

Learning what goes awry in the rare, inherited form of migraine may shed light on how more common forms of the disease begin and progress. The researchers who discovered the gene for familial hemiplegic migraine are already using the clues in the calcium channel protein to develop a drug that can prevent migraine attacks.

Current drug treatments, although very effective, must be taken as soon as symptoms begin. The first drug to directly stop a migraine in its tracks was Imitrex (sumatriptan), which became available in 1996. Im-itrex mimics the action of the neuro-transmitter serotonin, levels of which fluctuate during an attack. The drug constricts blood vessels in the brain, decreasing blood flow to certain areas. Newer drugs, called triptans, more precisely target the neurons that are affected in a migraine attack — specifically, those in an area called the trigeminal nucleus. These neurons control cerebral blood vessel dilation. Imitrex can cause cardiac side effects because it also binds to serotonin receptors on blood vessels in the heart.

Drugs can help about 85% of migraine sufferers. With several new drugs in development and a new understanding of how this painful condition develops, the future is bright for vanquishing migraine. ■

The larger axons of peripheral neurons are encased in lipid-rich sheaths formed by layers of cell membranes of neuroglial cells called Schwann cells, which wind tightly, somewhat like a bandage wrapped around a finger. The layers, called myelin (mi'e-lin), have a higher proportion of lipid than other surface membranes. This coating is called a myelin sheath. The portions of the Schwann cells that contain most of the cytoplasm and the nuclei remain outside the myelin sheath and comprise a neurilemma (nu"ri-lem'mah), or neurilemmal sheath, which surrounds the myelin sheath (fig. 10.4). Narrow gaps in the myelin sheath between Schwann cells are called nodes of Ranvier (fig. 10.4).

Schwann cells also enclose, but do not wind around, the smallest axons of peripheral neurons. Consequently, these axons lack myelin sheaths. Instead, the

Shier-Butler-Lewis: I III. Integration and I 10. Nervous System I: I I © The McGraw-Hill

Human Anatomy and Coordination Basic Structure and Companies, 2001

Physiology, Ninth Edition Function

Chromatophilic substance (Nissl bodies)

Cell body

Dendrites

Cell body

Dendrites

Nissl Bodies

-Synaptic knob of axon terminal

Myelin w

Schwann Cell Nucleus

^ Nucleus of Schwann cell

0 0

Responses

  • lassi
    Where is chromatophilic substance found?
    2 years ago
  • Jasmin Hongisto
    Are blood vessle in the nervous system?
    2 years ago
  • LAURIE MILNE
    What is the main function of the central nervous system?
    1 year ago
  • johanna adler
    What type of neuroglia cell surrond blood vessels in the brain?
    1 year ago
  • claire
    What vessel type is most affected by the nervous system?
    1 year ago
  • longo
    Are there blood vessels in the peripheral nerves?
    1 year ago
  • Katharina
    Is small vessel part of the central nervous system?
    1 year ago
  • merimas
    What is the function of blood vessel in thenerve?
    1 year ago
  • ULRICH WEISZ
    What do the blood vessels do in a spinal nerve?
    1 year ago
  • Alfrida
    How does nervous system communicate w blood vessels?
    10 months ago
  • Sinit
    Which fibers of the nervous system supply the most blood vessels?
    10 months ago
  • MEBRAT
    Are nerves inside or outside blood vessel?
    10 months ago
  • gloriana
    What blood vessel is involved with rbb and hemi?
    9 months ago
  • berhane
    Which nervous system controls blood vessels?
    9 months ago
  • ronald
    Which nerves control the function of superficial blood vessels?
    8 months ago
  • roope
    What are blood vessels to the nerves called?
    6 months ago
  • SEBHAT
    Can nerves close blood vessels?
    5 months ago
  • lete
    Where are blood vessels and nerves not found?
    4 months ago
  • jessamine brown
    Do nerves innervate blood vessels?
    4 months ago
  • mirren fraser
    Do dentrites on neurons contain blood vessells?
    4 months ago
  • Rocco
    Which of the following areas lacks blood vessels and nerves?
    2 months ago
  • Zahra
    Which cell connects neurons to blood vessels?
    1 month ago
  • selam
    Are sxchwann cells blood vessels?
    23 days ago
  • yi herrera
    How does nervous system is related to blood?
    16 days ago
  • petri
    Where are blood vessels found within the spinal nerves?
    8 days ago

Post a comment