The Nervous System, Third Edition

F. Evans-Martin

Description

The nervous system allows us to move, feel, and think, and it is involved in nearly all of the functions of the human body. Nerves communicate signals between the brain and muscles, allowing us to move our hands and feet. Or, they relay messages about the environment through touch, taste, sight, and smell. Nerves can also communicate information about how we are feeling at any particular time and help to maintain homeostasis, or a stable state of equilibrium. The Nervous System, Third Edition discusses the development and organization of this diverse system, its functions, and potential injuries and complications. Packed with full-color photographs and illustrations, this absorbing book provides students with sufficient background information through references, websites, and a bibliography.

Sujets

• Life Sciences
• Nervous System
• Medical
• Sciences et techniques
• Biology
• Medicine
• Anatomy
• Health
• Human
• Science

Informations

• Publié par: Infobase Publishing
• Date de parution: 01 janvier 2022
• Nombre de lectures: 0
• EAN13: 9781646937226
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

The Nervous System, Third Edition
Copyright © 2022 by Infobase
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:
Chelsea House An imprint of Infobase 132 West 31st Street New York NY 10001
ISBN 978-1-64693-722-6
You can find Chelsea House on the World Wide Web at http://www.infobase.com
Contents Chapters Our Amazing Nervous System Development of the Nervous System Organization of the Nervous System Sensation and Perception Movement Learning and Memory Emotions and Reward Systems Neuroendocrine and Neuroimmune Interactions Sleep and Wakefulness Diseases and Injuries of the Nervous System Support Materials Glossary Bibliography Further Resources About the Author Index
Chapters
Our Amazing Nervous System
Joshua poked at the embers of his campfire as he stared at the myriad of stars in the evening sky. The display of sunset colors had long faded from the sky, but the taste and aromas of his evening meal still lingered. Wildflowers filled the air with fragrance, and Joshua remembered noticing their beauty as he passed them during the day. A nearby stream trickled over the rocks, and the sounds of frogs and crickets filled the air. Rustling leaves and an occasional call from a night creature revealed the presence of forest animals.
Joshua nestled into his sleeping bag and soon fell asleep, dreaming of the natural wonders he had experienced that day. While Joshua slept, another natural wonder was actively at work, directing his dreams and regulating his breathing, his heartbeat, his body temperature, and the digestion of his evening meal. His amazing nervous system had received all the information he had observed during the day; interpreted it as beautiful sights, sounds, and aromas; and stored it for him to remember and enjoy. Every movement his body had made during his active day on the mountain trails had been under the control of his nervous system.
Protected within their bony casings of the skull and spinal column, the brain and spinal cord are the central core of the nervous system. A network of nerves branches out from them and acts as a fiber highway system for information coming in from the environment and commands going out to the muscles, glands, and body organs. Virtually every cell in the body is influenced by the nervous system. In turn, the nervous system is heavily affected by hormones and other chemicals produced by cells of the body.
Neuron Theory
Beginning with the ancient Greek philosophers, there have been centuries of debate over the brain and its functions. It was not until the end of the nineteenth century that the structure and function of the nervous system began to become clear. Because nervous tissue is so soft, fragile, and complex, it was very difficult to study. Although scientists had observed and drawn nerve cells, they could not view all of their connections under a microscope.
In 1838, the German botanist Matthias Jakob Schleiden introduced the theory that all plants are made up of individual units called cells. The next year, the German physiologist Theodor Schwann introduced the theory that all animals are also made up of cells. Together, Schleiden's and Schwann's statements formed the basis of cell theory, which states that the cell is the basic unit of structure in all living organisms. Although cell theory quickly became popular, most scientists of the nineteenth century believed that the nervous system was a continuous network, or reticulum, of fibers, and was therefore an exception to cell theory. This concept of the organization of the nervous system became known as reticular theory.
A breakthrough came in 1873, when the Italian scientist Camillo Golgi reported his discovery of a special stain that made neurons (nerve cells) and their connections easier to study under a microscope. However, since his technique was not refined enough to show the connections between individual neurons, Golgi continued to adhere to reticular theory. He believed the nervous system was a vast network of cytoplasm with many nuclei.
In 1886, the Swiss anatomist Wilhelm His suggested that the neuron and its connections might, in fact, be an independent unit within the nervous system. Another Swiss scientist, August Forel, proposed a similar theory a few months later. Using Golgi's staining technique and improving upon it, Spanish scientist Santiago Ramón y Cajal showed in 1888 that the neuron and its connections were indeed an individual unit within the nervous system. In a paper published in 1891, the German anatomist Wilhelm Waldeyer coined the term neurone and introduced the neuron doctrine. Known today as neuron theory, Waldeyer's concept extended cell theory to nervous tissue. However, it was not until after the invention of the electron microscope in the early 1930s that definitive evidence became available to show that neurons could communicate between themselves.
Golgi and Cajal were awarded a shared Nobel Prize in Physiology or Medicine in 1906 for their scientific studies of the nervous system. At the ceremony, each man gave a speech. Golgi's speech adhered to the reticular theory of nervous system structure. Cajal, on the other hand, spoke in enthusiastic support of neuron theory and gave evidence to contradict reticular theory. Since then, scientific studies have continued to support the neuron theory and have revealed more details that show how amazingly complex the nervous system really is. Although many questions remain to be answered, it is now clear that the nervous system is, in fact, made up of individual cells, just like the rest of the body.
Neurons
The basic signaling unit of the nervous system is the neuron. Neurons are found in the brain, spinal cord, and throughout the body. Scientists estimate that there are 86 billion neurons in the brain 1 and about one billion neurons in the spinal cord. Neurons come in many shapes and sizes and perform many different functions.
The number of different types of neurons may be as high as 10,000. Neurons are classified by either structure or function. Neurons can be divided into structural types based on the arrangement of their branches—the dendrites and axons. These structural types include unipolar neurons, bipolar neurons, pseudounipolar neurons, and multipolar neurons (see figure below).

The processes of neurons extend from the cell body in three basic patterns. Unipolar neurons (not shown) have only one process, an axon, that has multiple terminal processes. Because there are no dendrites, the cell body receives incoming information. Bipolar neurons have an axon and a dendrite that arise from opposite ends of the cell body. The pseudounipolar neuron, a type of bipolar neuron, has one fused process that branches near the soma into an axon and a dendrite. Most central nervous system neurons are multipolar neurons, which have multiple dendritic trees and usually one axon. Pyramidal cells are a type of multipolar neuron.
Source: Infobase Learning.
Functional types of neurons include sensory neurons, motor neurons, and interneurons. Sensory neurons generate nerve impulses in response to stimuli from the internal and external environments. These nerve impulses are transmitted to the brain, where they are interpreted. Motor neurons send impulses from the brain and spinal cord to the muscles and glands, resulting in movements and glandular secretions. Interneurons relay information between two other neurons, to which they are connected.
Like other cells, the cell body, or soma, of a neuron has an outer plasma membrane, or cell membrane, that encloses the watery cytoplasm in which the cell nucleus (plural: nuclei) and a variety of organelles are found (see figure below). The nucleus is the control center of the cell. It directs the activities of the other organelles, which are responsible for all of the cell's functions. Unlike most other cells, neurons do not divide to reproduce themselves. Also unlike most other cells, neurons are able to transmit an electrochemical signal.

Neurons, or nerve cells, are the signaling units of the nervous system. The myelin sheath, composed of the wrapping of individual Schwann cells in peripheral neurons and processes of oligodendrocytes in central neurons around the axon, insulates the axon and helps the electrical impulses travel faster. The gaps between the wrapped segments of myelin are called nodes of Ranvier.
Source: Infobase Learning.
Neurons, or nerve cells, are the signaling units of the nervous system. The myelin sheath, composed of the wrapping of individual Schwann cells in peripheral neurons and processes of oligodendrocytes in central neurons around the axon, insulates the axon and helps the electrical impulses travel faster. The gaps between the wrapped segments of myelin are called nodes of Ranvier.
Most cells in the body have geometric shapes, they are squarish, cubical, or spherical. Neurons, on the other hand, are irregular in shape and have a number of spiderlike extensions, or "processes," from the cell body. The neuron's processes send and receive information to and from other neurons.
In most neurons, extending from one end of the cell body are short processes called dendrites that branch in a treelike manner. In fact, their arrangement is referred to as the "dendritic tree." A single neuron can have anywhere from 1 to 20 dendrites, each of which can branch many times. Dendrites receive messages from other neurons and carry them toward the cell body.
Dendritic spines are short, thornlike structures that appear on the dendrites. There may be thousands of dendritic spines on the dendrites of just one neuron. This greatly increases the surface area that the dendritic tree has available for receiving signals