Neuron Structure and Function

4 Key excerpts on "Neuron Structure and Function"

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  eBook - ePub

  Psychopharmacology

  • R. H. Ettinger, R. H. Ettinger(Authors)
  • 2017(Publication Date)
  • Psychology Press

    (Publisher)

  The average human brain weighs approximately 1,400 grams (or roughly three pounds) and contains nearly 200 billion neurons. Each of these neurons may in turn communicate with just a few or as many as tens of thousands of other neurons. How the structure and organization of neurons and their surrounding environment allows for such communication will be the topic of the first part of this chapter. We will then describe the structures and functions of systems within the brain that allow humans and other organisms to function in, and adapt to, their continuously changing environments. This background will be necessary for us to understand how psychological disorders may arise and just how drugs might help to alleviate them.

  The Structure and Function of Neurons

  As mentioned above, the brain contains approximately 200 billion individual nerve cells or neurons. These neurons are the basic units of the brain as well as the rest of the nervous system. Neurons vary in shape, size, and other characteristics according to their location and their specific function.

  There are three major classes of neurons: sensory neurons, motor neurons, and interneurons. Sensory or afferent neurons carry ascending messages to the central nervous system (CNS) from receptors in the skin, ears, nose, eyes, as well as some organs, muscles, and joints. The brain and sometimes the spinal cord interpret these messages and send appropriate responses through descending motor or efferent neurons, which lead to sensory organs, muscles, glands, and other peripheral tissues to control movement and the functioning of glands, sensory organs, and other tissues. Interneurons reside only within the CNS and function to bridge communication between sensory and motor neurons. Without these connecting neurons, sensory messages would never result in the appropriate bodily response. Interneurons also communicate with each other throughout the nervous system. Although neurons vary in size, shape, and function, they share four common structures: the cell body, the dendrites, the axon, and the terminal buttons (see Figure 1.1 ).

  p.2

  Cell Body or Soma

  The cell body or soma is the largest part of the neuron. It contains structures that control the cell’s metabolic functions (cell respiration and metabolism). It also contains the nucleus which contains the cell’s genetic information encoded in DNA. The membrane of the cell body can have receptors and receive messages from other neurons, although the cell body is not typically the cell’s primary receiving target.

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  eBook - ePub

  The Student's Guide to Cognitive Neuroscience

  • Jamie Ward(Author)
  • 2019(Publication Date)
  • Routledge

    (Publisher)

  Chapter 2 Introducing the brain

  DOI: 10.4324/9781351035187-2

   CONTENTS

  • Structure and function of the neuron
  • The gross organization of the brain
  • The cerebral cortex
  • The subcortex
  • The midbrain and hindbrain
  • Summary and key points of the chapter
  • Example essay questions
  • Recommended further reading

  It is hard to begin a chapter about the brain without waxing lyrical. The brain is the physical organ that makes all our mental life possible. It enables us to read these words, and to consider thoughts that we have never considered before—or even to create thoughts that no human has considered before. This book will scratch the surface of how this is all possible, but the purpose of this chapter is more mundane. It offers a basic guide to the structure of the brain, starting from a description of neurons and working up to a description of how these are organized into different neuroanatomical systems. The emphasis is on the human brain rather than the brain of other species.

  Structure and Function of the Neuron

  All neurons have basically the same structure. They consist of three components: a cell body (or soma), dendrites and an axon, as shown in Figure 2.1 . Although neurons have the same basic structure and function, it is important to note that there are some significant differences between different types of neurons in terms of the spatial arrangements of the dendrites and axon.

  Figure 2.1 Neurons consist of three basic features: a cell body, dendrites that receive information and axons that send information. In this diagram the axon is myelinated to speed the conduction time.

  The cell body contains the nucleus and other organelles. The nucleus contains the genetic code, and this is involved in protein synthesis. Proteins serve a wide variety of functions from providing scaffolding to chemical signaling (they can act as neurotransmitters and receptors in neurons). Neurons receive information from other neurons and they make a “decision” about this information (by changing their own activity) that can then be passed on to other neurons. From the cell body, a number of branching structures called dendrites enable communication with other neurons. Dendrites receive information from other neurons in close proximity. The number and structure of the dendritic branches can vary significantly depending on the type of neuron (i.e., where it is to be found in the brain). The axon, by contrast, sends information to other neurons. Each neuron consists of many dendrites but only a single axon (although the axon may be divided into several branches called collaterals).

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  Neurology for the Speech-Language Pathologist - E-Book

  • Wanda Webb, Richard K. Adler(Authors)
  • 2016(Publication Date)
  • Mosby

    (Publisher)

  Fig. 2-2 for a refresher of the basic structure of the neuron.)

  Neurons vary greatly in size, but most of the billions of neurons of the central nervous system (CNS) are small. Classifying on the basis of shape, three types of neurons may be identified: multipolar , pseudounipolar , and bipolar cells . Figure 4-1 outlines diagrammatically the various types of neurons. Each nerve cell contains a nucleus and one to a dozen projections of varying length. These projections receive neural signals and conduct neural impulses. Those receiving neural stimuli are called dendrites and are the shorter and more numerous projections of the nerve cell. The dendrites of a neuron generally are no more than a few millimeters in length.

  Dendrites usually branch extensively, giving an appearance similar to a tree branch configuration. The dendrites have specialized receptors through which they receive signals from other neurons at the synapse (the contact point with another neuron). Information travels from a distal to a proximal point along dendrites to the cell body.

  Dendrites often display several thorn-like protuberances called dendritic spines (see Fig. 4-1 ), usually on the most distal branches of the dendrite “tree.” They are most prolific in the central nervous system (CNS) and are typically the sites of the synaptic contacts, forming an anastomosis , or connection.

  The other process of a neuron is the axon , a longer single fiber that conducts nerve impulses away from the neuron to other parts of the nervous system, glands, or muscle (see Fig. 4-1 ). Axons arise from the cell body at an area called the axon initial segment or the axon hillock. Axons do not contain ribosomes, which is how scientists distinguish them from dendrites at the ultrastructural level. Axons range in length from several micrometers to more than 2 m. The diameter of individual axons varies greatly, and conduction velocity along the axon ranges from 2 to 100 m/s depending on the fiber size. The larger the diameter, the greater is the conduction velocity. In a physiologic sense, the term axon

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  King's Applied Anatomy of the Central Nervous System of Domestic Mammals

  • Geoff Skerritt(Author)
  • 2017(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  5

  The Neuron

  The Anatomy of Neurons

  5.1 General Structure

  A typical neuron has dendrites, a cell body (perikaryon), an axon, collateral branches, axon terminals, and synaptic end bulbs.

  5.1.1 Relationships of the Dendrites and the Axon to the Cell Body

  The usual arrangement of neurons lying within the neuraxis is for the dendrites to lead directly to the cell body, and for the axon to lead from the cell body (Figure 5.1 (a)). However, the position of the cell body can vary. A primary afferent neuron (the first neuron in a sensory pathway from the periphery) has its cell body in a dorsal root ganglion of a spinal nerve or the equivalent ganglion of a cranial nerve. In this type of neuron, the cell body has relocated along the axon and, in functional terms, is now near the end of the axon instead of its beginning (Figure 5.1 (b)). The peripheral end of a primary afferent neuron typically consists of fine branching receptor terminals, which correspond to the dendrites of an ordinary neuron; the extreme tip of a receptor terminal is specialised, structurally and electrophysiologically, as a receptor ending. The re‐siting of the neuronal cell body of a primary afferent neuron reflects the evolutionary advantage of removing the cell body as far as possible from the surface of the animal’s body, where it could be too easily damaged. No matter what the position of the cell body, the nerve impulse normally travels from either the dendritic or the receptor endings towards the axon terminals, and ends at the synaptic end bulbs.

  Figure 5.1

  Two types of neuron with the cell body in different positions on the axon. (a) Typical unipolar neuron within the neuraxis. The numerous dendrites lead directly into the cell body, and the whole of the axon follows the cell body. (b)

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