Classification of Neurons

On the basis of structural differences, neurons can be classified into three major groups, as figure 10.6 shows. Each type of neuron is specialized to send a nerve impulse in one direction, originating at a sensitive region of the axon called the trigger zone.

1. Bipolar neurons. The cell body of a bipolar neuron has only two processes, one arising from either end. Although these processes are similar in structure, one is an axon and the other is a dendrite. Such

Dendrites

Dendrites

Unipolar Bipolar Multipolar Nerves
(a) Bipolar (b) Unipolar (c) Multipolar

Figure

Structural types of neurons include (a) the bipolar neuron, (b) the unipolar neuron, and (c) the multipolar neuron. Note in each case the "trigger zone" at the initial portion of the axon.

neurons are found within specialized parts of the eyes, nose, and ears.

2. Unipolar neurons. Each unipolar neuron has a single process extending from its cell body. A short distance from the cell body, this process divides into two branches, which really function as a single axon: One branch (peripheral process) is associated with dendrites near a peripheral body part. The other branch (central process) enters the brain or spinal cord. The cell bodies of some unipolar neurons aggregate in specialized masses of nerve tissue called ganglia, which are located outside the brain and spinal cord.

3. Multipolar neurons. Multipolar neurons have many processes arising from their cell bodies. Only one is an axon; the rest are dendrites. Most neurons whose cell bodies lie within the brain or spinal cord are of this type. The neuron illustrated in figure 10.3 is multipolar.

Neurons can also be classified by functional differences into the following groups, depending on whether they carry information into the central nervous system (CNS), completely within the CNS, or out of the CNS (fig. 10.7).

1. Sensory neurons (afferent neurons) carry nerve impulses from peripheral body parts into the brain or spinal cord. These neurons have specialized

Central nervous system Peripheral nervous system

Central nervous system Peripheral nervous system

Nervous Impulse Pathway Effector
Sensory (afferent) neurons carry information into the central nervous system (CNS), interneurons are completely within the CNS, and motor (efferent) neurons carry instructions to the peripheral nervous system (PNS).

Types of Neurons

Types of Neurons

A. Classified by Structure

Type

1. Bipolar neuron

Structural Characteristics

Cell body with a process, arising from each end, one axon and one dendrite

Location

In specialized parts of the eyes, nose, and ears

2. Unipolar neuron

Cell body with a single process that divides into two branches and functions as an axon

Cell body in ganglion outside the brain or spinal cord

3. Multipolar neuron

Cell body with many processes, one of which is an axon, the rest dendrites

Most common type of neuron in the brain and spinal cord

B. Classified by Function

Type

1. Sensory neuron

Functional Characteristics

Conducts nerve impulses from receptors in peripheral body parts into the brain or spinal cord

Structural Characteristics

Most unipolar; some bipolar

2. Interneuron

Transmits nerve impulses between neurons within the brain and spinal cord

Multipolar

3. Motor neuron

Conducts nerve impulses from the brain or spinal cord out to effectors—muscles or glands

Multipolar

receptor ends at the tips of their dendrites, or they have dendrites that are near receptor cells in the skin or in certain sensory organs.

Changes that occur inside or outside the body are likely to stimulate receptor ends or receptor cells, triggering sensory nerve impulses. The impulses travel on sensory neuron axons into the brain or spinal cord. Most sensory neurons are unipolar, as shown in figure 10.7, although some are bipolar.

2. Interneurons (also called association or internuncial neurons) lie within the brain or spinal cord. They are multipolar and form links between other neurons. Interneurons transmit impulses from one part of the brain or spinal cord to another. That is, they may direct incoming sensory impulses to appropriate regions for processing and interpreting. Other incoming impulses are transferred to motor neurons.

3. Motor neurons (efferent neurons) are multipolar and carry nerve impulses out of the brain or spinal cord to effectors—structures that respond, such as muscles or glands. For example, when motor impulses reach muscles, they contract; when motor impulses reach glands, they release secretions.

Two specialized groups of motor neurons, accelerator and inhibitory neurons, innervate smooth and cardiac muscles. Accelerator neurons increase muscular activities, whereas inhibitory neurons decrease such actions.

Motor neurons that control skeletal muscle are under voluntary (conscious) control. Other motor neurons, such as those that control glands and smooth and cardiac muscle, are largely under involuntary control.

Table 10.1 summarizes the classification of neurons.

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Responses

  • Simone
    Which structural type of neuron specialized for the parts of the eye nose and ears?
    6 years ago
  • t
    How neurons conduct nerve impulses?
    6 years ago
  • Lisa
    What carries sensory nerve impulses?
    6 years ago
  • MANUELA
    Which one of the neurons sends the impulse to the effector?
    6 years ago
  • FALCO
    What is the difference between unipolar neurons and multipolar neurons?
    5 years ago
  • kimberly
    What structural type of neuron is a motor neuron?
    5 years ago
  • william
    What is the difference between a multipolar neuron and a bipolar neuron?
    5 years ago
  • Niklas
    What type of neuron lies completely in the cns?
    5 years ago
  • FRANCIS
    What are the three groups that neurons classified in?
    4 years ago
  • Connie
    What nerve cells are classified into three groups?
    4 years ago
  • trenton
    What cells are classified in three groups of neuron?
    3 years ago
  • Katie
    What specialized neurons are on the eyes,ears and nose?
    3 years ago
  • martin
    How are bipolar neuron classified?
    3 years ago
  • LUIGI
    What are the 3 major structural groups into which neurons are classified?
    2 years ago

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