Lab 1: Nervous Tissue Histology
Neurons -- Multipolar Neurons


The neuron, the structural and functional unit of the nervous system, is the key cell-type of nervous tissue. An individual neuron consists of a perikaryon (cell body) and cell processes, typically one axon and a number of dendrites. The cell body consists of the cell nucleus and cytoplasm immediately surrounding the nucleus.

Three major categories of neurons are recognized:
      Unipolar (pseudo-unipolar) neurons are sensory neurons with cell bodies in spinal and cranial nerve ganglia. (Note: unipolar neurons are sometimes called “pseudo-unipolar” because they originate embryologically as bipolar neurons.)
      Bipolar neurons are relatively rare. They are sensory neurons found in olfactory epithelium, the retina of the eye, and ganglia of the vestibulocochlear nerve.
      Multipolar neurons are the most common type of neuron. They are located in the central nervous system (brain and spinal cord) and in autonomic ganglia. Multipolar neurons have more than two processes emanating from the neuron cell body.

Fig. 1. Schematic drawing of bipolar, unipolar, and multipolar neurons. Arrows indicate the direction of excitation flow, from input to output.

Multipolar neurons:

Search for multipolar neurons in glass slide 48 in your Histology slide box, cerebellar cortex of pig (Golgi stain).

Note: Three layers are recognized in the cerebellar cortex (superficial gray matter of the cerebellum):
    (1) an outer molecular layer composed of few cells and many nonmyelinated fibers,
    (2) an intermediate layer comprised of flask-like cell bodies called Purkinje cells,
          which are multipolar neurons, and
    (3) an inner granular layer composed of tightly-packed cells and fibers.

On glass slide 48, find a section similar to that shown below.

Fig. 2. Pig cerebellar cortex: M = molecular layer, P = layer of Purkinje cell bodies, G = granuale cell layer. The arrow points to an axon of a Purkinje neuron.
      At middle of the slide, cell bodies of three Purkinje cells (neurons) are visible as black, rounded, profiles. Cell processes, termed dendrites, extend superficially from the cell body into the molecular layer. A small axon (arrow) emerges from each cell body and runs through the granue cell layer. (Note: Axons and dendrites of neurons are normally visible only in special stained sections such as this Golgi preparation.)

Question: Why is the Purkinje cell considered a multipolar neuron?
All multipolar neurons have two characteristics:
    -- more than two processes emanate from the cell body, and
    -- the cell body receives synaptic input just like the dendrites

Fig. 3. Schematic drawing of a ventral horn SE multipolar neuron. A number of dendrites and one axon emanate from the cell body. The axon has few if any branches until its termination in a profusion of branches. The axon is specialized for conducting excitation from the input region to the output region of the neuron. The cell body plus the dendrites of a mulltipolar neuron constitute a "dendritic zone" which receives synaptic input from other neurons. Axonal terminal branches (telodendria) make synaptic contact with other neurons or with effector organs (neuromuscular synpases are shown above).

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