Neurons

Three major categories of neurons are recognized:
      Bipolar neurons are relatively rare. They are sensory neurons found in olfactory epithelium, the retina of the eye, and ganglia of the vestibulocochlear nerve.
      Unipolar (pseudo-unipolar) neurons are sensory neurons with cell bodies located in spinal and cranial nerve ganglia. (Note: unipolar neurons are sometimes called pseudo-unipolar because embryologically they originate as bipolar neurons and subsequently become unipolar.)
      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 cartoon of bipolar, unipolar, and multipolar neurons. Arrows indicate the direction of excitation flow along the neuron, from input to output.

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Multipolar neurons:

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.

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

Somatic Efferent Neuron:

Fig. 3. Schematic cartoon of a ventral horn Somatic Efferent (SE) multipolar neuron. SE neurons have their cell bodies in the CNS; their axons join peripheral nerves and innervate skeletal muscle. As a multipoar 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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