DEFINITION OF A NEURON
Neurons are excitable cells that are specialized for the reception of stimuli and the conduction of the
nerve impulse.
They vary considerably in size and shape, but each possesses a cell body from whose surface project
one or more processes called neuritis. Those neurites responsible for receiving information and
conducting it toward the cell body are called dendrites. The single long tubular neurite that conducts
impulses away from the cell body is called the axon. The dendrites and axons are often referred to as
nerve fibers.
Although the cell body of a neuron may be as small as 5 µm or as large as 135 µm in diameter, the
processes or neurites may extend over a distance of more than 1 m.
CLASSIFICATION OF NEURON
The number, length, and mode of branching of the neurites provide a morphologic method for
classifying neurons.
1. Unipolar neurons are those in which the cell body has a single neurite that divides a short
distance from the cell body into two branches, one proceeding to some peripheral structure and
the other entering the central nervous system. The branches of this single neurite have the
structural and functional characteristics of an axon. In this type of neuron, the fine terminal
branches found at the peripheral end of the axon at the receptor site are often referred to as the
dendrites. Examples of this form of neuron are found in the posterior root ganglion.
2. Bipolar neurons possess an elongated cell body, from each end of which a single neurite
emerges Examples of this type of neuron are found in the retinal bipolar cells and the cells of
the sensory cochlear and vestibular ganglia.
3. Multipolar neurons have a number of neurites arising from the cell body With the exception of
the long process, the axon, the remainder of the neurites are dendrites. Most neurons of the
brain and spinal cord are of this type.
Neurons may also be classified according to size:
Golgi type I neurons have a long axon that may be 1 m or more in length in extreme cases. The
axons of these neurons form the long fiber tracts of the brain and spinal cord and the nerve
fibers of peripheral nerves. The pyramidal cells of the cerebral cortex, the Purkinje cells of the
cerebellar cortex, and the motor cells of the spinal cord are good examples.
2. Golgi type II neurons have a short axon that terminates in the neighborhood of the cell body or
is entirely absent. They greatly outnumber the Golgi type I neurons. The short dendrites that
arise from these neurons give them a star-shaped appearance. These neurons are numerous in
the cerebral and cerebellar cortex and are often inhibitory in function.
1.
STRUCTURE OF NEURON
NEUROGLIA AND CLASSIFICATION.
Neuroglia is the non excitable support of the nervous system
Classification based on the function
NEUROGLIA
ASTROCYTES
OLIGIDENDROCYTES
MICROGLIA
PROVIDE SUPPORT
PRODUCE MYELIN
ACTIVE DURING
INFECTION
EPENDYMA
EPENDYMOCYTES
TANYCYTES
CHOROIDAL
EPITHELIAL CELLS
ABSORPTION AND
CIRCULATION OF CSF
TRANSPORT CSF TO
HYPOPHYSEAL
PORTAL SYSTEM
PRODUCE CSF