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Nervous tissue

(textus nervosus)

Excitability is one of the basic properties of living cells. Each neuron has only one axon that transmits the
Phylogenetic development has led to the differentiation of signal away from the cell body. Axons may divide in the
cells in which the capacity for excitability is particularly periphery, sending out collateral branches. At the target
pronounced. These highly specialised nerve and sensory organ, axons typically exhibit terminal branching, form-
cells have the capacity to receive signals (stimuli), to direct ing the telodendritic zone. Individual free nerve endings,
these stimuli centrally, to process them and, with the input which vary in number and form, terminate in end bulbs
of higher centres or via reflexes, to initiate a response. The (boutons). These represent the effector component of the
diverse functional capacity of these cells can be ascribed to neuron.
their neuro-ectodermal origin.
During embryonic development the epithelial charac- Classification of nerve cells
teristics of this tissue are lost, but the deeper cell layers of The diverse morphology of neuronal components is deter-
the neuro-ectoderm remain in structural and functional mined by the site of embryonic development of the nerve
contact through intimate interconnections and particularly cell. Epithelial cells of the neural tube differentiate into bi-
long cell processes. potential progenitor cells, which give rise to neurons and
Nerve cells are so highly differentiated that they have glial cells. After the glial cell line has split off, neuroblasts
lost the capacity for metabolic self-sufficiency. They require eventually differentiate into mature neurons. These cells
assistance in this regard from supporting, or glial, cells. are not capable of division. Based on the number of cell
Nervous tissue is thus composed of two structural processes, neurons are classified as:
elements:
· unipolar,
· nerve cells or neurons and · bipolar,
· glial cells, or neuroglia. · pseudo-unipolar or
· multipolar nerve cells (Figures 5.1 to 5.8).
Nerve cells (neuron, neurocytus)
Nerve cells have elongated processes (dendrites and Unipolar neurons
axons) that form complex connections (synapses) with Unipolar neurons have a single cell process (axon). The
other cells. Dendrites detect changes in their environment existence of this type of neuron in differentiated nervous
and transfer these signals to the cell body (perikaryon), tissue is the subject of debate. The cells of the first neural
from which the stimulus is transmitted along the axon. layer of the retina (rods and cones) are frequently referred
While nerve cells exhibit exceptional variation in size to as specialised unipolar neurons (Figure 5.6).
and shape, they are consistent in their basic structure (peri-
karyon, and a number of variably branching cell processes Bipolar neurons
[axons and dendrites]). The nerve cell inclusive of its pro- In bipolar neurons, two processes (a dendrite and an axon)
cesses represents a genetic, morphologic, functional and arise from opposite sides of the perikaryon. These are rel-
trophic unit referred to as a neuron. atively uncommon, constituting the middle neural layer
The perikaryon, comprising the nucleus and surround- of the retina, the sensory cells of the olfactory mucosa
ing cytoplasm, and the numerous dendrites constitute the and the cells of the spiral and vestibular ganglia of the
receptive portion of the neuron. Dendrites (Gk dendron = inner ear.
tree) are extensively branched, elongated processes. At their
extremities, they feature specific sensory receptors or form
synapses with other neurons, from which they receive stim-
uli. Dendrites conduct impulses towards the perikaryon.

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