Page 84 - Headlines Histology2024_Neat

• The axon originates from a region in the cell body called axon hillock that is
a cone-shaped portion devoid of Nissl bodies.

• Some axons are invested by a segment of myelin sheath separated by gaps
called nodes of Ranvier.

• The myelin sheath shows cone-shaped clefts (of Schmidt-
• Lantermann) that are actually helical tunnels extend from outside the sheath

to inside. They are formed by portions of Schwann cell cytoplasm trapped
within the myelin during its formation.
• Few collateral branches of the same diameter extend perpendicularly from
the axon at nodes of Ranvier.
• The region of the axon located just beneath the axon hillock is called initial
segment. It is not invested with myelin sheath and at these region nerve
impulses is initiated.
• The axon is surrounded by plasmalemma (axolemma). The cytoplasm of the
axon (axoplasm) is devoid of ribosomes and rER (Nissl bodies). It contains
abundant neurofilaments and microtubules.
• In fixed state, a central band of parallel neurofilaments, which are
continuous with those of the cell body, are found embedded in the
axoplasm. Other organelles include numerous mitochondria and sER.
• The axon ends in small branches called telodendria which end in small
swelling called terminal buttons that contain synaptic vesicles and
mitochondria and participate in synapse.
• The axons are commonly refers as nerve fibers and they function as a major
site for information output from the neuron.
Dendrites

• The dendrites are highly branched treelike, with broad origin and tapering ends
with many fine spines called gemmules.

• They are not myelinated and end either as specialized sensory receptors or
form synapses with other neurons.

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