Page 338 - Atlas of Histology with Functional Correlations

the nose (the latter two are found in the PNS).

Unipolar neurons. Most neurons in the adult organism that exhibit only one
process leaving the cell body were initially bipolar during embryonic
development. The two neuronal processes fuse during later development and
form one axon process. This process then divides close to the cell body into
two long axonal branches. One of these branches continues to the CNS,

whereas the other branch extends to the peripheral organ. The unipolar
neurons (formerly called pseudounipolar neurons) are also sensory. The cell
bodies of unipolar neurons are found in numerous dorsal root ganglia of

spinal nerves and cranial nerve ganglia. The ganglia represent a collection of
neurons surrounded by their supportive cells and can be either sensory or
motor.

MYELIN SHEATH AND MYELINATION

OF AXONS

Specialized cells in both the CNS and the PNS surround and wrap around the
axon multiple times. This process builds up successive layers of modified cell
membrane and forms a lipid-rich, insulating sheath around the axon called the

myelin sheath. As the wrapping around the axon continues, the cells’ cytoplasm
is gradually forced or squeezed out from between the membranes of the
concentric layers. Along the myelin sheath are small islands of Schwann cell
cytoplasm between the myelin membranes that were not squeezed out during the

myelination process. These are Schmidt-Lanterman incisures or clefts seen
best with electron microscopy as they pass obliquely across the width of the
myelin sheath.

The myelin sheath extends from the initial segments of the axon to the

terminal branches. Interspersed along the length of a myelinated axon are small
gaps or spaces in the myelin sheath because the myelin sheath is formed by
numerous cells. Where the myelinating cells meet is a tiny area that is devoid of
myelin. These gaps in myelin sheath are called nodes of Ranvier. Axons in both
the CNS and the PNS can be either myelinated or unmyelinated.

In the PNS, all axons are surrounded by Schwann cells that either myelinate
the axons or envelope the unmyelinated axons with their cytoplasm. Schwann
cells myelinate individual peripheral axons and extend along their entire length,
from their origin at the cell body to their termination in the muscle or gland. In

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