1. CELLULAR ORGANIZATION OF THE
NERVOUS SYSTEM
DR. BRIAN LISHENGA
DEPT. OF MEDICAL PHYSIOLOGY
EGERTON UNIVERSITY

2. NEURONS
• Neurons are the functional unit of the
communication network.
• Parts of a neuron:
 Soma or cell body: contains the nucleus and
nucleolus and a well-developed biosynthetic
apparatus for manufacturing membrane
constituents, synthetic enzymes,
neurotransmitters.
 Nissl bodies, (found in the soma) are stacks
of RER, and a prominent Golgi apparatus.
 The soma also contains numerous
mitochondria and cytoskeletal elements,
including neurofilaments and microtubules.

3. NEURONS
• Parts of a neuron:
 Dendrites: Extensions of the neuronal cell body.
They can be more than 1 mm long, and can
account for more than 90% of the surface
area. They contain Nissl bodies, neurofilaments
and microtubules.
 Axon: Arises from the soma (or sometimes from
a dendrite) in a specialized region called the
axon hillock.
 Doesn’t have Nissl bodies so it degenerates
when disconnected from the soma.
 Its length varies with neuron type from a few
mm to greater than a meter.

4. NEURON
Schematic diagram of an idealized neuron and its major components. Most afferent
input from axons of other cells terminates in synapses on the dendrites (d), although
some may terminate on the soma (S). Excitatory terminals tend to terminate more
distally on dendrites than do inhibitory ones, which often terminate on the soma

5. NEUROGLIA
• These are supportive cells.
• Neuroglial cells in the human CNS
outnumber neurons by an order of
magnitude: there are about 10 trillion
neuroglia and 1 trillion neurons.
• Neuroglial cells in the peripheral nervous
system (PNS) include;
 Schwann cells and satellite cells,
• In the central nervous system (CNS) they
include;
 Astrocytes, oligodendroglia, microglia and
ependymal cells.

6. Schematic representation of nonneural elements in the central nervous system. Two astrocytes (darker colour)
are shown ending on a neuron's soma and dendrites. They also contact the pial surface and/or capillaries. An
oligodendrocyte (lighter colour) provides the myelin sheaths for axons. Also shown are microglia (darker colour)
and ependymal cells (lighter colour). (Redrawn from Williams PL, Warwick R: Functional neuroanatomy of man,
Edinburgh, 1975, Churchill Livingstone.)

7. ASTROCYTES
• Astrocytes (named for their star shape) regulate the
microenvironment of neurons in the CNS. They can
actively take up K+ ions and neurotransmitter substances,
which they metabolize
• Astrocytes form part of the BBB. They have foot
processes that contact the capillaries and the pia mater.
These foot processes may help limit the free diffusion of
substances into the CNS. The processes also surround
groups of synaptic endings and isolate these from adjacent
synapses.
• Astrocytes provide mechanical support to the CNS. The
cytoplasm of astrocytes contains glial filaments, which
provide mechanical support for CNS tissue.
• After injury, the astrocytic processes that contain these
glial filaments hypertrophy and form a glial "scar."

8. OLIGODENDROGLIA AND SCHWANN CELLS
• Oligodendrocytes myelinate nerve axons in the
CNS while Schwann cells provide myelin sheath
in the PNS.
• Myelin increases the speed of action
potential conduction, in part by limiting the
flow of ionic current during action potentials to
the nodes of Ranvier (the unmyelinated
portions of the axon at the junctions between
adjacent sheath cells).
• This action results in saltatory conduction,
which is the skipping of nerve impulses from
node to node

9. OTHER NEUROGLIA
• Satellite cells:
• Encapsulate dorsal root and cranial nerve ganglion cells and
regulate their microenvironment in a fashion similar to that
employed by astrocytes.
• Microglia are latent phagocytes. When the CNS is damaged,
the microglia help remove the cellular products of the damage.
They are assisted by neuroglia and by other phagocytes that
invade the CNS from the circulation.
• Ependymal cells form the epithelium that separates the CNS
from cerebrospinal fluid (CSF) in the ventricles. Many
substances diffuse readily across the ependyma, which lie
between the extracellular space of the brain and the CSF.
• CSF is secreted by specialized ependymal cells of the choroid
plexuses, located in the ventricular system.

10. CLINICAL CORRELATES
• Most neurons in the adult nervous system are postmitotic cells
(although there may also remain some precursor neurons or stem cells).
• Many glial precursor cells are present in the adult brain, and they
can still divide and differentiate.
• Therefore, the cellular elements that give rise to most intrinsic brain
tumors in the adult brain are the glial cells. For example: Astrocyte
Tumors: Vary in malignancy from the slowly growing astrocytoma to the
rapidly fatal glioblastoma multiforme
• Oligodendroglia Tumours Oligodendroglioma,
• Ependymal cells: Ependymoma.
• Meningeal cells can also give rise to slowly growing tumors
(meningiomas) that compress brain tissue,
• Schwann cells (e.g., "acoustic neurinomas," which are tumors formed by
Schwann cells of the eighth cranial nerve).
• In the brain of infants, neurons that are still dividing can sometimes
give rise to neuroblastomas (e.g., of the roof of the fourth ventricle) or
retinoblastomas (in the eye).