This document discusses neurons and glial cells in the nervous system. It states that the human CNS contains about 100 billion neurons and 10-50 times as many glial cells. Neurons transmit nerve impulses while glial cells provide support and communication. The two main types of glial cells are microglia and macroglia. Macroglia include oligodendrocytes, Schwann cells, and astrocytes. Neurons contain a cell body, dendrites, an axon, and axon terminals. Most neurons are myelinated by oligodendrocytes or Schwann cells to aid impulse conduction. Materials are transported along microtubules in the axon from the cell body to axon

1. Neurons/Nerves
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2. INTRODUCTION
• The human CNS contains about 1011(100 billion)
neurons
• Also contains 10–50 times this number of glial cells
• About 40% of the human genes participate, at least
to a degree, in formation of CNS
• The neurons are the basic building blocks of the
nervous system
• Neurons perform the specialized function of
integration & transmission of nerve impulse

3. • Neurons and glial cells along with brain capillaries
form a functional unit that is required for normal
brain function, including synaptic activity, ECL fluid
homeostasis, energy metabolism, and neural
protection
• Disturbances in the interaction of these elements
are the pathophysiological basis for many
neurological disorders (eg, cerebral ischemia,
seizures, neurodegenerative diseases, and cerebral
edema)

4. GLIAL CELLS
• Previously, glial cells (or glia) were viewed as CNS
connective/ supporting tissue; also called neuroglia
or glia (glia = glue)
• Today theses cells are recognized for their role in
communication within the CNS in partnership
with neurons
• non-excitable and do not transmit nerve impulse
(action potential)
• Unlike neurons, glial cells continue to undergo
cell division in adulthood and their ability to
proliferate is particularly noticeable after brain
injury (eg, stroke)

5. • Most commonly, neuroglial cells constitute the
site of tumors in nervous system
• Two major types of glial cells in the nervous system;
microglia and macroglia
• Microglia; smallest neuroglial cells; derived from
macrophages outside of the nervous system; often
called the macrophages of CNS
• Scavenger cells that remove debris resulting from
injury, infection, and disease (eg, multiple sclerosis,
AIDS-related dementia, Parkinson disease, and
Alzheimer disease)

6. • Three types of macroglia: oligodendrocytes,
Schwann cells, and astrocytes
• Oligodendrocytes and Schwann cells are involved in
myelin formation around axons in the CNS and
peripheral nervous system, respectively
• Astrocytes are star-shaped neuroglial cells present
in all the parts of the brain; Two types of astrocytes
are found in human brain:
• i. Fibrous astrocytes- occupy mainly the white
matter
ii. Protoplasmic astrocytes- found in gray matter
and have a granular cytoplasm

8. • Both send processes to blood vessels of brain,
particularly, the capillaries, forming tight junction
with capillary membrane; Tight junction in turn
forms the blood-brain barrier
• Also send processes that envelop synapses and the
surface of nerve cells
• Protoplasmic astrocytes have a membrane potential
that varies with the external K+ concentration but
do not generate propagated potentials
• Produce substances that are tropic to neurons, and
help maintain the appropriate conc. of ions and
neurotransmitters by taking up K+ and the
neurotransmitters glutamate and γ-aminobutyrate

11. Neurones
• Neurons occur in a variety of sizes and shapes
• Most of them contain four parts: (1) a cell body, (2)
dendrites, (3) an axon, and (4) axon terminals
• The cell body(soma) contains the nucleus & ribosome
and is the metabolic center of the neuron
• The dendrites form a series of highly branched
outgrowths from the cell body; receive most of the
inputs from other neurons
• The branching dendrites (some neurons may have as
many as 400,000!) increase the cell’s receptive surface
area and thereby increase its capacity to receive signals
from a myriad of other neurons

13. • Particularly in the cerebral and cerebellar cortex,
the dendrites have small knobby projections called
dendritic spines
• The axon, sometimes also called a nerve fiber, is
a single long process that extends from the cell
body to its target cells (from μm to m in length)
• The part of the cell body where the axon is joined is
known as the initial segment, or axon hillock
• The initial segment is the “trigger zone” where, in
most neurons, the electric signals are generated &
are then propagated away from the cell body along
the axon or, sometimes, back along the dendrites

15. • The main axon may have branches, called
collaterals, along its course
• Near the ends both the main axon and its collaterals
undergo further branching; The greater the degree
of branching of the axon and axon collaterals, the
greater the cell’s sphere of influence
• Each branch ends in an axon presynaptic terminal;
composed of a number of synaptic knobs which are
also called terminal buttons or boutons
• They contain granules or vesicles in which the
synaptic transmitters secreted by the nerves are
stored

16. • The axons of many neurons are myelinated, that is,
they acquire a sheath of myelin, a protein–lipid
complex that is wrapped around the axon
• In the PNS, myelin forms when a Schwann cell wraps its
membrane around an axon up to 100 times
• The myelin is compacted when the extracellular
portions of a membrane protein called protein zero (P0)
lock to the extracellular portions of P0 in the apposing
membrane
• Various mutations in the gene for P0 cause peripheral
neuropathies; 29 different mutations have been
described that cause symptoms ranging from mild to
severe

17. • Myelin sheath is not a continuous sheath; absent at
regular intervals (1-μm constrictions that are about 1
mm apart); the area where myelin sheath is absent is
called node of Ranvier
• Segment of the nerve fiber between two nodes
is called internode
• Myelin sheath is responsible for white color of nerve
fibers
• Unmyelinated neurones are simply surrounded by
Schwann cells without the wrapping of the Schwann
cell membrane that produces myelin around the axon
• In the CNS, most neurons are myelinated; the myelin-
forming cells are the oligodendroglia

18. • Here oligodendrocytes emit multiple processes that
form myelin on many neighboring axons
• In multiple sclerosis, a crippling autoimmune
disease, patchy destruction of myelin occurs in the
CNS
• The loss of myelin is associated with delayed or
blocked conduction in the demyelinated axons
• Neurons are classified by three different methods.
A. Depending upon the number of poles
B. Depending upon the function
C. Depending upon the length of axon

19. • Depending Upon The Number Of Poles:
• Based on the number of poles from which the nerve
fibers arise, neurons are divided into three types:
• 1. Unipolar Neurons: Neurons that have only one
pole; from a single pole, both axon and dendrite
arise; this type of nerve cells is present only in
embryonic stage in human beings
• 2. Bipolar Neurons: Neurons with two poles are
known as bipolar neurons; Axon arises from one
pole and dendrites arise from the other pole
• 3. Multipolar Neurons: Neurons which have many
poles; one of the poles gives rise to axon and all
other poles give rise to dendrites

22. • Depending Upon The Function
• On the basis of function, nerve cells are classified into
two types:
1. Motor or efferent neurons
2. Sensory or afferent neurons
• 1. Motor or Efferent Neurons: Neurons which carry
the motor impulses from CNS to peripheral effector
organs like muscles, glands, blood vessels, etc.
Generally, each motor neuron has a long axon and
short dendrites
• 2. Sensory or Afferent Neurons: Neurons which
carry the sensory impulses from periphery to CNS;
generally, each sensory neuron has a short axon and
long dendrites

23. • Depending Upon The Length Of Axon
• Depending upon the length of axon, neurons are
divided into two types:
1. Golgi type I neurons
2. Golgi type II neurons
• 1. Golgi Type I Neurons: They have long axons; cell
body of these neurons is in different parts of CNS
and their axons reach the remote peripheral organs
• 2. Golgi Type II Neurons: Such neurons have short
axons; present in cerebral cortex and spinal cord

24. AXONAL TRANSPORT
• Neurons-secretory cell; differ from other secretory
cells in that the secretory zone is generally at the
end of the axon, far away from the cell body
• Protein synthesis occurs in cell body & transported
to the axonal ending by axoplasmic flow
• The functional and anatomic integrity of the axon is
very important; if the axon is cut, the part distal to
the cut degenerates (wallerian degeneration)
• Orthograde transport occurs along microtubules
that run along the length of the axon and requires
two molecular motors, dynein and kinesin

26. • Orthograde transport moves from the cell body toward
the axon terminals
• Has both fast and slow components; fast axonal
transport occurs at about 400 mm/day, and slow
axonal transport occurs at 0.5 to 10 mm/day
• Retrograde transport, which is in the opposite
direction (from the nerve ending to the cell body),
occurs along microtubules at about 200 mm/day
• Synaptic vesicles recycle in the membrane, but some
used vesicles are carried back to the cell body and
deposited in lysosomes
• Some materials taken up at the ending by endocytosis,
including nerve growth factor (NGF) and some viruses,
are also transported back to the cell body