By:

Dr. Maria Simplicia E. Flores

At the end of this lesson, the student should be able to:
1. To define the neuron and name its processes
2. To learn the varieties of neurons and identify them in
different parts of the nervous system
3. To review the cell biology of a neuron and understand
the function of a nerve cell and its processes
4. To review the structure of the plasma membrane as it is
related to its physiology
5. To learn the transport of materials from the cell body
to the axon terminals
6. To understand the structure and function of synapses
and neurotransmitters
7. To review the supporting function of the neuroglial
cells for nerve cells and the possible role that they play
in neuronal metabolism, function, and neuronal death
Neuron - nerve cell and all its processes.
 Excitable cells that are specialized for the
reception of stimuli and the conduction of the
nerve impulse.
 Varies in size and shape, one or more neurites
project from each cell body
◦ Neurites responsible for receiving information
and conducting it toward the cell body are
dendrites.
◦ The single long tubular neurite that conducts
impulses away from the cell body is the axon.
  Dendrites and axons are
commonly referred to as
nerve fibers.

 Neurons are found in the

brain and spinal cord and
in ganglia.
 Normal neurons in the
mature individual do NOT
undergo division and
replication.
Schematic Drawing
of a Motor Neuron
 Cell body of a neuron may be as small as 5 μm
or as large as 135 μm in diameter, but the
processes or neurites may extend over a
distance of more than 1 m.
 Neurons can be CLASSIFIED MORPHOLOGICALLY
based on the NUMBER, LENGTH, and MODE OF
BRANCHING OF THEIR NEURITES.
1. UNIPOLAR neurons have a single neurite that
divides a short distance from the cell body into
two branches, one proceeding to some
peripheral structure and the other entering the
CNS.
 The branches of this single neurite have the
structural and functional characteristics of an
axon.
 In this type of neuron, the fine terminal branches
found at the peripheral end of the axon at the
receptor site are often referred to as the
dendrites.
 Example: posterior root ganglion.
2. BIPOLAR neurons have an elongated cell body,
with a single neurite emerging from each end.
 Examples: found in the retinal bipolar cells and
the cells of the sensory cochlear and vestibular
ganglia.
3. MULTIPOLAR neurons have a number of
neurites arising from the cell body.
 With the exception of the axon, the remainder of
the neurites are dendrites.
 Examples: neurons of the brain and spinal cord
 Neurons may also be CLASSIFIED ACCORDING
TO SIZE.
1. GOLGI TYPE I neurons have a long axon that
can stretch 1 m or more in length in extreme
cases.
 The axons of these neurons form the long fiber
tracts of the brain and spinal cord and the nerve
fibers of peripheral nerves.
 Examples: pyramidal cells of the cerebral
cortex, Purkinje cells of the cerebellar cortex,
and the motor cells of the spinal cord
2. GOLGI TYPE II neurons have a short axon that
terminates in the neighborhood of the cell
body or is entirely absent.
 They greatly outnumber the Golgi type I
neurons.
 The short dendrites that arise from these
neurons give them a star-shaped appearance.
 Examples: numerous in the cerebral and
cerebellar cortex and are often inhibitory in
function.
Different Types
of Neurons
A. Neuronal Structure
 Cell body - consists essentially of a mass of
cytoplasm in which a nucleus is embedded,
bounded externally by a plasma membrane.
1. Nerve Cell Body
 The volume of cytoplasm within the nerve cell
body is often far less than the total volume of
cytoplasm in the neurites.

A. NUCLEUS - stores the genes

 In mature neurons, the chromosomes NO
longer duplicate themselves and function only
in gene expression. Therefore, the
chromosomes exist in an uncoiled state.
 The large size of the nucleolus probably is due
to the high rate of protein synthesis.
 Barr body – composed of sex chromatin for
female chromosome.
 Nuclear envelope is continuous with the
cytoplasmic RER.
◦ The envelope is double layered and possesses
fine nuclear pores, through which materials
can diffuse into and out of the nucleus.
 Newly formed ribosomal subunits can be
passed into the cytoplasm through the nuclear
pores.
B. CYTOPLASM - rich in RER and SER, and
contains the following organelles and
inclusions:
(a) Nissl substance
(b) Golgi complex
(c) mitochondria
(d) microfilaments
(e) microtubules
(f) lysosomes
(g) centrioles
(h) lipofuscin, melanin, glycogen, and lipid
 Nissl substance – basophilic in color,
responsible for protein synthesis, composed
of RER (ribosomes contain RNA) arranged in
the form of broad cisternae stacked one on
top of the other.
 Consists of granules that are distributed
throughout the cytoplasm of the cell body,
except for the region close to the axon hillock.
 The granular material also extends into the
proximal parts of the dendrites but is NOT
present in the axon.
 Fatigue or neuronal damage causes the Nissl
substance to move and become concentrated
at the periphery of the cytoplasm. The
phenomenon which gives the impression that
the Nissl substance has disappeared is called
chromatolysis.
 Golgi complex – appears as network of
irregular wavy threads around the nucleus;
appears as clusters of flattened cisternae and
small vesicles made up of SER
 The protein produced by the Nissl substance is
transferred to the inside of the Golgi complex
in transport vesicles, where it is temporarily
stored and where carbohydrate may be added
to the protein to form glycoproteins.
 Each cisterna of the Golgi complex is
specialized for different types of enzymatic
reaction.
 The Golgi complex is also active in lysosome
production and in the synthesis of cell
membranes.
 Mitochondria - found scattered throughout
the cell body, dendrites, and axons
 Spherical or rod shaped
 Walls are double membrane; inner membrane
is thrown into folds or cristae
 It possess many enzymes which take part in
the tricarboxylic acid cycle and the
cytochrome chains of respiration.
 Important in the production of energy.
 Neurofibrils - are numerous and run parallel
to each other through the cell body into the
neurites.
 Resolve into bundles of neurofilaments

Neurofilaments - form the main component

of the cytoskeleton.
 Chemically, neurofilaments are very stable
and belong to the cytokeratin family.
 Microfilaments - measure about 3 to 5 nm in
diameter and are formed of actin.
 Concentrated at the periphery of the
cytoplasm just beneath the plasma
membrane where they form a dense network.
 Together with microtubules, these play a key
role in the formation of new cell processes
and the retraction of old ones.
 These also assist the microtubules in axon
transport.
 Microtubules - measures about 25 nm in
diameter; are found interspersed among the
neurofilaments
 Extends throughout the cell body and its
processes.

Microtubules and microfilaments provide a

stationary track that permits specific
organelles to move (stop-and-start
movement) by molecular motors.
 Cell transport - involves the movement of
membrane organelles, secretory material,
synaptic precursor membranes, large dense
core vesicles, mitochondria, and SER.
 Can take place in both directions, in the cell
body and its processes.
 RAPID TRANSPORT (100 to 400 mm/day) is
mediated by two motor proteins associated
with the microtubule ATPase sites.
 SLOW TRANSPORT (0.1 to 3.0 mm/day) involves
the bulk movement of the cytoplasm, and
includes the movement of mitochondria and
other organelles
RAPID TRANSPORT
 In anterograde (away from the cell) movement,
kinesin-coated organelles are thought to move
toward one end of the tubule
 In retrograde (toward the cell) movement,
dynein-coated organelles are thought to move
toward the other end of the tubule.
 The direction and speed of the movement of
an organelle are brought by the activation of
one of the motor proteins or both
simultaneously.
SLOW TRANSPORT
 Slow axonal transport occurs only in the
anterograde direction.
 Due to one of the kinesin families.
 Lysosomes - membrane-bound vesicles
measuring about 8 nm in diameter.
 Acting as intracellular scavengers and contain
hydrolytic enzymes.
 Formed by the budding off of the Golgi
apparatus.
 Exist in three forms:
(1) primary lysosomes
(2) secondary lysosomes - contain partially
digested material (myelin figures)
(3) residual bodies - the enzymes are inactive
and the bodies have evolved from digestible
materials such as pigment and lipid.
 Centrioles - small, paired structures found in
immature dividing nerve cells.
 Each centriole is a hollow cylinder whose wall is
made up of microtubule bundles.
 These are associated with the formation of the
spindle during cell division and in the formation
of microtubules.
 Involved also in the maintenance of
microtubules in mature nerve cells.
 Lipofuscin - pigment material; occurs as
yellowish brown granules within the
cytoplasm.
 Forms as the result of lysosomal activity
 It represents a harmless metabolic by product.
 Accumulates with age.
 Melanin granules - are found in the cytoplasm of
cells in certain parts of the brain (e.g., substantia
nigra of the midbrain).
 Their presence may be related to the
catecholamine-synthesizing ability of these
neurons, whose neurotransmitter is dopamine.
C. PLASMA MEMBRANE - forms the continuous
external boundary of the cell body and its
processes.
 Site for the initiation and conduction of the
nerve impulse.
 It is about 8 nm thick; appears as two dark
lines with a light line between them.
 Composed of an inner and an outer layer of
very loosely arranged protein molecules, each
layer being about 2.5 nm thick, separated by a
middle layer of lipid about 3 nm thick.
 Lipid layer is made up of two rows of
phospholipid molecules arranged so that their
hydrophobic ends are in contact with each other
and their polar ends are in contact with the
protein layers.
 Certain protein molecules lie within the
phospholipid layer and span the entire width of
the lipid layer
◦ Provide the membrane with hydrophilic
channels through which inorganic ions may
enter and leave the cell.
 Carbohydrate molecules are attached to the
outside of the plasma membrane and are linked
to the proteins or the lipids, forming as cell coat
or glycocalyx
 Plasma membrane and the cell coat together
form a semipermeable membrane that allows
diffusion of certain ions through it but restricts
others.
B. Nerve Cell Processes
 Neurites- the processes of a nerve cell (divided
into dendrites and an axon).
1. Dendrites - are the short processes of the cell
body which conduct the nerve impulse toward
the cell body; its diameter tapers as they extend
from the cell body, and often branch profusely.
 The finer branches bear large numbers of small
projections called dendritic spines.
 Dendrites are extensions of the cell body to
increase the surface area for the reception of
axons from other neurons.
 The cytoplasm of the dendrites closely
resembles that of the cell body and contains
Nissl granules, mitochondria, microtubules,
microfilaments, ribosomes, and SER.
 During early embryonic development, dendrites
are overproduced. Later, they are reduced in
number and size in response to altered
functional demand from afferent axons.
 Dendrites remain plastic throughout life and
elongate and branch or contract in response to
afferent activity.
2. Axon - the longest process of the cell body.
 Arises from a small conical elevation on the cell
body called axon hillock (no Nissl granules)
 It may arise occasionally from the proximal part
of a dendrite.
 Tubular and uniform in diameter; it tends to
have a smooth surface.
 It does not branch close to the cell body;
collateral branches occurs along their length;
branch profusely before their termination.
 Terminals – enlarged distal ends of the terminal
branches of the axons.
 Varicosities - series of swellings resembling a
string of beads near the termination (especially
those of autonomic nerves).
 Axons may be very short (0.1 mm) as seen in
many neurons of the CNS, or extremely long
(3.0 m) as seen when they extend from a
peripheral receptor in the skin to the spinal cord
and thence to the brain.
 Larger axon diameter conduct impulses rapidly,
while those smaller diameter conduct impulses
very slowly.
 Axon always conduct impulses away from the
cell body EXCEPT for the axons of the sensory
posterior root ganglion because it carries the
impulse toward the cell body.

 Axolemma - plasma membrane bounding the

axon
 Axoplasm - cytoplasm of the axon; lacks Nissl
granules and Golgi complex; RNA and
ribosomes are absent (sites for the production
of protein)
 Axonal survival depends on the transport of
substances from the cell bodies.

 Initial segment of the axon - first 50 to 100 um

after it leaves the axon hillock; most excitable
part of the axon and is the site in which an AP
originates. Under normal conditions, an AP does
not originate on the plasma membrane.
3. Myelin – covers the axon; consists of multiple
concentric layers of lipid-rich membrane
produced by glial cells namely: Schwann cells
(PNS) and oligodendrocytes (CNS).
 Nodes of Ranvier - small gaps (1 μm long) in
the segments of myelin sheaths (axon) where
myelin is absent.
 Smallest axons are unmyelinated.
 Function of myelin: insulator
 Myelination - increases the speed of impulse
conduction along the axon.
C. Axon Transport
 Materials are transported from the cell body to
the axon terminals (anterograde transport) and
to a lesser extent in the opposite direction
(retrograde transport).
 Fast anterograde transport (100 to 400 mm/day)
- refers to the transport of proteins and
transmitter substances or their precursors.
 Slow anterograde transport (0.1 to 3.0 mm/day)
- refers to the transport of axoplasm and
includes the microfilaments and microtubules.
 Retrograde transport - explains how the cell
bodies of nerve cells respond to changes in the
distal end of the axons.
◦ Examples: activated growth factor receptors,
pinocytotic vesicles arising at the axon
terminals, worn-out organelles which goes
back to the cell body for breakdown by the
lysosomes.

 Axon transport is brought about by microtubules

assisted by the microfilaments.
D. Synapses
 Nervous system consists of a large number of
neurons that are linked together to form
functional conducting pathways.
 Synapse or synaptic junction- the site where two
neurons (or a neuron and a skeletal muscle or
gland cell) come into close proximity and
functional interneuronal communication occurs.
 Communication between neurons takes place in
one direction only, usually occurs from the axon
terminal of the transmitting neuron (presynaptic
side) to the receptive region of the receiving
neuron (postsynaptic side).
 Synaptic spines – extensions of the surface of a
neuron form receptive sites for synaptic contact
with afferent boutons.

 Synapses occur in a number of forms:

1. between an axon of one neuron and the
dendrite or cell body of the second neuron
(most common type)
2. Depending on the site of the synapse:
axodendritic, axosomatic, or axoaxonic.
Different Types of Chemical Synapses
 Two types of synapses: chemical and electrical.
1. Chemical synapse- most common, in which a
chemical substance called neurotransmitter,
passes across the narrow space between the
cells and becomes attached to a protein
molecule in the postsynaptic membrane called
the receptor.
o Have several distinctive characteristics:
synaptic vesicles on the presynaptic side, a
synaptic cleft, and a dense thickening of the
cell membrane on both the receiving cell and
the presynaptic side.
◦ Presynaptic and postsynaptic membranes – are the
apposed surfaces of the terminal axonal expansion
and the neuron; separated by a synaptic cleft
measuring about 20 to 30 nm wide.
◦ The synaptic cleft contains polysaccharides.
◦ Presynaptic terminal contains many small
presynaptic vesicles containing neurotransmitters.
◦ One neurotransmitter is usually the principal
activator and acts directly on the postsynaptic
membrane, while the other transmitters function as
modulators and modify the activity of the principal
transmitter.
◦ The vesicles fuse with the presynaptic membrane
and discharge the neurotransmitter(s) into the
synaptic cleft by a process of exocytosis.
◦ The plasticity of synapses may be of great
importance in the process of learning and in the
development and maintenance of memory.
2. Electrical synapse– are gap junctions
containing channels that extend from the
cytoplasm of the presynaptic neuron to that of
the postsynaptic neuron.
◦ The neurons communicate electrically which
involves current that passes directly from cell
to cell through specialized junctions, hence
chemical transmitter is not present.
◦ The bridging channels permit ionic current
flow to take place from one cell to the other
with a minimum of delay.
◦ The rapid spread of activity from one neuron to
another ensures that a group of neurons perform
an identical function acting together.
◦ Electrical synapses are bidirectional; chemical
synapses are not.
E. Neurotransmitter
 Presynaptic vesicles containing neurotransmitter
substance and mitochondria which provide ATP
for the synthesis of new transmitter substance
play a key role in the release of neurotransmitter
substances at synapses.
 Most neurons produce and release only one
principal transmitter at all
 Neurotransmitters: ACh, norepinephrine,
epinephrine, dopamine, glycine, serotonin,
GABA, enkephalins, substance P, and glutamic
acid.
 ACh - found at the neuromuscular junction of
all skeletal muscle, in autonomic ganglia, and at
parasympathetic nerve endings.

 Norepinephrine - found at sympathetic nerve

endings.

 Dopamine - found in high concentration in

different parts of the CNS, such as in the basal
nuclei (ganglia).
Neuroglia – are nonexcitable cells supporting the
neurons of CNS.
 Generally smaller than neurons and outnumber
them by 5 to 10 times; comprise about half the
total volume of the brain and spinal cord.
 FOUR types namely:
(1) astrocytes
(2) oligodendrocytes
(3) microglia
(4) ependyma
1. ASTROCYTES - have small cell bodies with
branching processes that extend in all
directions.
 Two types: fibrous and protoplasmic
◦ Fibrous astrocytes are found mainly in the
white matter, where their processes pass
between the nerve fibers
◦ Protoplasmic astrocytes are found mainly in
the gray matter, where their processes pass
between the nerve cell bodies
 Functions:
1. form a supporting framework for the nerve
cells and nerve fibers
2. for the embryo, this serve as a scaffolding for
the migration of immature neurons
3. serve as electrical insulators preventing axon
terminals from influencing neighboring and
unrelated neurons.
4. barriers for the spread of neurotransmitter
substances released at synapses
5. limits the influence of GABA and glutamic acid
neurotransmitters secreted by the nerve terminals
6. take up excess K+ ions from the extracellular
space
7. store glycogen within their cytoplasm
8. serve as phagocytes by taking up degenerating
synaptic axon terminals
9. serve as a conduit for the passage of
metabolites or raw materials from blood capillaries
to the neurons through their perivascular feet.
10. produce substances that have a trophic
influence on neighboring neurons (e.g. cytokines)
11. blood-brain barrier.
2. OLIGODENDROCYTES - have small cell bodies
and a few delicate processes; their cytoplasm
does not contain filaments.
 Found in rows along myelinated nerve fibers and
surrounds nerve cell bodies
 Functions:
1. responsible for the formation of the myelin
sheath of nerve fibers in the CNS. However, the
myelin of peripheral nerves in the PNS is formed
from by Schwann cells.
provides an insulating coat and greatly
increases the speed of nerve conduction
2. Oligodendrocytes unlike Schwann cells, can
each form several internodal segments of
myelin on the same or different axons
◦ Oligodendrocytes are not surrounded by a
basement membrane.
◦ Myelination begins at about the 16th week of
intrauterine life and continues postnatally until
all the major nerve fibers are myelinated by the
time the child is walking.
3. Oligodendrocytes also surround nerve cell
bodies (satellite Oligodendrocytes).
4. It influence the biochemical environment of
neurons.
3. MICROGLIAL CELLS - embryologically unrelated
to the other neuroglial cells; derived from
macrophages outside the nervous system.
 Smallest of the neuroglial cells and are found
scattered throughout the CNS
 Wavy branching processes arise from their small
cell bodies that give off numerous spinelike
projections.
 Migrate into the nervous system during fetal life.
 Microglial cells increases in number in the
presence of damaged nervous tissue resulting
from trauma and ischemic injury and in the
presence of diseases including Alzheimer disease,
Parkinson disease, multiple sclerosis, and AIDS.
 Many of these new cells are monocytes that have
migrated from the blood.
Functions:
1. Microglial cells in the normal brain and spinal
cord appear to be inactive and are sometimes
called resting microglial cells. In inflammatory
disease of the CNS, they become the immune
effector cells. They retract their processes and
migrate to the site of the lesion. Here, they
proliferate and become antigen-presenting cells,
which, together with the invading T lymphocytes,
confront invading organisms.
2. Active phagocytic cell; their cytoplasm
becomes filled with lipids and cell remnants.
The microglial cells are joined by monocytes
from neighboring blood vessels.
4. EPENDYMAL CELLS - line the cavities of the
brain and the central canal of the spinal cord.
 They form a single layer of cells that are
cuboidal or columnar in shape and possess
microvilli and cilia.
◦ The cilia are often motile, and their movements
contribute to the flow of the cerebrospinal fluid
(CSF).
 Three groups of Ependymal cells :
1. Ependymocytes - line the ventricles of the
brain and the central canal of the spinal cord
and are in contact with the CSF. The adjacent
surfaces have gap junctions but the CSF is in
free communication with CNS intercellular
spaces.
2. Tanycytes - line the floor of the third ventricle
overlying the median eminence of the
hypothalamus. These cells have long basal
processes that pass between the cells of the
median eminence and place endfeet on blood
capillaries.
3. Choroidal epithelial cells - cover the surfaces
of the choroid plexuses. The sides and bases of
these cells are thrown into folds; near their
luminal surfaces, the cells are held together by
tight junctions that encircle the cells. The
presence of tight junctions prevents the
leakage of CSF into the underlying tissues.
 Functions:
1. Ependymocytes assist in CSF circulation within
the cavities of the brain and the central canal
of the spinal cord by the movements of the
cilia. The microvilli on the free surfaces of the
ependymocytes indicate that they also have an
absorptive function.
2. Tanycytes are thought to transport chemical
substances from the CSF to the hypophyseal
portal system. In this manner, they may play a
part in the control of the hormone production
by the anterior lobe of the pituitary.
3. Choroidal epithelial cells are involved in the
production and secretion of CSF from the
choroid plexuses.
 Accounts 20% of the total volume of the brain
and spinal cord.
 Very narrow gap that separates the neurons and
the neuroglial cells wherein these gaps are
linked together and filled with tissue fluid.
 It is in direct continuity with the CSF in the
subarachnoid space externally and with the CSF
in the ventricles of the brain and the central
canal of the spinal cord internally.
 It surrounds the blood capillaries in the brain
and spinal cord (CNS does not have lymphatic
capillaries).
 Provides a pathway for the exchange of ions and
molecules between the blood and the neurons
and glial cells. The plasma membrane of the
endothelial cells of most capillaries is
impermeable to many chemicals, and this forms
the blood—brain barrier.
 CEREBRAL EDEMA - there is an increase in the
bulk of the brain.
◦ It can be either vasogenic (extracellular
primarily) or cytotoxic (intracellular primarily).
◦ Because of the limited size of the cranial vault
within the skull, cerebral edema must be
treated emergently.
 Wallerian degeneration - if the axon is cut, the
part distal to the cut degenerates, because
materials for maintaining the axon (mostly
proteins) are formed in the cell body and can no
longer be transported down the axon
(axoplasmic transport).
 Distal to the level of axonal transection when a
peripheral nerve is injured, Schwann cells
dedifferentiate and divide. Together with
macrophages, they phagocytize the remnants of
the myelin sheaths, which lose their integrity as
the axon degenerates.
 Successful axonal regeneration does not
commonly occur after injury to the CNS.
 Many neurons appear to be dependent on
connection with appropriate target cells; if the
axon fails to regenerate and form a new
synaptic connection with the correct
postsynaptic cells, the axotomized neuron may
die or atrophy.
REGENERATION
A. Peripheral Nerves
 Regeneration denotes a nerve’s ability to regrow
to an appropriate target, including the
reestablishment of functionally useful
connections.
 Nerve crush versus Nerve transection
◦ Crush injury - axons may be severed, but the
Schwann cells, surrounding basal lamina, and
perineurium maintain continuity through the
lesion, facilitating regeneration of axons
through the injured nerve.
 ◦ Nerve transection (nerve is cut) – the
continuity of these pathways is disrupted.
Regeneration is less likely to occur.

B. Central Nervous System

 Axonal regeneration is typically abortive due to
glial scar formed by astrocytic processes,
properties of the oligodendroglial cells, and
inhibitory factor produced by oligodendrocytes,
CNS myelin, or both.
C. Remyelination
 In a number of disorders of the PNS (Guillain–
Barré syndrome), there is demyelination which
interferes with conduction. This condition is
often followed by remyelination by Schwann
cells, which are capable of elaborating new
myelin sheaths.
 Occurs much more slowly in the CNS.
 Little remyelination occurs in demyelinated
plaques within the brain and spinal cord
(Multiple Sclerosis).
D. Collateral Sprouting
 Occurs when an innervated structure has been
partially denervated.
 The remaining axons then form new collaterals
that reinnervate the denervated part of the end
organ.
 This kind of regeneration demonstrates that
there is considerable plasticity in the nervous
system and that one axon can take over the
synaptic sites formerly occupied by another.
 Capability to produce neurons from
undifferentiated, proliferative progenitor cells.
 After pathological insults that result in neuronal
death, the number of neurons is permanently
reduced.