Download Nerve tissue

Nerve tissue
1. General Characteristics






A. Made up of 2 types of cells: nerve cells and glial cells .
① nerve cells ( neurons) -- structural and functional unit.
②glial cells ( neuroglia )-- supporting, protecting and nourishing
neurons.
B. Neurons have unique processes and contact with each other via
synapses forming neural network and circuit.
C. Nervous tissue makes up nervous system. The central nervous
system (CNS) consists of the brain and spinal cord; the peripheral
nervous system (PNS) comprises ganglia, nerves and nerve endings.
D. Function -- integrating and coordinating body activity.
2. Neuron
1)Morphology:
Most neurons consist of three parts:
---cell body: spherical, pyramidal, fusiform or stellate
in shape, 5µm-150µm
---dendrite: like branches of tree
---axon: long thin cord-liked
2) Structure of neuron
① Cell body (perinuclear cytoplasm, perikaryon)
---Nucleus: large, spherical, and pale-staining with a
prominent nucleolus, indicating intense synthetic
activity.
---cytoplasm:
a. Nissl body:
LM: coarse or fine basophilic particles
EM: clusters of RER plus free ribosomes
Function: synthesis of proteins
structural proteins
proteins for transport
b. Neurofibrils
•LM: Thin threads with silver
staining;
• EM:
Neurofilaments and
microtubules;
• Function:
serve as cytoskeleton, and
are involved in
transportation of substances.
c. other organelles

Golgi complex
Mitochondria
Lysosome

Lipofuscin is increased in number with age.


② Dendrites:





One or more dendrites / neuron.
Usually short and thick, and
tapered as they branch and
rebranch like a tree.
Have numerous dentritic spines on
the surfaces, representing sites of
synaptic contact.
Contain similar organelles to
perikarya, especially Nissl bodies.
Main function is to receive
information from other neurons
and conduct it to the parent cell
body.
③Axons:





Only one axon / neuron.
Usually long and thin with uniform
diameter, does not branch profusely, but
may have collaterals.
Arises from a conical region called the
axon hillock that derives from the
perikaryon. The axon and axon hillock
are devoid of Nissl bodies.
Ends in several terminal branches called
axon terminals or buttons, which contain
vesicles with neurotransmitters in them.
Conduct impulses away from one
neuron to other neurons or to effector
cells such as muscle or gland cells.
* Axonal transports:




slow anterograde: proteins and
actin filaments.
Intermediate anterograde:
mitochondria.
fast anterograde: transports the
substances contained in synaptic
vesicles.
A retrograde flow: transports the
several molecules, inclding material
taken up by endocytosis( including
viruses and toxins), to the cell body.
3) Classification
---According to number of processes
 multipolar neuron, which have more than two cell
processes, one process being the axon and the others dendrites;
 bipolar neuron, with one dendrite and one axon.

pseudounipolar neuron,
single process, soon divides
into two in a T-shape, with
one branch extending to
a peripheral ending and
the other toward the CNS.
---According to the functional roles

sensory neuron (afferent neuron): are involved
in the reception of sensory stimuli from the
environment and from within the body.

motor neuron (efferent neuron): control effector
organs.

interneuron: establish relationships among other
neurons, forming complex functional network.
3. Synapse
---Definition: synapses are sites of functional contact
between neurons or between neuron and other effector cells
---Classification:
 Chemical synapse: use a chemical mediator
(neurotransmitter) to transmit impulses in one direction;

Electrical synapse: permit direct flow of electrical current
between two neurons (gap junction).
Neurotransmitters and Neuromodulators
 Neurotransmitters are chemicals that , when
combined with a receptor protein, either open or
close ion channels or initiate second-messenger
cascades.
 Neuromodulators are chemical messengers
that do not act directly on synapses but modify
neuron sensitivity to synaptic stimulation or
inhibition.
Types of synapses
Axosomatic
 Axodendritic
 Axoaxonic

Ⅰ.Chemical synapse
---structure
LM: in silver preparation, there are many button-liked
structures on the surface of dendrites and cell body,
called synaptic button
Synaptic boutons
EM:
Presynaptic terminal
Synaptic cleft
Postsynaptic terminal
1)Presynaptic terminal
Axon terminal
 presynaptic membrane
 synapse vesicle:
--round or flattened,
--clear or with electron dense core
--contain neurotransmitters
or neuromodulators

Mit, SER, microtubule and
microfilament
2)Synaptic cleft:
Extracellular space between pre- and
postsynaptic membranes.
3)Postsynaptic terminal:
 postsynaptic membrane
 receptors
The sequence of events during synapse activity.



When the nerve impulse reaches
the presynaptic terminal, the
synaptic vesicle fused with the
presynaptic membrane and
discharges the neurotransmitters
into the synaptic cleft by
exocytosis.
The neurotransmitter then
diffuses across the cleft and
combines with specific receptor
molecules in the postsynaptic
membrane.
The reaction between the
transmitter and the receptor
molecules induces an increase in
the permeability of the
postsynaptic membrane and
causes a change in the membrane
potential of the postsynaptic
neurons.
Ⅱ. Electrical synapse
This type of synapse transmit ionic signals through gap junctions
that cross the pre- and postsynaptic membranes, thereby
conducting neuronal signals directly.
Ⅲ.Classification of chemical synapse:
/According to function:

Excitatory synapse
The neurotransmitters react with receptors present at the
postsynaptic region, promoting a transient electrical activity
(depolarization) at the postsynaptic membrane. These
synapses are called excitatory, because their activity
promotes impulses in the postsynaptic cells membrane.

Inhibitory synapse
The neurotransmitter-receptor interaction has an opposite
effect, promoting hyperpolarization with no transmission of
the nerve impulse. These are called inhibitory synapses.

Thus , synapses can excite or inhibit impulse transmission
and thereby regulate nerve activity.
4. Glial cell (neuroglia)

Glial cells are supporting cells
within the nervous system. They
are non-excitable and do not
conduct nerve impulses.

The nunber of glial cells is 5 to 10
times the number of neurons.

Most glial cells possess processes.
However, unlike neurons, the
processes of glial cells cannot be
classified as dendrites and axon.

In routine HE histological
preparation, only the nuclei of
glial cells can be seen. Such nuclei
are smaller than those of neurons
and lack of nucleoli.
Ⅰ. Astrocyte



star-shaped cells with
multiple radiating
processes.
Have bundles of
intermediate filament
that reinforce their
structure.
end feet: to form glia
limitans or vascular feet-constitute blood brain
barrier
Fibrous astrocytes:
Located in the white matter
 with few long, thin processes

Protoplasmic astrocyte:
Found in the gray matter
 With many short, thick
processes with more branch

Function:

Serve as structural support and insulation to the neurons;

Repairing: when the CNS is damaged, astrocytes
proliferate to form cellular scar tissue.

The end feet constitute blood brain barrier and the
barrier regulates the diffusion of many substances
between the blood and brain.

It is believed that through the end feet, astrocytes transfer
molecules and ions from the blood to the neurons.
Ⅱ. Oligodendrocyte:
---structure: smaller,
fewer process
---function:
their processes form
myelin-sheath of NF
in CNS
Ⅲ. Microglia:
---structure: small,
elongated cells with short
irregular processes.
---function:
They are phagocytic and
derived from mesoderm
(monocytes).
Ⅳ. Ependymal cell:
---structure:
 cuboidal
or low columnar epithelial cells
 apical: microvilli and cilia
---function:
 produce
cerebrospinal fluid
 ---distribution: ventricle of brain and central
canal of spinal cord
Ⅴ.Schwann cell
Produce the myelin sheath
that provides the
insulation of neurons in
the PNS. One Schwann
cells forms myelin around
a segment of one axon.
(the same oligodendrocyte
forms myelin sheaths for
several nerve fibers.).
5. Nerve fibers
---definition: a nerve fiber is composed of an
axon and a surrounding
---classification: according to myelin-sheath
 myelinated nerve fiber (MNF)
 unmyelinated nerve fiber
Ⅰ. Myelinated nerve fiber in PNS
1) Basic structure: axon surrounded by myelin sheath and
neurolemma.
2) Along its length the nerve fiber is segmented:
a. The region devoid of myelin sheath and with bare portion of the
axon is known as the node of Ranvier.
 b. Each segment between two consecutive nodes of Ranvier is
called an internode.
 c. The neurolemma is the outermost layer of cytoplasm, cell
membrane and basal lamina of Schwann cells.

Longitudinal section and cross section
of myelined nerve fiber
3) Each internode consists of one Schwann cell:

The myelin sheath results from Schwann cell
plasmalemma winding around the axon many times
and so contains lipids and proteins.( Schwann cell
→invagination and envelop the axon →form mesaxon
→ mesaxon become longer and longer →spiral around
the fiber →form myelin sheath.)
In EM myelin sheath
shows a series of
concentrically
arranged lamellae,
4)The functional role
 a.
Enhancing the speed of conduction along
them via saltatory conduction, i.e., impulses
jumping from node to node, because myelin
sheath serves as an insulator.
 b. The thicker axon has the thicker myelin
sheath and longer internode, and in turn has
greater conduction velocity.
Ⅱ. MNF in CNS
---structure:
 similar to in PNS
 myelin-sheath is formed by flattened ending of
oligodendrocyte’s processes
 one oligodendrocyte can envelop many axons
Ⅲ.N-MNF in PNS
---structure:
no myelin-sheath and Ranvier node
 one Schwann cell envelops more axons

Ⅳ.N-MNF in CNS

* nothing to envelop the axon---naked axon
6.Nerves
Nerves : made up of nerve fibers and connective tissue.


1. Most nerves are mixed, i.e., contain both sensory
(afferent) and motor (efferent) nerve fibers, and both
myelinated and unmyelinated fibers.
2. There are 3 connective tissue sheets:
a.Epineurium, a fibrous connective tissue encloses the
entire nerve and also fills the space between bundles of
nerve fibers.
b.Perineurium is a continuous sheet of flattened
epithelium-like cells surrounding each nerve bundles.
c.Endoneurium envelops each nerve fiber. It is a very
thin layer of loose connective tissue.
E: epineurium
P: perineurium
F: fascis
7. Nerve Ending -classified into sensory and motor nerve endings
1)Sensory nerve ending
---including free and
encapsulated nerve
endings
①free nerve ending-responsible for heat, cold,
and pain.
---structure: NF→lose
myelin-sheath → branch
→ distribute in epidermis,
cornea, hair follicle
epithelial cell and CT
---function: feel cold, hot,
pain and slight touch
② Encapsulated N ending
---have CT capsule
a. Tactile corpuscle
---structure:
CT capsule
 flattened cell--transverse arranged


NF→lost myelin sheath→spiral flattened
cells
---distribution: dermal papillae, especially in tip of finger or toe,
palms, soles and lips
---function: touch receptors
b. lamellar corpuscle (Pacinian)
 Distributed in subcutaneous tissue, mesentery, ligament
Composed of concentric lamellae of flattened cells and
internal cylinder with the naked axon inserted in it.
---function: feel deep or heavy pressure

c. Muscular spindles
---structure:
 Distributed in skeletal
muscle.
 Formed by CT capsule,
intrafusal muscle fibers
(thin, striated, nuclei
arranged in chain or
cluster).
 Nerve fibers end as flowerspray endings.---function:
detect muscle length and
change in muscle length
2)Motor nerve ending :
 muscular T and gland
 Somatic motor nerve
ending :
 motor end plate,
myoneural junction
A moter nerve fiber innervates many muscle fibers comprising a
motor unit, whereas a muscle fiber is innervated by only one axon
branch.
LM: nerve fibers ramify with each terminal dilating
as a plate-like mass and touching a muscle fiber.
Ultrastructure of the MEP. The drawing at the upper right shows branching of
a small nerve with a MEP for each muscle fiber. The structure of one of the
bulbs of an end-plate is highly enlarged in the center drawing. Note that the
axon terminal bud contains synaptic vesicles. The region of the muscle cell
membrane covered by the terminal bud has clefts and ridges called junctional
folds. The axon loses its myelin sheath and dilates, establishing close, irregular
contact with the muscle fiber. Muscle contraction begins with the release of
acetylcholine from the synaptic vesicles. This neurotransmitter causes a local
increase in the permeability of the sarcolemma. The process is propagated to
the rest of the sarcolemma, including the T tubules, and is transferred to the SR.
The increase of permeability in this organelle liberates calcium ions that trigger
the sliding filament mechanism of muscle contraction. Thin filaments slide
between the thick filaments and reduce the distance between the Z lines,
thereby reducing the size of all bands except the A band.