Transcript Axon 轴突

Nervous tissue
Department of Histology and Embryology
Medical college in Three Gorges University
Nervous tissue, composed of neurons and
neuroglia, is richly supplied with blood.
The specialized cells that constitute the
functional units of the nervous system are
called neurons(神经元).
Within the brain and spinal cord neurons are
supported by a special kind of connective
tissue that is called neuroglia(神经胶质细
Cell body Cytoplasm 细胞
Nucleus 细胞核
Dentrites 树突
or neurites
Axon 轴突
• Cell body(胞体): Perikaryon
The cell body,also called perikaryon, is the
part of neuron that contains nucleus and
surrounding cytoplasm.
Microscopic examination shows that the cell
bodies of neurons are located only in grey
matter which also contains dendrites and
axons starting from or ending on the cell
bodies. They can be pyramidal, spherical,
ovoid or pear-shaped. Measuring 5-150 um in
(1)Cell membrane: the structure is as the
same as the normal cell. It functions in
getting the stimuli and integration and
conducting the nerve impulse.
(2) The nucleus is large and pale with H-E
(3)Cytoplasm: the cytoplasm has some
distinctive characteristics not seen in other
cells. The cytoplasm is basophilic and full of
Nissl bodies: The cytoplasm shows the
presence of a granular material that stains
intensely with basic dyes; this material is the
Nissl substance (also called Nissl bodies or
When examined by EM, these bodies are seen
to be composed of rough surfaced
endoplasmic reticulum. The presence of
abundant granular endoplasmic reticulum is
an indication of the high level of protein
synthesis in neurons.
The proteins are needed for maintenance and repair, and
for production of neurotransmitters and enzymes.
The processes arising from the cell body of a
neuron are called neurites. These are of two
kinds. Most neurons give off a number of
short branching processes called dendrites
and one longer process called an axon.
Dendrites 树突
The dendrites are characterized by the fact
that they terminate near the cell body. They
are irregular in thickness, and Nissl granules
extend into them. They bear numerous small
spines which are of variable shape.
Dendrites can distinguished
immunocytochemically from axons because
of the presence in them of microtubule
associated protein (MAP-2) not present in
• Axon 轴突:
• Every neuron has only one long thin process
or axon which arises from a special region
or axon hillock, which is devoid of Nissl
bodies. It carries the impulse received by
the neuron to distant region. The axon may
be up to one meter in length.
An axon may give off a variable number of
branches. Some branches, that arise near the
cell body and lie at right angles to the axon
are called collaterals. At its termination
the axon breaks up into a number of fine
branches called telodendria which may end in
small swellings (terminal boutons or bouton
terminaux). An axon (or its branches) can
terminate in two ways.
The axon is identified according to the axon
hillock with LM.The part of the axon just
beyond the axon hillock is called the initial
irregular in thickness
uniform diameter
Nissl granules
No Nissl substance
axon hillock
In addition to these differences in structure,
there is a fundamental functional difference
between dendrites and axons. In a dendrite,
the nerve impulse travels towards the cell
body whereas in an axon the impulse travels
away from the cell body.
• The classification of neurons:
• According to the number of process
The shape of the cell body is dependent on the
number of processes arising from it. The most
common type of neuron gives off several
processes from the cell body is,
therefore, multipolar. Some neurons
have only one axon and one dendrite and are
Another type of neuron has a single process
(which is highly convoluted). After a very
short course this process divides into two.
One of the divisions represents the axon;
the other is functionally a dendrite, but its
structure is indistinguishable from that of an
axon. This neuron is described as unipolar,
but from a functional point of view it is to be
regarded as bipolar. (To avoid confusion on
this account this kind of neuron has been
referred to, in the past, as a pseudounipolar
neuron but this term has
now been
discarded). Depending on the shapes
of their cell bodies some neurons are referred
to as stellate (star shaped) or pyramidal.
• According to the number of the processes:
unipolar,or pseudounipolar neuron
bipolar neuron
multipolar neuron
According to the size of cell body and the
length of axon: According to Cajal (1889):
Golgi type I neurons: long axon
Golgi type II neurons: short axon
• According to their function:
Sense ( afferent )neurons:
Motor( efferent ) neurons
• According to the neurotransmitter they
• Cholinergic neurons: acetylcholine
• Aminergic neurons: adrenaline, non
• Peptidergic neurons: neuropeptids
The Synapse 突触:
Concept: Synapses are highly specialized
intercellular junctions which link the
neurons of each nervous pathway.
Similar intercellular junctions link neurons
and their effector cells such as muscle
fibers;where neurons synapse with skeletal
muscle they are referred to as
neuromuscular junction or motor end plate.
• Classification of synapses:
According the constitution:
axodendritic synapse 轴树突触
axosomatic synapse 轴体突触
axoaxonal synapse 轴轴突触
dendro-axonic 树轴突触
dendro-dendritic 树树突触
somato-somatic synapse 体体突触
somato-dendritic synapse 体树突触
Structure of synapse:
(1) The axon may terminate in a single bulb-like
end called a presynapse bouton (or synaptic bag).
There are several microstructure in the synapse
synapse vesicle,microtubules
Mitochondria and lysosomes
Dense presynaptic membrane which is broken
up into several bits (or dense projection )in EM.
(2) synapse cleft:
Delicate fibres or granular material may be
seen within the cleft. On either side of the cleft
there is a region of dense cytoplasm.
(3)postsynaptic cleft: On the postsynaptic
side the dense Cytoplasm is continuous and
is associated with a meshwork of filaments
called the synaptic web. The thickened
areas of membrane on the presynaptic and
postsynaptic sides constitute the active zone
of a synapse. Neurotransmission takes
place through this region.
The postsynaptic process may also show
membranous structures of various shapes,
microtubules, filaments and endoplasmic
• Neuroglia:
Within the central nerve system,supporting
cells are designated neuroglia , or glial
cells.the four of glial cells are:
ependymal cells
• Oligodendrocytes,small cells that are active
in the formation and maintenance of myelin
in the CNS
• Astrocytes, cells that provide physical and
metabolic support for the neurons of the
CNS, Astrocytes are of two types,
protoplasmic and fibrous
• Microglia, inconspicuous cells with
small,dark, elongated nuclei that possess
phagocytotic properties
• Ependymal cells, column-shaped cells that
line the ventricles of the brain and the
central canal of the spinal cord
• Peripheral neuroglia:
Schwann cells are active in the formation
and maintenance of myelin in the PNS.
Satellite cells:The neuronal cell bodies of
ganglia are surrounded by a layer of small
cuboidal cells called satellite cells. (spinal
and autonomic ganglia)
• Myelinated and non-myelinated nerve fibers
In the peripheral nervous system, all axon are
enveloped by highly specialized cells called
Schwann cells which provide both structure and
metabolic support. In general, small diameter
axon (e.g. those of the autonomic nervous
system and small pain fibers) are simply
enveloped by the cytoplasm of Schwann cells.
• These nerve fibers are said to be non-
myelinated. Large diameter fibers are
wrapped by a variable number of concentric
layers of the Schwann cell plasma membrane
forming a myelin sheath; such nerve fibers
are said to be myelinated.
• Within the central nerve system,myelination
is similar to that tin the peripheral nervous
system except that the myelin sheaths are
formed by cells called oligodendrocytes.
• Nerve fibers 神经纤维:
These are process of neurons and are
collected to form nerve trunk.
In longitudinal section: The nerve fiber
is seen to consist of the central axis cylinder
or axon arising from the neuron. It is
surrounding by layer myelin which consist
of phospholipids which acts as an insulator.
• It is interrupted at places. At these points,
the points are called nodes of Ranvier.
Outside the myelin sheath is a thin cell of
Schwann, which are neurilemma.The cells
of neurilemma are also known as cells of
Schwann, which are neuroectodermal in
origin. These cells are responsible for laying
down the myelin sheath of the peripheral
Nodes of Ranvier
• Transverse section
• epineurium神经外膜: nerve truck
perineurium神经束膜: Each of fascicles
endoneurium神经内膜:each nerve fibers.
Peripheral nerve ending 神经末梢
• Exteroceptive receptors:
Free Nerve Endings
Lamellated Corpuscles (of Pacini)
Tactile Corpuscles (of Meissner)
Proprioceptive receptors:
Muscle Spindles
When the terminals of sensory nerves do not
show any particular specialization of structure
they are called free nerve endings. Such
endings are widely distributed in the body:
connective tissue,epithelial lining of the skin,
cornea, alimentary canal, and respiratory
Lamellated Corpuscles (of Pacini) :环层小体
Pacinian corpuscles are circular or oval
structures. These are much larger than tactile
corpuscles. They may be up to 2 mm in length,
and up to 0.5 mm across. They are found in the
subcutaneous tissue of the palm and sole, in
the digits, and in various other situations.
Tactile Corpuscles (of Meissner) 触觉小体
These are small oval or cylindrical structures
seen in relation to dermal papillae in the hand
and foot, and in some other situations. These
corpuscles are believed to be responsible for
Muscle Spindles 肌梭
These are spindle-shaped sensory end organs
located within striated muscle .The spindle is
bounded by a fusiform connective tissue
covering (forming an external capsule)
within which there are a few muscle fibres of
a special kind. These are called intrafusal
• Intrafusal fibers contain several nuclei that
are located near the middle of the fiber.
nuclear bag fiber
nuclear chain fibers
• Motor end plate 运动终板
In most neuromuscular junctions the
nerve terminal comes in contact with a
specialized area near the middle of the
muscle fiber. This area is roughly oval or
circular, and is referred to as the sole
plate. The sole plate plus the axon
terminal constitute the motor end plate.
Neurons vary considerably in the size and
shape of their cell bodies (somata) and in
the length and manner of branching of their
processes. The cell body varies in diameter
from about 5 um, in the smallest neurons, to
as much as 120 um in the largest ones.
Histology of cerebra, cerebellum
and spinal cord
Chengying Fu
Nervous system
Central nervous
Peripheral nervous
Brain and
spinal cord
The nerves outside
the CNS and their
associated ganglia.
• Cerebral cortex:
• The neurones of the cerebral cortex are
divided into five different morphological
types which are arranged in several layers.
• Neurone types in the cerebral cortex:
• The five types of cortical neuron:
Pyramidal cells
Stellate cells
Cells of Martinotti
Fusiform cells
Horizontal cells of Cajal:
Horizontal cells of Cajal
Cells of Martinotti
Stellate cells
Pyramidal cells
Fusiform cells
• Layers of the neocortex:
Plexiform layer
Outer granular layer
Pyramidal cell layer
Inner granular layer
Ganglionic layer
Multiform cell layer
Plexiform layer
Outer granular layer
Pyramidal cell layer
Inner granular layer
Ganglionic layer
Multiform cell layer
• 1. Plexiform layer: containing dendrites and
axons of cortical neurone making synapses
with one another, and small interneuron.
• 2. Outer granular layer:A dense population
of small pyramidal cells and stellate cells
make up this thin layer which also contains
various axons and dendritic connection
from deeper layer.
• 3. Pyramidal cell layer:
• Pyramidal cell of moderate size
predominate in this broad layer, the cells
increasing in size deeper in the layer.
• 4. Inner granular layer: This layer consists
mainly of densely packed stellate cells.
• 5. Ganglionic layer: large pyramidal cells
and smaller number of stellate cells and
cells of Martinotti make up this layer.
• 6. Multiform cell layer: This is so named for
the wide variety of differing morphological
forms found in this layer.
Plexiform layer
Outer granular layer
Pyramidal cell layer
Inner granular layer
Ganglionic layer
Multiform cell layer
• Cerebellum:
• The cerebellum has enormous amount of
surface area due to the folds, or folia. These
folds are small gyri. White matter and gray
matter can be identified at low
• The gray matter includes the granular cell
layer, purkinje cell layer ( with Purkinje
cells and cell processes), and the molecular
layer. The pale region, which is most
superficial, is the molecular layer. This is
made up primarily of small axonal
processes and a few small dark cell bodies.
• These are mainly a single layer of Purknjie
cell which is deep to the molecular layer, on
the boundary between the molecular and
granular cell layers. The Purkinje neurons
are the large irregularly spaced cells without
much of their broad dendritic arborization
evident in this preparation. The granular cell
layer is made up of many tightly packed
small dense staining nerve cell bodies.
• Purkinje cell is impregnated with silver in
this preparation.The flask-shaped Purkinje
cells are arranged in single row. Each
Purkinje cell possesses several main
dendrites that enter the molecular layer and
direct toward surface of cerebellum.
• The dendrites arborize richly in the
molecular layer. The arborization is fanshaped and extends at right angles to the
cerebellar folia.An axon projects out of the
bottom of the Purkinje cell.
• Spinal cord:
• In cross section,the spinal cord exhibits a
butterfly-shaped grayish tan inner substance,
the gray matter surrounding the central
canal and a whitish peripheral substance,
the white matter.
• The white matter contains only myelinated
and unmyelinated axons traveling to and
from other pars of the spinal cord and to and
from the brain. Functionally related bundles
of axons in the white matter are called tracts.
The white matter contains nerve fiber that
form ascending and descending tracts.
• The gray matter contains neuronal cell
bodies and their dendrites, along with axons
and neuroglia.
• The anterior and posterior prongs are
referred to as ventral horns and dorsal horns
respectively. They are connected by the
gray commissure.