Nervous tissue

　　　 Department of Histology and Embryology Medical college in Three Gorges University

Content Neuron Synapses Neuroglia Nerve fiber Nerve ending

　　　 Outline

More pay attention to the microstructure of

neuron and its classification

Master the microstructure of the synapses

Know the micrograph and function of glia

Familiar the microstructure of nerve fiber

Nervous tissue is composed of nerve cells

and neuroglia. The specialized cells that

constitute the functional units of the

nervous system are called neurons.

The function of neuron:generate nerve

impulses in response to stimuli and

transmit them along cellular processes.

Neuroglia or glia: neurons are supported by

a special kind of connective tissue within

the brain and spinal cord, that is called

neuroglia,it also located in the PNS.

Function: support, protect, connect

Central nervous system

Peripheral nervous system

Brain and spinal cord

The nerves and their associated ganglia.

Nervous system

Microstructure LM: EM: FunctionClassification

Neuron

1.Microstructure of neuron

Cell body

Axon

Dendrites

Neurons

Cell body

Processes or neurites

Dendrites

Axon

Nucleus

Cytoplasm

Cell membrane

The cell body,soma, is the part of neuron that

contains nucleus and surrounding cytoplasm,

also called perikaryon. It is the trophic center

of the neuron. The protein and enzymes

synthesis in this area.

Cell body: Perikaryon

Where is cell body

Cell body: Perikaryon

Position: only in grey matter in CNS which also contains dendrites and axons starting from or ending on the cell bodies,ganglia in PNS

Shape:They can be pyramidal, spherical, ovoid or pear-shaped.

Size: Measuring 5-150 um in diameter.

Microscopic examination

(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

stain,prominent nucleoli are very clear.

Cell body: Perikaryon

nucleus

nucleolus

(3)Cytoplasm: the cytoplasm has some

distinctive characteristics not seen in other

cells. The cytoplasm is basophilic and

full of neurofibrils.

Cell body: Perikaryon

Nissl body

Neurofibril

H-E stains Silver nitrate

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

granules).

Cell body: Perikaryon

细颗粒样的尼氏体

Nissil body

axon hilllock

Neurofibrils

Neurofibrils are thin black fibers

observed in LM with silver nitrate slides,

which is composed of microtubule and

filaments in EM.

Cell body: Perikaryon

EM

RER

ribosome

pigment

synapse

microtubemitochondria

EM: rough surfaced endoplasmic reticulum.

The presence of abundant granular endoplasmic

reticulum is an indication of the high level of

protein synthesis in neurons. Mitochondria,

SER,lysosomes,Golgi complexes,ribosome etc.

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.Most neurons

give off a number of short branching

processes called dendrites and one longer

process called an axon.

Neurites or processes

Cell body

Axon

Dendrites

collaterals

Nissl bodyNeurofibril

H-E stains Silver nitrate不同形态

Axon

Dendrites

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

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.

Axon

axon hilllock

An axon may have not much branches than

that of dendrites. If 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.

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 segment.

Neurites or processes

Dendrites Axons

many one

short long

irregular in thickness uniform in diameter

Nissl granules No Nissl substance

spines axon hillock

impulse towards the soma away from the cell body

1) morphology

2) Axon hillock in LM

3) Dendrites have microtubule associated protein

(MAP-2 ) in immunocytochemically, not present in

axons.

Identification of dendrites from axon

distinguished from axons

2. Classification of neuron

Cell body

Axon

Dendrites

• 1)According to the number of the processes

unipolar,or pseudounipolar neuron

bipolar neuron

multipolar neuron

multipolar neuron

bipolar neuron

Unipolar neuron

Unipolar neuron

Spinal ganglia

Satellite cells

Unipolar neuron

Bipolar neuron

Retina of eye

axon hilllock

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

Sense ( afferent )neurons:

Interneurons

Motor( efferent ) neurons

•According to their function:

• 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 bipolar.

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.

Depending on the shapes of their cell bodies some neurons are referred to as stellate (star shaped) or pyramidal.

• According to the neurotransmitter they release

• 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.

synapse

Processes

Cell body

• Classification of synapses:

According the constitution:

axodendritic synapse

axosomatic synapse

axoaxonal synapse

dendro-axonic

dendro-dendritic

somato-somatic synapse

somato-dendritic synapse

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 bag:

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 reticulum.

presynaptic elements

postsynaptic elements

synaptic cleft

presynpatic membrane

visicle

mitochondria

postsynaptic membrane

presynaptic elemen

Synaptic cleft

Neuroglia

• Neuroglia:

Within the central nerve system:

Oligodendrocytes

astrocytes

microglia

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:

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 nerves.

Myelin sheath

Nodes of Ranvier

Axon

internode

施 -万切迹

• Transverse section

• epineurium: nerve truck

• perineurium: Each of fascicles

endoneurium:each nerve fibers.

Peripheral nerve ending

• 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 system.

Lamellated Corpuscles (of Pacini) ：

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) :

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

touch.

Muscle Spindles

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

fibers.

• Intrafusal fibers contain several nuclei that are located near the middle of the fiber.

nuclear bag fiber

nuclear chain fibers

• Motor end plate

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.