Faculty of Dental Medicine and Surgery system... · 2019-03-25 · Dr. Nafisa Bakri Idris. Content...

Faculty of Dental Medicine and Surgery

Sem 2

Man and his environment

Nervous tissue

Dr. Nafisa Bakri Idris

Content

• Neuron

• Synapses

• Neuroglia

• Nerve fiber

Objectives

By the end of this lecture the students should be able

to:

• Understand 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

1.Microstructure of neuron

Cell body

Axon

Dendrites

Microstructure

LM:

EM:

Function

Classification

Neuron

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

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

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 :

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

Structures of the synapse

(1) The axon may terminate in a single bulb-like end called a presynapse bouton (or synaptic bag).

(2) synapse cleft

(3)postsynaptic cleft

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

• 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

• Transverse section

• epineurium: nerve truck

• perineurium: Each of fascicles

endoneurium:each nerve fibers.

Thank you