What is Nervous Tissue ? | Histology | Anjani Mishra

Nervous tissue

Written By Anjani Mishra

Nervous tissue is a specialized tissue, the main component of the nervous system are- the brain, spinal cord, and nerves which regulates and controls body function.

Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about responses to the stimulus.

  
                                         Fig:- Sehematic diagram of a typical neuron

1. Dendrites
2. Nucleus
3. Nissl body
4. Neurofibril
5. Axis Cylinder
6. Myelin sheath
7. Nucleus of neurolemma
8. Collateral brances
9. Nucleus of neurolemma
10. Axis Cylinder
11. Neurolemma
12. Node of ranvier 
13. Myelin incision
14. Myelin sheath
15. Terminal branches
16. Terminal plate 

 

Component parts of the nervous tissue

Structurally, nerve tissue consists of two types of cells

A. Nerve cells, or neurons

B. Supporting cells or glial cells

v                   Neuroglial cells and ependymal (CNS)

v                   Schwann cells and satellite cells (PNS)

           A. Neurons

The structural and functional unit in both the CNS and PNS is the neuron or nerve cell.

Composition

A neuron consists of

i)                   A cell body, or perikaryon, and

ii)                 Cell processes:-             

                            Axon 
                            Dendrites

i) A cell body

Most neurons cell bodies are situated in the grey matter of the brain and spinal cord.

Size: 5-100m                                       

Shape: may be pyramidal, fusiform(Spindle shape), flask shaped or polygonal 

                                          Fig:-Multipolar Neurons, Spinal cord, sheep

1. Axon hillock
2. Dendrite
3. Nissl granules 

Features of cell body

a) Cell membrane

It is trilaminar membrane: outer and inner protein layer, and intermediate lipid layer.

b) Nucleus

It is large, spherical and central in position with a prominent nucleolus. Some nerve cells may be binucleated.

c) Cytoplasm

It contains the following structures;

Nissl bodies- Aggregation of R.E.R & free ribosome (Protein synthesis)

Neurofibrils- Neurofilament (Stability & support to the cell)

Microtubules

Mitochondria

Golgi-complex

Lysosomes

Centrosomes and centrioles

Cytoplasmic inclusions

ii) Cell processes

Axon

An axon is a single long cylindrical process of neuron that varies in length and diameter according to the type of neuron.

Features

•   Axons are usually very long process for example, axons of the motor cells of the spinal cord that innervate the foot muscles may have a length of upto 100cm.
•   Most neurons have only one axon.
•   The axon arises from the cell body at the axon-hillock which is pyramid-shaped region.
•   The plasma membrane of axon is called axolemma and its contents are known as axoplasm.

                                              Fig:-  Multipolar neurons, spinal cord, cow

1. Axon hillock
2. Nissl granules
3. Nucleus
4. Dendrite
5. Nucleolus

 
Dendrites

One or more, short branching processes of neuron, specialized in receiving stimuli from the environment, sensory epithelial cells, or other neurons are called dendrites.

Features

•   One or more in number, each of which arise from the cell body
•   They are usually short and divide like the branches of a tree
•   Their cytoplasm contains Nissl bodies, mitochondria and neurofibrils

Function

Convey impulses from periphery to the cell body. They are the principal signal reception and processing sites on neurons.

Classification of neurons

1. According to the number of cell process, it is of three types; 

i) Unipolar neurons

They have only one process arises from one pole of the cell which divides into two branches one which conducts impulses from sensory ending towards the cell body and another which conveys these impulses to the brain or spinal cord. It is also known as pseudo unipolar neurons.

Location:- Spinal ganglia

                  Ganglia of certain cranial nerve 

                                        Fig:- Dorsal root ganglion, spinal cord, chicken

1. Axon
2. Unipolar neuron

ii) Bipolar neurons

They have two processes arises from two opposite poles of the cell. These processes are axon and dendrite.

Location:- Ganglia of inner ear

                - Retina of eye

                -Olfactory mucous membrane

iii) Multipolar neurons

Which have one axon and two or many dendrites arises from different poles of the cell.

Location:- Neuron of brain

                  -Spinal Cord

                  -Autonomic ganglia

2. According to the functional role

i) Motor neuron (efferent)

It carries impulses from the CNS to the periphery which control effector organs such as muscle fibre or glands.

ii) Sensory neuron (afferent)

It involved in the reception of sensory stimuli from the environment and from within the body. They conduct the impulses to the CNS for processing.

B. Supporting cells or glial cells 

Glial cells (Neuroglial cells) and ependyma (CNS)

Glial cells are non-excitable supporting cells of the CNS, furnishing a micro-environment ideal for neuronal activity.

Classification/types of glial cells

 Glial cells:- 3 types

1. Macroglia:- 2 types

        a. Astrocyte:-

               Fibrous

               Protoplasmic

        b. Oligodendrocyte

2. Microglia

3. Ependymal

      a. Astrocytes: 2 types

• Shape- star shaped
• Cell body- small (but largest of the 4 types)
• Cell processes- have a number of radiating processes
• Nucleus- oval 

             Fibrous:- Few long processes
                   Distribution:- White matter in the CNS

             Protoplasmic:- Many short branched processes

                   Distribution:- Gray matter in the CNS

                                               Fig:- Fibrous Astrocyte, medulla, cat

1. Astrocyte, cell body
2. Astrocyte, process
3. Neuron

b. Oligodendrocyte:

•   Smaller than astrocytes
•   They have small cell bodies with a few delicate processes
•   Nucleus- rounded and condensed. No microfilaments in their cytoplasm

Distribution:- Predominant glial cells in the white matter in the CNS.

2. Microglia:

• Small elongated cells with short irregular processes
• Nucleus- dense elongated
• Somewhat less numerous than astrocytes and oligodendrocytes
• They originate from precursors cell in the bone marrow

Distribution:- Both white and gray matter, usually near blood vessels

Acts as a phagocytic cells in CNS, invade micro-organisms

3. Ependymal cells:

•   Ependymal cells are low columnar or cuboidal cells that line the ventricles of the brain and central canal of the spinal cord
•   In some CNS location, these cells are ciliated
•   Tanycytes- special type of ependymal cell which extend processes into the hypothalamus

                                         Fig:- Central canal, spinal cord, sheep

1. Gray matter
2. Central canal
3. Cilia 

 Function

v  Responsible for secreting CSF

v  Facilitate the movement of CSF

Schwann cells and satellite cells (PNS)

Schwann cells (Neuroloemmocytes)

• These are flattered cells
• Nucleus- ovoid or flattened
• Cytoplasm- Contains Small golgi complex and a few mitochondria

Function:- Allow myelination of axon in PNS. One schwann cell forms myelin around a segment of  one axon. 

                                     Fig:- Nerve fascicles, unmyelinated , left ventricle, pig

1. Perineurium
2. Axon
3. Schwann cell, nucleus

Satellite cells

•   Small cells, derived from the embryonic neural crest like neurolemmocytes
•   They form a covering layer over the large neuronal cell bodies in PNS ganglia
•   Closely associated with the neurons and exert a supportive or tropic role

                                                   Fig:- Dorsal root ganglion ,dog 

1.  Nerve fiber
2.  Neuron cell body
3.  Satellite cell 
4.  Nisll granules

Myelin sheath

Definition:

Myelin sheath is lipoprotein complex that covers most of the axons and certain dendrites in the central nervous system and PNS.

It is responsible for the white matter of CNS and for white colour of many peripheral nerve.

Formation:

Myelin sheath is formed by:

v                 Oligodendrocyte in the CNS, and

v                 Schwann cell in the PNS

Structure:

Myelin sheath consists of many layers of modified cell membranes

Nodes of ranvier:

v    Myelin sheath is interrupted at regular intervals by gap, called the nodes of Ranvier, where the adjacent Schwann cells/oligodendrocyte meet.

v       Collateral branches of the axon arise at the nodes of Ranvier.

v       Nodes of Ranvier are the sites for the exchange of ions between the axoplasm and extra cellular fluid.

Internode:

It is the distance between two nodes of Ranvier and consists of one Schwann cell.

Functions: (Myelin sheath)

1. It acts as an insulator of nerve fibre, and reduces the loss of electrical activities into the surrounding tissue by dispersion

2. It is responsible for the faster conduction of impulse through the nerve fibres. The impulse jump from one node to another, the larger the intermodal segment, the faster is the rate of conduction.

Myelin sheath is absent in

i) Nodes of Ranvier

ii) Proximal part of axon (pre axon) close to cell body.

iii) Near the termination of axon

iv) At the cleft of Schmidt-Lanterman

Neurilemma (sheath of Schwann)

Surrounding the myelin sheath, there is a thin membrane of Schwann cell that form a continuous sheath and encloses the peripheral nerve whether myelinated or non-myelinated, called neurilemma or sheath or Schwann.

v  It is absent in the CNS

v  In non-myelinated nerve fibre, the neurilemma continuously surrounds axolemma.

Functions:

1. It helps in protection and insulation.

2. It is responsible for the regeneration of peripheral nerve fibres.

3. It is necessary for the formation of myelin sheath (myelinogenesis)

Nerve fibres

A nerve fibre is an axon (or dendrite) of a nerve cell with its covering, the function of which is to conduct nerve impulses.

                                                Fig:- Dorsal root ganglion, dog

1. Neuron cell body
2. Nerve fibers

Classification

     A. (Structural basis)

1. Myelinated fibres- Covered by myelin Sheath, and is white in colour.

         Distribution:- White matter (CNS)

                                  Peripheral nerves

                                              Fig:- Axons, myelinated, medulla, horse

1. Myelin sheath
2. Axon

 2. Unmyelinated fibres- No myelin sheath, and is gray in colour

         Distribution:- Gray matter (CNS)

                                  Post-ganglionic (ANS)

                                  Somatic fibres (PNS)

                                        Fig:- Nerve fascicles, unmyelinated, uretar, pig

1. Adipose tissue
2. Nerve fibers x.s.
3. Nerve fibers l.s. 

 B. (Source of orgin)

1. Cranial nerves- Arising from brain

2. Spinal nerves- Arising from spinal cord.

C. (Functional basis)

1. Sensory (Afferent) nerve fibres- carry sensory impulses from different parts of the body to the CNS.

2. Motor (efferent) nerve fibres- carry motor impulses from CNS to different parts of the body.

D. (Distrubution basis)

1. Somatic nerve fibres- these supply the skeletal muscles of the body

2. Visceral or antonomic nerve fibres- these supply the various internal organs

E. (Chemical basis):- Neurotransmitter substance secreted

1. Adrenergic nerve fibre- which secrete adrenalin

2. Cholinergic nerve fibre- which secrete acetylcholine 
 

 (Myelinated PNF) Structure:-

A myelinated peripheral nerve fibre is made up of the following structures from within out wards:

1. Axopalsm- forms the central core of axis cylinder

2. Axolemma- semipermeable membrane that covers the axoplasm

3. Myelin sheath- it sorrounds the axis cylinder

4. Neurilemma- it sorrounds the myelin sheath

Connective tissue covering

5. Endoneurium- it is a delicate C.T. sheath which sorrounds the neurolemmal sheath. It  

                             consists of a thin layer of reticular fibres produced by Schwann cells.

6. Perineurium- it is a C.T. sheath which sorrounds each bundles of nerve fibres. It is formed     

                           by layer of flattened epithelium like cells.

7. Epineurium- it is the most external fibrous coat of dense C.T. which sorrounds the   

                          bundles of nerve fibres to form nerves

Nerve trunk

A collection of bundles of nerve fibres(funiculi) enclosed in a connective tissue sheath(epineurium) called nerve trunk. The main stem of a nerve fibres bound together by a tough sheet of C.T.

Synapse

Synapses are the sites of functional contact between neurons or between neurons and other effector cells (eg; muscles and gland cells)

The synapse is responsible for unidirectional transmission of nerve impulse from neuron to another cell.

Classification

A) Anatomical (morphological)

1. Axosomatic synapse- An axon of one neuron forms a synapse with a cell body of another neuron.

2. Axodendritic synapse- Axon — dendrite

3. Axoaxonic- Axon — Axon

4. Dendrodendrite synapse- Dendrite — dendrite

B) Functionally

1. Excitatory synapse

2. Inhibitory synapse

C) Ultra structurally

1. Type—I (asymmetrical synapse):

In which the synaptic cleft is 30mm with pronounced thickening of the postsynaptic membrane. Synaptic vesicles are spherical in this type.

2. Type—II (Symmetrical synapse)

In which the synaptic cleft is 20mm with apparent equal thickening of pre and post synaptic membranes.

Synaptic vesicles are flattened in this type.

Neurotransmitter is a chemical substance that act as the mediator for the transmission of nerve impulses from one neuron to another neuron through a synapse.

Neurotransmitters are release at nerve endings and transmit impulses from nerve to nerve or from nerve to effector tissue (muscle or glands)

 

If you have any questions you can ask me on :
mishravetanatomy@gmail.com 

Facebook Veterinary group link -  https://www.facebook.com/groups/1287264324797711/ 

Twitter - @MishraVet
Facebook - Anjani Mishra

Website: mishravetanatomy.blogspot.com