CLASSIFICATION OF

NERVE FIBERS

PRESENTER

RAJ NIDHIROLL NO 119PARA G2G.S.V.M. MEDICAL COLLEGE, KANPUR

NEURON

• It is the structural and the functional unit of nervous system.

• The human nervous system contains approximate 1012 neurons.

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STRUCTURE OF NEURON

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INTRODUCTION ABOUT NERVE FIBER• A nerve fiber is a thread like

extension of a nerve cell and consists of an axon and myelin sheath (if present) in the nervous system.

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BASIS OF CLASSIFICATI

ON

DEPENDING UPON

DISTRIBUTION

ERLANGER AND GRASSER’S

CLASSIFICATION

NUMERICAL CLASSIFICATI

ON

DEPENDING UPON

STRUCTURE

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Depending upon STRUCTURE1RAJ NIDHI 6

• NERVE FIBERS THOSE ARE COVERED BY MYELIN SHEATH

MYELINATED NERVE FIBERS

• THOSE ARE NOT COVERED BY MYELIN SHEATH

UNMYELINATED NERVE FIBERS

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MYE

LIN

SH

EATH

In peripheral nervous system it is formed byschwann’s cell. While in case of central nervous system it is formed by oligodendroglia.

COMPOSITION

PROTEINSLIPIDS(CHOLESTEROL,

LECITHIN & SPHINGOMYELIN)+

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MYE

LIN

SH

EATH

The places ,where myelin sheath is absent are called node of ranvier(2-3µm) and these are present once about 1-3 mm distance along the myelin sheath.

FACTS

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IT PREVENTS LEAKAGE OF IONS BY 5000 FOLDS. IT INCREASES VELOCITY OF CONDUCTION BY 5-50 FOLDS DUE TO SALTATORY CONDUCTION i.e. ABOUT 100 m/s IN CASE OF MYELINATED NERVE FIBERS WHILE IN NONMYELINATED IT IS ABOUT 0.25 m/s. SALTATORY CONDUCTION CONSERVES ENERGY BECAUSE ONLY

NODES OF RANVIER GET DEPOLARISED.

About the myelinsheath

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Depending upon DISTRIBUTION2RAJ NIDHI 11

• Supply the skeletal muscles of the body.

SOMATIC NERVE FIBERS

• Supply the various internal organs of body.

VISCERAL OR AUTONOMIC NERVE FIBERS

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SOM

ATIC

NER

VE F

IBER

S These are α type motor nerve fibers. The neurotransmitter released at the neuron endings is

acetylcholine(Ach). It always leads to muscles excitation . Inhibition takes

place centrally due to participation of interneurons.

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AUTO

NO

MIC

NER

VE F

IBER

S they innervate smooth muscles , cardiac muscles and

glands. Their main work is to maintain homeostasis with the

help of autonomic nervous system. they can lead to either excitation or inhibition of

effector organs.

Autonomic nerve fibers

Sympathetic nerve fibers

Parasympathetic nerve fibers

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SYM

PATH

ETIC

AN

D

PARA

SYM

PATH

ETIC

NER

VE F

IBER

SPREGANGLIONIC POSTGANGLIONIC

Release acetylcholine in both. Release acetylcholine in case of parasympathetic.

Release either acetylcholine or norepinephrine in case of sympathetic.

Myelinated B fibers. Unmyelinated C fibers are present in the case of sympathetic .

Terminate on the postganglionic cyton

Terminate on visceral effector.

In case of sympathetic it is smaller than postganglionic nerve fiber and vice versa for parasympathetic nerve fiber.

In case of parasympathetic it is smaller than preganglionic nerve fiber and vice versa for sympathetic nerve fibre

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3 Depending upon diameter and velocity of conduction (Erlanger and Grasser’s classification)

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ERLANGER AND GRASSER’S CLASSIFICATION

• Erlanger and Grasser studied the action potential of mixed nerve trunk by means of cathode ray oscilloscope and they obtained the compounded spike. So they divided nerve fibers into 3 groups. They observed that the main cause of difference in nerve fibers is diameter

PROPERTIES CORELATED WITH DIAMETER

AS Diameter increases• Velocity of conduction increases.• Magnitude of electrical response increases.• Threshold of excitation decreases.• Duration of response decreases.• Refractory period decreases.

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• A GROUP

• B GROUP

• C GROUP

GROUPS OF NERVE FIBERS

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A GROUP• A group is composed of largest fibers.• The fibers of this group are myelinated.• Both sensory and motor in function.• It is found in somatic nerves as SCIATIC AND

SAPHENOUS nerve. It is further classified into 4 sub groups.• Aα (afferent and efferent fibers)• Aβ (afferent and efferent fibers) • Aγ (efferent fibers)• Aδ (afferent fibers)

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B GROUP

• The fibers of this group are myelinated.• The B fibers are found solely in preganglionic

autonomic nerve.

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C GROUP

• It is composed of smallest fibers.• All the fibers of this group are unmyelinated.• Mostly found in visceral and cutaneous nerve.• They have high threshold i.e. 30 folds that of A group.• Generally they are found in postganglionic

sympathetic nerve.

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About the nerve fibers

Numerical classification4RAJ NIDHI 25

NUMERICAL CLASSIFICATION OF SENSORY NERVE FIBERS

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CLINICAL ASPECTS

MULTIPLE SCLEROSIS• It is autoimmune disorder.• Causes may be genetic or

environmental.• Antibodies & white blood cells in

the immune system attack myelin causing inflammation and injury of sheath.

• So the loss of myelin leads to leakage of k+ through voltage gated channels, hyperpolarisation and failure to conduct action potential.

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Treatment for multiple sclerosis

NO TREATMENT of this disease but some drug like β-INTERFERON suppresses the immune response, reduce the severity & slow the progression of disease.

PERIPHERAL NEUROPATHY

Peripheral neuropathy is damage to nerves of peripheral nervous system which may be caused by diseases or trauma.

It is classified according to number of nerves affected or types of nerve cells affected (motor ,sensory, autonomic).

MONONEUROPATHY It is a type of neuropathy that only affects single nerve. The most common cause of mononeuropathy is physical compression

of the nerve known as compression neuropathy.

MONONEURITIS MULTIPLEX It is the simultaneous or sequential involvement of the individual

noncontiguous nerve trunk either partially or completely. the pattern of involvement is asymmetric.

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POLYNEUROPATHY It is pattern of nerve damage which is a quite different from

mononeuropathy and often more serious and affecting more areas of body. The pattern of involvement is symmetric.

AUTONOMIC NEUROPATHY It is a form of polyneuropathy which affects the involuntary system

i.e. autonomic nervous system affecting mostly the internal organs. Most commonly it is seen in person with long standing diabetes mellitus type1 and 2.

NEURITIS It is a general term of inflammation of a nerve or the general inflammation of the peripheral nervous system.

REFERENCES

GUYTON GANONG ’S REVIEW OF MEDICAL PHYSIOLOGY INTERNET

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