8/14/2019 Deem An

1/13

Physiologic Classification Of NervePhysiologic Classification Of Nerve

FibersFibers

1.1. General Classification Of Nerve FibersGeneral Classification Of Nerve Fibers

The fibers are divided in to types A and C fibers, andThe fibers are divided in to types A and C fibers, and

the type A fibers are further subdivided in to alphathe type A fibers are further subdivided in to alpha

,beta , gamma and sigma fibers.,beta , gamma and sigma fibers.

Type A fibers are the typical large and medium sizedType A fibers are the typical large and medium sized

myelinated fibers of spinal nerves, that transmitmyelinated fibers of spinal nerves, that transmit

impulses at velocities as great as 120 m/sec.impulses at velocities as great as 120 m/sec. Type C fibers are the small unmyelinated nerveType C fibers are the small unmyelinated nerve

fibers, that transmit impulses as slowly as 0.5 m/sec.fibers, that transmit impulses as slowly as 0.5 m/sec.

8/14/2019 Deem An

2/13

2. Alternative Classification2. Alternative Classification Group Ia : Fibers from the annulospiral endings ofGroup Ia : Fibers from the annulospiral endings of

muscle spindles (average about 17 microns inmuscle spindles (average about 17 microns in

diameter).diameter). Group Ib : Fibers from the Golgi tendon organGroup Ib : Fibers from the Golgi tendon organ

(average about 16 microns in diameter).(average about 16 microns in diameter).

Group II : Fibers from most discrete cutaneousGroup II : Fibers from most discrete cutaneous

tactile receptors (average about 8 microns intactile receptors (average about 8 microns indiameter).diameter).

Group III : Fibers carrying temperature, crude, touch,Group III : Fibers carrying temperature, crude, touch,and pricking pain sensations (average about 3and pricking pain sensations (average about 3

microns in diameter).microns in diameter). Group IV : Unmyelinated fibers carrying pain, itch,Group IV : Unmyelinated fibers carrying pain, itch,

temperature, and crude touch sensations (0.5 to 2temperature, and crude touch sensations (0.5 to 2microns in diameter).microns in diameter).

8/14/2019 Deem An

3/13

Effect of Local Anaesthesia on

Afferent Nerve Fiber Impulses

The local anaesthetic agent prevents the

increase in sodium permeability of the nerve

membrane.In a normal afferent nerve-fiber an effective

stimulus will cause depolarization and give rise

to an impulse.When the axonal limiting membrane has been

stabilized by a local anaesthetic agent

depolarization is prevented and no impulse is

8/14/2019 Deem An

4/13

8/14/2019 Deem An

5/13

PainPain Pain is a protective mechanism.Pain is a protective mechanism.

Types of Pain and Their QualitiesTypes of Pain and Their Qualities

1- Fast pain:1- Fast pain:

Is felt with in about 1 second after a pain stimulus isIs felt with in about 1 second after a pain stimulus is

applied.applied.

2- Slow pain :2- Slow pain :

Begins only after 1 second or more and thereBegins only after 1 second or more and thereincreases slowly over many seconds and sometimesincreases slowly over many seconds and sometimes

even minutes.even minutes.

8/14/2019 Deem An

6/13

Pain Receptors and Their StimulationPain Receptors and Their Stimulation

Pain receptors are free nerve endings.Pain receptors are free nerve endings.

The pain receptors in the skin and other tissues areThe pain receptors in the skin and other tissues areall free nerve endings. They are wide spread in theall free nerve endings. They are wide spread in the

superficial layers of the skin as well as in certainsuperficial layers of the skin as well as in certain

internal tissues such as periosteum, the arterial walls,internal tissues such as periosteum, the arterial walls,

the joint surfaces and the flax and tentorium in thethe joint surfaces and the flax and tentorium in thecranial vault.cranial vault.

Pain Stimuli :Pain Stimuli :

PainPaincan be elicited by multiple types of stimuli. Theycan be elicited by multiple types of stimuli. Theyclassified as mechanical, thermal, and chemical painclassified as mechanical, thermal, and chemical pain

stimuli.stimuli.

8/14/2019 Deem An

7/13

In general , fast pain is elicited by the mechanical andIn general , fast pain is elicited by the mechanical and

thermal types of stimuli, whereas slow pain can bethermal types of stimuli, whereas slow pain can be

elicited by all three types .elicited by all three types .

Some of the chemicals that excite the chemical type ofSome of the chemicals that excite the chemical type of

pain are bradykinin, serotonin, histamin, potassiumpain are bradykinin, serotonin, histamin, potassium

ions, acids, and proteolytic enzymes.ions, acids, and proteolytic enzymes.

The chemical substances are especially important inThe chemical substances are especially important in

stimulating the slow, suffering type of pain that ocurrsstimulating the slow, suffering type of pain that ocurrs

after tissue injury.after tissue injury.

8/14/2019 Deem An

8/13

Pain Theory

Gate Control Theory

The passage of afferent pain impulses is eitherinhibited or facilitated in the gelatinosa of the

spinal cord or the caudal nucleus of thetrigeminal nerve.

Cells transmitting via the anterior and/or lateralspinothalamic tracts must be activated.

The Gate Control Theory postulates that anintermediary cell acts as a gate to eachtransmitting cell and normally inhibits its

activity.

8/14/2019 Deem An

9/13

Gate control theory

The gate control theory of pain, is the idea that

the perception of physical pain is not a direct

result of activation ofnociceptors, but instead is

modulated by interaction between differentneurons, both pain-transmitting and non-pain-

transmitting. The theory asserts that activation of

nerves that do not transmit pain signals can

interfere with signals from pain fibers and inhibitan individual's perception of pain.

http://en.wikipedia.org/wiki/Painhttp://en.wikipedia.org/wiki/Nociceptorhttp://en.wikipedia.org/wiki/Neuronhttp://en.wikipedia.org/wiki/Neuronhttp://en.wikipedia.org/wiki/Nociceptorhttp://en.wikipedia.org/wiki/Pain

8/14/2019 Deem An

10/13

Contd

Gate control theory asserts that activation of nerves which do not transmitpain signals, called nonnociceptive fibers, can interfere with signals frompain fibers, thereby inhibiting pain. Afferent pain-receptive nerves, those thatbring signals to the brain, comprise at least two kinds of fibers - a fast,relatively thick, myelinated"A" fiberthat carries messages quickly withintense pain, and a small, unmyelinated, slow "C" fiberthat carries thelonger-term throbbing and chronic pain. Large-diameter A fibers arenonnociceptive (do not transmit pain stimuli) and inhibit the effects of firingby A and C fibers.

The peripheral nervous system has centers at which pain stimuli can beregulated. Some areas in the dorsal horn of the spinal cord that are involvedin receiving pain stimuli from A and C fibers, called laminae, also receiveinput from A fibers.[3] The nonnociceptive fibers indirectly inhibit the effectsof the pain fibers, 'closing a gate' to the transmission of their stimuli.[3] In

other parts of the laminae, pain fibers also inhibit the effects ofnonnociceptive fibers, 'opening the gate'.[3]

An inhibitory connection may exist with A and C fibers, which may form asynapse on the same projection neuron

http://en.wikipedia.org/wiki/Afferent_nervehttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/A_delta_fiberhttp://en.wikipedia.org/wiki/C_fiberhttp://en.wikipedia.org/wiki/Chronic_painhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Dorsal_hornhttp://en.wikipedia.org/wiki/Spinal_cordhttp://en.wikipedia.org/wiki/Lamina_of_the_vertebral_archhttp://en.wikipedia.org/wiki/Synapsehttp://en.wikipedia.org/wiki/Projection_neuronhttp://en.wikipedia.org/wiki/Projection_neuronhttp://en.wikipedia.org/wiki/Synapsehttp://en.wikipedia.org/wiki/Lamina_of_the_vertebral_archhttp://en.wikipedia.org/wiki/Spinal_cordhttp://en.wikipedia.org/wiki/Dorsal_hornhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Chronic_painhttp://en.wikipedia.org/wiki/C_fiberhttp://en.wikipedia.org/wiki/A_delta_fiberhttp://en.wikipedia.org/wiki/A_delta_fiberhttp://en.wikipedia.org/wiki/A_delta_fiberhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Afferent_nerve

8/14/2019 Deem An

11/13

The A and B fibers tranismit impulsesconveying common sensation and areinhibitory, whilst the C fibers transmit

impulses concerned with pain and diminishthe inhibitory effect of the inter mediarycell.

It is suggested that the activity of theintermediary cell is governed by the

balance between the afferent impulsescarried in the axons of the rapidlyconducting thick myelinated A and B fibersand the more slowly conducting thin non-m elinated C fibers.

8/14/2019 Deem An

12/13

Pathways for Transmission of PainImpulses in Orofacial Region

Impulses originating in the nerve-endings of the dental pulp &the supporting structures of the teeth are conveyed to the CNSby the second(maxillary) & the third (mandibular) divisions ofthe TRIGEMINAL nerve.

From cell bodies in the Gasserian ganglion , these neuralpathways pass to the sensory nucleus of the V nerve which issituated in the medulla oblangata & extends to the level of thesecond cervical segment of the spinal cord.

Then they pass via the trigeminal lemniscus to the postero-ventral nucleus of the thalamus &then via connecting neuronsto the brain.

Interruption of these neural pathways at any level may abolishthe sensation of pain.

8/14/2019 Deem An

13/13