7/29/2019 3rd lecture on nerve physiology by Dr. Roomi.pptx

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Nerve physiology3rd lecture

By

Dr. Mudassar Ali Roomi (MBBS, M. Phil)

7/29/2019 3rd lecture on nerve physiology by Dr. Roomi.pptx

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Inhibition of Excitability-

Stabilizers and Local Anesthetics

the factors that decreasenerve excitability arecalled as membrane-stabilizing factors.

Calcium

local anesthetics: Examples: procaine and

tetracaine.

Mechanism of action of localanesthetics: acts directly onthe activation gates of thesodium channels, making itmuch more difficult for thesegates to open.

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Role of calcium in membrane excitability

Hypercalcemia decreasemembrane excitability.***

In hypercalcemia decreasedexcitability of membrane (alsosevere constipation).**

Hypocalcemia increasedexcitability of nervous tissue.**

Inner side of sodium channels ishighly negatively charged.

Calcium ions bind with negativeinner surface of sodium

channels complete closure ofactivation gates of thesechannels at rest.

So, Calcium (normal levels)st

abilizes the membrane anddecreases its exciability.

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Why tetany occurs in hypocalcemia??

Decreased calcium in ECFno complete closure ofactivation gates ofsodium channels at rest

sodium ions leak intomembrane from ECFhypo-polarization(membrane potential

becomes less negative &near to threshold) & onslight stimulationaction potential (tetany).

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Why tetany is aggravated in alkalosis?

Protein molecules behaveas acid (proton donorsbecome anions) inalkaline pH.

At 7.4 (alkaline body pH),protein anions bindpositive ions (sodium,potassium, calcium).

Protein bound form of

calcium increases & ionicform thus decreasestetany.

7/29/2019 3rd lecture on nerve physiology by Dr. Roomi.pptx

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NERVE FIBER /SKELETAL MUSCLE

CARDIAC

MUSCLE

SMOOTH

MUSCLE

Multiple peripheral

nuclei.

Single central

nucleus.

Single central

nucleus.

Triad is at the

junction of A & Ibands.

Intercalated discs /

gap junctions are atthe level of Zmembranes. (Diad)

In some places

(intestine), randomlydistributed thick &thin filamentsinterdigitate.

Few mitochondria.Major source of energyis CHO.

Glycolysis Citric acid

cycle.

More mitochondria

(25% mass). Majorsource of energy isfat (60%) at rest.

Fewer mitochondria.Mostly glycolyticmetabolism.

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Propagation of action potential

Propagation of actionpotential is an exampleof positive feedbackmechanism.

velocity (m/sec) ofmyelinated fiber =diameter (in mm) x 4.5

velocity of (m/sec)unmyelinated fiber =Square root of diameter

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Propagation of action potential in

un-myelinated nerve fibers

Point to point conduction. Local circuit of current is formed

between depolarized point &adjacent polarized point.

Current flowing out throughdepolarized point, activates Na+

channels at polarized pointdepolarization action potential.

Then a new circuit of current isformed between this depolarizedpoint & adjacent polarized point.

In case of unmyelinated nervefiber, velocity of conduction isslow, because it is point to point.

Synapses only allow propagationbetween pre synaptic to postsynaptic neuron inside the body(law of forward conduction). butin vitro it is in both directions.

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Myelination

Schwann cells surround the nerve

axon forming a myelin sheath

Sphingomyelin decreases

membrane capacitance and ion flow5,000-fold

Sheath is interrupted every 1-3 mm

: node of Ranvier

Figure 5-16; Guyton & Hall

7/29/2019 3rd lecture on nerve physiology by Dr. Roomi.pptx

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Propagation of action potential

along a myelinated nerve fiber

SALTATORY CONDUCTION

Saltare: to jump

Node to nodeconduction.

Internodes aremyelinated and act asinsulators.

Myelin sheath is absent

at the nodes of Ranvier& neurilemma at thenodes has got ionchannels.

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Local circuit of saltatory conduction

depolarized node

adjacent polarized node

conduction of

current throughaxoplasm and ECF

next node is also

depolarized

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Benefits of saltatory conduction

1. Faster velocity ofconduction in largemyelinated nerve fiberse.g A-alpha fibers (120m/sec ). It is very slow(0.5-2 m/sec) in smallunmyelinated nervefibers e.g. type C fibers.

2. Less energy expenditure

due to less ionic change3. Insulation of nerve fibersprevents the shortcircuiting.