nerve-physiology 3
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Transcript of nerve-physiology 3
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Nerve physiology 3rd lecture
By
Dr. Mudassar Ali Roomi (MBBS, M. Phil)
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Inhibition of Excitability- “Stabilizers” and Local Anesthetics
• the factors that decrease nerve excitability are called as membrane-stabilizing factors.
• Calcium
• local anesthetics: • Examples: procaine and
tetracaine.
• Mechanism of action of local anesthetics: acts directly on the activation gates of the sodium channels, making it much more difficult for these gates to open.
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Role of calcium in membrane excitability
• Hypercalcemia decrease membrane excitability.***
• In hypercalcemia decreased excitability of membrane (also severe constipation).**
• Hypocalcemia increased excitability of nervous tissue.**
• Inner side of sodium channels is highly negatively charged.
• Calcium ions bind with negative inner surface of sodium channels complete closure of activation gates of these channels at rest.
• So, Calcium (normal levels) stabilizes the membrane and decreases its exciability.
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Why tetany occurs in hypocalcemia??
• Decreased calcium in ECF no complete closure of activation gates of sodium channels at rest sodium ions leak into membrane from ECF hypo-polarization (membrane potential becomes less negative & near to threshold) & on slight stimulation action potential (tetany).
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Why tetany is aggravated in alkalosis?
• Protein molecules behave as acid (proton donors become anions) in alkaline pH.
• At 7.4 (alkaline body pH), protein anions bind positive ions (sodium, potassium, calcium).
• Protein bound form of calcium increases & ionic form thus decreases tetany.
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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 & I
bands.
Intercalated discs /
gap junctions are at
the level of Z
membranes. (Diad)
In some places
(intestine), randomly
distributed thick &
thin filaments
interdigitate.
Few mitochondria.
Major source of energy
is CHO.
Glycolysis Citric acid
cycle.
More mitochondria
(25% mass). Major
source of energy is
fat (60%) at rest.
Fewer mitochondria.
Mostly glycolytic
metabolism.
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Propagation of action potential
• Propagation of action potential is an example of positive feedback mechanism.
• velocity (m/sec) of myelinated 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 through depolarized point, activates Na+ channels at polarized point depolarization action potential.
• Then a new circuit of current is formed between this depolarized point & adjacent polarized point.
• In case of unmyelinated nerve fiber, velocity of conduction is slow, because it is point to point.
• Synapses only allow propagation between pre synaptic to post synaptic neuron inside the body (law of forward conduction). but in 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 flow
5,000-fold
• Sheath is interrupted every 1-3 mm
: node of Ranvier
Figure 5-16; Guyton & Hall
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Propagation of action potential along a myelinated nerve fiber
SALTATORY CONDUCTION
• Saltare: to jump • Node to node
conduction. • Internodes are
myelinated and act as insulators.
• Myelin sheath is absent at the nodes of Ranvier & neurilemma at the nodes has got ion channels.
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Local circuit of saltatory conduction
• depolarized node adjacent polarized node conduction of current through axoplasm and ECF next node is also depolarized
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Benefits of saltatory conduction
1. Faster velocity of conduction in large myelinated nerve fibers e.g A-alpha fibers (120 m/sec ). It is very slow (0.5-2 m/sec) in small unmyelinated nerve fibers e.g. type C fibers.
2. Less energy expenditure due to less ionic change
3. Insulation of nerve fibers prevents the short circuiting.