lab 3 - The Compound Action Potential(CAP Of the Frogs...

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The Compound Action Potential (CAP) Of the Frog’s Sciatic Nerve These notes are mainly concerned with the practical aspects of the experiment but they include a brief review of the theory. The experimental set-up and the theoretical background are described in more detail in the lab lecture and in the Virtual Lab . Introduction (Fig 1, Table 1) A) Structure/Function of large, mixed, compound nerves (See Fig. 1 and 1b) a) In mammals, large peripheral nerves, for example the vagus, the sciatic and the ulnar typically are bundles of thousands of individual axons enclosed in a loose connective tissue sheath, the Epineurium. Within the Epineurium, the axons are grouped in fascicles, encased in a more structured epithelial sheath (the perineurium). Each axon is surrounded by a thin connective tissue sheath, the endoneurium. Note that the frog’s sciatic consists of only a single bundle of fibres, surrounded by the perineurium and loose epineurium. b) The axons within a compound nerve include afferent (sensory) nerves and efferent (motor and autonomic) nerves. Individual axons vary in diameter, myelination, excitability, threshold and conduction speed. Figure 1b Cross section of a frog’s sciatic nerve NOTE: The classification of different axons on the basis of diameter, myelination, electrical properties and conduction speed (etc.) is summarized in Table 1. B) The Compound Action Potential (CAP) a) The CAP is the sum of “all-or-none” action potentials arising more or less simultaneously in individual axons in a compound nerve. The CAP does not occur naturally. It is elicited experimentally or clinically by stimulating the whole nerve with extracellular stimulating electrodes and is recorded by means of extracellular recording electrodes . b) A typical nerve such as the sciatic contains efferent ( α and γ motor axons , and post-
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ganglionic autonomic axons) and afferent (sensory) axons. The properties of the CAP: threshold, amplitude, duration, conduction velocity are determined by the type and number of individual axons which are recruited (excited) by the stimulus. The number and type of axons excited depend on the intensity of the stimulus. c) The CAP differs from action potential of a single axon in several important ways. -The individual action potential demonstrated by intracellular recording from a single axon is an “all-or-none” response which does not change with stimulus intensity (above threshold) . -The CAP is a graded response whose magnitude increases with the intensity of stimulation. This is because different axons have different thresholds of excitation . The largest axons have the lowest threshold of excitation i.e. , they are the most excitable. (Thus,
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This note was uploaded on 02/16/2011 for the course PHGY 212 taught by Professor Juliomartineztrujillo during the Fall '10 term at McGill.

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lab 3 - The Compound Action Potential(CAP Of the Frogs...

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