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Western University
Physiology 3130Z
Tom Stavraky

Physiology Lab 3130Y Exam 1NotesNerve Lab Sciatic nerve contains many nerve fibers axons all of which can conduct action potentialsoSome fibres in nerve trunk normally conduct from the spinal cord to periphery efferent nerves oSome fibres conduct normally conduct from the periphery to the spinal cord afferent nervesIf an electrical stimulus of sufficient intensity is applied to the nerve trunk at a point near the spinal cord all fibers will become excited and conduct action potentials peripherallyOnly action potentials in motoneurons however will cause gastrocnemius muscle to contract Why With electrode placed on nerve trunk a CAP can be recorded when nerve is electrically stimulatedIntracellular recordingoRecords an action potential from a single nerve fiberoAction potential is a positivegoing signal with an amplitude of 7090 mVExtracellular electrodeoRecording the extracellular electrical signal resulting from the summation of individual action potentials in many of the nerve fibers making up the compound nerveoSignal usually has an amplitude of only 510 mV and is negative going oCAP CAPoShape and amplitude of the CAP changes with changes in stimulus intensity graded and is a function of the threshold and conduction characteristics of the nerve fibers that are stimulatedoDifferences in the size diameter and myelination of the nerve fibers within the sciatic nerve trunk give rise to differences in threshold and conduction velocityoAlgebraic sum of all individual fibre action potentials of the nerveoAs stimulus strength increases recruit more fibres therefore more APs add up to produce a larger bellshaped curveoConduction velocity of single fibres depends on fibre diameterthat nerve bundle is composed of fibres of varying diameterFast fibres large diameter low thresholdcontribute APs that fall towards start of CAPSlow fibres small diameter high thresholdcontribute APs that fall towards tail of CAPoGradually increase stimulus strength recruit more fibres giving rise to a wider CAP with longer durationoSize of the CAP no longer increases past maximal because all of the Aalpha fibres making up the nerve have been excited and are conducting action potentialsoEach neuron within nerve bundle reaches action potential threshold at a different point in timeoIn vivoSmall diameter neurons are associated with greater resistance and thus at the axon hillock have a greater voltage depolarization forceCurrent is constantlarger voltage comes from neuron with higher resistance Explains why small diameter neurons reach voltage threshold 55 mV before large alphamotor neuronsoIn vitroLarge diameter neurons or low resistance neurons generate larger currents and reach threshold fasterVoltage constant higher current responselow resistance oPlateauing of the CAP amplitude appears when all motor units have been recruited Action potentialsoInitiated as excitatory signals are received at the postsynaptic celloExhibit an allornothing characteristic
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