LABORATORY SIX Skeletal Muscle Physiology 1

Contraction of a Skeletal Muscle Begins with electrical excitation of muscle, called a stimulus The stimulus must be of certain strength to generate a response Subthreshold stimulus - too weak to generate a response Thershold stimulus – strong enough to generate a minimal response (muscle contraction) 2

Muscle Twitch Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Single stimulus, single response Has three phases lag phase - delay between stimulus & contraction contraction (twitch) relaxation - caused by transport of Ca 2+ back into SR Phases of a Muscle Twitch 3

Increased stimulus intensity (higher voltage) leads to increased muscle contraction (stronger signal) Increases in stimulus strength will activate additional motor units (neuron + fibers) Maximum contraction occurs when all motor units in a muscle are stimulated to contract In this experiment, the muscle will get a chance to relax before the next stimulus is applied Graded Muscle Response 4

Increased Frequency of Stimulation (Same Stimulus Intensity, Graded Muscle Response) Wave Summation (the additive effect of these twitches on contraction) Increasing the rate at which an stimuli is applied (frequency) Successively greater twitch height (amplitude) Muscle does not get a chance to relax before the next stimulus arrives Tetanus (sustained contraction) One contraction is immediately followed by another The muscle does not return to a resting state due to increased frequency of stimuli; therefore, the effects are added Fatigue: Muscle eventually runs out of ATP & oxygen, so fibers cannot contract 5 Question #7, p114: Indicate all phases and all details listed on #4, 6B Procedure, p110 For Fatigue phase, the line falls down.

Electromyogram Recording Biopac software is used to look at graded response in a whole muscle Calibrate the software for each volunteer –Eliminate electrical noise –Normalize for each individual muscle action To see a graded response, the volunteer does a minimal (1/4), a small (1/2), a medium (3/4), and a full (4/4) clench force By increasing the clench force, you are activating more motor units 6

Locate Motor Points (Fig 6.4 & 6.5, p 110) Use Student Stimulator to electrically stimulate the muscle at locations on the surface of the skin called motor points Apply electrodes on the surface of body at motor points to induce muscle contraction directly or to induce the related motor neuron which then causes the muscle to contract Begin with the anterior surface of the forearm Turn the voltage down to 1 before locating motor points on face and neck Read the instructions on 6C Procedure, p111, carefully 7

Laboratory Report Six (p113) Complete in the lab: –Report 6A, Q1-4 –Report 6C, Q8-23 For Q18-23, remember to indicate the voltage used for each motor point Complete at home: –Report 6B, Q5-7 For Q7, remember to refer to p110, #4 to label all the assigned details 8