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Lab 1: Electrical and Mechanical Properties of Skeletal Muscle

Electrophysiology of Skeletal Muscle

When a nerve action potential reaches the neuromuscular junction, the neurotransmitteracetylcholine is released from

each motor neuron of the motor unit. The acetylcholine binds to postsynaptic receptors causing a simultaneous increase

in permeability of the postsynaptic membrane to ions (Na+, K+ and Cl-)

An endplate potential is generated which normally depolarizes the muscle fiber membrane to threshold with consequent

generation of an action potential that is propagated over the entire fiber.

This triggers excitation-contraction coupling.

o Release of intracellular Ca2+ and generation of a twitch contraction

EMG: electromyogram; recordings of summated responses from individual fibers in an in vivo functioning muscle

Lab Experiment

1. Preparation of the frog muscle

2. Recording of total and passive muscle force

3. Recording summated muscle action potentials

4. Measurement of time delay between the muscle electrical and mechanical response to nerve stimulation.

a. Why is there a finite delay time between the electrical and mechanical response of the muscle?

b. Is this delay time variable or reasonably constant?

c. What mechanisms contribute to the contraction and relaxation times?

d. Are there any significant differences between the recorded contractions after the first and fifth stimuli?

e. Does the submaximal stimulus strength affect the properties of the contraction response?

5. Temporal summation and tetany6. Measurement of influence of muscle length on isometric passive and action tension of skeletal muscle (measurement of

passive and active components of the total muscle tension as a function of muscle length)

Total tension generated along the length of a muscle fiber upon contraction is the sum of two components (an

active and a passive component)

ACTIVE COMPONENT

The active component is a result of an active mechanism that causes an increased interdigitation of the

actin and myosin filaments relative to one another. This results in an internal shortening within the

fiber.

The active mechanism is initiated by the action potential of the muscle fiber release of Ca2+ from

the sarcoplasmic reticulumPASSIVE COMPONENT

The passive component of the total tension component is the resistive force generated by the elasticity

possessed by the muscle fiber. This force is generated only when the muscle fiber is stretched beyond

its equilibrium length.

o Equilibrium length (le): the length it would assume if the connective tissue is freed from one

of its bony attachments in the body.

o Optimal length (lo): optimum bridge formation, peak active tension

7. Study of fatigue

a. Can active contraction be produced by continuous stimulation?

b. Is the electrical as well as the mechanical activity attenuated with time? Mechanical vs. Electrical tetanyc. Does the activity recover with time between the periods of stimulation?

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d. Where and why does the failure occur?

Wrap-Up Session

Quantal summation: as we increase the voltage, we increase the number of muscle fibers firing action potentials

Motor unit= nerve + muscle fibers

To increase the control of a muscle, smaller motor units are used

Every single branch of a nerve has the same amplitude of action potential

In the lab:

The stimulating electrode changed the transmembrane field (nerve stimulus) release of Acetylcholine at the

neuromuscular junction open Na+ channels (ionotropic channels) depolarization action potential fires in the musclefiber action potential propagates down T tubule Action potential reaches the terminal cisternae (sarcoplasmic

reticulum) SR releases Ca2+ Calcium binds with Troponin C releasing inhibition Actin and Myosin bridges form

Contraction

In order to stop the contraction, an ATPase calcium pump actively transports calcium back into the SR.

Fatigue

Muscle fibers fall out (Type II cannot sustain contraction, Type I use aerobic energy and are able to sustain 100% of

initial force)

Electrical events are less

o ATP shortage cannot run Na+/K+ Pump

o Cant release calcium

o Lactic acid buildup

o K+ accumulation depolarization Lose ability to open Na+ channels

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Relative to the length-tension curve generated in the skeletal muscle lab:A. active tension was zero when passive tension was zero.B. total tension = active tension + passive tension at all muscle lengths.C. active tension was maximal when passive tension was maximal.D. passive tension increases linearly with passive stretch.E. active tension is maximal at the equilibrium length of the muscle.

Explanation: B is correct. By definition, total tension is the sum of active plus passive tension.The plot of these tensions as a function of muscle length is the length tension curve determinedfor the frog gastrocnemius muscle in the lab. None of the other answers is correct.

Assuming no movement in the force transducer, the active contraction of the frog gastrocnemiusmuscle (in the skeletal muscle lab):

A. was an isotonic contractionB. was a combination of an isotonic and isometric contractionC. was an isometric contractionD. did not produce internal workE. did not produce heat

Explanation: C is the correct answer because, by definition, an isometric contraction is one wherethe external dimensions of the muscle do not change (i.e. there is no velocity of shortening). Thisis the type of contraction that was measured with the isometric transducer in the skeletal musclelab exercise. None of the other answers is correct.

Which of the following statements concerning contraction of the frog gastrocnemius muscle (inthe skeletal muscle lab) is NOT TRUE?

A. Complete (fused) tetanus normally was observed at a stimulus frequency of 5 to 10 HzB. The amplitude of the tetanic contraction was always greater than that of the twitch

contraction.C. Quantal summation cannot be elicited in a single motor unit.D. Temporal summation can be elicited in a single motor unit.E. The stimulus applied to the motor neurons to elicit muscle contraction is actually anelectrical current.

Explanation: A is not true because complete (fused) tetanus normally was observed at a stimulusfrequency of approximately 25 Hz or more. Incomplete (or partial) tetanus was observed at the 5Hz to 10 Hz range. Each of the other answers is true.

A major cause of the time delay between the electrical and mechanical response observed in thefroggastrocnemius muscle is the time required for:

A. the electrical stimulus to initiate the gastrocnemius nerve action potentials.B. propagation of the action potentials along the gastrocnemius nerve fibers to theneuromuscular junction.C. neurotransmitter release.D. diffusion of neurotransmitter to the muscle fiber end plates.E release and diffusion of Ca2+ from the sarcoplasmic reticulum.

Explanation: E is correct because both the electrical and mechanical responses were recordedfrom the muscle. Each of the other events occurs prior to the muscle fiber action potential.