Lab 1: Electrical And Mechanical Properties Of Skeletal Muscle

Lab 1: Electrical and Mechanical Properties of Skeletal Muscle Electrophysiology of Skeletal Muscle • When a nerve action potential reaches the neuromuscular junction, the neurotransmitter acetylcholine 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 tetany 6. 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 reticulum PASSIVE 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 tetany c. Does the activity recover with time between the periods of stimulation?

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 muscle fiber  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 Can’t release calcium o Lactic acid buildup o K+ accumulation  depolarization  Lose ability to open Na+ channels

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 determined for 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 gastrocnemius muscle (in the skeletal muscle lab): A. was an isotonic contraction B. was a combination of an isotonic and isometric contraction C. was an isometric contraction D. did not produce internal work E. did not produce heat Explanation: C is the correct answer because, by definition, an isometric contraction is one where the external dimensions of the muscle do not change (i.e. there is no velocity of shortening). This is the type of contraction that was measured with the isometric transducer in the skeletal muscle lab exercise. None of the other answers is correct. Which of the following statements concerning contraction of the frog gastrocnemius muscle (in the skeletal muscle lab) is NOT TRUE? A. Complete (fused) tetanus normally was observed at a stimulus frequency of 5 to 10 Hz B. 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 an electrical current. Explanation: A is not true because complete (fused) tetanus normally was observed at a stimulus frequency of approximately 25 Hz or more. Incomplete (or partial) tetanus was observed at the 5 Hz 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 the frog gastrocnemius 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 the neuromuscular 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 recorded from the muscle. Each of the other events occurs prior to the muscle fiber action potential.