Lab05 Muscle

Introduction

During these experiments you will examine the mechanical aspects of the contraction of a typical skeletal muscle, the frog’s gastrocnemius muscle. Specifically, you will use a Force transducer hooked up to the PowerLab to examine a) how the membrane properties of muscle cells and the organization of the muscle tissue are similar to and different from those of nerve, b) the contraction time, ½ relaxation time, and tension of an individual skeletal muscle contraction, c) how the duration of a muscle action potential relates to the duration of a muscle contraction and the implications this relationship has for the development of tension in an intact muscle.

I. Physiology Background

A. Twitch: For the purposes of this lab, a muscle twitch is defined as a single contraction of a muscle tissue resulting from a single stimulus or a single compound neural action potential. When a skeletal muscle is stimulated directly or by a nervous impulse, the mechanical events which follow constitute a single twitch. Generally, nerve impulses arrive at muscle fibers in volleys and not singly.

B. Cellular Mechanisms of the Twitch and its control:

1. Development of tension: results from the coordinated shortening of sarcomeres along the length of a muscle fiber. Note the orientation of actin and myosin in the figure to the right. During a contraction, cross-bridges from the thick myosin filaments swing out and pull actin attached to z-line proteins at each end of the sarcomere toward the middle of the sarcomere (also see figs. 9-8, 9-9 in text).

2. Cross-Bridge cycling (see fig 9-8, 9-9, 9-12 in the textbook):

a) In a sarcomere of a relaxed muscle fiber the energized myosin cross-bridge (yes it is energized in the muscle’s relaxed state) is unable to bind to functional sites on actin filaments because these sites are covered by a filamentous protein known as tropomyosin.

b) During contraction, actin sites are...