Frog Muscle Protocol

Physiology of Skeletal Muscle

In this experiment, students will investigate the physiological properties of skeletal muscle from the isolated amphibian gastrocnemius. Concepts include the single twitch, graded response and the relationship between muscle length and tension generated. Students will also explore tetanus and muscle fatigue.

Written by staff of ADInstruments.

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Background

The physiology of skeletal muscle was not fully understood until the early 20th century. Some of the earliest experiments on muscle physiology were performed between 1661 and 1665 by Jan Swammerdam, who demonstrated that an isolated frog muscle could be made to contract when the sciatic nerve was irritated with a metal object. Later, Luigi Galvani (1737–1798) demonstrated that frog muscle responded to electrical currents (Fulton and Wilson, 1966).

The basic unit of a muscle is the muscle cell, or fiber. Whole muscles are made up of bundles of these fibers. A single muscle fiber has a very regular structure, defined by myofibrils. Each myofibril consists of an arrangement of the contractile proteins actin and myosin, which are able to slide past each other in the presence of Ca2+ and ATP.

Skeletal muscle is similar to nerve tissue in that it responds to a stimulus in an all-or-none fashion. This response is called a twitch. Depending on the intensity and frequency of stimulation, greater numbers of fibers are activated. By increasing the number of active muscle fibers, the muscle is able to increase the force it generates. Muscles with large cross-sectional areas are able to generate larger forces than those with small cross-sectional areas.

The muscle fibers and their associated motor nerves are called a motor unit. Greater numbers of motor nerves associated with a muscle give finer control over that muscle. Motor nerves release the neurotransmitter acetylcholine from their synaptic bulbs onto the muscle. This junction between a nerve and a...