Describe the properties of muscles observed on electrical stimulation.

Properties of muscles observed on electrical excitation: 

1. Single muscle twitch: It is a muscle contraction initiated by a single briefstimulation. It occurs in 3 stages: a latent period of no contraction, a contraction period and a relaxation period. 

2. Summation: If the muscle is stimulated before the end of the twitch, it generates greater tension i.e., summation or addition of effect takes place. Repeated stimuli will produce increasing strength of contraction (stair case phenomenon).

3. Tetanus: If stimulation is very rapid and frequent the muscle does not have time to relax. It remains in a state of contraction called tetanus. 

4. Rcfraètory period: Immediately aller one stimulus, the muscle fibre cannot respond to another stimulus. This resting or refractory period is about 0.02 seconds. 

5. Threshold stimulus: For a muscle fibre to contract, a certain minimum strength or intensity of stimulus is required. This is called threshold stimulus. 

6. All or none principie: A stimulus below threshold will not result in contraction. A threshold stimulus will result in contraction. This contraction leads to maximum force. Higher stimulus will not increase force of contraction i.e. a muscle libre contracts either ftilly or not at all. This is ‘all or none’ principle. All types of muscle and nerve fibres obey this law.

7. Oxygen debt: During strenuous exercise, the oxygen supply of muscle rapidly becomes insufficient to maintain oxidative phosphorylation of respiratory substrate. At this stage, muscles contract anaerobically and accumulate lactic acid produced by anaerobic glycolysis. Lactic acid produces less AlP and is toxic. It causes tiredness, pain and muscle cramps. During recovery, oxygen consumption of the muscle far exceeds than that in the resting state. This extra oxygen consumed during recovery is called oxygen debt of the muscle.

Note: The duration of refractory period varies with the muscle involved. Cardiac muscles have a longer refractory period of about 250 msec whereas skeletal muscles have a short refractory period of about I msec