With muscle contraction there is a:
Latent Period (zero): the brief delay between the application of a stimulus and the beginning of the contraction. During this time, the muscle AP propagates over the sarcolemma and Ca++ ions are release from the SR. Contraction Period (10-100msec): Ca++ binds to troponin, myosin-binding sites on actin are exposed, crossbridges form, and peak tension is developed in the muscle fibre. Relaxation Period (10-100msec): Ca++ is actively transported back into the SR, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fibre decreases. If two stimuli are applied (one immediately after the other), the muscle will respond to the first but not the second. During contraction, the muscle temporarily loses its excitability and cannot respond for a period of time. This period of lost excitability is called the Refractory Period and is a characteristic of all muscle and nerve cells.
Frequency of Stimulation
Twitch Contraction
A brief contraction of all the muscle fibres in a motor unit in response to a single action potential in its motor neuron.
Wave Summation
When a second stimulus occurs after the refractory period of the first stimulus is over BUT before the skeletal muscle fibre has relaxed, the second contraction will actually be stronger than the first.
Unfused (Incomplete) Tetanus
When a skeletal muscle fibre is stimulated at a rate of 20-30 times per second, it can only partially relax between stimuli. This results in a sustained but wavering contraction.
Fused (Complete) Tetanus
When a skeletal muscle fibre is stimulated at a much higher rate of 80-100 times per second. The muscle fibre DOES NOT relax at all. This is a sustained contraction in which individual twitches cannot be detected. **Note: tetanus in horses can be forceful enough to break bones or tear out tendon insertions
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