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16 terms

Define Terms/Lab Sheet Chapter 12

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Motor Unit
A motor neuron and all the skeletal muscle fibers it innervates (stimulates).
Muscle Tone
A small number of motor units are involuntarily activated to produce a sustained contraction of their muscle fibers.
Fused (complete) tetany
A sustained contraction with no relaxation observed between twitches. Individual twitches can not be observed.
Unfused (incomplete) tetany
When a skeletal muscle fiber is stimulated at 20-30/second, it can only partially relax between stimuli, resulting in a sustained, but wavering contraction.
Muscle twitch
A brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron.
Motor unit summation
Contraction strength increases by increasing the number of motor units stimulated.
Motor unit recruitment
Increasing the number of motor units contracting at the same time; also increases the amount of force generated.
Wave summation
Increased muscle force due to an increase in the frequency of nerve impulses. (stimuli arriving, one after the other causes larger contractions)
Latent period
Lasts about 2 milliseconds (msec) and is the time between stimulation of muscle cells and force generation.
Contraction period
Lasts about 10 to 100 milliseconds (msec) and is the period during which force is increasing.
Relaxation period
This period, which lasts 10 to 100 milliseconds, (msec) is the period when muscle force is decreasing. Acetylcholine is rapidly broken down by the enzyme acetylcholinesterase. This ends the generation of muscle action potentials and the calcium channels in the sarcoplasmic reticulum membrane close.
Threshold stimulus
The voltage at which the first perceptible contractile response is obtained.
Fatigue
The inability of a muscle to contract forcefully after prolonged activity. Due to depletion of creatine-phosphate, glycogen and ATP; buildup of lactic acid and ADP; failure of nerve implulses to release enough acetylcholine.
Myogram
Recording of a muscle contraction.
Electromyography
Electromyography (EMG) is a test that checks the health of the muscles and the nerves that control the muscles
Electromyogram
An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction.