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10.4 Nervous System Control of Muscle Tension
10.4 OBJECTIVES
1. Describe the length-tension relationship
A motor unit is formed by a motor neuron and all of the muscle fi- bers that are innervated by that same motor neuron. A single con- traction is called a twitch. A mus- cle twitch has a latent period, a contraction phase, and a relaxa- tion phase. A graded muscle re- sponse allows variation in muscle tension. Summation occurs as successive stimuli are added to- gether to produce a stronger mus- cle contraction. Tetanus is the fu- sion of contractions to produce a continuous contraction. Increas- ing the number of motor neurons involved increases the amount of motor units activated in a muscle, which is called recruitment. Mus- cle tone is the constant low-level contractions that allow for pos- ture and stability
Skeletal muscles are rarely com-
pletely relaxed, or flaccid. Even if
a muscle is not producing move-
ment, it is contracted a small
amount to maintain its contractile
proteins and produce muscle tone.
muscle tone is referred to as hypotonia or atrophy, and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and dis- play functional impairments, such as weak reflexes. Conversely, excessive muscle tone is referred to as hypertonia, accompanied by hyperreflexia (excessive reflexresponses), often the result of damage to upper motor neurons in the CNS. Hypertonia can present with mus- cle rigidity (as seen in Parkinson’s disease) or spasticity, a phasic change in muscle tone, where a limb will “snap” back from passive stretching (as seen in some strokes).
Contraction of a Muscle Fiber
A cross-bridge forms between actin and the myosin heads triggering contraction.
As long as Ca++ ions remain in the sarco- plasm to bind to troponin, and as long as ATP is available, the muscle fiber will con- tinue to shorten.
MOVIE 1.23 Neuromuscu- lar junction (NMJ) Motor end- plate 8:59 minutes Khan Academy
The absence of the low-level contractions that lead to
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State of Alaska EMS Education Primer - 2016
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