Anatomy & Physiology All-in-One For Dummies (+ Chapter Quizzes Online)
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When it comes to contraction of a muscle fiber, it’s an all-or-nothing affair. Nonetheless, the stimuli may vary in frequency and spatial distribution. It has been demonstrated that an increase in the frequency of stimulatory nerve impulses gives a stronger contraction. Maximum stimulus brings all motor units to bear together.

Conversely, slower or fewer action potentials — a weaker stimulus, as it were — causes fewer motor units to become involved in a contraction. So it’s true that a muscle organ can have graded degrees of contraction depending on the level of stimulation.

As for how this can be so, one theory proposes that individual fibers have specific thresholds of excitation; thus, those with higher thresholds respond only to stronger stimuli. Another theory holds that the deeper a fiber is buried in the muscle, the less accessible it is to incoming stimuli.

In physiology, a muscle contraction is referred to as a muscle twitch. A twitch is the fundamental unit of recordable muscular activity. The twitch consists of a single stimulus-contraction-relaxation sequence in a muscle fiber. The contraction is the period when the crossbridges are active.

The relaxation phase occurs when crossbridges detach and the muscle tension decreases. Complete fatigue occurs when no more twitches can be elicited, even with increasing intensity of stimulation.

The short lapse of time between the application of a stimulus and the beginning of muscular response is called the latent period or phase. In mammalian muscle, latency is about .001 second, or one one-thousandth of a second.

Several types of muscle contractions relate to tone:

  • Isometric: Occurs when a contracting muscle is unable to move a load (or heft a piece of luggage or push a building to one side). It retains its original length but develops tension. No mechanical work is accomplished, and all energy involved is expended as heat.

  • Isotonic: Occurs when the resistance offered by the load (or the gardening hoe or the cold can of soda) is less than the tension developed, thus shortening the muscle and resulting in mechanical work. There are two main types of isotonic contraction:

    • Concentric contraction: Occurs when the force generated by the muscle is less than the maximum tension. The muscle shortens with increased velocity. An example is weight lifting that performs work on a load.

    • Eccentric contraction: Occurs when the force of a load is greater than the maximum muscle tension, causing the muscle to elongate and performing work on the muscle. Muscle injury and soreness may result.

But muscles aren’t independent sole proprietors. Each muscle depends upon companions in a muscle group to assist in executing a particular movement. That’s why muscles are categorized by their actions. The brain coordinates the following groups through the cerebellum.

  • Prime movers: Provide the major force for producing a specific movement. Just as it sounds, these muscles are the workhorses that produce movement.

  • Antagonists: These muscles exist in opposition to prime movers, regulating the motion by contracting and providing resistance.

  • Fixators or fixation muscles: These muscles serve to steady a part while other muscles execute movement. They don’t actually take part in the movement itself.

  • Synergists: These muscles control movement of the proximal joints so that the prime movers can bring about movements of distal joints.

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