12.8B: Overview of Motor Integration
A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates.
- Describe motor integration and how it affects motor activity
Key Points
- Motor units contain muscle fibers of all the same type; these may be many muscle fibers (as in the case of quadriceps) or a few muscle fibers (as in the case of the muscles that control eye movement).
- Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units that subserve a single muscle are considered a motor unit pool.
- Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.
- The smaller the motor unit, the more precise the action of the muscle.
Key Terms
- Henneman’s size principle : According to this principle, motor unit recruitment is always in the same order from smallest to largest motor unit. Additionally, the motor unit action potential is an all-or-none phenomenon—once the recruitment threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully.
- alpha motor neuron : Alpha motor neurons (α-MNs) are large, lower motor neurons of the brainstem and spinal cord. They innervate the extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate the intrafusal muscle fibers of muscle spindles.
- motor unit : A neuron with its associated muscle fibers.
Rectus femoris : The rectus femoris muscle is one of the four quadriceps muscles of the human body. These muscles may have as many as a thousand fibers in each motor unit.
A motor unit consists of a single alpha motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type (either fast twitch or slow twitch).
When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle. All of the motor units that subserve a single muscle are considered a motor unit pool.
The number of muscle fibers within each unit can vary. Thigh muscles, for example, can have a thousand fibers in each unit, eye muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle’s need for refined motion.
The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.
Motor unit recruitment is the progressive activation of a muscle by the successive recruitment of motor units to accomplish increasing gradations of contractile strength. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction.
Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle. It is therefore a measure of how many muscle fibers of that muscle are activated. The higher the recruitment, the stronger the muscle contraction will be.
Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.
The orbicularis oris (eye) muscle : These small motor units may contain only 10 fibers per motor unit. The more precise the action of the muscle, the fewer fibers innervated.
Motor Unit Categories
Motor units are generally categorized based upon the similarities between several factors such as:
Physiological
Contraction speed in isometric contractions:
- Rate of rise of force.
- Time to peak of a twitch contraction (response to a single nerve impulse).
Biochemical
Histochemical (the oldest form of biochemical fiber typing):
- Glycolytic enzyme activity.
- Oxidative enzyme activity.
- Sensitivity of myosin ATPase to acid and alkali.
Immunohistochemical (a more recent form of fiber typing):
- Myosin heavy chain (MHC).
- Myosin light chain—alkali (MLC1).
- Myosin light chain—regulatory (MLC2).
FUNCTIONAL EXAMPLE
The quadriceps muscles contain many thousands of muscle fibers in general, both slow and fast twitch, to produce sufficient force for body movements such as standing, walking, running, and jumping.
The eye muscles, on the other hand, contain few muscle fibers, enabling them to be more exact in movement so that vision is not jumpy, but consequently they produce very little force.
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