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Motor Unit
Motor Units
Summary
Definitions
Motor unit
- A single motor neuron and the many skeletal muscle fibers it innervates.
Motor neuron
- Motor neuron comprises a cell body and an axon.
See: Neuron to see a spinal motor neuron histological slide.
Motor axon
- Axon of the motor neuron leaves the cell body of the motor neuron, which lies in the gray matter of the spinal cord, and travels to its target muscle.
- The axon forms branches, which innervate individual muscle fibers.
Neuromuscular junction
- The neuromuscular junction is the site where motor axons terminate on individual muscle cells.
Innervation ratio
- The number of muscle cells a single motor neuron innervates; in general, the smaller this number, the finer the motor control.
- Small innervation ratios produce fine motor control, such as for eye muscle control.
- Large innervation ratios produce gross motor control, such as for hamstring muscle knee flexion.
- According to the "all or nothing" principle: all motor unit fibers fibers contract simultaneously.
Muscle twitch on a Myogram
Muscle twitch
- Muscle twitch is the response of a motor unit to a single action potential.
- A myogram traces the three phases of a muscle twitch.
- X-axis = "Time (milliseconds)."
- Y-axis = "Percentage of Maximum Tension."
- Intercept = zero, which is the point of motor neuron stimulation.
3 Phases of a Muscle Twitch
Three phases of a muscle twitch from time 0 outward:
- Latent period; depends on the distance from the motor neuron to the skeletal muscle fiber as well as the speed of transmission down the axon.
- Contraction period: tension rises steadily to reach maximum tension.
- Relaxation period; is typically longer than the contraction phase: