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Motor Units

Notes

Motor Units

Sections


Summary

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.
    Axons from a single neuron innervate muscle cells throughout the muscle; any given muscle has multiple motor units.

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

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.
No change in muscle tension during latent period.

Contraction period: tension rises steadily to reach maximum tension.

Relaxation period; is typically longer than the contraction phase:
The tension decreases. Muscles typically shorten during the contraction phase, and re-lengthen during the relaxation phase.

Muscle fiber types

The velocity and duration of contraction is in part determined by the type of skeletal muscle fiber involved.

Each skeletal muscle comprises muscle fiber types in varying proportions.

Both genetics and athletic training influence the proportion of muscle fiber types.

Slow

  • Slow fibers are red, due to high levels of myoglobin
  • Rely on oxygen for ATP generation, and are therefore aerobic
  • Slow contraction velocity; they have the highest resistance to fatigue
  • Contribute primarily to endurance and posture maintenance

Intermediate

  • Intermediate fibers are also red and rely on aerobic metabolism
  • They have a faster contraction velocity than do slow fibers
  • They display relatively high resistance to fatigue
  • They contribute to activities of moderate intensity and duration.

Fast

  • Fast fibers are white, because they have small amounts of myoglobin
  • Anaerobic metabolism
  • Fast contraction velocity
  • Fatigue quickly
  • Contribute to activities that are intense but of short duration.

Full-Length Text

  • Here we will learn the anatomy of motor units, draw a myogram (a graph of the motor unit's response to a single action potential), and learn key differences between the three skeletal muscle fiber types.

Let's begin with the motor unit.

  • First, start a table.
  • Denote that a motor unit comprises a single motor neuron and the many skeletal muscle fibers it innervates.
  • Now, draw a bundle of skeletal muscle fibers (aka cells).
  • Next, draw the motor neuron, which comprises a cell body and an axon.
  • Show that the 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.
  • Then, show that the axon forms branches, which innervate individual muscle fibers.
  • Next label the neuromuscular junction, which is the site where motor axons terminate on individual muscle cells.
  • Axons from a single neuron innervate muscle cells throughout the muscle; any given muscle has multiple motor units.
  • Now, denote that the innervation ratio refers to the number of muscle cells a single motor neuron innervates; in general, the smaller this number, the finer the motor control.
  • Denote that small innervation ratios produce fine motor control, such as for eye muscle control.
    • Whereas large innervation ratios produce gross motor control, such as for hamstring muscle knee flexion.
  • Next, denote that according to the "all or nothing" principle: all motor unit fibers fibers contract simultaneously.

Now, let's address the muscle twitch.

  • Denote that the muscle twitch is the response of a motor unit to a single action potential.

Let's draw a myogram (a graph) to see the three phases of a muscle twitch.

  • First, draw the x-axis.
  • Label it "Time (milliseconds)."
  • Then, draw the y-axis.
  • Label it "Percentage of Maximum Tension."
  • Now, at the intercept, show that the axes begin at zero, which is the point of motor neuron stimulation.
  • Next, divide the graph into the three phases of a muscle twitch from time 0 outward:
    • Latent period, which 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.
    • Relaxation period, which is typically longer than the contraction phase.

Now, draw a curve to show that:

  • During the latent period, there is no change in muscle tension.
  • Then, in the contraction phase, tension rises steadily to reach maximum tension.
  • Next, in the relaxation phase, the tension decreases.
    • Muscles typically shorten during the contraction phase, and re-lengthen during the relaxation phase.
  • The velocity and duration of contraction is in part determined by the type of skeletal muscle fiber involved.
  • Denote that each skeletal muscle comprises muscle fiber types in varying proportions; however, return to the top of our table and denote that within a single motor unit, all fibers are of the same type.
  • Both genetics and athletic training influence the proportion of muscle fiber types.

Let's learn key features of the different muscle fiber types.

  • Muscle fibers are classified as slow, fast, or intermediate, according to metabolic and structural characteristics and their functional consequences.
  • Denote that we will divide skeletal muscle into:
    • Type I, slow oxidative fibers.
    • Type II A, fast aerobic fibers.
    • Type II B, fast anaerobic fibers.
      (Intertextual variation exists regarding the division of skeletal muscle fibers).

Now, let's learn specifics of their biology and physiologic properties.

  • Create a 4 x 6 table.
  • From left to right, write in the following table headers:
    • Characteristic,
    • Slow Oxidative (aka Type I),
    • Intermediate (aka Fast Aerobic, aka Type IIa), and
    • Fast (aka Fast Anaerobic Type IIb).
  • Next, in the first column, indicate the following characteristics of muscle fibers:
    • Fiber color, which is an indication of oxygen consumption
    • Metabolism, which indicates whether oxygen is used to generate ATP
    • Contraction velocity
    • Fatigue resistance
    • Function
  • Now, denote that slow fibers are red, due to high levels of myoglobin; they rely on oxygen for ATP generation, and are therefore aerobic; they have a slow contraction velocity; they have the highest resistance to fatigue; and, they contribute primarily to endurance and posture maintenance.
  • Next, denote that intermediate fibers are also red and rely on aerobic metabolism; they have a faster contraction velocity than do slow fibers; they display relatively high resistance to fatigue; and, they contribute to activities of moderate intensity and duration.
  • Now, denote that fast fibers are white, because they have small amounts of myoglobin; they engage in anaerobic metabolism; they have fast contraction velocity; they fatigue quickly; and, they contribute to activities that are intense but of short duration.