Three types of neurons innervate arthropod muscle fibers.
All insect muscle fibers are innervated by fast motor neurons.
Here we see a fast motor neuron with three branches.
Each muscle fiber is innervated by a branch of the fast neuron.
Fast motor neurons produce rapid, total contraction of the muscle fiber.
Some fibers of insect muscles are also innervated by slow motor neurons.
Slow neurons control the partial graded contractions of the fibers.
The differences is in action by fast and slow motor neurons are based on the
amount of glutamate neurotransmitter that is released by each type of neuron,
and the amount of summation effects.
Fast neurons release large amounts of
glutamate resulting in intense depolarizations with little facilitation
or summation as compared to slow motor neurons.
Slow motor neurons must fire repeatedly to cause a given muscle fiber to attain
the levels of depolarization and contraction that are comparable to those
for a fast motor neuron and facilitation and summation are common.
Note that since only one muscle fiber in our muscle unit has slow innervation,
not only is the contraction rate of the entire muscle slow and graded in response to
stimulation by the slow neuron, but the overall maximum contraction of the
entire muscle is only about one third of that observed for stimulation by the fast neuron.
Some muscle fibers are also innervated
by inhibitory neurons that block the stimulatory effects of both the fast and the slow neurons.
Inhibitory neurons release gamma-aminobutyric acid as their neurotransmitter chemical.
Gamma-aminobutyric acid hyper-polarizes the fiber membrane to neutralize the
depolarizing action of glutamate released by the stimulatory neurons.
Let us now see how inhibitory neurons interact with stimulatory neurons for
adding additional controls to insect muscle contraction.
In our first example, the fast neuron stimulates all three muscle fibers to
provide a maximum contraction.
Next, firing the inhibitory neuron
hyper-polarizes muscle fibers 2 and 3 at the same time as they are depolarized by the fast neurons.
The inhibitory neuron hyper-polarization
block the stimulatory depolarizations to prevent muscle fibers 2 and 3 from responding.
Note that only muscle fiber one undergoes depolarization and contraction
so that total muscle shortening can only be one-third of the shortening which is
seen when all three muscle fibers are stimulated.
In the second case, we see the graded contraction of the muscle by the slow neuron.
Note that the contraction is only one-third shortening of the entire
muscle, since only muscle fiber two has the slow innervation.
When a slow neuron stimulus is combined with stimulus by the inhibitory neuron, the
slow neuron stimulation is blocked completely. and muscle fiber 2 is
completely suppressed so there is no muscle contraction.
Finally, in the last example, both the fast and slow neurons fire
simultaneously with the inhibitory neurons.
Again, the muscle shortens by only one- third since only muscle fiber 1 is
free to respond to its fast neuron.
Polyneuronal innervation of single muscle fibers, and multiterminal end
plates over the surface of the muscle fiber are adaptations to small size that
allow small arthropod animals, such as insects, to compensate for having fewer
muscle fibers than are found in the larger vertebrate muscles.
In this way, arthropods minimized the number of neurons that are needed to innervate the
muscles while preserving the range of graded and fast contractions that are
essential for normal movements and activity.