Muscles must be capable of fine regulated contractions to allow animals
to move their limbs smoothly with coordination.
Without fine control, muscles would respond in a
spastic manner.
Let us see how vertebrates accomplish fine muscle control.
The objectives of this tutorial are twofold.
The first objective is to describe the structural relationships between motor neurons and
the muscle fibers of vertebrate skeletal muscles.
The second objective is to explain how motor neurons control the contractions of vertebrate skeletal muscles.
Muscles contract in response to electrical stimulation by motor neurons.
Muscles are composed of bundles of filamentous contractile cells called fibers.
The whole muscle contracts in proportion to the number of fibers that undergo stimulation.
Let us observe a maximum shortening of the muscle in response to all the fibers contracting.
If a few fibers are stimulated, the muscle contracts part way.
If all the fibers are stimulated, the muscle contracts fully.
A muscle fiber of vertebrate animals has
only a single motor neuron that controls its contraction.
However a single neuron may control more than one fiber within a muscle.
A neuron plus all the muscle fibers that it controls are referred to as a motor unit.
In this model, the muscle is comprised of six fibers and four motor units.
The motor units are: neuron R and fibers 1 and 3;
neuron B and fiber 2; neuron Y and fibers 4 and 6
and neuron P and fiber 5.
The point of contact between a neuron and a muscle is termed
the neuromuscular junction or motor endplate.
A single motor neuron is usually branched and has multiple motor endplates
in one or two restricted areas on a vertebrate muscle fiber, but for
simplicity, we will show only one endplate for each neuron.
The motor endplate of a neuron with a muscle fiber is comparable in structure
and function to the synapse between two neurons.
The neuron terminus of the motor endplate contains vesicles filled with neurotransmitter chemicals.
Acetylcholine is the neurotransmitter chemical for motor endplates of vertebrate animals.
The muscle fiber surface is differentiated in the region of the endplate
to possess acetylcholine receptors associated with membrane sodium channels.
The membrane also contains the enzyme acetylcholinesterase.
Acetylcholinesterase breaks down the
vertebrate muscle neurotransmitter chemical, acetylcholine.
An action potential in the motor neuron causes calcium
channels to open, and calcium ions to enter the motor endplate.
The calcium ions assist in the migration of the neurotransmitter vesicles and
their fusion with the endplate membrane.
The acetylcholine from the neuron
diffuses across the space and activates muscle fiber acetylcholine receptors to
open sodium channels in the fiber membrane and allow sodium ions entry into the fiber.
Sodium ions diffuse into the muscle fiber and depolarize the
fiber membrane in the region of the end plate.
This depolarization is called the "endplate potential".
If sufficiently strong, the endplate potential generates
a self-propagating action potential to form, and spread over the surface of the muscle fiber.
After generating the endplate potential, the acetylcholine is degraded
by the acetylcholinesterase enzyme to remove the stimulus to the sodium channels.
The sodium channels close, and the endplate potential dies out.
In the muscles of vertebrate animals, the action potential spreads over the
surface of the muscle fiber and results in contraction of the fiber.
Most vertebrate muscle fibers produce an all-or-nothing contraction in response
to the nervous stimulation of the muscle.
The number of individual fibers that contract determines
the amount of overall muscle contraction.
Here we see all fibers being stimulated in a total contraction of the muscle.
If only motor neuron R is active, it produces action potentials in fibers 1 and 3.
The action potentials spread from the neuromuscular endplate over the surface
of fibers 1 and 3,
and since only two of the six fibers are stimulated,
it produces a contraction equivalent to one-third the muscles maximum contraction.
Input over other neurons also results in muscle contractions proportional to the
number of fibers stimulated.
Let us observe several combinations of active motor units,
and the resulting degree of muscle contractions.
A single vertebrate muscle fiber is innervated by only one motor neuron.
A single motor neuron may innervate many muscle fibers.
A motor neuron plus the fibers that it innervates are called a motor unit.
Motor neurons have endplate contacts with the fiber in only a single localized region
and release acetylcholine as their neurotransmitter chemical.
Motor neurons stimulate fiber contraction by a propagated action potential.
Fine control of muscle contraction is a function of the proportion of motor units
that are active within the muscle.