The vertebrate motor unit the vertebrate motor unit

The vertebrate motor unit

The vertebrate motor unit
•
muscles performing powerful, coarsely controlled
movements have larger number of fibers per motor unit

example : a single calf muscle motor neuron may
connect to ~2000 muscle fibers
•
muscles that produce more precise, delicate movements
contain fewer fibers per motor unit

example : extraocular muscles have only ~12 muscle
fibers per motor neuron
A single motor unit consists of a single motor neuron and all
the muscle fibers that it controls .
The numbers of muscle fibers per motor unit and numbers
of
motor units per muscle vary widely.

Motor units in extraocular muscles
Extraocular muscles (which control the direction of gaze)
produce very precise, delicate movements.
Each motor neuron connects to only ~12 muscle fibers.

The vertebrate motor units
Increasing the number of active motor units is recruitment .
Asynchronous recruitment of motor units delays fatigue:
alternating motor activities gives motor units that have been
active the opportunity to rest while others take over.

Action potentials and muscle fiber contractions
A single action potential in the muscle fiber causes an all-or-
none muscle fiber contraction (a twitch ). The length and the
strength of a sustained contraction ( tetanus ) depends on the
frequency of action potentials.

Treppe and summation
The frequency of action potentials determines the tension .
The graduated series of increasingly vigorous contractions
that results when a series of identical stimuli are applied to
a rested muscle is called treppe . It does not result in
tetanus, and is important in cardiac muscle activity.
When a muscle is stimulated by the motor neuron more than
once in a short period (more frequently than in treppe),
the successive twitches add to each other so the force
generated is greater. This is called summation .

Summation Treppe
Treppe and summation

High frequency stimulation can cause a smoothly fused
contraction called tetanus . This extreme shortening is
because all of the elastic muscle fibers of the muscle
become completely shortened.
•
incomplete tetanus : there are brief relaxation phases between
stimulations but the muscle does not relax completely
•
complete tetanus : relaxation phases are eliminated
Tetanus

Incomplete tetanus Complete tetanus
Tetanus

Fatigue
In skeletal muscles, repetitive stimulation leads to fatigue
when the fibers run out of enough energy, evident as a drop
in tension. Rest overcomes fatigue, but it will reoccur sooner
if inadequate recovery time is allowed.