An alpha motor neuron and all the muscle fibers it innervates
A pool of motor neurons lie in an elongated cluster (motor nucleus) over 1-4 spinal
Motor neurons for one muscle are clustered together
The dendritic trees of alpha motor neurons receive synaptic inputs from sensory
neurons, spinal interneurons, and descending motor neurons.
A typical muscle consists of many thousands of muscle fibers working in parallel, and
organized into a smaller number of motor units.
When an axon reaches the muscle, it branches and innervates from a few fibers up to
several thousand muscle fibers.
The muscle fibers that belong to a motor unit reside in a portion of the muscle.
A motor unit has a certain territory –where the muscle fibers from that motor unit
Long (as fraction of muscle length)
Most of muscle depth as well
In cross section we see muscle fibers from different motor units are
Motor unit properties
Motor units vary in their response properties (speed, force)
One action potential in neuron produces one twitch in the muscle fibers
Rapid mechanical event
Different motor units produce different forces in one twitch
Motor neurons usually fire repeatedly – not just one twitch
Tetany – twitches occur so rapidly it does not decrease between twitches
Increase tetanic force by increasing firing rate
Unfused tetanus – can still see individual forces
Fused tetanus – can no longer see individual twitches
Motor units vary in their twitch properties (speed, force) Some have faster or slower twitches, greater or lesser twitch forces
An action potential in a motor neuron results in a motor unit “twitch”
Individual motor unit twitch forces summate to produce a more constant tetanic force
output that is dependent on the frequency of the motor unit twitches.
The force output from the individual motor units that are currently active (or firing) is
summed, resulting in a larger force that is transmitted through connective tissue to the
tendon and bone.
Activation of a motor unit
Depolarization of alpha motor unit