Muscle III: Control of Movement
Hierarchy of Muscle Control
Sensorimuscle cortex is the middle level of motor control and includes the parts
of the cerebral cortex that act together to control skeletal muscle activity –
premotor area, primary motor cortex, supplementary motor cortex, somatosensory
cortex, parietal-lobe association cortex.
Other middle levels of motor control are the basal nuclei (part of subcortical
nuclei), thalamus, brainstem, and cerebellum.
Local levels of motor control – brainstem and spinal cord, receptors, muscle
2 pathways from brain/CNS to spinal cord
Corticospinal pathway – long axons, most cross in the brainstem and go down
opposite side of spinal cord – crosstalk
Brainstem Pathway – mostly uncrossed, go straight down spinal cord
Each muscle is connected to only one motor neuron – it doesn’t need any other nerves
But neurons are biforlated – each neuron regulates several muscle cells
A motor unit is a motor neuron and all of its attached muscle fibers
A complete muscle is a mixture of motor units
Motor units are controlled according to their size – cross section and pass from the cortex
to the spinal cord
Tiny motor units – slow-oxidative fibers, recruited first so they fire first
Intermediate motor units – fast-oxidative fibers, recruited after slow-
oxidative, so they fire next.
Gigantic motor units – fast-glycolytic fibers – recruited last, fire last and
die off quickly
Varying degrees of muscle tension for different sizes of motor units
Small motor units can’t cause much rise in muscle tension, intermediates cause a
little more, and the fast-glycolytic fibers cause the greatest rise because they have
the largest-diameter fibers and have the most number of fibers per motor unit.
The size of the motor unit is directly related to the size of the muscle fibers and
the number of the muscle fibers that the motor neuron is connected to, as well as
the size of the motor neuron
Frequency of the AP determines which size motor units activated
Small frequency = small motor units activated, as the frequency increases, larger
motor units are activated
The other two types of motor units keep firing even when the large motor units
are activated (the slow-oxidative never gives up – keeps firing throughout)
Small motor units are activated first because they have smaller neurons, which
means that the spike initiation zone is smaller.
In larger motor units with larger
neurons, the spike initiation zone is larger, so the synaptic current is more spread
out – requiring a larger frequency.