Motor homunculus: the motor map
– similar to the sensory map in that it distorts the representations of the body
– the areas that require the most detailed control and allow for finely graded movements
(mouth, throat, hand) are most highly represented
The processing areas of the motor cortex (listed below) interact with sensory processing areas in
the parietal lobe and the basal ganglia and cerebellar areas to identify where we want to move, plan
the movement and execute our actions.
- Primary motor cortex (MI) + 4 premotor cortical areas:
a) Supplementary motor area (SMA or MII)
b) Cingulate motor area (in the cingulate gyrus, inferior to the SMA)
c) Lateral ventral premotor area
d) Lateral dorsal premotor area
The same muscle can be activated from several sites in the cortex. There is not a one-to-one
correspondence between a site and a muscle, as was once believed. The benefit of having this type of
activation is that muscles that required more stimulation at once can receive it.
The corticospinal tract separates near the junction between the medulla and the spinal cord.
The two aspects that are controlled by the Primary Motor Cortex are:
1. Absolute force
2. The speed of a movement
Difference in functions between the premotor cortex and the supplementary motor cortex:
- Movements that are initiated internally are controlled primarily by the SMA
- Movements that are initiated externally (by some sensory stimulus) are controlled primarily
by the PMA.
Premotor and supplementary motor areas differ in their activity depending on how the movement is
initiated and guided. Premotor neurons are more active when a sequential task is visually guided,
while supplementary motor area neurons are more active when the sequence is remembered and self-
Learning of sequences involves the supplementary motor cortex, yet once the sequence gets over
learned, control of movement can be transferred to the primary motor cortex.
Monkeys that are able to pick up bananas, yet cannot do so as a response to external stimuli, such a