NEUROSCIENCE: Motor Systems
Premotor Cortex & Supplementary Motor Cortex

PREMOTOR CORTEX and SUPPLEMENTARY MOTOR CORTEX

PREMOTOR CORTEX (Premotor Area, PMA)

A. General Characteristics

1. Location: anterior to M-1, occupying the more lateral segment of cortical area 6

2. Output: mainly to M-1; also to the brainstem reticular system

3.  Premotor cortex also contributes to the pyramidal tract but the primary motor cortex is required for fine, discrete movements

B. Role: planning or anticipation of movement by facilitating appropriate M-1 columns

1. Premotor neurons become active before the onset of anticipated movements, particularly in response to external cues

2. Lesions of the premotor cortex lead to slowing of anticipated movements but do not lead to paralaysis

3. Thus, the premotor cortex acts to prepare the primary motor cortex for planned movements by facilitating appropriate M-1 columns

SUPPLEMENTARY MOTOR CORTEX (Supplementary Motor Area, SMA)

A. General Characteristics

1. Location: anterior to M-1, occupying the medial segment of cortical area 6 and extending into the medial cortical surface within the longitudinal fissure

2. Output: mainly to M-1

B. Role: programming M-1 for the execution of complex movements

1. Stimulation of SMA neurons elicits complex movements involving several muscles, particularly movements involving bilateral coordination

2. SMA neurons become active when voluntarily executing complex movements but not when executing simple acts

3. SMA neurons become active when thinking about complex movements even if the movement is not executed particularly in response to internal cues

4. Lesions of the SMA inhibit the ability to perform complex movements

SUMMARY: the PMA and the SMA function to transform the intention to perform a complex motor act into the specific sequence of movements necessary to accomplish the act