Overview
This lecture covers the somatotopic organization of the motor cortices, their vascular supply, associated clinical syndromes, and important anatomical features such as the motor homunculus.
Somatotopic Organization of Motor Cortex
- The primary motor cortex has a somatotopic map: leg and foot medially, trunk, arm, hand laterally, and face/mouth most lateral and inferior.
- Pre-motor and supplementary motor areas (SMA) also display somatotopic mapping for different body parts.
- Motor cortex regions work together to plan and initiate movement.
Vascular Supply and Stroke Syndromes
- The anterior cerebral artery (ACA) supplies the medial cortex, especially the leg and foot area.
- ACA stroke can cause contralateral leg weakness.
- The middle cerebral artery (MCA) supplies lateral cortex, covering face, hand, and arm areas.
- MCA strokes typically affect contralateral face, hand, and arm more than lower limbs.
Functional Features of Motor Cortices
- Stimulation of primary motor cortex produces discrete, non-preventable, contralateral muscle movements.
- All movements are contralateral except some oral muscles (tongue, masseter, pharynx).
- Lesions cause initial flaccid paralysis; then hemiparesis, mild spasticity, most affecting fine movements.
Clinical Correlates: Jacksonian Seizure & Homunculus
- Jacksonian seizure: motor symptoms spread along the somatotopic map in the cortex, supporting mapped organization.
- The motor homunculus visually represents the cortical area devoted to each muscle group; hands and tongue have largest representation.
Functions of Premotor and Supplementary Motor Areas
- Premotor cortex: higher threshold for movement; involves larger muscle groups; important for externally-guided movements.
- SMA: associated with planning/learning of complex, internally-generated movements; stimulation can cause posture changes or speech arrest.
Key Anatomical Landmarks
- Central sulcus separates precentral (motor) and postcentral (sensory) gyri.
- The hand knob ("hand hook") is a recognizable omega-shaped feature on the precentral gyrus, useful for surgical navigation.
- Paracentral lobule includes leg/foot areas on either side of the central sulcus.
Key Terms & Definitions
- Somatotopy — the mapping of body regions to specific locations in the brain cortex.
- Primary Motor Cortex — precentral gyrus, initiates voluntary movement.
- Premotor Cortex — area involved in planning movements in response to external cues.
- Supplementary Motor Area (SMA) — region involved in planning complex, self-initiated movements.
- Motor Homunculus — distorted figure showing amount of cortex devoted to body parts.
- Jacksonian Seizure — type of focal seizure where movements progress along body regions mapped in motor cortex.
- Central Sulcus — brain landmark separating frontal and parietal lobes.
Action Items / Next Steps
- Review motor cortex maps and major vascular territories (ACA vs MCA) for clinical relevance.
- Identify key gyri and the hand knob on brain images.
- Prepare for potential discussion or questions on motor homunculus and Jacksonian seizure.