Motor Cortex — Function, Location & Quiz
Primary motor cortex is the narrow strip where a movement plan becomes descending command. Its position on the precentral gyrus is the landmark to fix first, because the almost mirror-like postcentral gyrus just behind it is sensory rather than motor.
The motor homunculus turns this strip into a clinical map: a small cortical lesion can weaken a particular contralateral body region, and vascular territory predicts which part is most exposed. That link from folded surface to bedside weakness is why the region appears so often in localization questions.
Test this anatomy in the Lesion Localization Quiz or open the interactive 3D brain model.
Location and anatomical map
Voluntary movement execution, motor planning.
Atlas category: cortical. Brodmann areas or landmark: BA 4 (primary motor cortex, M1).
Functions
- Direct control of voluntary movements via corticospinal tract
- Somatotopic organization — motor homunculus
- Force and direction coding of movements
- Motor learning and adaptation
Pathways and connections
- Corticospinal tract → spinal motor neurons (direct pathway)
- M1 ← Premotor cortex, SMA (movement planning)
- M1 ← Cerebellum via thalamus (motor correction)
- M1 ← Basal ganglia via thalamus (movement initiation)
Clinical relevance
- Lesion → contralateral hemiparesis or hemiplegia
- Upper motor neuron signs: spasticity, hyperreflexia, Babinski sign
- Stroke in MCA territory often affects face and arm (lateral homunculus)
- Penfield's stimulation studies mapped the motor homunculus
Key facts
- Motor homunculus: hands and face have disproportionately large representation
- Contains giant Betz cells — among the largest neurons in the CNS
- Penfield & Boldrey (1937) — mapped the homunculus during epilepsy surgery
- M1 uses population coding — movement direction encoded by population vector
Exam tip
Motor homunculus = upside down, with feet at the medial surface (ACA territory) and face lateral (MCA territory). Know the vascular implications!
Common questions
Where is the primary motor cortex?
It lies in the precentral gyrus of the frontal lobe, immediately anterior to the central sulcus. It corresponds mainly to Brodmann area 4.
What does the motor cortex control?
It sends commands for voluntary movement through descending pathways, especially the corticospinal tract. Its body map gives the hands and face disproportionately large cortical representation.
What are the signs of a motor cortex lesion?
A lesion can cause contralateral weakness with upper-motor-neuron signs such as spasticity, hyperreflexia and a Babinski response. The affected body part depends on the lesion’s place on the motor homunculus.