Motor System Quiz

See Path of UMNs. Option D is correct because many upper motor neurone cell bodies lie in the grey matter of the precentral gyrus, the strip of frontal lobe just in front of the central sulcus that makes up the primary motor cortex. Grey matter is the part of the nervous system made of nerve cell bodies, in contrast to white matter, which is made of their axons. The cell bodies in the precentral gyrus are arranged like a map of the body, called the motor homunculus (Latin for little man): the leg is represented medially, on the inner surface of the hemisphere, and the hand more laterally. Their axons head downwards through the corona radiata (the fan of white matter beneath the cortex), the internal capsule, the midbrain, the pons and the medullary pyramids, and most cross in the lower medulla before running down the spinal cord as the lateral corticospinal tract. Other cortical areas and brainstem centres also give rise to descending motor pathways, which is why the precentral gyrus is the main rather than the only source. A memory aid: precentral means in front of the central sulcus, and that front strip is the motor strip.

• Cell bodies: many upper motor neurone cell bodies lie in the grey matter of the precentral gyrus, the primary motor cortex.
• Grey matter: it is made of nerve cell bodies, whereas white matter is made of their axons.
• Motor homunculus: the leg is represented medially and the hand more laterally.
• Course: axons pass through the corona radiata, internal capsule, midbrain, pons and medullary pyramids, and most cross in the lower medulla.