Posterior parietal cortex contains a command apparatus for hand movements

Significance

The primate hand has evolved into a specialized sensorimotor device that can grasp, explore, and manipulate objects with extraordinary skill. The frontal lobe is generally thought to be the exclusive source of descending commands to the spinal cord to control hand movements. Here, we identify a region within the parietal lobe that could also contribute commands to control hand movements directly at spinal levels. Intracortical stimulation in a lateral region in area 5 of posterior parietal cortex reliably evokes hand movements. Corticospinal neurons in this region make disynaptic connections with hand motoneurons. These observations suggest that a region within lateral area 5 contains a unique command apparatus that could assist in generating dexterous finger movements required during haptic behavior.

Abstract

Mountcastle and colleagues proposed that the posterior parietal cortex contains a “command apparatus” for the operation of the hand in immediate extrapersonal space [Mountcastle et al. (1975) J Neurophysiol 38(4):871–908]. Here we provide three lines of converging evidence that a lateral region within area 5 has corticospinal neurons that are directly linked to the control of hand movements. First, electrical stimulation in a lateral region of area 5 evokes finger and wrist movements. Second, corticospinal neurons in the same region of area 5 terminate at spinal locations that contain last-order interneurons that innervate hand motoneurons. Third, this lateral region of area 5 contains many neurons that make disynaptic connections with hand motoneurons. The disynaptic input to motoneurons from this portion of area 5 is as direct and prominent as that from any of the premotor areas in the frontal lobe. Thus, our results establish that a region within area 5 contains a motor area with corticospinal neurons that could function as a command apparatus for operation of the hand.