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A reaching movement is coded by a population of neurons in the arm motor map. A. Raster plots show the firing pattern of a single primary motor cortex
neuron during movements in eight directions. The neuron discharges at the maximal rate for movements near 135 degrees and 180 degrees and at lesser
intensities for movements in other directions. The cell's lowest firing rate is for movements opposite the cell's preferred direction. Different cells have
different preferred directions, and their broad directional tuning curves overlap extensively. The plots are from a study in which a monkey was trained to
move a handle to eight targets arranged radially on a horizontal plane around a central starting position. Each row of tics in each raster plot represents the
activity in a single trial, aligned at the time of movement onset (time zero). (Reproduced, with permission, from Georgopoulos et al. 1982.)
Source: Voluntary Movement: The Primary Motor Cortex, Principles of Neural Science, Fifth Editon
B. Many primary motor cortex neurons with a broad range of preferred movement directions respond at different intensities during reaching movements in
Citation: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, Mack S. Principles of Neural Science, Fifth Editon; 2012 Available
a particular direction. The overall directional bias of the activity within the population of neurons shifts systematically with movement direction so that the
at: http://mhmedical.com/ Accessed: May 11, 2017
vectorial sum of the activity of all cells is a population vector that closely matches that of the direction of movement. This shows that the motor command
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for a movement is generated by a widely distributed population of cells throughout the arm motor map, each of which fires at a different intensity for
movement in a particular direction.