output from the cortex to the spinal cord. this mistake is old, dating back at least to campbell in 1905. yet the betz cells compose only about 2-3% of the neurons that project from the cortex to the spinal cord, and only about 10% of the neurons that project specifically from the primary motor cortex to the spinal cord. a range of cortical areas including the premotor cortex, the supplementary motor area, and even the primary somatosensory cortex, project to the spinal cord. even when the betz cells are damaged, the cortex can still communicate to subcortical motor structures and control movement. if the primary motor cortex with its betz cells is damaged, a temporary paralysis results and other cortical areas can evidently take over some of the lost function. clinical significance lesions of the precentral gyrus result in paralysis of the contralateral side of the body (facial palsy, arm-/leg monoparesis, hemiparesis) - see upper motor neuron. movement coding evarts suggested that each neuron in the motor cortex contributes to the force in a muscle. as the neuron becomes active, it sends a signal to the spinal cord, the signal is relayed to a motorneuron, the motorneuron sends a signal to a muscle, and the muscle contracts. the more activity in the motor cortex neuron, the more muscle force. georgopoulos and colleagues suggested that muscle force alone was too simple a description. they trained monkeys to reach in various directions and monitored the activity of neurons in the motor cortex. they found that each neuron in the motor cortex was maximally active during a specific direction of reach, and responded less well to neighboring directions of reach. on this basis they suggested that neurons in motor cortex, by "voting" or pooling their influences into a "population code", could precisely specify a direction of reach. the proposal that motor cortex neurons encode the direction of a reach became controversial. scott and kalaska showed that each motor cortex neuron was better correlated with the details of joint movement and muscle force than with the direction of the reach. schwartz and colleagues showed that motor cortex neurons were well correlated with the speed of the hand. strick and colleagues found that some neurons in motor cortex were active in association with muscle force and some with the spatial direction of movement. todorov proposed that the many different correlations are the result of a muscle controller in which many movement parameters happen to be correlated with muscle force. the code by which neurons in the primate motor cortex control the spinal cord, and thus movement, remains debated. some specific progress in understanding how motor cortex causes, movement has also been made in the rodent model. the rodent motor cortex, like the monkey motor cortex, may contain subregions that emphasize different common types of actions. for example, one region appears to emphasize the rhythmic control of whisking. neurons in this region project to a specific subcortical nucleus in which a pattern generator coordinates the cyclic rhythm of the whiskers. this nucleus then projects to the muscles that control the whiskers. additional images precentral sulcus central sulcus the motor tract.