The Motor Cortex Map
A simple view, that is almost certainly too limited and that dates back to the earliest work on the motor cortex, is that neurons in motor cortex control movement by a feed-forward direct pathway. In that view, a neuron in motor cortex sends an axon or projection to the spinal cord and forms a synapse on a motoneuron. The motoneuron sends an axon to a muscle. When the neuron in cortex becomes active, it causes a muscle contraction. The greater the activity in motor cortex, the stronger the muscle force. Each point in motor cortex controls a muscle or a small group of related muscles. This description is only partly correct.
Most neurons in the motor cortex that project to the spinal cord synapse on interneuron circuitry in the spinal cord, not directly onto motoneurons. One suggestion is that the direct, cortico-motoneuronal projections are a specialization that allows for the fine control of the fingers.
The view that each point in motor cortex controls a muscle or a limited set of related muscles was debated over the entire history of research on the motor cortex, and was suggested in its strongest and most extreme form by Asanuma on the basis of experiments in cats and monkeys using electrical stimulation. However, almost every other experiment to examine the map, including the classic work of Ferrier and of Penfield showed that each point in motor cortex influences a range of muscles and joints. The map is greatly overlapping. The overlap in the map is generally greater in the premotor cortex and supplementary motor cortex, but even the map in the primary motor cortex controls muscles in an extensively overlapped manner. Many studies have demonstrated the overlapping representation of muscles in the motor cortex.
It is believed that as an animal learns a complex movement repertoire, the motor cortex gradually comes to coordinate among muscles.
The clearest example of the coordination of muscles into complex movement in the motor cortex comes from the work of Graziano and colleagues on the monkey brain. They used electrical stimulation on a behavioral time scale, such as for half a second instead of the more typical hundredth of a second. They found that this type of stimulation of the monkey motor cortex often evoked complex, meaningful actions. For example, stimulation of one site in cortex would cause the hand to close, move to the mouth, and the mouth to open. Stimulation of another site would cause the hand to open, rotate until the grip faced outward, and the arm to project out as if the animal were reaching. Different complex movements were evoked from different sites and these movements were mapped in the same orderly manner in all monkeys tested. Computational models showed that the normal movement repertoire of a monkey, if arranged on a sheet such that similar movements are placed near each other, will result in a map that matches the actual map found in the monkey motor cortex. This work suggests that the motor cortex does not truly contain a homunculus-type map of the body. Instead, the deeper principle may be a rendering of the movement repertoire onto the cortical surface. To the extent that the movement repertoire breaks down partly into the actions of separate body parts, the map contains a rough and overlapping body arrangement noted by researchers over the past century.
A similar organization by typical movement repertoire has been reported in the posterior parietal cortex of monkeys and galagos and in the motor cortex of rats.
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