his work has become so popular in neuroscience. 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 the motor cortex control movement by a feed-forward direct pathway. in that view, a neuron in the motor cortex sends an axon or projection to the spinal cord and forms a synapse on a motor neuron. the motor neuron sends an electrical impulse to a muscle. when the neuron in the cortex becomes active, it causes a muscle contraction. the greater the activity in the motor cortex, the stronger the muscle force. each point in the 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 motor neurons. 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 the 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 the 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. map of the body in the human brain. 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 the 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 and mice. evolution of the motor cortex mammals evolved from mammal-like reptiles over 200 million years ago. these early mammals developed several novel brain functions most likely due to the novel sensory processes that were necessary for the nocturnal niche that these mammals occupied. these animals most likely had a somatomotor cortex, where somatosensory information and motor information were processed in the same cortical region. this allowed for the acquisition of only simple motor skills, such as quadrupedal locomotion and striking of predators or prey. placental mammals evolved a discrete motor cortex about 100 mya. according to the principle of proper mass, "the mass of neural tissue controlling a particular function is appropriate to the amount of information processing involved in performing the function." this suggests that the development of a discrete motor cortex was advantageous for placental mammals, and the motor skills that these organisms acquired were more complex than their early-mammalian ancestors. further, this motor cortex was necessary for the arboreal lifestyles of our primate ancestors. enhancements to the motor cortex (and the presence of opposable thumbs and stereoscopic vision) were evolutionarily selected to prevent primates from making mistakes in the dangerous motor skill of leaping between tree branches (cartmill, 1974; silcox, 2007). as a result of this pressure, the motor system of arboreal primates has a disproportionate degree of somatotopic representation of the hands and feet, which is essential for grasping (nambu, 2011; pons et al., 1985; gentilucci et al., 1988).