The motor cortex is an area of the frontal lobe located in the posterior precentral gyrus immediately anterior to the central sulcus.

The primary motor cortex is the main contributor to generating neural impulses that pass down to the spinal cord and control the execution of movement.

These cells were mistakenly thought to be the main outputs from the cortex, sending fibers to the spinal cord.

A set of acronyms are commonly used: PMDr (premotor dorsal, rostral), PMDc, PMVr, PMVc.

Electrical stimulation of these neurons causes an apparent defensive movement as if protecting the body surface.

[26][27] This premotor region may be part of a larger circuit for maintaining a margin of safety around the body and guiding movement with respect to nearby objects.

In the monkey brain, neurons in the SMA are active in association with specific learned sequences of movement.

[39] Others have suggested that, because the SMA appears to control movement bilaterally, it may play a role in inter-manual coordination.

[37][41] On the basis of the movements evoked during electrical stimulation, it has been suggested that the SMA may have evolved in primates as a specialist in the part of the motor repertoire involving climbing and other complex locomotion.

In 1870, Eduard Hitzig and Gustav Fritsch demonstrated that electrical stimulation of certain parts of the dog brain resulted in muscular contraction on the opposite side of the body.

[43] This confirmed experimentally the existence of a cortical motor center, which was inferred by Jackson a few years earlier on the basis of clinical observations.

[47] A little later, in 1874, David Ferrier,[48] working in the laboratory of the West Riding Lunatic Asylum at Wakefield (at the invitation of its director, James Crichton-Browne), mapped the motor cortex in the monkey brain using electrical stimulation.

He also found that when electrical stimulation was maintained for a longer time, such as for a second, instead of being discharged over a fraction of a second, then some coordinated, seemingly meaningful movements could be caused, instead of only muscle twitches.

After Ferrier's discovery, many neuroscientists used electrical stimulation to study the map of the motor cortex in many animals including monkeys, apes, and humans.

However, Penfield drew a picture of a human-like figure stretched over the cortical surface and used the term "homunculus" (diminutive of "homo", Latin for "man") to refer to it.

We looking at the brain chart of the text book may never forget the unspeakable complexity of the reactions thus rudely symbolised and spatially indicated´.

While pictures of an orderly representation of limb segments across the cortical surface (such as the one shown above) have endured in textbooks, they are erroneous and misleading.

[54][55] 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[56] 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[48] and of Penfield[7] showed that each point in the motor cortex influences a range of muscles and joints.

[64] It is believed that as an animal learns a complex movement repertoire, the motor cortex gradually comes to coordinate among muscles.

[65][66] 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 found that this type of stimulation of the monkey motor cortex often evoked complex, meaningful actions.

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.

[72] Notwithstanding, direct tests of the idea that the motor cortex contains a movement repertoire have not corroborated this hypothesis.

[73] Varying the initial position of the forelimb does not change the muscle synergies evoked by microstimulation of a motor cortical point.

The evoked movement trajectory is most natural when the forelimb lays pendant ~ perpendicular to the ground (i.e., in equilibrium with the gravitational force).

The paths of the paw are curved with changes and reversals of direction and the passive influence of the gravitational force on the movements is obvious.

This allowed for the acquisition of only simple motor skills, such as quadrupedal locomotion and striking of predators or prey.

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).