page_title: Primary motor cortex
section_title: Alternative maps
hierarchical_section_title: Primary motor cortex / Function / Alternative maps
caption_attribution_description: Map of the body in the human brain. The cerebral cortex has been “inflated” to show the results more clearly. Movements of different body parts evoked activity in the primary motor cortex (left of the central sulcus, shown by the arrow) and primary somatosensory cortex (right of the central sulcus). The map also shows eye-movement activity in the frontal eye field and the parietal cortex. The map is a winner-take-all. Although different body parts activated overlapping areas of cortex, only the strongest activations are indicated. The map in the primary motor cortex contains two common complexities. First, the hand representation (red) is surrounded on three sides by a representation of the wrist. Second, there are two distinct hand representations possibly corresponding to areas 4a and 4p.
context_page_description: The primary motor cortex is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex, and several subcortical brain regions, to plan and execute movements. Primary motor cortex is defined anatomically as the region of cortex that contains large neurons known as Betz cells. Betz cells, along with other cortical neurons, send long axons down the spinal cord to synapse onto the interneuron circuitry of the spinal cord and also directly onto the alpha motor neurons in the spinal cord which connect to the muscles.
  At the primary motor cortex, motor representation is orderly arranged from the toe to mouth along a fold in the cortex called the central sulcus. However, some body parts may be controlled by partially overlapping regions of cortex. Each cerebral hemisphere of the primary motor cortex only contains a motor representation of the opposite side of the body.
context_section_description: At least two modifications to the classical somatotopic ordering of body parts have been reported in the primary motor cortex of primates.
  First, the arm representation may be organized in a core and surround manner. In the monkey cortex, the digits of the hand are represented in a core area at the posterior edge of the primary motor cortex. This core area is surrounded on three sides (on the dorsal, anterior, and ventral sides) by a representation of the more proximal parts of the arm including the elbow and shoulder.  In humans, the digit representation is surrounded dorsally, anteriorly, and ventrally, by a representation of the wrist.
  A second modification of the classical somatotopic ordering of body parts is a double representation of the digits and wrist studied mainly in the human motor cortex. One representation lies in a posterior region called area 4p, and the other lies in an anterior region called area 4a. The posterior area can be activated by attention without any sensory feedback and has been suggested to be important for initiation of movements, while the anterior area is dependent on sensory feedback. It can also be activated by imaginary finger movements and listening to speech while making no actual movements. This anterior representation area has been suggested to be important in executing movements involving complex sensoriomotor interactions. It is possible that area 4a in humans corresponds to some parts of the caudal premotor cortex as described in the monkey cortex.
  In 2009, it was reported, that there are two evolutionary distinct regions, an older one on the outer surface, and a new one found in the cleft. The older one connects to the spinal motorneurons through interneurons in the spinal cord. The newer one, found only in monkeys and apes, connects directly to the spinal motorneurons. The direct connections form after birth, are dominant over the indirect connections, and are more flexible in the circuits they can develop which allows the post-natal learning of complex fine motor skills. "The emergence of the 'new' M1 region during evolution of the primate lineage is therefore likely to have been important for the enhanced manual dexterity of the human hand."
caption: Map of the body in the human brain
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