Motor cortex
Motor cortex is a term that describes regions of the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 involved in the planning, control, and execution of voluntary motor
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...


Anatomy of the motor cortex

The motor cortex can be divided into four main parts:
  • the primary motor cortex
    Primary motor cortex
    The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

     (or M1), responsible for generating the neural impulses controlling execution of movement
  • and the secondary motor cortices, including
    • the posterior parietal cortex
      Posterior parietal cortex
      The posterior parietal cortex plays an important role in producing planned movements. Before an effective movement can be initiated, the nervous system must know the original positions of the body parts that are to be moved, and the positions of any external objects with which the body is going to...

      , responsible for transforming visual information into motor commands
    • the premotor cortex
      Premotor cortex
      The premotor cortex is an area of motor cortex lying within the frontal lobe of the brain. It extends 3 mm anterior to the primary motor cortex, near the Sylvian fissure, before narrowing to approximately 1 mm near the medial longitudinal fissure, which serves as the posterior border for...

      , responsible for motor guidance of movement and control of proximal and trunk
      Trunk or torso is an anatomical term for the central part of the many animal bodies from which extend the neck and limbs. The trunk includes the thorax and abdomen.-Major organs:...

       muscles of the body
    • and the supplementary motor area
      Supplementary motor area
      The supplementary motor area is a part of the sensorimotor cerebral cortex . It was included, on purely cytoarchitectonic arguments, in area 6 of Brodmann and the Vogts...

       (or SMA), responsible for planning and coordination of complex movements such as those requiring two hands.

Other brain regions outside the cortex are also of great importance to motor function, most notably the cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

 and subcortical motor nuclei
Nucleus (neuroanatomy)
In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...


Early work on motor cortex function

In the 19th century Eduard Hitzig
Eduard Hitzig
Eduard Hitzig was a German neurologist and neuropsychiatrist born in Berlin.He studied medicine at the Universities of Berlin and Würzburg, and had as instructors, famous men such as Emil Du Bois-Reymond , Rudolf Virchow , Moritz Heinrich Romberg and Karl Friedrich Otto Westphal...

 and Gustav Fritsch
Gustav Fritsch
Gustav Theodor Fritsch was a German anatomist, anthropologist, traveller and physiologist from Cottbus, best known for his work with neuropsychiatrist Eduard Hitzig on the electric localization of the motor areas of the brain...

 demonstrated that electrical stimulation of certain parts of the brain would result in muscular contraction on the opposite side of the body.

In 1949 Canadian neurosurgeon Wilder Penfield
Wilder Penfield
Wilder Graves Penfield, OM, CC, CMG, FRS was an American born Canadian neurosurgeon. During his life he was called "the greatest living Canadian"...

 developed a surgical procedure to relieve epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

. His initial procedure was to electrically probe the surface of the patient's cortex to find the problem area. During such investigations, he discovered that stimulation of Brodmann's area
Brodmann area
A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...

 4 readily elicited localised muscle twitches. Furthermore, there appeared to be a “motor map” of the body surface along the gyrus that comprises area 4. Area 4 is therefore now known as the primary motor cortex. Following this discovery, he discovered that stimulation of regions which are in front of the M1 caused more complicated movements; however, more electrical current was required to initiate movements from these areas. These 'premotor' cortical areas are located in Brodmann's area 6.

The motor cortical areas are now typically divided into three regions which have 2 different functional roles:
  1. primary motor cortex
    Primary motor cortex
    The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...

  2. pre-motor area (PMA)
  3. supplementary motor area (SMA)

Penfield's experiments have made everything seem pretty straightforward: the purpose of M1 is to connect the brain to the lower motor neurons via the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 in order to tell them which particular muscles need to contract. These upper motor neurons are found in layer 5 of the motor cortex and contain some of the largest cells in the brain (Betz cell
Betz cell
Betz cells are pyramidal cell neurons located within the fifth layer of the grey matter in the primary motor cortex, M1. They are named after Vladimir Alekseyevich Betz, who described them in his work published in 1874. These neurons are the largest in the central nervous system, sometimes reaching...

s whose cell bodies can be up to 100 micrometres in diameter. For comparison, rod photoreceptors are about 3 micrometres across). The descending axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s of these layer 5 cells form the cortico-spinal or pyramidal tract. However, a single layer 5 forms synapses with many lower motor neurons which innervate different muscles. Furthermore, the same muscle is often represented over quite large regions of the brain's surface, and there is an overlap in the representation of different regions of the body. These facts mean that M1 neurons do not form simple connections with lower motor neurons. The activity of a single M1 neuron could cause contraction of more than one muscle; this suggests that M1 may not simply be coding the degree of contraction of individual muscles.

Non-activity responses in the motor cortex

Functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...

 (fMRI) scans of persons reading words have shown that the act of reading a verb that refers to a face, arm, or leg action causes increased blood flow and activity in the motor cortex. The areas of the motor cortex that are active correspond to sites of the motor cortex that are associated with that activity. For example, reading the word lick would increase blood flow in sites corresponding to tongue and mouth movements. While reading the verbs, blood flow also increases in premotor regions, Broca's area
Broca's area
Broca's area is a region of the hominid brain with functions linked to speech production.The production of language has been linked to the Broca’s area since Pierre Paul Broca reported impairments in two patients. They had lost the ability to speak after injury to the posterior inferior frontal...

 and Wernicke's area
Wernicke's area
Wernicke's area is one of the two parts of the cerebral cortex linked since the late nineteenth century to speech . It is involved in the understanding of written and spoken language...

. Based on this information, it has been proposed that word understanding hinges on activation of interconnected brain areas that assimilate information about a particular word and its associated actions and sensations.
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