Definitions

Motor_cortex

Motor cortex

Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.

Areas of the motor cortex

The motor cortex can be divided into five main areas:

  • the primary motor cortex (or M1), responsible for generating the neural impulses controlling execution of movement
  • and the secondary motor cortices, including

Other brain regions outside the cortex are also of great importance to motor function, most notably the cerebellum and subcortical motor nuclei.

Early work on motor cortex function

In the 1950s Canadian neurosurgeon Wilder Penfield experimented with removing parts of epileptic's brains. To check crucial sections of the cortex, he electrically stimulated cortical surface and observed results; he found that stimulation of Brodmann's area 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 (M1)
  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 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 cells whose cell bodies can be up to 100 micrometres in diameter. For comparison, rod photoreceptors are about 3 micrometres across). The descending axons 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 inervate 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. To understand what is going on, we need to record from the motor cortex of unaesthetised monkeys while they are performing a task.

More recent work: coding of reach direction in M1

Much of the seminal work in this field has been done by Georgopoulos, who recorded from neurons in M1 while a monkey used its arm to point to targets at various locations in space. He found that "single neurons would respond to a broad but restricted range of reach directions." This made him think that cells in M1 were "encoding the direction of reach" and also the activity of single muscles. However, a single cell can't accurately encode reach direction because its resolution is too coarse. He recorded the preferred reach direction of hundreds of cells in M1 and found there to be a broad range of preferred directions. The question now arises: how does the M1 make accurate reaching motions when individual neurons code direction fairly coarsely? Georgopolous found that (for a particular reach direction) when the preferred direction and firing of the neurons are added together as a vector, the sum accurately predicts the final reach direction of the monkey. Furthermore, he found evidence of a mental rotation of the population vector when a monkey was asked to point at an angle to a target: the population vector initially pointed directly at the target, and then "rotated" rapidly until it pointed at the desired angle to the target.

Is M1 really coding reach direction?

The occurrence of the population vector in M1 is undisputed but its purpose is uncertain. It turns out that single neuron responses in M1 do not correlate only with reach direction but also with a number of other co-variables such as initial arm position, acceleration, movement preparation, target position, distance to target, joint configuration, muscular force, etc. Recent work has suggested that the population vector in M1 codes motion around single joints and that the apparent reach-direction code is an epiphenomenon due partly to the physical restrictions of the musculoskeletal system.

What is needed for the execution of a correctly coordinated movement?

Whatever M1 is really coding, it would appear to be relatively low-level. Looking at M1 responses doesn't show how complex movements are planned or how they are guided. For this, the premotor areas must be investigated into, regarding how they interact with the basal ganglia to initiate movement and the cerebellum for the visual guidance of movement. A key feature in both of these tasks is sensory feedback. Without sensory feedback, normal initiation and (particularly) guidance of movement are not possible. Initiating a movement at the right time is usually based upon sensory information. Correct completion of a movement (such as reaching) requires you to know a lot of your environment, such as the initial length of the muscle, muscle velocity, load on the muscle, and posture. You also need to know how the movement is progressing in the form of visual and proprioceptive feedback in order to correct the movement as it happens or improve it on subsequent occasions through trial-and-error learning.

The role of the premotor areas, PMA and SMA

Stimulation of SMA and PMA causes initiation of complex, often bilateral, movements. Initiation of these movements requires more electrical current to be delivered than that required for initiation of movements from M1. This is because the premotor areas are only indirectly connected with lower motor neurons.

PMA receives input from the cerebellum and is involved in planning movements based on external (especially visual) cues; it is involved mostly in control of postural and proximal limb muscles. Lesions of PMA disrupt learned responses to visual cues. SMA has strong reciprocal connections with the basal ganglia and is involved in planning learned sequences of movements; it is the site of readiness potential which begins one second before a movement is initiated. Stimulation of SMA creates an urge to move. Bilateral SMA lesions block all movement and cause flaccid paralysis. More minor, unilateral lesions disrupt learning of movement sequences independent of external cues.

PMA and SMA are the routes through which the cerebellum and basal ganglia form parallel processing loops with the motor-cortical areas. It is through these loops that movements are initiated and guided. Damage to structures in these loops causes specific movement impairments.

Non-activity responses in the motor cortex

Functional magnetic resonance imaging (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 (Hauk, et al., 2004).

While reading the verbs, blood flow also increases in premotor regions, Broca's area and Wernicke's area. 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 (de Lafuente & Romo, 2004).

References

  • de Lafuente V, Romo R. (2004). "Language abilities of motor cortex". Neuron 41 (Jan. 22): 178–180.
  • Hauk O, Johnsrude I, Pulvermuller F (2004). "Somatotopic representation of action words in human motor and premotor cortex". Neuron 41 (Jan. 22): 301–307.

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