The Human Brain : From Neurone to Nervous System

MOTOR CONTROL: Projections to the Motor Areas

  1. The cerebral cortex consists of columns of neurones, each concerned with one particular function; these are linked with other parts of the cortex by bundles of subcortical association fibres, and indirectly through connections with the basal ganglia and thalamus.

  2. The pre-motor cortex receives information necessary for planned movements from other areas of association cortex that integrate sensory and motor functions, or those concerned with conscious thought, intelligence and attention.

  3. The pre-motor areas utilise information provided by the basal ganglia, the posterior parietal cortex (PPC - concerned with spatial awareness) and other areas of association cortex .

  4. These are all interconnected, and the basal ganglia's role is to initiate movement: this the part of the CNS that become active before any other in the performance of a movement.

  5. Events in 3 dimensional space involve vision, touch and proproception and the role of the posterior parietal cortex (situated between the somatosensory and visual areas) is to integrate these inputs, which are essential if movements are to achieve a conscious objective.

  6. Key Words: pre-motor cortex, posterior parietal cortex, association cortex,

The diagram shows the route taken by information travelling from the posterior parietal cortex to the pre-motor and supplmentary motor areas the control complex movements. The posterior parietal association cortex integrates visual and tactile space, i.e. the position of the limb as signalled by proprioceptors and by the visual system.
Association Cortex
The Primary Somatosensory area (S1) reports about proprioceptive and cutaneous events, such as the position of a limb, and any contact with objects.
The pre-motor cortex utilises all these pieces of information about the position of the body in 3-dimenional space relative to external objects, and organises appropriate movements. In addition, the frontal cortex is concerned with conscious thought, attention, initative and decision-making.
In order to organise complex movements the pre-motor areas have to be connected with all these areas, using bundles of subcortical association fibres, and connections through the basal ganglia and thalamus.
Areas of association cortex, including the PPC, commuicate with the Pre-motor (lateral) and Supplementary Motor Cortex (medial), as well as other sites in the frontal lobe.

Image source:
The diagram indicates the connections achieved by bundles of short (arcuate)or long subcortical association fibres (fasciculi). These connect areas of the cortex concerned with different functions, and allow areas such as the pre-motor area to utilise proprioceptive information concerned with the position of the limbs and the position of objects in the visual field.

The arcuate fasciculus is a long band of association fibres concerned with relaying auditory messages (concerned with understanding of the spoken word; in the temporal lobe) to the motor areas concerned with speech, in the frontal lobe.
Motor skills are learned throughout life, probably within the cerebellum, and this structure provides feedback - an error signal that allows the motor cortex to adjust movements and provide greater precision.
Finally, the sequence of movements organised by these complex circuits is executed by the motor cortex and the corticospinal tract which carries motor commands to the alpha motoneurones and therefore the musculature.