Pediatric Movement Disorders - Anatomy and Physiology

Cortex

Relatively little is known about the types of computations performed in the motor cortex. Most knowledge concerns the representation of movement information. It is clear that, for arm movements, cells within motor cortex respond selectively to different features of movement, including direction, force, joint motion, and limb position.

Some cells also respond to sensory information, including tactile or visual information. Much research has investigated the "maps" of motor function that determine which areas of the brain control specific, distinct parts of the body. It is clear that these maps are not fixed; rather, they change with time and throughout life. In particular, increased use of one part of the body tends to increase the cortical area devoted to that part of the body. Some authors believe that this is related to the acquisition of skill. Certainly there are changes following nerve injury, limb amputation, or stroke. These changes suggest that plasticity of the maps may be responsible for some degree of recovery of function or at least re-allocation of uninjured neurons after an injury to part of the cortex.

The connection between the motor cortex and the motor cells of the anterior horn of the spinal cord is not simple. It appears that control of the fingers is performed under relatively direct cortical control, in that...

• motor cortex neurons send axons via the corticospinal tract directly to the spinal cord alpha-motoneurons and
• alpha-motoneurons send signals to the muscles controlling the fingers.

For almost all other parts of the body, the cortex either sends signals to...

• intermediate layers within the spinal cord, where the signals modulate the tone or reflex gain and also cause direct contraction of the muscles, or
• intermediate processing stages in the midbrain or pons of the brainstem.

It is difficult to construct simple models of the response to cortical injury. In particular, the hypothesized origin of spasticity from a possible decrease in cortical input, is difficult to reconcile. This is particularly obvious with the knowledge that most control of the trunk and proximal leg muscles proceeds via connections in the brainstem. Thus, the neurons that connect to the spinal cord are not directly affected by injury above the brainstem; however, if the cortex and basal ganglia are injured or disconnected from the brainstem, normal control signals may be absent or inappropriate. This observation again lends support to the idea that it is an abnormal pattern of the motor command that causes symptoms, rather than the absence of a motor command.

In monkeys with dystonia, there is an increase in the area of the cortical map devoted to the fingers in dystonia. This finding has generated considerable interest. There is a suggestion that a similar increase in size as well as confusion of different fingers may occur in adult focal dystonia. This implies that some cases of dystonia may be reflected in changes in the cortical map. In addition, this may be an alternative cause of these symptoms that only indirectly involves the basal ganglia. Whether this is important in childhood-onset generalized dystonia is not known.