Normal Physiology ofMotor Control

The intention to contract a skeletal muscle begins in the premotor area of the frontal lobes. This is where we plan our behaviour, the neurons compiling a program for the degree and sequence of muscle contractions required to perform an action.

This program is then transmitted to the neurons of the precentral gyrus. The precentral gyrus exhibits somatotopy, and the map of which is called the motor homonculus.

Basal Ganglia

Almost all areas of the cerebral cortex send signals to the basal ganglia. The basal ganglia process these signals and direct their output to the thalamus, which relays the signals back to the cerebral cortex. Thus the basal ganglia are involved in the feedback circuit for the planning and execution of movement.

More specifically the basal ganglia help to plan and control complex patterns of muscle movements, controlling the relative intensities, directions and sequences of multiple successive and parallel movements. They also control the onset and cessation of planned movements and the repetitive movements at the hip and shoulder that occur when walking.

The Putamen circuit

The putamen circuit is involved in organising learned patterns of movement such as writing, riding a bike and virtually any other skilled movement, most of which are performed subconsciously.

It is not well known how the putamen circuit functions to help execute patterns of movements. However when a portion of it is damaged certain patterns of movement become very abnormal.• Globus pallidus – Athetosis (writhing)• Subthalamus – Hemiballismus (sudden flail)• Putamen – Chorea (flicking movements)• Substantia nigra – Parkinson’s Disease

The Caudate circuit

Most motor actions occur as a result of conscious thoughts generated in the mind, a process called cognitive control of motor activity. The caudate circuit plays a major role in this cognitive control.

It is this circuit that enables us to achieve specific conscious movement goals.

NeurotransmittersIt should be noted that in general terms GABA always functions as an inhibitory agent, thus making all of these loops negative feedback loops.

Dopamine is also an inhibitory neurotransmitter in most areas of the brain , so it too functions as a stabiliser under some conditions.

In summary the basal ganglia have two vital functions:

• To help the cortex execute subconscious but learned patterns of movment

• To help plan multiple parallel and sequential patterns of movement that the mind must put together to accomplish a purposeful task.

The cerebellum is also involved in normal motor control, its role is to control the timing of motor activities and ensure rapid, smooth progression of one muscle movement to the next. It also helps to control the intensity of muscle contraction when the muscle load changes as well as the instantaneous interplay between agonist and antagonist muscle groups.