The most complex movements that we can perform, include voluntary movements that require conscious planning and involve control of these basic spinal mechanisms by the brain. At present, we are only beginning to understand the complex interactions between brain regions that occur when voluntary movements are being performed.

The Motor Cortex

One important brain area in control of voluntary movements is the motor cortex, which exerts powerful control over the spinal cord, partly through direct control of its alpha motor neurons. The neurons in this motor cortex can be divided in two groups based on their function:

• Neurons that appear to specify the coordinated action of many muscles, producing an organized movement of a limb.
• Neurons that control only two or three related muscles, such as those of the hand, to perform finely tuned, skilled movements.

Other Brain Regions

In addition to the motor cortex, several other brain regions, such as the basal ganglia, thalamus, cerebellum and a large number of neuron groups located in the midbrain and brainstem, are involved in the control of voluntary movements. The brain regions devoted to such control are often relatively large, containing millions of intricately interconnected neurons.

The Cerebellum

The cerebellum receives direct and powerful sensory information from the muscle receptors and the sense organs of the inner ear, which signal head position and movement, and signals from the cerebral cortex. Apparently, the cerebellum acts as an integrative structure for all this information to ensure smooth muscle coordination and action, enabling us to perform skilled movements more or less automatic.

Evidence suggests that, when we learn to walk, speak, or play a musical instrument, the necessary, detailed control information is stored within the cerebellum. From here, it can be called upon by commands from the cerebral cortex.

Disturbances in the Brain

The basal ganglia have widespread connections with sensory and motor cortex areas. Dopamine depletion can lead to a loss of regulation of the basal ganglia, which can cause serious movement disorders, such as Parkinson’s disease. A major factor of this disease is the loss of dopamine neurons in the substantia nigra of the midbrain.

The cerebellum is also crucial for skilled movements and for the learning of new movements. A disturbance of cerebellar function can, for example, lead to poor coordination of muscle control, disorders of balance and even difficulties in speech, which represents one of the most intricate forms of movement control.