The sensorymotor system is responsible for all voluntary body movements. It activates a broad combination of cortical and subcortical structures in the brain through a tightly orchestrated interaction between voluntary thoughts and existing knowledge. Once the ascending input from the sensory system is processed and analyzed, the sensorymotor system issues commands that descend through neural pathways to the appropriate areas for action.
Every motor movement begins with the activation of the sensory system as it provides information about the surrounding environment. Information is first received through the sensory receptors for all five of our senses. This information ascends through appropriate pathways to a corresponding nucleus of the thalamus (except for olfaction, which is the only sensory system whose major sensory pathway reaches the cerebral cortex without first passing through the thalamus). The thalamus relays the information from the receptors to the corresponding primary sensory cortex, which processes and analyzes the input. The signals from the thalamus are often relayed to a secondary sensory cortex as well - if it serves a parallel function to the primary sensory cortex. For example, in the case of a sound stimulus, the thalamus would relay the information to the primary auditory cortex for processing the input, while simultaneously sending the information to a secondary visual cortex for locating the sound, and to the prefrontal cortex for identifying what the sound is. This combination of segmented information would then be forwarded to corresponding association cortices to integrate it into usable feedback for an appropriate cognitive reaction.
This particular association cortex plays an important role for integrating the information of the position of the parts of the body that are to be moved as well as the position of any external objects with which the body is going to interact. In order to direct behaviour and attention, this cortex receives substantial information from more than one sensory system: the visual system (for localization of body and external objects in space), the auditory system, and the somatosensory system.
Various brain imaging studies have shown that the posterior parietal cortex comprises a mosaic of small areas, each specialized for guiding particular movements of the eyes, head, arms, or hands.
Much of the output of the posterior parietal cortex goes to areas of motor cortex: the dorsolateral prefrontal association cortex, various areas of the secondary motor cortex, and to the frontal eye field (an area that controls eye movements).
The dorsolateral prefrontal cortex receives information from the posterior parietal cortex, and it plays an important role in the evaluation of external stimuli and the initiation of voluntary reactions to them. The properties of the neurons in this cortex suggest that decisions to initiate voluntary movements are made in this area, but these decisions depend on important interactions with the posterior parietal cortex. The activity of some neurons depends on the characteristics of objects; the activity of others depends on the locations of objects; and the activity of still others depends on a combination of both. The activity of other dorsolateral prefrontal neurons is related to the response, rather than to the object.
Once these reactions have been initiated, the projections are relayed to areas of the secondary motor cortex, to the primary motor cortex, and to the frontal eye field, as mentioned in the previous section.
Areas of the secondary motor cortex receive much of their input from the posterior parietal cortex and the dorsolateral prefrontal cortex, and send much of their output to primary motor cortex. The secondary motor cortex consists of four principal areas in each hemisphere, each with their own subdivisions: the supplementary motor area, the premotor cortex, and two cingulate motor areas.
In general, areas of secondary motor cortex are thought to be involved in the programming of specific patterns of movements after taking general instructions from dorsolateral prefrontal cortex. Evidence of such a function comes from brain-imaging studies in which the patterns of activity in the brain have been measured while the subject is either imagining his or her own performance of a particular series of movements or planning the performance of the same movements.
The primary motor cortex is the major point at which sensorimotor signals from the cerebral cortex converge. The conventional view of the primary motor cortex is that it is organized somatotopically (according to a map of the body) as is commonly described with this illustration. As for the neuronal activity in the cortex, it was thought that each neuron encodes a different preferred direction of movement. However, the conventional view that many of these neurons are tuned to movement in a particular direction has been challenged. New studies suggest that, rather than direction, primary motor cortex neurons are sensitive to a particular target location. The importance of the target of a movement, rather than the direction of a movement, is that:
The primary motor cortex can produce innumerable patterns of muscle contraction required to get a body part from any starting point to a target location. The key point is that the route that neural signals follow from a given area of primary motor cortex is extremely plastic and is presumably determined at any point in time by somatosensory feedback.
The cerebellum and the basal ganglia are both important sensorimotor structures, but neither is a major part of the pathway by which signals descend through the sensorimotor hierarchy. Instead, both the cerebellum and the basal ganglia interact with different levels of the sensorimotor hierarchy and, in so doing, coordinate and modulate its activities.
Although the cerebellum constitutes only 10% of the mass of the brain, it contains more than half of the brain’s neurons. The cerebellum receives information from primary and secondary motor cortex, information about descending motor signals from brain stem motor nuclei, and feedback from motor responses via the somatosensory and vestibular systems. The cerebellum is thought to compare these three sources of input and correct ongoing movements that deviate from their intended course. By performing this function, it is believed to play a major role in motor learning
On the other hand, the basal ganglia do not contain as many neurons as the cerebellum, but in one sense they are more complex. The basal ganglia are a complex diverse collection of interconnected nuclei and their anatomy suggests that, like the cerebellum, they perform a modulatory function. They contribute few fibers to descending motor pathways; instead, they are part of neural loops that receive cortical input from various cortical areas and transmit it back to the cortex via the thalamus.