Motor control includes conscious voluntary movements, subconscious muscle memory and involuntary reflexes,[1] as well as instinctual taxes.
To control movement, the nervous system must integrate multimodal sensory information (both from the external world as well as proprioception) and elicit the necessary signals to recruit muscles to carry out a goal.
This pathway spans many disciplines, including multisensory integration, signal processing, coordination, biomechanics, and cognition,[2][3] and the computational challenges are often discussed under the term sensorimotor control.
Some researchers (mostly neuroscientists studying movement, such as Daniel Wolpert and Randy Flanagan) argue that motor control is the reason brains exist at all.
[5] All movements, e.g. touching your nose, require motor neurons to fire action potentials that results in contraction of muscles.
[8] For tasks requiring small forces, such as continual adjustment of posture, motor units with fewer muscle fibers that are slowly-contracting, but less fatigueable, are used.
[9] All organisms face the computational challenges above, so neural circuits for motor control have been studied in humans, monkeys,[10] horses, cats,[11] mice,[12] fish[13] lamprey,[14] flies,[15] locusts,[16] and nematodes,[17] among many others.
Here, larval and adult fish have been useful in discovering the functional logic of the local spinal circuits that coordinate motor neuron activity.
Further research has provided evidence that these stages do exist, but that the response selection period of any reaction time increases as the number of available choices grows, a relationship known as Hick's law.
The system has no compensatory capability.” Some movements, however, occur too quickly to integrate sensory information, and instead must rely on feed forward control.
Reflexes are typically characterized as automatic and fixed motor responses, and they occur on a much faster time scale than what is possible for reactions that depend on perceptual processing.
[27] Reflexes play a fundamental role in stabilizing the motor system, providing almost immediate compensation for small perturbations and maintaining fixed execution patterns.
Some reflex loops are routed solely through the spinal cord without receiving input from the brain, and thus do not require attention or conscious control.
Others involve lower brain areas and can be influenced by prior instructions or intentions, but they remain independent of perceptual processing and online control.
These loops may include cortical regions of the brain as well, and are thus slower than their monosynaptic counterparts due to the greater travel time.
These compensatory actions are reflex-like in that they occur faster than perceptual processing would seem to allow, yet they are only present in expert performance, not in novices.
Coordinating the numerous degrees of freedom in the body is a challenging problem, both because of the tremendous complexity of the motor system, as well as the different levels at which this organization can occur (neural, muscular, kinematic, spatial, etc.).
Motor programs are executed in an open-loop manner, although sensory information is most likely used to sense the current state of the organism and determine the appropriate goals.
He observed that the redundancy of the motor system made it possible to execute actions and movements in a multitude of different ways while achieving equivalent outcomes.
Perception is extremely important in motor control because it carries the relevant information about objects, environments and bodies which is used in organizing and executing actions and movements.
[44] Many models of the perceptual system assume indirect perception, or the notion that the world that gets perceived is not identical to the actual environment.
Forward models structure action by determining how the forces, velocities, and positions of motor components affect changes in the environment and in the individual.
In the reaching task mentioned above, the persistence of bell-shaped velocity profiles and smooth, straight hand trajectories provides evidence for the existence of such plans.
[48] Under this understanding of behavior, actions unfold as the natural consequence of the interaction between the organisms and the available information about the environment, which specified in body-relevant variables.
There are several mathematical models that describe how the central nervous system (CNS) derives reaching movements of limbs and eyes.
Hence, the minimum torque-change model was introduced as an alternative, where the CNS minimizes the joint torque change over the time of reaching.
[54] Later it was argued that there is no clear explanation about how could the CNS actually estimate complex quantities such as jerk or torque change and then integrate them over the duration of a trajectory.
In response, model based on signal-dependent noise was proposed instead, which states that the CNS selects a trajectory by minimizing the variance of the final position of the limb endpoint.
[9] Another type of models is based on cost-benefit trade-off, where the objective function includes metabolic cost of movement and a subjective reward related to reaching the target accurately.
Longer reaching distances have a similar effect, since more error is accumulated in the initial sub-movement and thus requiring more complex final correction.