The motor system is the part of the central nervous system that is responsible for movement.
Much of the brain and nervous system are dedicated to the processing of sensory information, to build detailed representations of the external environment.
Through vision, hearing, touch and other senses, we perceive the world and interact with it. However, all this processing would have very little value if we did not have an effective way of acting on it.
In some cases, the relationship between sensory input and motor output is simple and straightforward; for example, touching a hot stove causes immediate hand removal. But usually, our actions are conscious and require not only sensory information but also a large number of various cognitive processes that allow us to choose the most appropriate motor production at all times. In any case, the final movement is a set of orders for certain muscles in the body to move in a certain way.
Motor behavior is one of the most important means of expression of people. All behavior, whether conscious or unconscious, is based on a set of muscle contractions orchestrated by the brain and spinal cord.
Characteristics of the motor system
The motor system is characterized by receiving constant sensory information and presenting a double organization: hierarchical and in parallel.
Our motor system can make three types of movement:
Voluntary movement (reading, playing the piano, etc.):
- Movements aimed at a specific motive or purpose.
- Its execution improves with practice.
- They can occur in response to an external stimulus, or not.
Reflex responses (hand removal when touching a burning cup):
- Rapid stereotyped and involuntary responses to eliciting stimuli.
Rhythmic motor patterns (walking, running, chewing, etc.):
- Combination of voluntary acts and reflexes.
- Normally the beginning and end of these movements are voluntary, but once initiated the movement continues in a more or less stereotyped manner.
The motor system receives constant sensory information
The operation of the motor system is closely related to the operation of the sensory systems.
Vision, hearing and receptors located on the body surface report the situation of objects in space and of our body regarding these objects. The proprioceptors of the musculature and joints, and the vestibular system report the length and tension of the muscles and the position of the body in space. The motor system uses this information to select the appropriate response (plan the movement) and to make the necessary adjustments while performing the movement (refine the movement).
The motor system needs to receive sensory information to plan and refine the movements that are carried out.
When we want to take an object by hand, the motor system uses the information provided by the sensory systems to correct, if necessary, the marked path (feedback or feedback processes). Sometimes it is more effective to use ante-feeding mechanisms. For example, when we want to catch a ball that has been thrown at us, we have to predict the trajectory it will follow in order to place the hands correctly. In this case, the feedback system must interpret the visual cues correctly to be able to tighten the muscles in anticipation of the impact of the ball.
Double organization of the motor system: hierarchical and in parallel
Hierarchical organization: the motor system is made up of different components related by tracks that follow a downward path. All movements are produced by motor neurons of the marrow and brain stem They innervate the muscles. These motor neurons are controlled and coordinated by the brain, by neurons of the cerebral cortex and the brain stem.
We found three main levels of motor control: spinal cord, brain stem and cerebral cortex.
Primary motor neurons or alpha type of the spinal cord and brain stem:
After the spinal cord is disconnected from the upper centers, proper stimulation can produce reflex motor responses.
- They occupy the lower level of the hierarchy of the motor system.
- On these converge all the motor orders of the upper levels.
- They send their axons out of the CNS to innervate the skeletal muscle fibers. They also synapt with interneurons.
- They have autonomy to make automatic stereotyped movements (reflex responses).
- It constitutes an intermediate level in the hierarchy of the motor system.
- In different nuclei of the brain stem originate descending pathways to the spinal cord.
- It is the upper level of the motor hierarchy.
- It includes the association areas of the parietal and prefrontal cortex and the actual motor areas (the premotor and primary motor areas).
- It is responsible for planning, initiating and directing voluntary movements.
- The cerebral cortex exerts this influence directly by means of projections on the medulla, and indirectly by projections in centers of the brain stem that project to the spinal cord.
The descending motor pathways originating in the cortex and brain stem are essential for the control of voluntary movements and constitute the link between thoughts and actions.
Parallel organization: From the upper levels of the motor hierarchy the orders reach the lower levels directly through the brain stem. This fact shows that the motor systems are not only organized in series, but also in parallel. Serial and parallel processing of the descending motorways provides greater processing and adaptation capacity in motor control.
The basal ganglia and the cerebellum
As we have mentioned, there are three levels related to motor control: the motor neurons of the marrow and the brainstem, the brainstem and the cerebral cortex. It is worth noting the existence of two other subsystems related to motor control:
- Basal ganglia
- The cerebellum
These systems do not have direct access to alpha motor neurons, but regulate the activity of motor neurons that give rise to the descending pathways.
One of the main functions is to correct the errors in the movement by comparing the motor orders produced in the cortex and the brainstem with the sensory feedback on the movements that are actually occurring.
The importance of the basal ganglia in the movement is evident when observing the motor alterations that accompany the dysfunctions of the basal ganglia, the Parkinson's disease and Huntington's disease.