The motor system is the part of the central nervous system that is in charge of the movement.
Much of the brain and nervous system is dedicated to processing sensory information to construct detailed representations of the external environment.
Through vision, hearing, touch and other senses, we perceive the world and relate to it. However, all this processing would be of little value if we did not have an effective way of acting upon it.
In some cases, the relationship between sensory input and motor output is simple and direct; for example, touching a hot stove causes an immediate withdrawal of the hand.
But, in general, our actions are conscious and require not only sensory information but also a large number of different cognitive processes that allow us to choose the most appropriate motor production at any given moment. In any case, the final movement is a set of commands for certain muscles in the body to move in a certain way.
Motor behaviour is one of people’s most important means of expression. All behaviour, whether conscious or unconscious, is based on a set of muscular 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 parallel.
Our motor system can make three types of movement:
- Voluntary movement (reading, playing the piano, etc.):
- Movements directed to a specific motive or purpose.
- Its execution improves with practice.
- They can be produced as a response to an external stimulus, or not.
Reflex responses (withdrawal of the hand 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 the end of these movements are voluntary, but once initiated the movement continues in a more or less stereotyped way.
The motor system receives constant sensory information
The functioning of the motor system is closely related to the functioning of sensory systems.
Vision, hearing and receptors on the body surface inform the situation of objects in space and of our body with respect to 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 the movement is being performed (refine the movement).
The motor system needs to receive sensory information to plan and refine the movements being performed.
When we want to catch an object by hand, the motor system uses the information provided by the sensory systems to correct, if necessary, the marked trajectory (feedback processes).
Sometimes it is more effective to use anterofeedback 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 our hands correctly. In this case, the anterofeeding system must interpret the visual signals correctly in order to tighten the musculature in anticipation of the impact of the ball.
Double organization of the motor system: hierarchical and parallel
Hierarchical organization: the motor system is made up of different components related by tracks that follow a downward trajectory. All movements are produced by motor neurons of the medulla and brainstem that innervate the muscles. These motor neurons are controlled and coordinated by the brain, by neurons of the cerebral cortex and the brainstem.
We find three main levels of motor control: spinal cord, brain stem and cerebral cortex.
Primary motor neurons or alpha type of the medulla and brainstem:
After the spinal cord is disconnected from the upper centers, appropriate stimulation can produce reflex motor responses.
- They occupy the lower level of the motor system hierarchy.
- On these converge all the motor commands of the upper levels.
- They send their axons out of the CNS to innervate the fibers of skeletal musculature. They also synapse 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 brainstem they originate descending pathways towards the spinal cord.
- It’s the top level of the motor hierarchy.
- It includes the association areas of the parietal and prefrontal cortex and the motor areas (the premotor and primary motor areas).
- She is responsible for planning, initiating and directing voluntary movements.
- The cerebral cortex exerts this influence directly through projections on the spinal cord, and indirectly through projections on brain stem centers that project to the spinal cord.
The descending motorways originating in the cortex and brainstem 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 commands reach the lower levels directly through the brainstem.
This shows that motor systems are not only organized in series, but also in parallel. Serial and parallel processing of the downhill motorways provides greater processing and adaptation capacity in the motor control.