Mastering motion- reflex, rhythmic and complex movements

Introduction

Movement is arguably the most fundamental aspect of human behaviour and is one of the most obvious features distinguishing plants and animals. The ability to physically respond to stimuli has enhanced our chances of survival an immeasurable amount. 

As such, our body’s ability to move has evolved and refined itself over many millennia, even developing new ways to move that protect us in many ways. For example, involuntary reflexes have reduced the computational demand on our brain to move parts of our body away from hot or painful objects, making the process almost instantaneous. Meanwhile, central pattern generators (CPGs) in our spinal cord have also reduced cognitive load by carrying out subconscious movement. 

This has allowed the motor cortex and cerebellum to focus on planning, coordinating, and refining purposeful movements in response to sensory feedback. 

While movement can be separated into even more categories, understanding the neural pathways of these three types can be beneficial to uncover core concepts of human neurophysiology, and even pave the way for treating movement disabilities. With that said, let’s take a deep dive into the circuitry and principles of reflex, rhythmic, and voluntary movement.

Reflex movements

Reflex movements are rapid, involuntary responses to stimuli that are commonly used to help us avoid danger or harm. An example includes touching a hot object and immediately jerking our hand away from it. The goal of this form of movement is to be as quick as possible in order to avoid injury. As such the neural pathway, known as a reflex arc, is simple and can take as few as three neurons. 

Firstly, sensory receptors detect a stimulus, such as heat, and send a signal up towards the central nervous system (CNS) through sensory neurons. Instead of going up to the brain for processing and movement planning, the sensory neuron connects with a relay neuron in the spinal cord, and then to motor neurons. This reduces the time taken to respond as it bypasses the brain’s processing circuitry. Motor neurons then carry a signal to relevant muscles to contract and move the body away from danger. 

Because the signal from the sensory receptors bypasses the brain, this movement is subconscious, meaning it happens without consciously deciding to move. This makes the movement rapid and stereotyped – the motion is predictable as there is minimal planning; just a need to move anywhere away from the stimulus.

Central Pattern Generators (CPGs)

CPGs are networks of neurons in the spinal cord that, when activated, produce rhythmic pattern-like movement such as walking or running. This type of movement is also subconscious as it does not require active focus to perform. However, unlike reflex movements, CPG output does not require sensory activation or feedback. Instead CPGs are activated by descending pathways from the medulla – a region of the brainstem that is responsible for performing involuntary movement. 

CPGs typically control movements that are necessary for survival such as breathing and heartbeats. The lack of need to consciously focus on these movements allows us to instead direct our attention to more complex situations, such as responding to stimuli or achieving a specific goal. This is where voluntary movements are required. 

Voluntary movements

Any movement performed via conscious decision-making requires activity from a range of areas in the brain. To respond to our environment, we firstly need information on what is around us. This is largely handled by the frontal lobe which perceives our external environment through sensory input and attention. Human fMRI studies have highlighted increased activity in the frontal lobe as we switch our attention, thus perceiving different parts of our external environment. This information of our environment is sent to the motor cortex which plans our next movement. Complex multi-limb movements may require additional processing from premotor and association areas. Once the movement has been planned, it then has to pass through the cerebellum, which refines specific parts of the movement, such as precise finger motion. After refinement, the movement signal is then sent to relevant muscles via motor neurons to carry out the intended movement.

An example of a complex movement is reaching out and grabbing an object. This seemingly simple task requires coordinated movement of the hand, arm, shoulder, and torso to ensure we move our arm the right amount – not too far so that we go past the object, and not too near so that we do not reach it. This also requires great precision to grab the object with appropriate force, to gain a firm grip while ensuring we do not break the object. 

A lot of planning goes into rudimentary movements, and yet sometimes we can still get things wrong. For instance, suppose we couldn’t see the object too well so we end up going too far and missing it. This will be picked up by our sensory organs, giving our brain feedback on what we ended up doing. By comparing the actual movement with our intended movement, we can create an error signal of how far we missed and in what direction. This drives learning – by using our previous errors, we can refine our future movements to eventually achieve our intended goal. In this example, we may learn that we keep extending our arm too far, and so with repetitive trials we eventually move the right amount in order to grab the object, as we intended. The cerebellum is largely seen as responsible for motor learning, however the deep underlying mechanism is still being researched.

When the same complex movement is performed again and again, it can be trained to become subconscious movements activated by spinal CPGs, gradually requiring less coordination from the motor cortex to perform. This is how common movements such as walking, go from being a strenuous task as a toddler to a simple ability requiring minimal focus as an adult. 

Conclusion

Overall, we can see a general trend of movements requiring more parts of the CNS as they become more complex. Precise, unfamiliar movements requiring multiple limbs are the most complex, thus recruiting decision-making and motor coordination areas in order to perform.

By repeating an action again and again, we can train ourselves to perform it with less and less input from higher brain regions, until it eventually becomes a subconscious coordinated act that can be performed on demand. 

Written by Ramim Rahman

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