How do we move?
What's actually going on in the brain and body, to allow us to move, like in reaching for a coffee? Trainee doctor Isabelle Cochrane has been brewing up an answer...
Movements are generated when a type of nerve, called a motor neurone, sends a chemical signal directly to a muscle, causing it to contract. This can occur in response to a sensory stimulus from the environment: contact with a hot surface, or a stinging nettle, for instance, causes a rapid withdrawal reflex!. Movements are also voluntary such as walking, or using tools, to do things like typing, or throwing a ball..
The parts of the nervous system that generate movements are collectively called the motor system. Over millennia, these have evolved to operate automatically. This means that most of our movements are not under direct conscious control: even though the decision to carry out a particular movement might be conscious, we do not have to think consciously about the sequence of nerve signals, muscle contractions, and joint movements that are going to be needed to carry out an action successfully. Instead, these calculations are carried out at various levels in the motor system.
So let’s start by looking at withdrawal reflexes.
These occur when a painful or unpleasant stimulus is experienced, for instance touching a piping hot oven dish. The result of this stimulus will be something we have all experienced: an almost instantaneous removal of the affected digit from the offending item. When researchers first studied this historically they realised that this reaction occurs far too rapidly for the movement commands to be going via the brain. Instead, sensory neurones that detect painful stimuli are linked, via short spinal nerve cells called interneurones, straight to the motor neurones responsible for withdrawing a limb from the noxious stimulus. In other words, the machinery of the spinal cord is sufficient to generate one of the less complex goal-directed movements. Of course, we know the story is not so simple: for instance, no matter how painful it is, we can resist dropping our favourite mug full of steaming coffee. Many of our spinal reflexes are ultimately still under the control of the brain, which can refuse permission for a particular action to take place.
So is the spinal cord able to generate movement other than reflexes? In vertebrates, it would appear that the answer is generally “yes” - the spine contains networks of neurones known as ‘central pattern generators’. These fire rhythmically to produce stereotyped movements, such as swimming, scratching and walking. However, the story is less clear cut in man and scientists aren’t sure whether there are central pattern generators in the adult human spinal cord, or whether these circuits are located elsewhere in the nervous system, such as the brain.
The part of the brain that is responsible for producing movement commands is called the primary motor cortex; this is a strip of brain tissue which sits roughly beneath where your headphones rest on your head. The primary motor cortex is organized like a map of the body, with different regions of the cortex responsible for the muscles of a particular part of the body. This part of the brain – and the bundles of nerve fibres that flow from it - are commonly affected by strokes, which is why one of the most prominent symptoms of a stroke is paralysis of the arm, leg or face - and based on which of these structures is paralysed, one can predict quite accurately which part of the motor cortex has been affected.
The nerve fibres that flow away from the motor cortex form two bundles that run down the front of the spinal cord and are called the corticospinal tracts; the nerves exit these bundles at the right locations along the spinal cord to connect to and activate target motor neurones and intermediate “interneurones” that control muscles.
So how does the motor cortex actually make a movement happen? At a simple level, a nerve cell in the motor cortex becomes active and fires a barrage of impulses along its fibre down the corticospinal tract. These impulses are transmitted to and activate a select group of motorneurones and interneurones in the part of the spinal cord that controls the relevant part of the body you want to move. When the motorneurones fire up, the muscles they activate contract, and you move.
In reality it is, of course, more complex than this. The motor cortex tends to be organised in terms of ‘motor synergies’. This means that the brain contains libraries of various useful movement combinations - for example, activating the muscles that stretch the elbow and the muscles that stretch the wrist simultaneously, in order to reach out and grab an object. Sensory feedback during a movement can help to modify these synergies, both at the time of the action, in order to refine the movement, but also in the future, enabling a learning process by which this ‘library’ can be expanded.
You might wonder, though, what controls the motor cortex? That role – in planning and refining movements before they happen falls to structures that sit just in front of the motor cortex, just behind your forehead. These are the supplementary motor area and pre-motor cortex. They are richly connected with the front parts of the brain that are involved in decision-making, as well as with other brain structures such as the cerebellum, which helps with the coordination of movement and in learning new movements, and the basal ganglia, which are involved in delivering the “go signal” that kicks off a movement in the first place.
So next time you reach for your cup of coffee, spare a thought also for the millions of neurons buzzing away in your motor cortex, cerebellum and spinal cord to ensure that the coffee ends up where it should and when it should!