At the lowest extreme of the motor hierarchy, in the spinal cord, hundreds of specialised nerve cells called motor neurons increase their rate of firing. The axons of these neurons project out to the muscles where they activate contractile muscle fibres. The terminal branches of the axons of each motor neuron form specialised neuromuscular junctions on to a limited number of muscle fibres within one muscle. Each action potential in a motor neuron causes the release of neurotransmitter from nerve endings and generates a corresponding action potential in the muscle fibres. This causes Ca2+ions to be released from intracellular stores inside each muscle fibre. This in turn triggers contraction of the muscle fibres, producing force and movement.
The electrical events in the muscles of the arm can be recorded with an amplifier, even through the skin, and these electro-myographic recordings (EMGs)can be used to measure the level of activity in each muscle.
The spinal cord plays an important part in the control of the muscles through several different reflex pathways. Among these are the withdrawal reflexes that protect you from sharp or hot objects, and the stretch reflexes that have a role in posture. The well-known ‘knee-jerk’ reflex is an example of a stretch reflex that is rather special because it involves only two types of nerve cell – sensory neurons that signal muscle length, connected through synapses to motor neurons that cause the movement. These reflexes combine together with more complex ones, in spinal circuits that organise more or less complete behaviours, such as the rhythmic movement of the limbs when walking or running. These involve coordinated excitation and inhibition of motor neurons.
Motor neurons are the final common path to the muscles that move your bones. However, the brain has a major problem controlling the activity of these cells. Which muscles should it move to achieve any particular action, by how much, and in what order
The motor cortex
At the opposite end of the motor hierarchy, in the cerebral cortex, a bewildering number of calculations have to be made by many tens of thousands of cells for each element of movement. These calculations ensure that movements are carried out smoothly and skillfully. In between the cerebral cortex and motor neurons of the spinal cord, critical areas in the brain stem combine information about the limbs and muscles ascending from the spinal cord with descending information from the cerebral cortex.
The motor cortex is a thin strip of tissue running across the surface of the brain, directly in front of the somatosensory cortex. Here is a complete map of the body: nerve cells that cause movements in different limbs (via connections onto the motor neurons in the spinal cord) are topographically arranged. By using a recording electrode, neurons may be found in any part of this map that are active about 100 milliseconds before activity in the appropriate muscles.
Quite what is coded in the motor cortex was the subject of a long debate – do the cells in the cortex code for actions that a person wants to perform or for the individual muscles that must be contracted to perform it. The answer to this question turned out to be somewhat different – individual neurons do not code for either. Instead a population code is used in which actions are specified by the firing of an ensemble of neurons.
Just in front of the motor cortex lie important pre-motor areas that are involved in planning actions, in preparing spinal circuits for movement, and in processes that establish links between seeing movements and understanding gestures. Striking new findings include the discovery of mirror neurons in monkeys that respond both when the monkey sees a hand movement and when the animal performs that same movement. Mirror neurons are likely to be important in imitating and understanding actions.
Behind the motor cortex, in the parietal cortex, a number of different cortical areas are concerned with the spatial representation of the body and of visual and auditory targets around us. They seem to hold a map of where our limbs are, and where interesting targets are with respect to us. Damage to these areas, for example after a stroke, can cause misreaching for objects or even neglect or denial of parts of the world around us. Patients with so-called parietal neglect fail to notice objects (often on their left side) and some even ignore the left side of their own body.
The basal ganglia
The basal ganglia are a cluster of interconnected areas located beneath the cortex in the depths of the cerebral hemispheres. They are crucial in the initiation of movements, though quite how they do this is far from clear. The basal ganglia seem to act rather like a complex filter, selecting information from amongst the enormous numbers of diverse inputs they receive from the anterior half of the cortex (the sensory, motor, prefrontal and limbic regions). The output of the basal ganglia feeds back to the motor cortical areas.
A common human motor disorder, Parkinson’s disease, is characterised by tremor and difficulty in initiating movements. It is as if the selective filter in the basal ganglia is blocked. The problem is the degeneration of neurons in an area of the brain called the substantia nigra (so-called because it is black in appearance) whose long, projecting axons release the neurotransmitter dopamine into the basal ganglia. The precise arrangement of the dopamineaxons onto their target neurons in the basal ganglia is very intricate, suggesting an important interaction between different neurotransmitters. Treatment with the drug L-Dopa, which is converted into dopamine in the brain, restores dopamine levels and restores movement.
The basal ganglia are also thought to be important in learning, allowing the selection of actions that lead to rewards.
The cerebellum is crucial for skillful smooth movements. It is a beautiful neuronal machine whose intricate cellular architecture has been mapped out in great detail. Like the basal ganglia, it is extensively interconnected with the cortical areas concerned with motor control, and also with brain-stem structures. Damage to the cerebellum leads to poorly coordinated movements, loss of balance, slurred speech, and also a number of cognitive difficulties.
Sounds familiar? Alcohol has a powerful effect on the cerebellum.
The cerebellum is also vital for motor learning and adaptation. Almost all voluntary actions rely on fine control of motor circuits, and the cerebellum is important in their optimal adjustment – for example with respect to timing. It has a very regular cortical arrangement and seems to have evolved to bring together vast amounts of information from the sensory systems, the cortical motor areas, the spinal cord and the brainstem.
The acquisition of skilled movements depends on a cellular learning mechanism called long-term depression (LTD), which reduces the strength of some synaptic connections (see chapter on Plasticity). There are a number of theories of cerebellar function; many involve the idea that it generates a “model” of how the motor systems work – a kind of virtual reality simulator of your own body, inside your head. It builds this model using the synaptic plasticity that is embedded into its intricate network. So, catch that ball again, and realise that almost all levels of your motor hierarchy are involved from planning the action in relation to the moving visual target, programming the movements of your limbs, and adjusting the postural reflexes of your arm. At all stages, you would need to integrate sensory information into the stream of signals leading to your muscles.