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Sunday, December 4, 2022
Dr. Brown: The brain’s motor system and how we move

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series at the Niagara-on-the-Lake Public Library. ____________

Toward the end of his BBC series “The Ascent of Man,” Jacob Bronowski turned his attention to the brain as the biological engine that made possible all of humanity’s achievements in the arts and sciences.

In one segment he uses the early development of the motor system in infancy to illustrate the unfolding march from birth (a few brainstem and spinal reflexes and total dependence on others for nourishment, warmth and protection), to later holding the head up, rolling over, crawling, sitting up, using a nearby prop to shakily stand, soon make the first wobbly steps and, after months of babbling, the first words emerge.

The biological choreography underpinning of such a developmental sequence is exquisite and accompanied by enormous unseen, unfolding changes in the brain as nerve cells specialize, multiply, migrate and make near and distant connections. And along the way to maturity the planning, imagining, creative, moral, language-speaking, story-telling and mystical parts of the nervous system begin to take hold and shape what we attend to then and for the rest of our lives.

The forebrain plays a large part in the skill with which we move. Without it, no dancing, no playing musical instruments, no writing or even something as simple as tapping the desk in front of me with my finger, are possible.

All depend on closely related regions of the brain working together, including the forebrain, primary and secondary associative cortex in the frontal and parietal lobes, the basal ganglia and cerebellum, whose relationships with one another are extraordinarily complex and poorly understood as yet.

But whatever our limits, deep brain stimulation (DBS) has proven to be an effective way to stop drug-induced or resistant movement and postures in Parkinson’s disease and the sometimes very disabling tremor in selected cases of benign essential tremor.

What I intend to concentrate on in the NOTL Library's brain series is much simpler – the upper motor neurons (UMNs) in the primary motor cortex (area 4) and the closely related lower motor neurons (LMNs), which innervate their target muscle fibres and muscles to make happen what was intended by the forebrain and other regions of the brain. Let’s begin with the LMNs.

LMNs, together with the muscle fibres they innervate, constitute the basic unit of the motor system – the motor unit (MU), a term coined by the father of modern neurophysiology, Sir Charles Sherrington. The neurons of which are found in the brain stem and spinal cord and supply all the somatic muscles of the body beginning at the rostral end with the neurons that control eye movements through to those neurons at the lower end of the spinal cord which innervate the pelvic floor muscles. MUs differ widely in their properties.

Those recruited with minimal effort (think here of a leisurely walk) are the most common, relatively small (fewest number of muscle fibres per motor neuron), generate the smallest forces and resist fatigue. Mid-range MUs are less common, harder to recruit, innervate more muscle fibres per motor neuron, generate larger forces and like the lowest threshold MUs, resist fatigue (think here of a brisk walk or climbing a hill).

The largest, least common MUs supply the largest number of muscle fibres per motor neuron, have the highest thresholds for recruitment, generate the largest forces but fatigue readily because unlike lower threshold units, they are much more dependent on glycogen for their energy (think here of sprinting).

This overly simplistic picture none-the-less, illustrates that even at the simplest level, the motor system offers a wide range of forces and endurance. MUs are also very trainable – the force generated by individual motor units and muscles and their fatigue resistance can easily be changed by training in experimental animals and humans.

UMNs are located in the primary motor cortex (area 4) but the areas of the neocortex associated with different regions of the body vary widely in size from one region of the body to another. For example, the areas of the motor cortex and thus the number of UMNs, are much larger for the face, tongue, mouth, forearm and hand muscles compared to areas representing the proximal arm, trunk and leg.

This observation and the finding that the hand and forearm areas of the primary motor cortex contain large nerve cells that are directly connected to the related motor neurons in the cervical spinal cord suggests that the primary motor cortex exercises a high degree of direct control over the forearm and hand muscles and might explain the skill with which we use our hands.

Chimpanzees possess few such direct connections, while other primates have none. Whether such direct connections exist for the facial muscles hasn’t been explored but given the large representation of the face in the motor cortex, I wouldn’t be surprised that similar direct connections exist for the oral-facial and tongue muscles given their key roles in speech.

Finally, just as exercise and training alter the physiological properties of muscles and MUs, so do they alter the motor cortex.

For example, training reshapes the contours and thickness of the neocortex in the thumb and finger areas of piano players. And if we can modify the motor areas of the brain with exercise training, no doubt it’s possible to alter other regions of the brain through conditioning – which might explain some of the impact of mindfulness and mediation on the brain.

Long-term changes in the brain in response to training, almost certainly involve strengthening connections between related nerve cells by thickening some connections and growing others. All of which leads to next week when we examine the sensory side of the nervous system.