“Movement is magical”, said my supervisor recently. Indeed, daily movements like walking, typing on a keyboard, or speaking feel smooth and effortless, but behind this seeming effortlessness lies an extended and complex machinery that encompasses many brain areas. Another thing is that some of those areas are activated by me (or at least it feels so) - I want to type that letter, and I do. Some parts of this machinery are known, while some remain a mystery. This time, I read a book, “The Brain in Motion,” by Sten Grillner, which describes recent advances in our understanding of the motor systems of animals.

Several parts of the nervous system work together to produce movements.

Spinal Cord and Midbrain/Brainstem.

The spinal cord is a collection of neurons and neuronal tracts that are located inside the spine. The spinal cord enters the cranium and merges into the brainstem. The brainstem consists of three parts (from bottom to top): the medulla oblongata, the pons, and the midbrain. Midbrain/Brainstem/Spinal cord contain neurons and neuronal circuits mostly responsible for innate standard movement patterns, such as respiration, walking, and chewing. Despite being innate, these circuits are quite flexible. Despite being standard, these circuits can bring about what appears to be cognitive behavior: freeze and escape responses, and maternal caring. Impressive fact: several times per hour, mammals, including humans, make a long and deep inhale without awareness. This deep inhale is needed to expand the lungs and ensure that all alveoli are expanded.

Tectum and Superior Colliculus (SC).

These brain regions are adjacent to the midbrain. Both are present in many animals and seem to be evolutionarily relevant. Tectum/SC function is to detect stimuli that are important, for example, an approaching threat. SC contains a 3D map of space, such that when a threat is detected, the next movement will follow in an appropriate manner. The neuronal circuits there may generate eye movements (saccades), head-neck turns, escape movements, or defense. Impressive fact: in SC of larvae, single neurons understand multi-sensory stimuli; if visual and electrosensory information is coming from the same point in space, these signals reinforce each other; when the same information is coming from different points in space, these signals cancel each other. 

Basal Ganglia.

The basal ganglia are a set of structures containing grey matter (neurons) and localized in the middle of the brain, on top of the brainstem. The main function of the basal ganglia is movement selection and action gating (influencing whether movements are facilitated or suppressed). All the motor centers that I have described so far are under modulatory inhibitory control from the basal ganglia, meaning they will be activated most exclusively in the case that the basal ganglia “allows”. The basal ganglia also function: 1) to select appropriate motor programs; 2) to control the amplitude of selected movement; 3) to learn new motor sequences. A part of the basal ganglia is subtantia nigra, the brain's main storage place for dopamine. Neurons in the substantia nigra deteriorate in Parkinson’s disease, which leads to motor deficits. Impressive fact: a single dopaminergic neuron in human substantia nigra may modulate a million synapses (there are nearly a quadrillion (10^15) synapses in the whole human brain).

Cerebellum.

The cerebellum is a brain region localized behind the brainstem and underneath the occipital lobe of the neocortex. The cerebellum receives motor-related signals from all structures involved in movement. These signals are processed in a vast number of cerebellar neurons. The information from the cerebellum is sent back to the motor system. The main goal of cerebellar processing is to make movements more precise. For example, if you start wearing new glasses, the cerebellum will adjust its circuits to make eye movements precise in these new settings. Impressive fact: there are more neurons in the cerebellum than in the rest of the brain combined.

Neocortex.

There are several movement areas in the cortex. The primary motor cortex directly drives voluntary movements. The Supplementary Motor Area (SMA) is crucial for planning movements. The premotor cortex serves the function of preparing and selecting movements. All these areas work in tight coordination with sensory areas, such as the visual cortex, to make the movement appropriate in the environment. Additionally to body movement areas, there are specialized areas for eye movement (frontal eye fields; FEF) and speech production area (Broca’s area). Impressive fact: mid-century experiments with cats showed that decorticated cats (who had their cortex removed) were able to carry on living, moving, and even mating and parenting. However, it seemed that their cognitive abilities were reduced as they were disinterested in their environment.

The book contains a good description of what is known about the motor system of animals, including humans. It’s quite impressive how many neural circuits are known down to very small details, such as neuron types and neurotransmitters involved. The author concludes with gaps in knowledge, one of which is of interest to me: voluntary movements. The author admits that we don’t know much about voluntary movements; in particular, about the subjective sense of volition and its neural basis. The book sometimes gets quite technical, which is unavoidable for an all-encompassing volume. I personally will keep this book on my desk, as it contains many useful figures and covers many important structures and their functions.

Favorite quote:

“with each month, the baby will successfully be able to balance the head, to sit and stand without support, to crawl, and finally to walk a few steps… this is essentially a biological maturation process; the child is not learning to walk“

April, 2026