This week we discussed about the Motor System – which are chapter 13 (Spinal Control of Movement) and 14 (Brain Control of Movement) of the book Neurocience: Exploring the Brain. Not without going back to chapter 10 (The Central Visual System) to explain something that had been left unsaid.
The motor system consists of all our muscles and the neurons that control them. The spinal cord contains specific motor “programs” for generating coordinated movements, being these programs accessed, executed and modified by descending commands from the brain. Motor control can be divided in two parts: the spinal cord’s command and control of coordinated muscle contractions and the brain’s command and control of the motor programs in the spinal cord.
The cells of skeletal muscle – which constitutes the bulk of the muscle mass of the body and function to move bones around joints, for example –, muscle fibers, are each innervated by a single axon. The ventral horn of the spinal cord contains lower motor neurons, that innervate these skeletal muscle fibers, and which are divided into two categories: alpha motor neurons and gamma motor neurons. The first ones trigger, directly, the generation of force by muscles. Muscle contraction results from the individual and combined action of motor units – which are an alpha motor neuron and all the muscle fibers it innervates.
If alpha motor neurons excite skeletal muscles, we need to understand what regulates motor neurons in order to understand the control of muscles. An interesting experiment to understand more about the innervation and the types of motor units (slow and fast) is one where you force slow motor units to innervate a fast muscle. What happens to the muscle? It will switch to assume slow properties; the types of proteins expressed by the muscle were altered by the new innervation.
The spinal cord contains an intricate network of circuits for the control of movement; it is far more than just a conduit for somatic sensory and motor information.
In this part of the lecture, we learned about movement and its spinal control from different points of view (biochemistry, genetics, biophysics, behaviour, etc.). And this knowledge, derived from every approach, made us achieve the most complete understanding of the topic, leaving us still with some questions that should be answered in the next part of the lecture.
How does the brain influence the activity of the spinal cord? First, it communicates with the spinal cord through axons that descend from the brain along two major groups of pathways: lateral pathways (2 tracts) – controlling voluntary movements of the distal musculature – and ventromedial pathways (4 tracts) – controlling postural muscles.
Several experiments were made on monkeys to prove the relation with the pathways and muscles, among other things. Next, this week we leave an interesting note on behavioural neurophysiology, taken from the book that we are following in this course.