Motor Control

This lecture had so many details :/

Describe the hierarchical organisation of motor system from a structural and functional perspective

I'm not really sure what we're supposed to know here, so here's a description of the diagram on the "Hierarchical organisation of motor control" slide. Motor areas in the cerebral cortex can send signals to the spinal cord, either directly or via the brainstem. From the spinal cord, motor neurons in the ventral horn can innervate muscles, causing movement. There are also accessory areas, such as the thalamus, basal ganglia, and cerebellum, which play roles in motor control.

There is also some kind of hierarchy of motor control when it comes to voluntary movement. The first step is the "strategy" (figuring out what needs to be done), which is carried out in the prefrontal cortex, posterior parietal cortex, and basal ganglia. The next step are "tactics" (figuring out how to do what needs to be done), which takes place in the pre-motor cortex and supplementary motor area (SMA). The SMA is important in mental rehearsal of actions (i.e. imagining what you need to do without actually doing it). The third and final step is execution (getting stuff done), which uses the primary motor cortex, brain stem, and spinal cord. Once again, other inputs are received by the somatosensory cortex, cerebellum, basal ganglia, and so on.

Describe the similarities and differences between voluntary movements, reflex movements and rhythmic motor patterns

Voluntary movements are, well, voluntary. We choose to do them, and we get better at them as we practice them. Reflex responses are rapid and involuntary, and the amplitude of response may depend on the eliciting stimulus. Rhythmic motor patterns combine aspects of both voluntary and reflex movements- initiation is usually voluntary, but continuation of the movement is usually reflexive. An example of a rhythmic motor pattern is walking- you usually choose to start walking somewhere, but then once you start walking, you can keep walking without having to think about what you're doing.

Describe the physiological properties of motor units and their recruitment during voluntary and reflex movements.

Motor units consist of a motor neuron and all of the muscle fibres that it innervates. The innervation ratio is the number of muscle fibres innervated by a single motor neuron. Different muscles have different innervation ratios: for example, eye muscles have a low innervation ratio for fine control, whereas muscles that need powerful (but not necessarily accurate) movements, such as the gastrocnemius, have a much higher innervation ratio.

Motor units may be made up of predominantly slow-twitch or fast-twitch muscle fibres. (All of the fibre types are described here.) Again, the types of muscle fibres depend on the muscle type. A muscle that plays a more postural role, such as the soleus, has more slow-twitch fibres. A muscle that requires more power, such as the gastrocnemius, has more fast-twitch fibres.

According to Henneman's size principle, smaller motor neurons have the lowest threshold for synaptic activation, so they are recruited first. As smaller motor neurons are more likely to innervate slow-twitch muscle fibres, slow motor units tend to be recruited first. As intensity increases, more fast units will be recruited.

Identify the muscle receptors and explain their role in spinal reflexes.

The main muscle receptors involved in spinal reflexes are muscle spindles (which detect muscle length) and Golgi tendon organs (which, if I remember correctly, detect the amount of force on the muscle). Cutaneous receptors may also be involved in these reflexes, and descending inputs from the brain stem and cortex may also provide input into the reflex response. Reflexes can be classified according to a three-level hierarchy: control of individual muscles, control of muscles around a joint, and control of muscles at several joints.

A common example of a spinal reflex is the muscle tendon reflex. When a muscle is stretched, it sends signals to the spinal cord via Ia afferent neurons. In the spinal cord, Ia afferent neurons synapse with motor neurons that stimulate the agonist muscle, causing reflex contraction. At the same time, Ia afferent neurons synapse with inhibitory motorneurons, which synapse with another motor neuron, causing reflex relaxation of the antagonist muscle.

Since rhythmic locomotor behaviour also has some things in common with reflex movements, I'm going to discuss it here. To my understanding, rhythmic locomotor behaviour is mainly due to inhibitory interneurons in the spinal cord. When the flexor is activated, the extensor is inhibited, and vice versa. Tonic descending input can also play a role, but it is not necessary: cats with a spinal transection will still display rhythmic locomotor behaviour.

Identify the principal descending pathways in the spinal cord.

Before I talk about descending pathways, I'm going to talk about the layout of the spinal cord itself. As I mentioned earlier, motor neurons are located in the ventral horn of the spinal cord. "Pools" of motor neurons refer to all of the neurons that go to a muscle or to a group of muscles. These "pools" of neurons are laid out in a certain way: motor neurons innervating proximal muscles are located medially, neurons innervating distal muscles are located laterally, neurons innervating extensor muscles are located ventrally (i.e. towards the front), and neurons innervating flexor muscles are located dorsally (i.e. towards the back).

Aside from motor neurons, there are also interneurons and propriospinal neurons within the spinal cord. Medial propriospinal neurons project bilaterally (i.e. on both sides), span large lengths of the spinal cord, and coordinate trunk muscles on both sides of the body. Lateral propriospinal neurons project ipsilaterally (i.e. on the same side) and over shorter distances, and are used to innervate distal limb muscles.

Now it's time to info-dump a bunch of stuff about descending pathways! Tighten your seatbelts, because there's a lot to learn.

Descending pathways can be divided into two main categories: indirect pathways, which run from the brainstem to the spinal cord, and direct pathways, which run from the cortex to the spinal cord. Indirect pathways, which include the vestibulospinal and reticulospinal tracts, are mostly important in complex polysynaptic pathways regulating posture. Direct pathways, which include the corticospinal tract, are mostly important in innervating lateral motor neurons that innervate distal limb muscles, which are important for voluntary movements.

Vestibulospinal Tract

The vestibulospinal tracts arise from the vestibular nucleus, which in turn receives input from the vestibular system (balance organs in the ear). The vestibular nucleus also has connections with the cerebellum and reticular formation. There are both lateral and medial vestibulospinal tracts. The lateral vestibulospinal tract acts ipsilaterally, and excites extensor muscles. The medial tract acts bilaterally, and excites axial muscles. The vestibulospinal tracts are largely responsible for reflexes that help align the head and body.

Reticulospinal Tract

The reticulospinal tracts receive input from the vestibular system, cerebellum, lateral hypothalamus, globus pallidus, and sensorimotor cortex. The medial, or pontine (i.e. arising from the pons) tract acts ipsilaterally, and excites axial and extensor muscles. The lateral, or medullary (i.e. arising from the medulla) tract acts bilaterally, and inhibits extensor muscles while it excites flexor muscles.

Corticospinal Tract

The corticospinal tracts receive input from the primary motor cortex, supplementary motor area (SMA), and primary somatosensory cortex. The lateral corticospinal tract crosses over at the medulla, so it innervates contralateral distal limb muscles. The ventral corticospinal tract projects ipsilaterally and innervates the axial and proximal limb muscles. The corticobulbar tract, which terminates in the brainstem, innervates the motor neurons of the head and face muscles.

Describe the role of the cerebellum and basal ganglia in movement control.

The basal ganglia had a very complicated slide, but I think all we need to know is that it regulates motor control by concentrating information from different structures and feeding back to the motor cortex and SMA.

The cerebellum is important for comparing executed movements with motor commands (i.e. comparing what you actually did with what you intended to do), timing and coordination of movements, balance, muscle tone, and regulating eye movements. There are three main regions of the cerebellum: the spinocerebellum, the cerebrocerebellum, and the vestibulocerebellum. The spinocerebellum is important in motor execution and signals to the vestibulospinal and reticulospinal tracts. The cerebrocerebellum is important in motor planning. Finally, the vestibulocerebellum has roles in balance, and signals to the vestibular nuclei.