Definition: A motor unit is an α-motoneuron, its axon and all muscle fibres to which it connects.
Summary:Motor Units are the final connection between CNS and muscle. They consist of two components, a lower motor neuron (α-motoneuron) and the muscle fibers which they innervate. These motor neurons create three different types of pathways. S type (slow switch) is the weakest and is able to be sustained for the longest amount of time. FF (fast twitch, fatiguable) fibres are strong and fast but fatigues quickly. FR (fast twitch, fatigue resistant) is a blend between FF and S, providing a middle ground of both strength and sustainability.
Found in the spinal cord and brainstem, Lower Motor Neurons are bundled into groups which form motor units. Each motor unit innervates only one type of skeletal muscle, and as there are three types of skeletal muscle there are three types of motor units.
Skeletal Muscle Types and Their Corresponding Motor Units:
Red Fibres: Thin muscle fibres with many mitochondria, red fibres are able to sustain a weak contraction for very long periods of time. These are innervated by the smallest motor unit, S-Type (Slow twitch, oxidative) which are active during activities such as standing.
White Fibres : Thick muscle fibres with few mitochondria, white fibres create fast and powerful contractions which are not sustained. They are innervated by the very thick FF (Fast twitch, Fatiguable) motor units. It requires a large stimulus to cause these to fire due to their large diameter.
Intermediate Fibres: As the name suggests, these are dead in the middle between red and white fibres; intermediate fibres are OK at sustaining contractions and are create mediocre contractions. These are innervated by the medium sized motor unit called FR (Fast twitch, fatigue Resistant)
Muscles contain varying amounts of each type of muscle fibre, and therefore varying amounts of each motor unit. A quadricep muscle, for example, which is used for standing, and walking (etc.) will contain mostly red and intermediate fibres whereas a hand which is used for fine, highly coordinated activities such as writing will contain mostly white fibres.
The Size Principle:
Since neurons are basically wires they are subject to the properties of conductivity, creating something very useful; The Size Principle. This is best illustrated through an example; When a person poises to jump a stimulus is sent to the motor units in the legs. This stimulus will first trigger the S-type pathways as they are the smallest; it will then progress to increasingly large motor units (FR) and finally to FF.
This is an extremely useful property, since it means the same type of stimulus can be used to stand, jump, and anything else. The only difference is the strength (quantity) of stimulus sent. During a powerful contraction, then, S, FR, and FF type motor units are stimulated causing all types of muscles to be activated.
So far only Lower Motor Neurons have been discussed, but now we must delve into Upper Motor Neurons and ascend the hierarchy of muscle control.
Note: Based on my sources we are currently unaware of how voluntary motions are made. We are only aware that certain structures are involved, but not how.
Upper Motor Neurons and Control of Motor Units
Summary: Upper Motor Neurons are those which end at the spinal cord. This is a much more complicated system than that of Lower Motor Neurons. Primarily, the motor cortex transmits signals either straight to the muscles or indirectly via the spinal cord pathways. The motor cortex is influenced by the cerebellum, basal ganglia, thalamus, prefrontal cortex; each of which in a sense “decide” which voluntary motion should be enacted.
The picture to the right depicts the divisions within the postcentral gyrus given to various parts of the body. In reality the lines aren’t this neat and there is a large amount of overlap, but this image provides a reasonable idea. The yellow region in the lower image is called the supplementary motor area, it too is divided about 4 times.
Origins of Stimuli and the Spinal Tracts – A Summary:
The corticospinal tract is the nerve bundle which carries upper motor neurons to their lower counterparts.
Stimuli present in the corticospinal tract originate roughly 1/3rd from the premotor cortex and supplementary motor area, 1/3rd from the primary motor cortex, and the rest from the somatosensory region of the postcentral gyrus (pictured above). The signals sent from these regions are transmitted via the Pyramidal Tract, Rubrospinal Tract,Tectospinal Tract and the Vestibulospinal Tract. The pyramidal tract originates in both the motor and sensory cortices. It controls the skilled movements of the hands, feet, and other extremities. The rubrospinal tract originates in the midbrain and controls large flexor muscles. The tectospinal tract originates in the contralateral superior colliculus and controls such handy reflexes as the “flinch”, a visual reflex. Vestibulospinal tracts originate from the ipsilateral vestibular nucleus and controls the opposite muscles of the rubrospinal tract – large extensor muscles. Finally, reticulospinal tracts originate in the pons and medula control muscle tone, heart beat, breathing (et al.) and reflexes.
Upper Motor Neurons:
Upper motor neurons join at the corticospinal tract, but have many different origins and are controlled by a broad array of systems. There are <1 000 000 fibres – a number which correlates to the number of lower motor neurons. These neurons have a massive range of sizes which relate to their purpose. Larger neurons are more likely to end up on lower motor neurons. There are many types of upper motor neurons, and I’ll start by explaining Betz cells due to their rather important history, then I will move on to the pyramidal tract – one of the most important things I will write about.
Originally thought to account for the majority of the motor system, Betz Cells actually only constitute roughly 3% of the corticospinal fibres. These are of the largest upper motor neurons with diameters ~10x that of the majority. They reside only in the primary motor cortex and connect directly with lower motor neurons.
The remainder of the upper motor neurons are unnamed (to my knowledge) and have an average diameter of ~2.5nm. These will accept stimuli primarily from the motor cortex, premotor cortex, or supplementary motor area. Signals are then transmitted either directly to lower motor neurons like Betz Cells (if they’re very large) or to the spinal cord and carried downwards (in the case of smaller neurons).
The Pyramidal tract, also known as the Corticospinal Tract, is a vast cluster of nerve fibres which travels from the motor area of the brain down to the final common pathways (lower motor neurons) and is responsible for the transmission of the signals for the majority of voluntary movements, primarily the fine coordinated movements of the extremities. This tract has a diffuse origin with the majority of cells coming from the pre- and postcentral gyrus. Upon leaving their origin, the axons travel through the pons until they reach the medulla oblongata. From this point on, the axons are trivially renamed pyramids. Most (~90%) of the pyramids then decussate in the caudal medulla and then again before their termination. Basically, the pyramids change sides twice, resulting in no change. This seems like rather pointless process, wasting both transmission time and resources on the increased length required to cross over. Regardless of this, about half of the nerves from this bundle terminate at the cervical level and the other half are split between the thoracic and lumbosacral levels roughly evenly.
This tract originates from the red nucleus in the midbrain, receiving it’s input from the cerebellum and motor cortex. The stimuli are the transmitted down the spinal cord to the large flexor muscles, producing an excitatory effect.
Tectospinal Tract: Fibres originate from the midbrain, primarily, and terminate mainly in the cervical level of the spinal cord. This tract is believed to produce reflexes in response to visual cues.
Vestibulospinal Tract: Originating in the vestibular nuclei of the midbrain, the vestibulospinal tract receives it’s input from the cerebellum and vestibular nerve. This tract is the primary control system of extensor muscles and posture.
Reticulospinal Tract: Axons from this tract descend in two different pathways, both located in the ventral funiculus. Stimuli from the reticulospinal tract has influences over alpha and gamma motoneurons, resulting in control of reflexes, voluntary movement, and muscle tone. In addition, this tract has partial control over the circulator and respiratory systems. Since they control voluntary action, nerves from this tract are likely responsible for conscious control over breathing, a useful yet often annoying trait.