The Neuromuscular System (Part 6) – Motor Units (Recruitment & Synchronization)

This is the final breakdown of anatomical structures within the NM system. We have so far covered: anatomy of the brain, spinal cord, neurons, neurotransmission via the tracts and skeletal muscle contraction. This article aims to discuss how the neuron is connected to multiple muscle fibers and how this determines the different types of skeletal muscle contractions.

In the last article, we discussed the role of the neuromuscular junction; emphasising the importance of events occurring at a single axon terminal. However, this was only a description of the events occurring at one axon terminal, which is not completely relevant at a gross level of muscle contraction.

What is a motor unit?

A motor unit consists of a single alpha motor neuron and all of the muscle fibers that it innervates. In other words, a motor neuron will have many axon terminals; all of which it is responsible for depolarizing at the same time. This may range between 5 and 2000, and will determine the type of motor unit it is and what type of contractions it is ideal for.

Muscle fibers are rarely ever innervated by more than one nerve fiber (not poly-neuronal). As Ach is liberated at the synaptic cleft, an action potential is propagated along the length of the whole muscle fiber at ~5 m/s-1, therefore activating the whole length of the muscle, but only at the fibers of the particular motor unit that has been depolarized. 

Motor units operate under the ‘All or Nothing Law‘, dictating that once a motor neuron is depolarized, adequate Ach must be released at every NM junction to innervate it’s associated muscle fibers. In the case that it does not, not a single fiber of the motor unit will be innervated. 

Motor unit recruitment

Much like a neuron action potential, muscle action potentials are the result of depolarization of ~100mV (-70 –> +30mV) and this occurs at the beginning of a contraction. The ability of the CNS to activate motor units or additional units to increase voluntary contraction of skeletal muscle is known as ‘motor unit recruitment‘. This is either achieved through the recruitment of additional motor units (spatial summation) or the rate at which action potentials are propagated down the axon terminal AKA ‘firing rate’ (temporal summation) (Figure 1).

Figure 1. Sequence of events leading to change in potential. Note how spatial and temporal summations result in identical depolarizations but how they differ in receiving signals.

Muscle fibers are generally activated by motor units based on size; this is referred to as the ‘Henneman size principle‘. Through observation of cat ventral rootlets, Henneman et al found that the larger diameter size, conduction velocity and size of the nerve cell was activated with increased levels of activity. As a result, it was then understood that the smaller motor units (smaller diameter, lower conduction velocities, smaller cell bodies and lower number of axon terminals) were recruited first in typical activity.

Motor units are therefore distinguished into 3 categories (Figure 2):

Figure 2. Classification of motor unit types from muscle fibers based on histochemical profile, size, and twitch (contractile) characteristics. It is important to note that the range of any single characteristic may vary considerably within any given motor unit (either within or between whole muscles).

1. Type I or Slow Fatigue resistant (S) – Smaller cell bodies in the ventral horn, smaller axon diameter, slower nerve conduction velocities, lower quantity of connected muscle fibers, therefore slower contractions, smaller yet sustained force production capacities and smaller twitch amplitudes.
2. Type IIa or Fast Fatigue resistant (FR) – Similar characteristics to Type I but also possesses characteristic of Type IIb.
3. Type IIb(x) or Fast Fatigable (FF) – Large cell bodies in the ventral horn, large axon diameter, high nerve conduction velocities, high quantity of connected fibers, therefore faster contractions, large yet short timed force production capacities and large twitch amplitudes.

In symmetrical fashion, the type of motor unit is connected to same muscle fiber type classifications (Table 1):

Table 1. Muscle fiber type characteristics. Type IIx is interchanged with IIb at times.

Hopefully it is now abundantly clear that motor units are also differentiated by their  associated fibers, which now allows for an easy walk through the general pattern of recruitment:

At lower forces, recruitment of Type S motor units will predominate until larger amounts of force or faster contractions are required. In the first instance, Type FR motor units will be recruited and if higher force outputs are still required, Type FF are then finally recruited. The secondary mechanism of increased recruitment may come after most motor units are activated and this would be through the increase of firing rates from the axon terminals to the NM junctions (although this occurs at lower rates during smaller contractions).

But muscles do not just simply contract once.. they are repeatedly stimulated for longer and more forceful contractions. So how would you explain that?

At higher rates of stimulation, the muscle does not relax before another contraction. Most motor units will have been activated and therefore an increased firing rate will become the key mechanism to increase contraction strength. This increase leads to 4-5x greater force production as there is complete mechanical fusion of the contraction and the muscle fibers are now in full ‘tetanus’, allowing for a ‘smooth’ contraction.

At lower rates of stimulation, mechanical fusion of muscle fibers are incomplete (lower firing rates), therefore the tension developed within the muscle is submaximal (Figure 3). This is due to smaller motor units having a longer refractory (hyperpolarization) period, wherein which they achieve tetanus at lower action potential frequencies (~20 Hz) in comparison to larger motor units (fast twitch fibers) at action potential frequencies of ~50 Hz.

Important note: Do not get twitch or tension waves confused with voltage waves. These are completely separate and do not follow the same trend! One represents the rate and size of action potentials, while the other represents mechanical tension at the muscle and tendon.

Figure 3. 1) Twitch response to single stimuli. 2) Spatial summation – responsible to multiple stimuli from various motor units. 3) Response to multiple stimuli from single motor uni at low frequency range. 4) Response to multiple stimuli from single motor uni at high frequency range.

Even during a fused tetanus, the muscle action potential develops in response to each action potential stimulus, thus revealing the rate at which the nerve is being stimulated (Figure 4).

Figure 4. Action potentials along side representative mechanical tension. Note how the same amount of depolarization is always needed; the only difference is now the rate at which they are received.

This ability to receive action potentials at a rapid rate relates to the ECC process discussed in the previous article. Simply put: mechanical fusion and speed of contraction is reliant upon rapid release & reuptake of Ca2+ from and to the terminal cisternae, thus allowing actin-myosin bonding, followed by myosin ATPase activity to release, cock back and re-bond with actin again. This process allows the muscle to continuously contract via consistent pulling of the sarcomeres towards the midline (therefore consistent development of mechanical tension = tetanus).

Motor unit synchronization

Just as we began to widen our scope from a single axon terminal to multiple within a single motor unit, we must widen the scope once more to factor in that contraction isn’t a result of one motor unit. Rather, it is the firing of multiple motor units at similar time frequencies to achieve muscular contraction! All of the motor units found within a single muscle are termed the motor pool. The question of how similarly timed these motor units within the pool fire off, is answered by observing the motor unit synchronization.

In most voluntary contractions, motor unit synchronization is thought to be the result of branched input of a single pre-synaptic neuron, therefore ensuring action potentials are arriving at multiple motor units via the same depolarized motor neuron at the same time. However, this has been seen to occur in peak durations of only a few milliseconds, hence it is named ‘short-term synchronization‘.  However, a great many studies that have examined motor unit synchronization in healthy human muscles show peak width durations of 10-20 milliseconds. This is assumed to be the combination of short term synchronization AND ‘broad-peak synchronization’, whereby motor units receive signals via independant inputs (interneurons) which are being synchronized via a common pre-synaptic input (Figure 5).

Figure 5. Short-term synchronizaton vs. broad-peak synchronization. Semmler JG, 2002

Broadly speaking, the overall synchrony of motor units within voluntary contraction is most likely reliant upon excitatory drive from the pre motor, motor & somatosensory cortex and the cerebellum, down the corticospinal pathway. Each of these synaptic inputs may arrive at different times and frequencies dependant upon required rate of force production, level of fine skill required, excitatory capacities of various muscles and direct/indirect pathways. It is possible that the CNS may be also to adapt and adjust descending drive, based on task complexity; indirect pathways may be priorotized for simple tasks while freeing up corticospinal direct pathways for more complex tasks.

While some improvements in motor unit synchronization have been seen in strength trained individuals, as well it’s supposed role in larger rate of force production; this area is not yet completely understood due to various methodological reasons and we will hopefully expand on this at a later point!

That concludes my summary of motor units for today! Stay tuned for the following articles/topics within the NM System series:

• Neurological adaptations to resistance training.

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