MN size and conduction velocity maps to number and type of its muscle fibers
MNs that control fast-twitch muscle fibers tend to innervate relatively large numbers of these fibers (e.g., 1000), and the MNs themselves have relatively large cell bodies and large-diameter axons that conduct action potential at higher speeds (e.g., 100 m/s).
MNs controlling slow-twitch (Type 1) muscle fibers are smaller, slower and innervate smaller numbers of thinner muscle fibers, resulting in slower force output. As might be expected, FR MNs and muscle units tend to be intermediate in size, speed and force output.
Can fiber composition change? This question, of special interest for study of athletic performance and of functional electrical stimulation, has been asked many times, and remains controversial.
Motor units, MN size, and orderly recruitment
In the 1960’s Henneman and colleagues introduced the concept of orderly recruitment of motor units, often called the size principle, which states that motor units are recruited in order of increasing size (Henneman et al 1965, Henneman and Mendell 1981). When only a small amount of force is required from a muscle with a mix of motor unit types, this force is provided exclusively by the small S units. As more force is required, FR and FF units are progressively recruited, normally in a remarkably precise order based on the magnitude of their force output. This serves two important purposes:
Smaller MNs have a smaller cell membrane surface area, resulting in a higher transmembrane resistance (Rhigh). Because of Ohm’s law (E = I R, where E is voltage, I is current, R is resistance), for a given synaptic current, smaller MNs produce a larger excitatory post-synaptic potential (EPSP), which reaches threshold sooner, resulting in an action potential.
Larger MNs may not be recruited: given a larger surface area (and thus lower overall transmembrane resistance), there may only be a subthreshold EPSP in response to the synaptic current input.
If we assume a common drive to the motor pool (i.e., a uniform synaptic current to MNs within this pool), and assume different MN sizes within this pool, as the amount of net excitatory synaptic input to a motor nucleus increases, the individuals MNs will reach threshold levels of depolarization in the order of their increasing size, with the smallest firing first — the size principle of orderly MN recruitment.
While the normal overall sequence of recruitment is S ® FR ® FR (with some overlap between types), recruitment has also been found to be orderly within the type categories. Such orderly recruitment is fairly robust, and has been seen for input drives ranging from transcortical stimulation to reflex-initiated excitation. However, the effect may be modified by systematic differences in the relative numbers and locations of synapses from a given source onto MNs of different unit types, usually related to dynamic task needs.
About half of the total surface area of the dedritic tree and the soma are covered by synaptic boutons, with relatively equal density on the dentrites, soma and axon hillock (Binder et al 1996). A typical MN is contacted by about 50,000 synaptic boutons representing about 10,000 presynaptic neurons; the vast majority of these connections are made upon the dendrites, which collectively account for 93-99% of the total cell surface area (Binder et al 1996), yet because many of these are farther from the axon hillock, their relative influence is smaller. MN recruitment and rate modulation depend on an integration of this barrage of converging synaptic current.
When a MN is depolarized just over its threshold for the initiation of action potentials, it tends to fire at a slow, regular rate (5-10 Hz), resulting in a partially fused train of contractions in its client muscle fibers. As its depolarization is increased by more net excitatory synaptic input, its firing rate increases. The mean level of force output increases greatly over this range of firing, saturating at a value that can be over 100 Hz (generally the smaller the MN, the higher the value). Simultaneously, other slightly larger MNs reach their thresholds for recruitment, adding their gradually increasing force levels as well. Because the relative timing of the individual action potentials in the various motor units is normally random and asynchronous (in a non-fatigued muscle), the various unfused contractions of all of the active motor units blend together into a smooth contraction.
It is this built-in smoothness that allows modelers to assume a lumped neuromotor drive to a lumped “macro-sacromere”-tendon unit. Yet it should be remembered that the overall force depends on both the number and size of active muscle units (recruitment) and their individual firing rates. In a typical muscle, the largest MNs are not even recruited until the muscle has generated about 50% of its peak force capacity.
There are several ways to study electrical excitation and properties of MNs. The effective synaptic current (IN) in MNs can be estimated by injecting current into an identified source of synaptic input and then recording the subsequent current required to voltage-clamp the membrane at the resting potential. One can then calculate IN at the threshold for repetitive discharge, and then once combined with the slope of the steady-state firing frequency-current relation, predict the effect of a given type of converging synaptic drive on steady-state MN firing behavior.
A key challenge in the area of functional electrical stimulation (FES) is to deal with the challenge of artificially exciting muscle without having the luxury of automatic orderly recruitment; fatigue in particular becomes an issue.
Orderly recruitment in multifunctional muscles and the concept of task groups
The concept and definition of a “motor pool” has been debated for many years. Are fan-like muscles such as the trapezius or deltoid best treated as one or multiple muscles? Often muscles receive MNs that exit from several spinal levels, and often certain muscles act as clear synergists for certain tasks and yet not for others. Loeb (1984) has noted that functional groupings of MNs during movements do not necessarily coincide with traditional anatomical boundaries between muscles (which we earlier called the motor pool), and has suggested that suggested categorization by task group subpopulations. An important component of the task group hypothesis is orderly recruitment of motor units within subpopulations; further research is needed in this area.