The Neuro-Muscular Unit

From Greg Mack

Conventional Description

A motor unit is classically defined as the alpha motor-neuron somi located in the spinal cord and all of the muscle fibers it innervates. A motor neuron communicates information via the action potential to its homonymous muscle fibers via a process called “cell depolarization”. The action potential propagates centrifugally from the soma via the axon to the muscle fiber’s sarcolemma through a process called saltatory conduction. This leads to the creation of muscle tension as previously described.

Information Organization for Motor Unit Activation = Muscle Contraction

The alpha motor neuron and all the muscle fibers it innervates represents the most reducible fully functional constituent of the central nervous system in that it is the smallest component of a muscle that can be activated at any given moment. Recent arguments assert that aggregates of alpha motor units that cooperatively control discrete aspects of the muscle motor system – called motor neuronal pools – and support discreet motor outputs should be called the most reducible functional unit. In both cases there is a 3-dimensional topography within the spinal cord of motor unit/pool location and specific propinquity that correspond to specific muscle fibers. (i.e. motor units of the cervical spinal cord are associated with the muscle fibers of the upper extremity not the lower extremity).

Generally, alpha motor pool organization is top down. As the spinal cord extends caudally from the brain the muscles associated with the motor neurons also extend caudally concomitant with that transition segment level by segment level. Modern anatomy textbooks identify specific spinal vertebral levels (i.e. C2, T1, L3) with alpha motor unit innervation of specific muscles (Supraspinatus, Diaphragm, Vastus Lateralis), with some noted overlap of innervation from the 1 or 2 segments immediately above and below.

It is observed that during early physical development (Embryologically) all neurons, including alpha motor neurons, and their axon outgrowth have specific pathway, target, and address selection assignments. Additionally, whole muscle structure has been observed to be separated by fascial compartments each of which may have its own uniquely assigned alpha motor pool;

Pattern Generation in the Nervous System

Vertebrate brain function depends not only on the differentiation and positioning of the neurons, but also on the specific connections these cells make among themselves and their peripheral targets. Nerves from a sensory organ such as the eye or nose must connect to specific neurons in the brain that can interpret stimuli from that organ, and axons from the central nervous system must cross large expanses of tissue before innervating their target tissue. How does the neuronal axon “know” how to traverse numerous potential target cells to make its specific connections?” Gilbert, S.F., “Developmental Biology”, 8th Edition, Pg. 426

“Each compartment must necessarily have its own nerve supply and individual

nerve fibers do not appear to supply muscle fibers in the adjacent compartment.

(see review by Monti et al., 2001) … Effective muscle contraction would rely on coordinated activation of motor units in each compartment of the muscle. Only by ensuring that contraction occurs fairly synchronously along the muscle belly can the compartmental innervation permit the muscle belly to shorten very rapidly.” Macintosh, Gardiner, McComas,“Skeletal Muscle: Form and Function”, 2006, Page 7

These observations lead to a conclusion that the general central nervous system network configuration (the network blueprint) is highly conserved across, and generally normative (unless genetic disorders alter the blueprint) within, humans resulting in specific muscle tissues having specified connectivity with the central nervous system. This connectional specificity is reflected in the fact that the alpha motor-neuron (and its associated muscle fibers) represent a final common convergent pathway for central and peripheral nervous system information – contract (this much) or don’t contract.

“Reflex-arcs show, therefore, the general features that the initial neurone of each is a private path exclusively belonging to a single receptive point (or small group of points); and that finally the arcs embouch into a path leading to an effector organ; and that their final path is common to all receptive points wheresoever they may lie in the body, so long as they have connection with the effector organ in question. Before finally converging upon the motor neurone the arcs converge to some degree. Their private paths embouch upon internuncial paths common in various degree to groups of private paths. The terminal path may, to distinguish it from internuncial paths, be called the final common path.” Charles S. Sherrington – “The Integrative Action of the Nervous System” 1906, Page 116

“… the motor neuron is the final common pathway of the motor system. As such, the variability of motor output is influenced by all neural processes before this point in the flow of behavior, and in many system perspectives, also by the anticipatory demands of the future states of the ongoing action.” Davids, Bennett, Newel – “Movement System Variability”, 2006, Page 8

Skeletal Muscle Fibre Classification (motor unit types)

Whole skeletal muscle contains a heterogenous combination of fast-contracting/high force/fast fatiguing fibres, slow contracting/low force/slow fatiguing fibres and intermediate types possessing some attributes of each of these two sets of general properties.

“Muscle fiber types can be described using histochemical, biochemical, morphological, or physiologic characteristics; however, classifications of muscle fibers by different techniques do not always agree. Therefore, muscle fibers that may be grouped together by one classification technique may be placed in different categories using a different classification technique. A basic understanding of muscle structure and physiology is necessary to understand the muscle fiber classification techniques.” Scott, W., Stevens J., Binder-Macleod, S., “Human Skeletal Muscle Fiber Type Classifications”, Physical Therapy November 2001 vol. 81 no. 11 1810-1816

There are three general grouping of muscle fibre types based on rate of force production, and energetic substrate utilization rate:

Slow Twitch Fatigue Resistant: Oxidative (Type I)

Fast Twitch Fatigue Resistant: Oxidative – Glycolytic (Type IIa)

Fast Twitch Fatigable: Glycolytic (Type IIb)

The terms ‘slow’ and ‘fast’ refer to the rate at which force is generated when action potentials act on the muscle fibre.

The term ‘twitch’ refers to the muscle fibre’s initial response to a single action potential.

“Thus, the reader should keep in mind the relative differences in contraction behavior of ‘fast twitch’ versus ‘slow twitch’ fibers, rather than specific absolute ‘time-to-peak’ values.” Schneck, D.J., “Mechanics of Muscle: Second Edition”, Pg. 21

The term ‘fatigue’ refers to the energy substrate system that is utilized by the motor unit and affect the continuity of the tension developed over time. i.e. creatine-phosphagen, glycolysis, oxidative, lipolysis.

Classification of fibre type can also be accomplished by observing a particular histochemical feature of all muscle types – myosin.

“Despite this compensation being less than perfect, quantitative histochemistry and single-fibre biochemical analysis point to the same general conclusion – that the majority of fibres in stable, mature skeletal muscle can be typed fairly decisively by the kind of myosin they contain, but the metabolic variables within each myosin-based type vary over wide ranges, often overlapping those of other myosin types.” Spurway, N., Wackerhage, H.,“Genetics and Molecular Biology of Muscle Adaptation”, Pg. 73.

Fibre Type Recruitment/Utilization Strategies

In early experiments exploring motor neuron electrical properties (Henneman & Mendel et. Al. 1957-1965) researchers noted how readily the motor neurons could be excited to elicit muscle contraction. It was noted though that the physical size of the neurons was associated with different time rates to contraction for given stimulus. Smaller motor neurons fired more readily than the larger ones. The researchers posited that is was due to the biophysics of the interaction. If the cell membrane properties per unit area are equal between different sized neurons, then the smaller neurons would change their electrical properties (polarity) faster than larger ones. The researchers called this phenomenon Henneman’s Principle of Orderly Recruitment or colloquially – The Size Principle.

Henneman’s Principle describes alpha motor unit recruitment as a progressive order based on the soma size – from the smaller to the larger – under most conditions. Type I alpha motor units have a smaller surface area as compared to the Type II and IIa types. Therefore, when force conditions require a small amount of muscle tension they reach their depolarization threshold first and dispatch an action potential. As muscle tension needs progressively increase, the next larger size motor neuron depolarizes and so on. As tension needs subside then the motor units repolarize in the reverse order.

“… it has been shown that at low levels of voluntary effort, slow contracting motor units with low tensions are recruited. As effort increases, faster motor units with higher tensions are recruited.” Lieber – Skeletal Muscle Structure, Function, & Plasticity, 2nd Edition, Page 101

“The order of recruitment is highly correlated with the diameter and conduction velocity of the axons and the size of the motor neuron bodies, as well as the size and strength f their muscle units.” Kandel, Schwartz, Jessel – Principles of Neural Science 4th Edition, Page 686

What Explains Henneman’s Principle?

Neurons and muscle cells (skeletal and smooth) are unique in that they are the only bodily cells that generate and propagate electrical signals, which may be carried over relatively long distances. Neurons possess both active and passive properties. The active property is the action potential delivered by saltatory conduction. The passive electrical properties are found in the soma membrane resistance and capacitance, in addition to the intracellular cytoplasmic resistance along a neuron’s axons and dendrites, and neurotransmitter influences. A neuron and its connection to other neurons create electrical circuits. As electrical circuits, a motor neuron’s electrical activity can be partially explained by Ohm’s Law.

Ohm’s Law (which is a misnomer as it is an idealized description of relationships based on observational – not derived – under specific conditions and only applies to those conditions) states that the current (I) through a conductor between two designated points is directly proportional to the potential difference across those two points (V), and inversely proportional to the resistance (R) between them (I = V/R). Materials subject to electric fields have fundamental properties; resistance, inductance, and capacitance. Capacitance is the property of being able to collect and store a charge of electricity and is a function of the total area of a material. More area more capacity. Less area less capacity.

The key idea relating Henneman’s Principle of Orderly Recruitment, Ohm’s Law, and Fibre/Motor Unit Recruitment is the capacitance of the cell membrane of motor neurons. Capacitance changes with a change in the area/size of the cell membrane. This leads to a proportional charge stored (large cell membrane = large charge stored, small cell membrane = small charge stored). Having a larger cell membrane requires a larger current necessary to increase the charge and alter the membrane potential that results in cell depolarization that then propagates an action potential toward a muscle fiber’s sarcolemma. Therefore, given all variables and conditions equal, for a given current the smaller motor neurons (Type I) will depolarize prior to the larger (Type II).

There is evidence to suggest that the idealized linear relationship described in Ohm’s Law and strictly applied to motor unit recruitment due to the relative cell membrane size does not hold under all conditions. Motor unit recruitment order is subject to nervous system modification by selective inhibition processes.

“Such evidence as there is (author note: for selective inhibition of motor units out of order) refers to special circumstances, such as high-force ballistic and eccentric efforts, and the involvement of certain muscles in actions different from their main ones. Spurway, N., Wackerhage, H.,“Genetics and Molecular Biology of Muscle Adaptation”, Pg 88.

“The classical view of the mammalian spinal motor neuron emerged from the laboratories of Eccles and Granit during the 1950s and 1960s. They held the view that the cell membrane in areas of synaptic contact was essentially passive, allowing a linear summation of synaptic inputs at the spike-initiating region. The stronger the net excitatory current, the higher the discharge rate. This situation would lead to a rather simple input-output relationship. It is now known that there are several active membrane properties that contribute to the electro-responsiveness of motor neurons. In addition, a great number of reports have appeared during the past 5 years showing that such properties are often strongly transmitter modulated. Taken together this implies that not only do motor neurons actively take part in information processing, but also that the input-output relation of individual motor neurons seems to be a flexible variable that may be adapted according to the external requirements. In this review we will focus on the most recent developments in this field.” Hultborn H., Kiehn O., “Neuromodulation of vertebrate motor neuron membrane properties” Current Opinion in Neurobiology 1992, 2:770-775

Motor Pools, Muscle Force, and Motor Neuron Firing Frequency/Rate Coding

Individual alpha motor neurons (including the Type I, IIa, IIb, and all the intermediary forms) in close vicinity to one another, linked via dendrites, and internuncial cells, that serve to innervate a specific whole muscle, compose what is referred to as a motor neuronal pool or motor nuclei.

“A motor pool consists of all individual motor neurons that innervate a single muscle. Each individual muscle fiber is innervated by only one motor neuron, but one motor neuron may innervate several muscle fibers. This distinction is physiologically significant because the size of a given motor pools determines the activity of the muscle it innervates: for example, muscles responsible for finer movements are innervated by motor pools consisting of higher numbers of individual motor neurons. Motor pools are also distinguished by the different classes of motor neurons that they contain. The size, composition, and anatomical location of each motor pool is tightly controlled by complex developmental pathways.” Wikipedia

“The contractile force of a motor unit depends on the force-generating capabilities of its fiber type multiplied by the number of muscle fibers innervated by the motor neuron.”  Kandel, Schwartz, Jessel – Principles of Neural Science 4th Edition, Pg. 685

The muscle force that a motor pool generates in the presence of a demand to do so is a function of several factors; the number of cross bridges formed, the number of fibers contracting, the size (physiological cross section) of the fibers contracting, the velocity of the cross bridging motion (as reflected in the Force: Velocity Curve), and what is termed “rate-coding” – the adjustment of the frequency of action potentials in the motor axons.

When a muscle sarcolemma reaches its depolarization threshold as the result of the alpha motor neuron’s action potential the Ca2++ stored in the terminal cisternae is liberated. The number and rate of cross bridge formation that creates tension is a result of the total amount of Ca2++ liberated and diffused amidst the myofilaments. The initial activation and subsequent reuptake of Ca2++ are competitive and time dependent processes. (Of note: A single action potential does not liberate enough Ca2++ to enable the cross bridging of all potential cross bridges). The frequency of action potentials arriving at, and depolarizing the, sarcolemma determines the amount of Ca2++ being liberated to the myofilaments. Therefore, lower frequency action potentials liberate less Ca2++. Due to the Ca2++ re-storage rates more Ca2++ is put back into the terminal cisternae prior to the next volley of action potentials which results in less cross bridging and leads to less tension generated.

As previously described “Rate Coding” is the number of action potentials (AP) generated and propagated by a motor neuron per unit time. The AP generation rate – termed depolarization – is proportional to the total input a motor neuron receives which ultimately alters its electrical field and excites the cell.

“Gradual increases in discharge rate produce linear increases in muscle force, with a strong relation between the rates of increase in discharge rate and force.” Enoka, Roger M., Neuromechanics of Human Movement, 4th Edition 2008, Pg. 225

 “Thus the rate, or frequency, of spikes indicates the intensity of the stimulus. To be a bit more precise, the number of spikes in a fixed time window following the onset of a static stimulus represents the intensity if that stimulus. This is the idea of rate coding.” Riker, Fred et al., “Spikes: Exploring the Neural Code”, 1996, Pg. 7

The origins of inputs to a motor neuron include: higher center neurons, a variety of interneurons, other motor neurons via dendrites, sensory neurons, neurotransmitters, and metabo-trophic second messengers. The net effect of all the concurrent inputs to a motor neuron at a particular point in time either add up to maintain or change its electrical field – called polarity – which directs the development of muscle force.

“During muscle contraction, there are only two ways to increase muscle force: either motor units can increase their firing rate (up to tetanic fusion frequency approximately 50 Hz), or additional motor units can fire. Normally, one increases force using a combination of these two processes, resulting in an orderly recruitment of motor units.” Preston, David C., Shapiro, Barbara E., “Electromyography and Neuromuscular Disorders: clinical-electrophysiologic correlations”, 2nd Edition, Page 221