Motor Function and the Motor Unit Organization of the Nervous System Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Afferent (sensory) division [periphery  CNS] Efferent (motor) division [CNS  periphery] Somatic [motor neurons] Autonomic Sympathetic Parasympathetic

Neurons Basic components of neurons Cell body Dendrites Axon Nucleus Dendrites Axon Myelination Nodes of Ranvier Axon terminals Synaptic end bulbs Neurotransmitter Acetylcholine (ACH)

Motor Unit The motor neuron and all the muscle fibers it innervates. Motor neuron determines fiber type Only ONE fiber type per motor unit FG FOG SO

MU Classifications

Motor Unit The number of muscle fibers in a motor unit (innervated by 1 motor neuron) varies Gastrocnemius 2,000 muscle fibers per motor neuron Extraocular muscles < 10 muscle fibers per motor neuron Ratio of muscle fibers to motor neurons Affects the precision of movement

More precise movements Less precise movements

Motor Units and Muscle Force Production The All-or-None Law (Bowditch’s Law) for motor units Applies to individual motor units, but not the entire muscle. The all-or-none law is based upon the difference between graded potentials and action potentials Stimulation threshold A motor unit is either activated completely or is not activated at all If there is enough graded potential to create an action potential that travels down the α-motor neuron of a motor unit, then all of the fibers in that motor unit will contract. The level of force production of a single motor unit is independent of the intensity of the stimulus, but it is dependent on the frequency of the stimulus This law implies a stimulation threshold  important for the Size Principle

Gradation of Muscle Force Two neural mechanisms responsible for force gradations: Recruitment Spacial summation Rate coding Temporal summation

Recruitment The Size Principle ↓ ↑ Varying the number of motor units activated. Number & Size of Motor Units Recruited Largest motor units Highest stimulus threshold Small motor units Low stimulus threshold Larger motor units Higher stimulus threshold The Size Principle Amount of Force Required During Movement ↓ ↑

Rate Coding Rate coding refers to the motor unit firing rate. Active motor units can discharge at higher frequencies to generate greater tensions. Recruitment vs. rate coding Smaller muscles (ex: first dorsal interosseous) rely more on rate coding Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment The firing of individual motor units occurs as a stochastic process Firing rate is a better term to describe the global changes in firing frequency (i.e., rate coding)

Rate Coding Rate coding Motor unit recruitment Motor unit recruitment Larger muscles % Maximal Voluntary Force Production 100 50 % Maximal Voluntary Motor Unit Recruitment Motor unit recruitment Smaller muscles Motor unit firing frequency

Rate Coding Rate coding occurs in two stages Treppe (the treppe effect) A phenomenon in cardiac muscle first observed by H.P. Bowditch; If a number of stimuli of the same intensity are sent into the muscle after a quiescent period, the first few contractions of the series show a successive increase in amplitude (strength) Tetanus A state of sustained muscular contraction without periods of relaxation Caused by repetitive stimulations of the α-motor neuron trunk (axon) at frequencies so high that individual muscle twitches are fused and cannot be distinguished from one another, also called tonic spasm and tetany Two forms of tetanus Incomplete tetanus – occurs when there are relaxation phases allowed between twitches Complete tetanus – occurs when the relaxation phases are completely eliminated between twitches

Important! Smaller muscles (ex: first dorsal interosseous) rely more on rate coding Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment

MMS Fiber Types Three general methods to determine or estimate muscle fiber type composition Invasive sampling of skeletal MMS tissue Biopsies Invasive and noninvasive analysis of motor unit recruitment strategies Needle, fine wire, and/or surface electromyography (EMG) Noninvasive field techniques for estimating fiber type composition Thorstensson test Based upon a fatigue index

MMS Biopsies From the biopsy sample, serial slices of the tissue can be treated Histochemical and Immunocytochemical treatments Histochemistry: Incubations with substrates or stains Immonocytochemistry The reaction between specific protein isoforms with an antibody to that isoform A common procedure is to characterize fibers based upon how different antibodies bind to different myosin heavy chain (MHC) isoforms Positive relationship between Myosin ATPase activity within a muscle fiber and contraction velocity (R. Close, 1965; M. Barany, 1967) Maximum velocity of shortening (dynamic) Time to peak tension (isometric) There are exceptions to this relationship (injury, distributional extremes, etc.); therefore, an immunoassay may simply be a test of Myosin ATPase activity, rather than contraction velocity Fast-twitch fibers react dark with Myosin ATPase when preincubated under alkaline conditions (i.e., pH ~10.3) “Acid-reversal” occurs when the reaction is reversed; fast-twitch fibers react light with Myosin ATPase when preincubated under acidic conditions (pH ~4.3) Staining with a succinic dehydrogenase (SDH) reactant can identify the oxidative fibers Fiber characteristics are then determined by light microscopy

MMS Fiber Typing TRADITIONALLY, four identified skeletal muscle fiber types Based upon MHC isoform reactants and enzymatic activity Type I Type IIa Type IIx Type IIb More sophisticated techniques, however, have identified more…

MMS Fiber Typing Comparison of fiber typing methods Histochemistry Qualitative, not quantitative False dichotomy Fiber typing exists on a continuum Gel electrophoresis and immunoblotting reveals a large number of separate MHC isoforms as well as myosin light chain (MLC) isoforms The combinations of MHC and MLC isoforms are numerous, but a more complex continuum has been suggested by Pette & Vrbova (1992): Type I Type Ic Type IIc Type IIac Type IIa Type IIab Type IIb

MMS Fiber Typing Genes are present to change MHC isoforms based upon a training stimulus The direction of fiber type transition seems to be from IIb  IIa Regardless of the training modality (Fry, JSCR, 2003) Baumann et al. 1987 Dudley, Tesch, Fleck, Kraemer, and Baechle, 1986 Fry, Schilling, Staron, Hagerman, et al. in press Staron et al. JAP, 1994, 1991, and 1990

Isometric muscle action at 30% MVC Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003. Displacement Sensor Laser Beam Accelerometer Bipolar EMG Electrodes Force Transducer Isometric muscle action at 30% MVC