Motor Function and the Motor Unit Organization of the Nervous System вЂ“ Central nervous system (CNS) Brain Spinal cord вЂ“ Peripheral nervous system (PNS) Afferent (sensory) division [periphery пѓ CNS] Efferent (motor) division [CNS пѓ periphery] вЂ“ Somatic [motor neurons] вЂ“ Autonomic Sympathetic Parasympathetic Neurons Basic components of neurons вЂ“ Cell body Nucleus вЂ“ Dendrites вЂ“ Axon Myelination Nodes of Ranvier вЂ“ Axon terminals вЂ“ Synaptic end bulbs вЂ“ Neurotransmitter Acetylcholine (ACH) Motor Unit The motor neuron and all the muscle fibers it innervates. вЂ“ Motor neuron determines fiber type Only ONE fiber type per motor unit вЂ“ FG вЂ“ FOG вЂ“ SO MU Classifications M o to r U n it C lassificatio n S lo w (S ) F ast fatigu e -resistan t (F R ) F R & F F co m b in a tio n F ast fatigab le (F F ) F ib er T yp e T yp e T yp e T yp e T yp e C lassificatio n I IIa IIx IIb M etab o lic F ib er T yp e C lassificatio n S lo w o x id ativ e (S O ) F ast o x id ativ e glyco lytic (F O G ) F O G & F G co m b in a tio n F ast glyco lytic (F G ) Motor Unit The number of muscle fibers in a motor unit (innervated by 1 motor neuron) varies вЂ“ Gastrocnemius 2,000 muscle fibers per motor neuron вЂ“ Extraocular muscles < 10 muscle fibers per motor neuron Ratio of muscle fibers to motor neurons вЂ“ Affects the precision of movement More precise movements Less precise movements Motor Units and Muscle Force Production The All-or-None Law (BowditchвЂ™s Law) for motor units вЂ“ вЂ“ 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 вЂ“ вЂ“ вЂ“ If there is enough graded potential to create an action potential that travels down the О±-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 пѓ important for the Size Principle Gradation of Muscle Force Two neural mechanisms responsible for force gradations: 1. Recruitment пЃ® Spacial summation 2. Rate coding пЃ® Temporal summation Recruitment Varying the number of motor units activated. Larger motor units Largest motor units Low stimulus threshold Higher stimulus threshold Highest stimulus threshold The Size Principle Amount of Force Required During Movement в†‘ Number & Size of Motor Units Recruited в†“ Small motor units Rate Coding Rate coding refers to the motor unit firing rate. вЂ“ Active motor units can discharge at higher frequencies to generate greater tensions. Recruitment vs. rate coding вЂ“ вЂ“ 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 100 Larger muscles Smaller muscles % Maximal Voluntary Motor Unit Recruitment 50 Motor unit firing frequency 0 0 50 % Maximal Voluntary Force Production 100 Rate Coding Rate coding occurs in two stages вЂ“ 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) вЂ“ Tetanus A state of sustained muscular contraction without periods of relaxation Caused by repetitive stimulations of the О±-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 вЂ“ Incomplete tetanus вЂ“ occurs when there are relaxation phases allowed between twitches вЂ“ Complete tetanus вЂ“ 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 вЂ“ Invasive sampling of skeletal MMS tissue Biopsies вЂ“ Invasive and noninvasive analysis of motor unit recruitment strategies Needle, fine wire, and/or surface electromyography (EMG) вЂ“ 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 вЂ“ Histochemical and Immunocytochemical treatments вЂ“ Histochemistry: Incubations with substrates or stains вЂ“ Immonocytochemistry The reaction between specific protein isoforms with an antibody to that isoform вЂ“ 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) вЂ“ Maximum velocity of shortening (dynamic) вЂ“ Time to peak tension (isometric) вЂ“ 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 вЂ“ Fast-twitch fibers react dark with Myosin ATPase when preincubated under alkaline conditions (i.e., pH ~10.3) вЂ“ вЂњAcid-reversalвЂќ occurs when the reaction is reversed; fast-twitch fibers react light with Myosin ATPase when preincubated under acidic conditions (pH ~4.3) вЂ“ Staining with a succinic dehydrogenase (SDH) reactant can identify the oxidative fibers вЂ“ Fiber characteristics are then determined by light microscopy MMS Fiber Typing TRADITIONALLY, four identified skeletal muscle fiber types вЂ“ Based upon MHC isoform reactants and enzymatic activity Type I Type IIa Type IIx Type IIb вЂ“ More sophisticated techniques, however, have identified moreвЂ¦ MMS Fiber Typing Comparison of fiber typing methods вЂ“ Histochemistry Qualitative, not quantitative False dichotomy вЂ“ Fiber typing exists on a continuum вЂ“ Gel electrophoresis and immunoblotting reveals a large number of separate MHC isoforms as well as myosin light chain (MLC) isoforms вЂ“ 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 вЂ“ The direction of fiber type transition seems to be from IIb пѓ IIa Regardless of the training modality (Fry, JSCR, 2003) вЂ“ вЂ“ вЂ“ вЂ“ 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 Displacement Sensor Accelerometer Laser Beam Bipolar EMG Electrodes Force Transducer Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003.