19. Motor Unit Recruitment - Texas A&M AgriLife

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Motor Unit RecruitmentANSC (FSTC) 607Physiology and Biochemistry of Muscle as a FoodMotor Unit RecruitmentI. Resting membrane potentialA. Definition – Potential electrical difference across living cellsB. Usually cell is negative inside relative to outsideII. Generation of the resting membrane potentialA. Resting state (steady state)requirements1. Equimolarity2. Electrical neutrality3. Zero electrochemicalgradientB. Basis for the resting membranepotential1. Ions responsible areprimarily Na and K .2. Factors influencingmagnitude of the actionpotential are primarily theconcentrations of Na and K .C. Calculation of the resting membrane potential1. Nernst equation: E (-RT)/zF)ln[Ki]/Ko]Where E potential difference across the membrane (usually in mV)R gas constantT absolute temperatureF Faraday's constant (# charges per mole ion)z valence of ion2. Modified Nernst equationR and F constantsT (20 C) 293 absolutez for K 1Convert ln to log10, so that: E (in mV) -58 log10[Ki]/[Ko]1

Motor Unit Recruitment3. Goldman equationE -58 log10 PK[Ko] PNa[Nai] PCl[Clo]PK[Ko] PNa[Nao] PCl[Cl1]Where P permeability coefficientII. Motor innervation of muscleA. Motor neuron1. Branched (can innervate many myofibers) à terminal axons.2. Absolute terminal innervation ratio: one myofiber innervated by one terminal axon.B. The motor unit1. Each motor neuron innervates several muscle fibers within a muscle.2. Size of motor unit varies with muscle and fineness of movement.a. There are 100 to 200 myofibers per motor neuron in larger muscles.-- rat soleus, 200 fibers/neuron; rat gastrocnemius, 1,000 fibers/neuronb. There are fewer in muscles such as ocular muscles.C. The neuromuscular junction1. Terminal axona. Vesicles (contain acetylcholine)b. Presynaptic membranec. Synaptic cleft2

Motor Unit Recruitment2. Myofibera. Postsynaptic membrane – sarcolemmab. Synaptic clefts – increase surface areaD. Transmission of impulse across the synaptic cleft – synaptic delay of the action potential1. Acetylcholinea. End-plate potentialsb. Quantal nature of transmitter release – each vesicle contains 103 to 104 moleculesof acetylcholine2. Acetylcholinesterasea. In synaptic cleft degrades acetylcholineb. Stops transmission signal, contraction3

Motor Unit RecruitmentIII. Motor unit recruitmentA. The motor unit1. A motor unit consists of onemotor neuron and all of themuscle fibers it contracts.2. Size of motor unit varieswith muscle and fineness ofmovement.3. All muscles consist of anumber of motor units and thefibers belonging to a motorunit are dispersed andintermingle amongst fibers ofother units.4. The muscle fibers belongingto one motor unit can be spreadthroughout part, or most of theentire muscle, depending on thenumber of fibers and size of themuscle4

Motor Unit RecruitmentIV. Force of contraction and motorunit recruitmentA. Motor unit recruitmentdepends on the force/resistance ofthe exercise.1. With light intensity exercisethe Type I (slow twitch)motor units are recruited.2. When the load is increased,the Type IIa (fast twitch)will be recruited with thehelp of the Type I fibers.3. When the load becomeseven greater, the Type IIb/xwill be recruited with thehelp of the Type IIa andType I motor units.B. Type I motor units are alwaysfiring no matter what theintensity.V. Motor unit recruitment, firing frequency, and fatigueA. The central nervous system can increase strength of muscle contraction by:1. Increasing the number of active motor units.B. During muscle fatigue, new motor units are recruited.1. A 30% decline in maximal voluntary contractions was associated with a 23% increasein motor unit recruitment.2. This occurs within 25 – 35 seconds.C. When nearly all motor units are recruited, increase in firing frequency becomes the primary me5

Motor Unit RecruitmentVI. ProprioceptorsA. Definition1. Provide information about the extent of muscle stretch (muscle spindles).2. Provide information about the extent of muscle contraction (Golgi tendon organs).B. Components1. Muscle spindles: small intrafusal contractile organs with afferent and efferent neurons2. Golgi tendon organs: small stretch receptors in tendons with afferent innervation only6

Motor Unit RecruitmentVII. Muscle spindlesA. Location and function1. Muscle spindles are in parallel with myofibers (intrafusal).2. Detect muscle stretch.B. Structure1. Small muscle fibers (4-7 mm long) that contain contractile proteins.2. Have afferent and efferent fibers (fusimotor neurons).3. Stretch of spindle signals muscle to contract (resists overstretching)a. Stimulates skeletal muscle neuron pool in spinal cord – monosynaptictransmission.b. Can be stimulated to contract when muscle relaxes by efferent motoneurons.7

Motor Unit RecruitmentVIII. Golgi tendon organsA. Location and function1. Golgi tendon organs are in serieswith myofibers, embedded intendons at ends of muscles.2. Detect contraction (tension).B. Structure1. Approximately 0.7 mm long2. Contain afferent fibers onlya. Contraction of muscle signalsmuscle to relax – polysynaptictransmission.b. Inhibit skeletal muscle neuronpool in spinal cord.8

Motor Unit RecruitmentIX. Muscle spindles vs Golgi tendon organsA. Muscle stretch increases discharge in muscle spindles1. Signals travel to spinal cord.2. Muscle is stimulated to contract; discharge in muscle spindle ceases.3. Muscle spindle contracts; discharges resume.B. Muscle contraction increases discharge in Golgi tendon organs.1. Signals travel to spinal cord.2. Inhibitory neurons cause muscle to relax.9

A. The motor unit 1. A motor unit consists of one motor neuron and all of the muscle fibers it contracts. 2. Size of motor unit varies with muscle and fineness of movement. 3. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. 4. The muscle fibers .File Size: 2MBPage Count: 9

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