Lecture 3 Motor Unit Recruitment and Rate Coding

Generation and control of skeletal muscle force Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the principles of motor unit recruitment and rate coding to the control of voluntary force Explain the principles of neuromuscular electric...

Generation and control of skeletal muscle force Michael Paris School of Kinesiology and Health Science York University, Toronto, ON Learning objectives Describe the principles of motor unit recruitment and rate coding to the control of voluntary force Explain the principles of neuromuscular electrical stimulation and force-frequency output Understand the basic principles of electromyography Explain the interactions between muscle mechanical properties and isometric and dynamic contractile output Major steps (control points) in pathway of Voluntary Movement Neural 30-35ms Muscular C.S. Sherrington 1852-1957 Co-recipient of the Nobel Prize in 1932 Motor Unit - The final common pathway The Motor Unit The motoneuron, and all muscle fibres it innervates - Liddell and Sherrington, 1925 Smallest or fundamental functional unit of contraction (ie., controlled by the CNS) vs. sarcomere Many muscle fibres innervated by each motoneuron, but each fibre is innervated by only 1 motoneuron The Final Common Pathway to create an output (muscle contraction); is from the lower motor neurons (spinal MNs) to the muscle, and depends on drive from higher centres (Upper MNs), plus sensory feedback. All fibres of one MU activated usually ~simultaneously Mosaic pattern of motor unit distribution Motor unit territories and size MUs have overlapping territories measured by the area of muscle in which motor unit supplies innervation to fibre Territory dispersed over 5-10 mm2 Size is dictated by # of fibres innervated largely responsible for differences in motor unit tension differences (i.e., slow/type 1 vs fast/type 2 Size of MU = innervation ratio Size varies across muscle, people, species M. tension is proportional to size of MU; fine control is inversely proportional to size Great variability among people in MU types per muscle Matching of motor unit and muscle fibres All myocytes for a given parent motor neuron are of the same fibre type i.e., a type 1 motor unit will contain only type 1 muscle fibres Size of motor unit (# of innervated fibres is type dependent) IIX > IIA > I Motor unit classification – cat medial gastrocnemius 3 criteria Speed Fast vs slow contraction time Maximal tetanic force Fatigue index ‘burke fatigue protocol’ From repetitive stimulation, how much force is lost 3 grouping of motor units observed Fast fatigable Fast fatigue resistance Slow fatigue resistance Muscle force gradation Two mechanisms used by the CNS 1. Change the number of active MUs (spatial summation) Recruitment or derecruitment = changing amount of active muscle mass 2. Modify the rate or frequency of MU action potential trains (temporal summation) recruitment derecruitment Muscle force gradation Two mechanisms used by the CNS 1. Change the number of active MUs (spatial summation) Recruitment or derecruitment = changing amount of active muscle mass Smaller MUs first recruited and last decrecruited – Orderly recruitment Why? Predictable: local spinal control vs. higher centres How? Related to size of motoneuron – Henneman size principle Elwood Henneman 1915 - 1995 Size does matter The Henneman Size Principle Small MUs Large MUs Smaller cells have higher synaptic density, and excitatory current is concentrated along a smaller membrane which reaches threshold of excitement sooner than larger cells Why orderly recruitment Limiting fatigue Smallest, fatigue resistant units recruited first During tasks requiring low force levels, limit the development of fatigue Fine force control Smallest, weakest units recruited first Smooth gradation of force with recruitment of new motor units >100 fold difference in force between weakest and strongest motor units Simplifies motor control schemes CNS is not burdened by increased degree of freedom associated with controlling individual motor units Implications of size principle on force output Cycling and vastus lateralis All MU types used, but Type IIB much less Glycogen depletion Muscle force gradation Two mechanisms used by the CNS 1. Change the number of active MUs (spatial summation) Recruitment or derecruitment = changing amount of active muscle mass 2. Alter frequency of excitation (temporal summation) – rate coding Changing the proportional output of a given motor unit How is temporal summation studied for force gradation? Record muscle force response from either artificial electrical stimulation, or record (electromyography (EMG)), firing rates of MU Act. Pot. trains during voluntary contractions Why is force increased with higher rates of stimulation, or firing rates? OR Why does a tetanus produce more force than a maximal twitch? 2 key factors More Ca2+ released from sarcoplasmic reticulum → more crossbridges formed Temporal summation of twitches – force produced per action potential can summate VOLUNTARY CONTROL Recruited MUs at 8-10Hz (low range) Upper range: 1-3 pulses at 100-200Hz (at start of FAST contractions) Sustained (2-4s) at MVC 25-60Hz (muscle dependent) Electromyography – surface EMG Summation of electrical activity of many asynchronously activated MUs recorded on the skin over the muscle. Pos. ~ linear relationship between force & EMG Intramuscular EMG Voluntary force Trains of APs Selective recording of individual activated MUs - trains of action potentials (APs) = rate of firing Voluntary neural drive Neuromuscular electrical stimulation Electrical stimulation of muscle Fused tetanus Input: stimulation in pulses per second (Hz) Output: contractile response Unfused tetanus Twitch Slow Fast Force / Frequency curves to describe the relationship between electrical stimulation frequency input and force output 100% 1 Hz 50 Hz 5 Hz 10 Hz 20 Hz Force / Frequency curves Motor unit/muscle fibre type influences the shape of the force-frequency curve 100% Type I motor unit Type II motor unit Can we recruit and maximally activate all motor units during voluntary efforts? The interpolated twitch technique Can all MUs of a muscle be recruited and activated maximally during voluntary efforts ? Maybe … for brief periods, most of the time, in most people and most muscles … Central limitation, number of muscles at once, trainable…? Interactions between activation and muscle mechanics Contractile state of the muscle is critical for force production The same activation (i.e., recruitment and rate coding) can produce dramatically different responses in the muscle Dependent upon the state of the muscle E.g., length-tension, type of contraction (isometric, concentric, eccentric), activation history (e.g., fatigue), etc. Properties of muscle related to movement Active tension curve Length / tension relationship at the sarcomere level Whole muscle: length / tension Total muscle tension active tension passive tension Dynamic contractile responses Concentric/shortening Muscle shortening while generating force Eccentric/lengthening Muscle lengthening while generating force Isokinetic vs. isotonic Isokinetic – fixed velocity throughout ROM Generally not experience in day-to-day activities Often limited to lab/rehabilitation settings Isotonic – fixed load (velocity varies) throughout ROM “More” characteristic of day-to-day activities Isotonic load / velocity relationship Force / velocity relationship eccentric isometric concentric Power is a functional measure for movement that combines speed (velocity) with force (torque). Power = force x velocity Rate of doing work – measured in Watts