MD137 Muscle Lecture3 PDF

Physiology School of Medicine 2024 Skeletal Muscle: Mechanics MD137: Principles of Lecturer: Dr K.McCullagh Physiology Understand factors that can determine muscle tension and strength: Motor Units and recruitment, Types of muscle contract...

Physiology School of Medicine 2024 Skeletal Muscle: Mechanics MD137: Principles of Lecturer: Dr K.McCullagh Physiology Understand factors that can determine muscle tension and strength: Motor Units and recruitment, Types of muscle contraction. The mechanics of single muscle contractions. Length- tension relationship. Muscle fibre Types.  Motor Unit A single motor neuron and all the muscle fibers it innervates - All the muscle fibers in a motor unit contract at once. The motor unit is the smallest functional unit of a muscle group. (We can’t selectively recruit a single muscle fibre alone. Instead, motor units are recruited that innervate clusters of muscle fibres)  Motor Unit – Contraction strength comes from motor unit recruitment. Finer muscle control requires smaller motor units (fewer muscle fibers). The eye muscles may have ~20 muscle fibers/motor units. Larger, stronger muscles may have motor units with thousands of muscle fibers e.g. quadriceps, biceps Control and strength are trade-offs. Types of muscle contraction  Types of muscle contraction Isometric – Muscles can’t shorten because the load is too great – Can be voluntary – cross bridges are forming and pulling. Sarcomeres may shorten but the muscle length does not change (see elastic elements) e.g. Postural muscles  Types of muscle contraction Isotonic contractions Concentric contraction Muscle fibers shorten when the tension produced is just greater than the load. Elastic elements: Titin,Connective sarcomeres may sarcomeres shorten a tissues (Endo-, Peri-, and Epi- shorten but the whole lot and the whole mysium) and tendons and muscle length does not muscle shortens aponeurosis change Human Physiology, Pearson 2019  Types of muscle contraction Isotonic contractions Eccentric: the muscle lengthens, despite contraction, if the load is too great. – Allows you to lower a weight gently after a full isotonic (concentric) contraction Eccentric contractions stimulate muscle hypertrophy with repeated use over time (Rhoades & Bell: Chapter 29). Some muscle damage related to compensatory growth  Types of muscle contraction Concentric (shortening) Eccentric (lengthening) Isometric (no change in length) Mechanics of Muscle  Mechanics of Single-fiber Contractions A muscle fiber generates force called tension in order to oppose a force called the load, which is exerted on the muscle by an object. The mechanical response of a muscle fiber to a single action potential is known as a twitch.  The Phases of a Twitch Contraction There are 3 major phases to a twitch contraction: 1. Latent Period Action potential to the onset of contraction. Time delay due to excitation-contraction coupling events. 2. Contraction Phase Time during tension development due to cross-bridge cycling. 3. Relaxation Phase Time for tension to decrease. Longer than contraction phase due to time for Ca2+ resequestering (pumping Ca2+ back into SR). Temporal summation of muscle twitches. (Stimulated Isolated Single Muscle fibre experiments) Single Twitch Fusion of twitches into a smooth tetanus (Stimulated Isolated Single Muscle fibre experiments) (AP Frequency) Full fused Tetanus can damage the muscle fibre and Single is rarely seen Twitch Motor unit summation (in a whole muscle bed) A+B A B Elasticity of Muscle: Passive force---Titin (largest known protein) I band The passive force produced by stretching a muscle cell is similar to releasing a stretched rubber band A simple apparatus for recording isometric contractions A single isotonic contraction of skeletal muscle If the force (weight) had been too large i.e. >3Units Concentric Eccentric Isometric Isometric A series of afterloaded isotonic contractions  Length-force Relationship Passive force in relaxed muscle fibers is mainly due to the largest known protein Titin. It has spring-like properties and run from Z-discs to centre of the sarcomere. Passive force (Pre-load) increases with increased stretching of the muscle, not from cross-bridge movements but from elongation of the titin filaments. If released, it will return to an equilibrium length (similar to releasing a stretched rubber band). Active force (After load) of a muscle fiber develops during contraction can be altered by changing the fiber length. If you stretch a muscle fiber to various lengths and tetanically stimulate it at each length (isometric contraction), the magnitude of the active tension will vary with length (Length-Tension Relationship) Note: The length at which the fiber develops the greatest isometric active force is termed the optimal length, L0. 20  Length-force Relationship Effect of filament overlap on force generation Active force Passive force Lo Physiological range (or Muscle Length) Elasticity of Muscle: Passive force---Titin (largest known protein) I band The passive force produced by stretching a muscle cell is similar to releasing a stretched rubber band A length-force curve for skeletal muscle Active Note Total force is the sum of Active and Passive force output. Sarcomere Length-Force Relationship Active Force is maximal when sarcomeres are at normal resting length. Increasing sarcomere length decreases muscle active force. – There are fewer interactions between myosin and actin. – At a certain point, no tension can be generated. Decreasing sarcomere length can also decrease muscle tension. Load (Force)-shortening Relationship Vander’s Human Physiology  Load(Force)-velocity curve Vmax Mathematically Derived value Fmax (Force) 26 Eccentric Contraction Vander’s Human Physiology In summary factors determining Muscle Tension Determined by: – Number of fibers recruited (determined by the size of the motor units and number of motor units recruited) – Frequency of stimulation (Action Potential frequency) – Thickness of each muscle fiber (thicker is stronger: more actin and myosin myofilament - more cross- bridges can be formed) – Initial length of the fiber at rest (length-tension relation) Muscle Fibre Types The major metabolic processes of skeletal muscle. MCK MCK: Muscle Creatine Kinase catalyzes the synthesis and breakdown of PCr.  Skeletal Muscle fibre diversity Red Muscle Slow fibre Slow Oxidative (Type I) Slow twitch Less tension High endurance Many mitochondria Mixed Red & Fast fibre Fast Oxidative Glycolytic White Muscle (Type IIa) Intermediate like fibre but with significant oxidative capacity White Muscle Fast fibre Fast Glycolytic (Type IIx) (In rodents Type IIb) Fast twitch More tension Fatigue rapidly Fewer mitochondria  A Closer Look 8.2 Immunohistochemical Staining of Skeletal Muscle Blue = Type I fibers Green = Type IIa fibers Black = Type IIx fibers Red = dystrophin (protein in sarcolemma)  Tension(force)-time for Skeletal Muscle Fiber Types 1 Type I 3 Type IIA 10 Type IIX  Whole-muscle Contraction In a motor unit, all muscle fibres innervated are the same Type i.e. Type I or IIa or IIx Type IIx fibres Type IIa fibres Type I fibres 33 Relative Abundance of Fiber Types Skeletal Muscle fibre diversity Type IIX & IIA Type I Sprint Long Distance Type I & IIA Middle Distance  Characteristics of three types of skeletal muscle fibres Blank Slow-Twitch Oxidative; Red Fast-Twitch Oxidative- Fast-Twitch Glycolytic; White Muscle (Type I) Glycolytic; Red Muscle (Type Muscle (Type IIB/IIX)* IIA) Speed of Development of Slowest Intermediate Fastest Maximum Tension Myosin ATPase Activity Slow Fast Fast Diameter Small Medium Large Contraction Duration Longest Short Short -ATPase Activity in S R Moderate High High Endurance Fatigue resistant Fatigue resistant Easily fatigued Use Most used: posture Standing, walking Least used: jumping; quick, fine movements Metabolism Oxidative; aerobic Glycolytic but becomes more Glycolytic; more anaerobic than oxidative with endurance fast-twitch oxidative-glycolytic training type Capillary Density High Medium Low Mitochondria Numerous Moderate Few Myoglobin High (Dark red) High (Red) Low (White Pale) *Type IIB are sometimes designated as Type IIX in human muscle physiology literature  Summary Motor Units: Small and large Types of Muscle Contraction: Isotonic and Isometric Mechanics of Muscle: Single fibre and whole muscle Length-Tension relationship: Active and Passive tension. Load-Velocity relationship Muscle Fibre Types: Contractile and Metabolic differences