NURS 207 (N01) Skeletal Muscle, Motor Unit, Muscle Fatigue PDF

NURS 207 (N01) Skeletal muscle Motor unit and Muscle fatigue October 24, 2024 Dr. P. Lee  Objectives: 1) Know the length-tension relationships for muscle fibers 2) Know the parameters that can affect muscle tension and how the tension i...

NURS 207 (N01) Skeletal muscle Motor unit and Muscle fatigue October 24, 2024 Dr. P. Lee  Objectives: 1) Know the length-tension relationships for muscle fibers 2) Know the parameters that can affect muscle tension and how the tension is being controlled 3) Know the structure and function of the three types of skeletal muscle fibers 4) Know the potential causes of muscle fatigue  Length-tension relationships for muscle fibers  Length-tension relationship ❖ Length-tension relationship stated that the tension developed during a muscle twitch is a direct reflection of the length of individual sarcomeres before contraction begins Sarcomere length is related to the overlap between the thick and thin filaments Sliding filament theory predicts: → Tension a muscle fiber generated is directly proportional to the number of crossbridges formed between the thick and thin filaments during muscle contraction  Length-tension relationship Optimal length (2 to 2.4 μm) gives Too much or too little overlap of thick and thin filaments in resting muscle results in decreased tension. optimal actin C myosin interaction. Maximum tension can be generated B D Tension (percent of maximum) 100 80 60 40 A E 20 0 1.3  m 2.0  m 2.3  m 3.7  m Decreased Increased length length Optimal resting length No room for If over stretching occurs, sliding to occur, no crossbridge can be no force can be formed. No force can be generated generated  Length-tension relationship Active force is at its maximum when the resting length of sarcomere is at its optimal which is about 2.0 to 2.4 μm → Normally, resting muscle fiber length is held very close to the optimum Passive force measured before muscle contraction → Muscle get stiffer as it is extended (rubber band effect), increasing amount of force is needed to elongate the muscle cell → The main contributing factor of this passive force is the titin in muscle fibers  Length-tension relationship Z disk Nebulin helps Actin Z disk align actin. M line Titin provides Myosin elasticity and stabilizes myosin. Control of muscle tension  Control of muscle tension Action potential (electrical impulse) is always the same size in a given neuron or muscle fiber Force of muscle fiber contraction does vary Force or tension that a given muscle fiber can generate depends on: ✓ Rate (frequency) of nerve impulses arriving at the neuromuscular junction ✓ Amount of stretch (passive tension) before contraction ✓ Energy substrates and oxygen availability ✓ Total number of muscle fibers are contracting in unison  Control of muscle tension Peak is the maximum magnitude of tension developed Slope is the rate of force or tension development Slope is the rate of Higher the rate relaxation (∆tension/∆time), stronger the force of contraction Twitch: contraction-relaxation from a single stimulation (single action potential)  Control of muscle tension (refractory period) If two consecutive stimuli are applied to a muscle fiber at a very close interval → The 1st stimulus will elicit a response (contraction) → The 2nd stimulus might not be able to elicit a response ▪ It is because the muscle fiber is still in its refractory period (about 5 msec for skeletal muscle) ▪ Absolute refractory period is the time interval after the initial stimulus that the muscle will not contract regardless how strong the stimulus is ▪ Relative refractory period is the time interval after the initial stimulus that the muscle will contract (weaker) when the stimulus applied is stronger that what normally would cause a contraction  Control of muscle tension (frequency of stimulation) If a second stimulus occurs after the refractory period is over and before the muscle fiber has fully relaxed, the 2nd contraction will be stronger than the 1st (phenomenon known as wave summation) Single twitches: Muscle relaxes Summation: Stimuli closer together completely between stimuli ( ) do not allow muscle to relax fully Summed twitches One twitch Tension Tension 0 100 200 300 400 500 0 100 200 300 400 500 Time (msec) Time (msec)  Control of muscle tension (frequency of stimulation) When the skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second, the result is a sustained but wavering contraction (phenomenon known as unfused or incomplete tetanus) Unfused tetanus Maximum tension Summation leading to unfused tetanus: Stimuli are far enough Tension apart to allow muscle to relax slightly between stimuli Time (msec)  Control of muscle tension (frequency of stimulation) When the skeletal muscle fiber is stimulated at a higher rate of 80 to 1000 times per second, the muscle fiber does not relax at all (phenomenon known as fused or complete tetanus) Summation leading to Complete tetanus complete tetanus: Maximum tension Muscle reaches Tension steady tension, or Fatigue causes muscle to lose if muscle fatigues, tension despite tension decreases Single-twitch tension continuing stimuli. rapidly 0 Time (msec) Control of muscle tension (motor unit recruitment) One muscle may have SPINAL CORD many motor units of different fiber types. Neuron 1 Neuron 2 Neuron 3 Motor nerve KEY Muscle Motor unit 1 fibers Motor unit 2 Motor unit 3 A motor unit consists of one motor neuron and all the muscle fibers it innervates A muscle may have many motor units  Control of muscle tension (motor unit recruitment) Motor unit is composed of a group of muscle fibers and this group of fibers is innervated only by a single somatic motor neuron → When the somatic motor neuron fires an electric signal (known as action potential), all muscle fibers in this motor unit contract → The number of muscle fibers in a single motor unit varies → All muscle fibers in a single unit are of the same fiber type  Control of muscle tension (motor unit recruitment) Typically, not all motor units of an entire muscle are contracted in unison → One unit may be contracting while other is relaxing → To delay muscle fatigue and allows the contraction of the muscle to be sustained for a longer duration Since muscle fiber type is varied with different motor unit, recruitment of specific motor unit can be employed for a specific motor task → The smaller muscles that produce fine and precise movements are made up of smaller motor units Diversity of skeletal muscle fibers  Diversity of skeletal muscle fibers Skeletal muscle fibers are classified into 3 main types → Slow-twitch oxidative fibers; fast-twitch oxidative- glycolytic fibers; and fast-twitch glycolytic fibers → Criteria of classification is based upon their speed of contraction and their resistant to fatigue → Skeletal muscle fibers at different locations will serve different demands for strength, speed, and fatigability → With ability to modify their speed of contraction and resistant to fatigue upon training (plasticity) Marathon runner Sprinter runner (endurance) (force & speed)  Diversity of skeletal muscle fibers Slow-twitch oxidative fibers (Type I) ✓ Fibers are smallest in diameter amongst the 3 types i.e. Least powerful type of muscle fibers ✓ Contain large amounts of myoglobin (color dark red) ✓ With high density of blood capillaries & mitochondria ✓ ATP generation mainly by aerobic cellular respiration (oxidative fibers) ✓ With low glycogen content in muscle fibers  Diversity of skeletal muscle fibers Slow-twitch oxidative fibers (Type I) ✓ ATPase in the myosin heads hydrolyzes ATP relatively slowly and therefore contraction cycle is at a slower pace (slow fibers → slow speed of contraction) ✓ Fatigue-resistant & capable for sustained contraction ✓ Primary functions include maintaining posture and endurance activities such as marathon  Diversity of skeletal muscle fibers Fast-twitch oxidative-glycolytic fibers (Type IIa) ✓ Intermediate fiber diameter amongst the 3 types (with moderate velocity of muscle shortening) ✓ Contain moderate amounts of myoglobin as compared to the slow-twitch fibers (color pink-red) ✓ With good supply of blood capillaries and large amount of mitochondria ✓ ATP generation mainly by aerobic glycolysis (oxidative-glycolytic → glucose is converted into pyruvate and then enters tricarboxylic acid cycle (TCA) to produce ATP with adequate oxygen supply) ✓ With moderate glycogen content in muscle fibers  Diversity of skeletal muscle fibers Fast-twitch oxidative-glycolytic fibers (Type IIa) ✓ ATPase in the myosin heads hydrolyzes ATP in a moderate to fast speed and therefore contraction cycle is also at a moderate pace compared to the other 2 types ✓ Moderately to high fatigue-resistant & capacity to resist fatigue increases with endurance training ✓ Primary functions include walking and sprinting  Diversity of skeletal muscle fibers Fast-twitch glycolytic fibers (Type IIb) ✓ Largest in fiber diameter amongst the 3 types and contain the most myofibrils (with fastest velocity of muscle shortening and most powerful contraction) ✓ Contain low level of myoglobin as compared to the other 2 types of fibers (color pale white) ✓ With relatively fewer supply of blood capillaries and lower amount of mitochondria ✓ ATP generation mainly by glycolysis (glycolytic fibers) ✓ Contain largest amount of glycogen content in muscle fibers  Diversity of skeletal muscle fibers Fast-twitch glycolytic fibers (Type IIb) ✓ ATPase in the myosin heads hydrolyzes ATP rapidly and therefore contraction cycle is the shortest compared to the other 2 types ✓ Adapted for intense anaerobic glycolytic mean to generate ATP ✓ With highest amount of creatine kinase (CK) CK PCr + ADP Creatine + ATP ✓ Fatigue quickly ✓ Primary functions rapid, intense movement of short duration (e.g. Weight-lifting) Muscle fatigue  Muscle fatigue Muscle fatigue is defined as condition in which the muscle is unable to generate or sustain force of contraction Muscle fatigue is a function of various parameters: ✓ Intensity of contractile activity ✓ Duration of contractile activity ✓ Aerobic vs. anaerobic cellular metabolism ✓ Composition of the muscle fibers ✓ Fitness level ✓ Psychological effects  Muscle fatigue Muscle fatigue can Types of Process Proposed fatigue map mechanisms either be centrally or peripherally, or both CNS together (mostly Central Psychological fatigue effects peripherally related) Somatic motor neuron Protective Central fatigue Neuromuscular junction reflexes → Usually precedes peripheral fatigue Excitation- contraction i.e. As a protective reflex to coupling prevent tissue injury Ca2+ signal → Possible triggered by the increase in acid production Contraction- relaxation during maximal exertion  Muscle fatigue Central fatigue Types of Process Proposed fatigue map mechanisms ✓ Could be a result of neural connection CNS Central Psychological failure or depletion fatigue effects of neurotransmitter Somatic motor neuron Protective centrally Neuromuscular junction reflexes ▪ Such a failure is usually Excitation- related to neuromuscular contraction coupling diseases rather than exercise exertion Ca2+ signal Contraction- relaxation  Sample questions 1) Length of individual sarcomere in skeletal muscle: a) is constant at all time under any given muscle tension. b) is equal to the length of the thick filaments. c) decreases progressively during muscle contraction. d) at rest is equal to the length of the thin filaments. 2) When a muscle is stimulated successively by two stimuli that the second stimulus (usually can cause the muscle to contract on its own) cannot cause the muscle to contract regardless how strong the second stimulus is. Such time interval between these stimuli is known as: a) Relative refractory period. b) Over stimulating period. c) Latent period. d) Absolute refractory period.  Muscle fatigue Types of Process Proposed fatigue map mechanisms CNS  Neurotransmitter release Peripheral fatigue  Receptor activation Somatic motor neuron Changes in muscle membrane potential Neuromuscular junction SR Ca2+ leak  Ca2+ release Excitation- contraction  Ca2+-troponin interaction coupling Ca2+ signal Depletion theories: PCr (creatine phosphate), ATP, glycogen Contraction- relaxation Accumulation theories: H+, Pi, lactate  Sample questions 3) The maximum tension that a muscle group can generate during: a) wave summation. b) complete tetanus. c) unfused tetanus. d) fatigue. 4) The group of muscle fibers that is activated by the same motor neuron is known as: a) somatic neural unit. b) fast-twitch glycolytic muscle group. c) motor unit. d) slow-twitch oxidative muscle group.  Sample questions 5) Which of the following types of muscle is most susceptible for muscle fatigue? a) Type IIb. b) Type IIa. c) Type I. 6) When the skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second: a) one can observe 20 to 30 well separated individual muscle twitches. b) it will produce a sustained but wavering contraction. c) stimuli are close enough and not to allow muscle relaxation to occur between stimuli. d) it will lead to complete tetanus.  Answer to sample questions 1) c 2) d 3) b 4) c 5) a 6) b