Rehab521 Autumn 2024 Skeletal Muscle Physiology Lecture Notes PDF

Normal (non-pathological) skeletal muscle physiology: Steps of contraction-relaxation Mary Beth Brown, PT, PhD UW Physical Therapy Proteins of the Myofibril Contractile protein (myofilaments) – Myosin (thick filament) – Actin (thin f...

Normal (non-pathological) skeletal muscle physiology: Steps of contraction-relaxation Mary Beth Brown, PT, PhD UW Physical Therapy Proteins of the Myofibril Contractile protein (myofilaments) – Myosin (thick filament) – Actin (thin filament) Regulatory protein – Tropomyosin (cover actin active site) – Troponin complex (Ca2+ binding protein, 3 subunits: T, C, I) Structural protein – Titin – Nebulin Nebulin: big protein with Titin as a force- big responsibilities. J generating muscle Muscle Res Cell Motil 41, 103–124 protein under regulatory (2020). https://doi.org/10.1007/ s10974-019-09565-3 control. Proteins of the Myofibril Contractile protein (myofilaments) – Myosin (thick filament) – Actin (thin filament) Regulatory protein – Tropomyosin (cover actin active site) – Troponin complex (Ca2+ binding protein, 3 subunits: T, C, I) Structural protein – Titin – Nebulin  Other Structural Proteins Dystroglyca n complex- serves as an anchor Sarcoglyca n Dystrophin Laminin All are important in force generation, and signaling  Duchenne Muscular Dystrophy  Progressive muscle weakness and early death resulting from dystrophin deficiency Position of Actin and Myosin During Relaxed and Contracted States Keep in mind that actin and myosin do not change length So what changes? Amount of actin and myosin OVERLAP  UNDERSTANDING THE PROCESS OF MUSCLE CONTRACTION 1. Neuromuscular Junction 2. Excitation-Contraction Coupling 3. Contraction-Relaxation cycle  Somatic motor neuron Calcium Acetycholine Acetycholine receptor Voltage-gated Na+ channels Sarcolemma Na+ T-tubule Sarcoplasmic Reticulum (SR) DHPR (a voltage-gated L type Ca++ channel) Ryanodine receptor (RyR, a Ca++ release channel on the SR membrane) Calcium Troponin C Troponin I Troponin T Tropomyosin Myosin Actin ADP + Pi Calcium pumps and exchangers Structure of the somatic motor neuron which controls skeletal muscles ACh binds to Cell Single long nicotinic body axon cholinergic in CNS receptor m u scul ar The ‘neuro nction’ ju Neuromuscular Junction Motor Unit One muscle may have SPINAL CORD many motor units of Each motor unit is a different fiber types. group of muscle fibers that will all contract together Neuron 1  Categorized by Neuron 2 Neuron 3 their contractile Motor and metabolic nerve properties into fiber types KEY Muscle fibers Motor unit 1 Each muscle fiber is Motor unit 2 only innervated by Motor unit 3 one somatic motor neuron Process of Excitation-Contraction Coupling Axon terminal of somatic motor neuron (terminal bouton) Muscle fiber 1 1 Somatic motor neuron releases ACh ACh at neuromuscular junction. 2 Na+ 2 Net entry of Na+ through ACh receptor-channel initiates a local muscle action potential Motor end plate RyR T-tubule Ca2+ Sarcoplasmic reticulum Z disk DHPR Troponin Actin Tropomyosin M line Myosin head Myosin thick filament Initiation of muscle action potential  Binding of ACh to the ACh Receptor is step 1 of membrane depolarization Closed channel Open channel K+ ACh Na + K+ Na+ This receptor is a channel that is chemically gated. Upon ligand (ACh) binding, channel opens to let Na+ pass through. It is the entry of Na+ that will then depolarize the membrane. Process of Excitation-Contraction Coupling Axon terminal of somatic motor neuron (terminal bouton) Muscle fiber 1 1 Somatic motor neuron releases ACh ACh at neuromuscular junction. 2 Na+ 2 Net entry of Na+ through ACh receptor-channel initiates a local muscle action potential 3 Motor end plate RyR 3 If sufficient Na+ entry, will trigger voltage T-tubule gated Na+ channels which will propagate Ca2+ Sarcoplasmic the action potential down into the t-tubule reticulum Z disk DHPR KEY Troponin Actin Tropomyosin M line DHPR = dihydropyridine receptor, an L-type Ca++ channel Myosin head Myosin thick filament RyR = ryanodine receptor, a Ca++ release channel Initiation of muscle action potential Process of Excitation-Contraction Coupling 3 4 Action potential in t-tubule alters conformation of DHP Receptor. 5 Excitation-contraction coupling DHPR opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 6 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 Myosin heads execute power 7 stroke. 6 Myosin thick filament 8 Actin filament slides toward center of sarcomere. Distance actin moves KEY DHPR = dihydropyridine receptor, an L-type Ca++ channel RyR = ryanodine receptor, a Ca++ release channel  Remember two action potentials required FIRST: Action potential sent down somatic motor neuron from the CNS Allows for the release of ACh from the terminal nerve endings SECOND: Action potential propagates along muscle fiber membrane Due to local action potential at site of AchR (transporting Na+) Propagated by transport of Na+ through voltage-gated Na+ channels Allows for the release of Ca++  Important: It is the release of Ca++ from the SR that is signal for initiating contraction  What if End-Plate Potential (EPP) is not sufficiently strong? End plate potential is sufficient at point B to allow membrane to depolarize The end plate potential is too weak at points A and C – Insufficient number of Na+ channels open to allow membrane to reach required threshold level to become depolarized – Subsequently, the DHPR will not be triggered to open, and the RYR will not be opened to release Ca++  And please remember… ACh acting on a skeletal muscles’ motor end plate is always excitatory and creates muscle contraction. There is no antagonistic innervation to relax skeletal muscles. Relaxation occurs when the somatic motor neurons are inhibited in the CNS, preventing ACh release. End plate potential is sufficient at point B to allow membrane to depolarize The end plate potential is too weak at points A and C – Insufficient number of Na+ channels open to allow membrane to reach required threshold level to become depolarized – Subsequently, the DHRP will not be triggered to open, and the RYR will not be opened to release Ca++  MALIGNANT HYPERTHERMIA Genetic mutation of calcium release channel RYR (or dihydropyridine receptor DHPR) Malignant hyperthermia episode is triggered by anesthetic agent, causes RYR to stay open. Consequently, an altered calcium balance between the lumen of the sarcoplasmic reticulum (SR) and the sarcoplasm occurs. Normally, muscle cell depolarization is sensed by the DHPR, which signals RYR1 opening. In malignant hyperthermia, accumulation of abnormally high levels of calcium in the sarcoplasm causes uncontrolled anaerobic and aerobic metabolism and sustained muscle cell contraction. Results in respiratory acidosis, metabolic acidosis, muscle rigidity, and hyperthermia. If not stopped, ATP depletion eventually causes widespread muscle fiber hypoxia (cell death, rhabdomyolysis), which manifests clinically as hyperkalemia and myoglobinuria and an increase in creatine kinase. Emergency treatment with dantrolene sodium which binds to RYR1, causing it to mostly close, thereby reversing the uninhibited flow of calcium into the sarcoplasm. http://video.nationalgeographic.com/video/kids/animals- pets-kids/mammals-kids/goat-fainting-kids/ And in case you can’t play the above goat video because a subscription is required, you can see a silly YouTube video compilation at this link: https://www.youtube.com/watch?v=YI4hzzepEcI Myotonia congenita Inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance resulting from a genetic defect in chloride channel CLCN1 Advances in Genetics vol 63 2008  Somatic motor neuron Calcium Acetycholine Acetycholine receptor Voltage-gated Na+ channels Sarcolemma Na+ T-tubule Sarcoplasmic. Reticulum (SR) DHPR (a voltage-gated L type Ca++ channel) Ryanodine receptor (RyR, a Ca++ release channel on the SR membrane) Calcium Troponin C Troponin I Troponin T Tropomyosin Myosin Actin ADP + Pi Calcium pumps and exchangers Position of Actin and Myosin During Relaxed and Contracted States Keep in mind that actin and myosin do not change length  When the head of a myosin molecule bends towards its arm it moves the actin filament along with it, and will then break free and create a new attachment http://www.youtube.com/watch?v=gJ 309LfHQ3M  Cross-Bridge Cycle Each cycle advances the myosin head by 2 actins, ~ 11 nm  Cross-Bridge Cycle Start with the muscle in the rigor or rigid state, which, at the cellular level, is an Attached state. The myosin head is fixed at a 45 deg angle relative to filaments. If no new ATP comes in (due to cell death) our muscles stay in this state- also known as RIGOR MORTIS.  Cross-Bridge Cycle Step 1: ATP binding to myosin head Reduces affinity of myosin for actin, causes myosin head to release from actin  Cross-Bridge Cycle Step 2: ATP hydrolysis Breakdown of ATP to ADP + Pi by myosin ATPase ADP + Pi stay on myosin head. Myosin head pivots to cocked position, perpendicular (90 deg) to filaments and lines up with a new actin molecule (but doesn’t attach yet!) Review of ATP (Adenosine triphosphate) e Pas AT s i n M yo  Cross-Bridge Cycle Step 3: Cross-bridge formation Cocked myosin head attaches weakly to its new position on actin. ADP + Pi are still on myosin head. Cross-Bridge Cycle Step 4: Release of Pi from myosin Triggers the powerstroke! Myosin head bends 45 deg and pulls actin filament toward tail of myosin. This generates force and motion! Cross-Bridge Cycle Step 5: Release of ADP from myosin Dissociation of ADP from myosin leaves actomyosin complex in a strongly attached rigid state.  Cross-Bridge Cycle So what regulates whether or not a crossbridge can form?  Steric Hindrance Model of Contractile Regulation Position of tropomyosin is associated with presence vs. absence of calcium Image when Image when Overlay of the calcium NOT calcium two images present present  Troponin G-Actin In a relaxed state tropomyosin partially blocks TN the actin-myosin Myosin head binding sites. Tropomyosin blocks binding This prevents site on actin Pi ADP myosin from completing a power stroke The function of tropomyosin These components working together allow actin-myosin interaction to be regulated by changes in [Ca++]  Cross-Bridge Cycle So what stops a crossbridge from forming?  Removal of Calcium Ions to Stop Muscle Contraction 1. Sodium-calcium exchanger and calcium pump of sarcolemma rid cell of calcium 2. Calcium pump located in wall of SR (Ca- ATPase) 3. Calcium binding proteins Function to remove free Ca++ in order to restore resting intracell. Ca++ levels Normal resting Ca++ levels are too low to initiate muscle contraction  Each round of the cross-bridge cycle consumes one molecule of ATP In skeletal muscle, the entire cellular store of ATP is sufficient to allow only a few seconds of continuous maximal contraction The muscle cell must resynthesize ATP from ADP very rapidly Remember in‘Metabolism’ section of the course- how we generate ATP through aerobic and anerobic means  Rigor Mortis Living muscle will be in a state of rigor temporarily each cross bridge cycle During a state of rigor there is no ATP bound to myosin Actin and myosin are locked in a state of rigor due to the absence of ATP Typically lasts for ~72 hours post death, until the muscle proteins start to degrade Energy Required for Skeletal Muscle Contraction The major energy consumers of muscle contraction are: To release myosin from actin, ATP is required. The energy from they hydrolysis of ATP is used to fuel the power stroke for contraction to occur (Mysosin ATPase). Ca-ATPase pump to return Ca++ back to the sarcoplasmic reticulum to allow muscle to relax Na-K pump to restore resting membrane potential of the skeletal muscle membrane Twitch and Tetanus With a twitch, notice how force lasts much A twitch is a longer than the duration single of the AP or Calcium transient contraction- relaxation cycle! Summation of twitches Tetany or tetanus  Twitch and Tetanus Increasing frequency of e-stim of skeletal muscle results in an increase in the force of contraction. How? Due to prolongation of the intracellular Ca2+ transient. Summation and tetany results from initiation of another intracellular Ca2+ transient before the muscle has completely relaxed- causes a summation of twitch forces.  Review of normal physiology: Somatic (motor) Reflexes  What does the term ‘efferent’ mean? E = causes The nerve signaling is in the direction of going an Effect from the CNS to a peripheral effector What does the term ‘afferent’ mean? The nerve signaling is in the direction of going away from the sensory receptor, to provide info to the CNS A = sensory signal is going Away from the source So, test yourself, which is which here? Simultaneous excitation of agonist muscle and inhibition of antagonistic muscles  Classification by Efferent Division: Autonomic (Visceral) Reflex Important autonomic reflexes (e.g. homeostasis) are integrated first in the CNS (hypothalamus, thalamus, and brain stem) Classificatio n by Efferent Division: Somatic (Muscle) Reflex  The Sensory Receptors of Skeletal Muscle Sensory receptors are AFFERENT neurons These receptors function almost entirely on a subconscious level 1. Muscle Spindle – Located in the belly of the muscle – Relays information to the CNS about changes in muscle length 2. Golgi Tendon Organ – Located between the tendon and the extrafusal fibers – Relays information to the CNS about tension in the tendon and changes in the rate of tension Location of the Sensory Receptors in Skeletal Muscle Location of the Sensory Receptors in Skeletal Muscle E, extrafusal muscle fiber I, intrafusal muscle fiber Arrow, connective tissue capsule Muscle spindle in rat soleus muscle Muscle Spindles and the Golgi Tendon Organ are important for regulating the intrinsic control of the muscle The Muscle Spindle Muscle Spindle Golgi Tendon Organ Reminder slide about classificati on of nerves Circled are the ones we are talking about regarding muscle Muscle Spindle Muscle spindle s are found in parallel with muscle fibers Sensory nerve fibers encircle the central region of each intrafus al fiber  Structural Organization of the Muscle Spindle Muscle spindles are oriented in parallel to the muscle fibers (extrafusal fibers) Intrafusal muscle fibers are surrounded by the extrafusal fibers Center of the muscle spindle contains no contractile elements Extrafusal fibers are innervated by alpha motor neurons, while intrafusal fibers are innervated by gamma motor neurons  Role of the Muscle Spindle To monitor muscle length and prevent overstretching – Muscle spindles fire even when muscle is relaxed (are constitutively active)  How are Muscle Spindles Excited? The receptor region of the muscle spindle responds to changes in stretch and can be activated in one of two ways: 1) Lengthening the whole muscle stretches the mid-portion of the spindle which will excite the sensory receptor 2) Even if the length of the muscle does not change, contraction of the end regions of the spindle’s intrafusal fibers stretches the mid-portion of the spindle and excites the sensory receptor Two sensory receptors are located in the center of a muscle spindle innervate the intrafusal fibers: – Primary endings (Type 1a)-wraps around central region – Secondary endings (Type II)-found on one or both sides of the primary endings Anterior Motor Neurons (Alpha and Gamma) and Interneurons (posterior) Motor neurons are located in the anterior portion of the cord Located in the gray matter of the Anterior (anterior) horn of the spinal cord, leave via the anterior root  Anterior Motor Neurons: Single  motor neuron Alpha Alpha (α ) motor neurons (efferent fibers) – Large type A-alpha motor Can excite 3-100 nerve fibers (~14 microns) individual muscle fibers Extensive branching when they innervate skeletal muscle fibers – Stimulation of a single type A-alpha fiber can excite 3 to 100 EXTRAFUSAL MUSCLE FIBERS collectively called a MOTOR UNIT Motor Unit Each motor unit is a group of muscle fibers that will all contract together  Categorized by their contractile and metabolic properties into fiber types Each muscle fiber is only innervated by one somatic motor neuron Alpha Motor Neurons Innervate Extrafusal Fibers  Anterior Motor Neurons: Gamma Gamma motor neurons (these are also efferent fibers) – Smaller type A-gamma motor fibers (~5 microns in diameter) – Stimulation of a gamma motor neurons excite INTRAFUSAL FIBERS Intrafusal fibers are much smaller than extrafusal fibers Help regulate muscle tone Central region of an intrafusal fiber contains no actin or myosin – Central region does not contract Intrafusal fibers are pointed at their ends Gamma Motor Neurons Innervate Intrafusal Fibers  Two Types of Intrafusal Fibers 1. Nuclear bag fiber: nuclei are located together in an expanded region of the center of the spindle – Excites the primary afferent (Type 1a) 2. Nuclear chain fiber: nuclei are arranged in a chain in the central region of the spindle – Excites the primary afferent (Type 1a) AND the secondary afferent (Type II) afferen afferent) t)  Control of Static and Dynamic Responses by Gamma Motor Neurons: Intrafusal Fibers Two types of gamma motor neurons that innervate the intrafusal muscle fibers: – Gamma-dynamic (Gamma-D): primarily excites the nuclear bag intrafusal fibers (enhances dynamic response) – Gamma-static (Gamma-S): primarily excites the nuclear chain intrafusal fibers (enhances static response) afferen afferent) t)  Primary afferent (Type 1a) Sends sensory information to CNS. Note how it wraps around the nuclear bag Nuclear bag fiber fiber This bag has several nuclei clumped together Location of Primary and Secondary Endings Primary and Secondary Endings of the Intrafusal Fiber PRIMARY ENDINGS SECONDARY ENDINGS Located in the center Located in center of of the muscle fiber muscle fiber on either side Type Ia fiber (~17 of the primary endings Type II fibers (~8 microns) microns) Transmit signals more Transmits signals to slowly spinal cord rapidly Static vs Dynamic Response for Muscle Spindles Static: Slow response Dynamic: Fast Response When the center of spindle When the center of the spindle is stretched slowly the is stretched rapidly the number of impulses number of impulses generated generated by the primary by the primary endings and secondary endings increases in proportion to the increases in proportion to rate of change in muscle the degree of stretch spindle length – Signals continue for – Signals stop as soon as lengthening/shortening stops several minutes  Positive and Negative Sensory Input Positive Signals – If there is an increased number of signals delivered to the CNS this provides information that the muscle is being stretched from normal Negative Signals – If there is a reduction in the number of signals delivered to the CNS this provides information that the muscle is being understretched from normal Muscle Spindle Function  Alpha-Gamma Co-activation When signals are sent to alpha motor neurons, gamma motor neurons are typically co-activated to allow extrafusal and intrafusal fibers to contract together – 1. Prevents sensory receptor portion of muscle spindle from changing during muscle contraction – 2. Allows the receptor to operate under optimal conditions  Without Gamma Motor Neurons (hypothetical situation) Extrafusal muscle contracts via alpha-motor neuron, however intrafusal fibers innervated by the gamma motor neurons are not activated – This results in a change in the activation of the sensory neurons (Type 1a and Type II)  Dynamic vs Static Stretch Reflex Dynamic: Occurs QUICKLY Static: Occurs SLOWLY Dynamic stretch reflex initiated by dynamic signals transmitted from primary Static stretch reflex is sensory endings initiated by primary and – Responding rapidly to secondary sensory endings changes in either stretch or – Typically takes over after a unstretch from normal dynamic stretch reflex resting length of the Allows for more fluid muscle spindle contractions that are not Will cause the muscle to jerky either increase or decrease – Constant muscle strength of contraction contractions Ia Ia & II  Patellar Tendon (Knee Jerk) Reflex Afferent path: Action Integrating center: potential travels through Sensory neuron sensory neuron. synapses in Receptor: Muscle spinal cord. spindle stretches and fires. Stimulus: Tap to tendon stretches muscle. Requires divergent pathways Activation of Efferent path 1: onto quadriceps Somatic motor neuron and inhibition of hamstring Effector 1: Quadriceps muscle Efferent path 2: Interneuron inhibiting somatic motor neuron Response: Quadriceps contracts, swinging lower leg forward. Effector 2: Hamstring muscle Response: Involves Hamstring stays relaxed, reciprocal allowing extension of leg inhibition (reciprocal inhibition).  Clinical Application of the Stretch Reflex  Knee jerk reflex Striking the patellar tendon with a hammer stretches the quads This initiates a stretch reflex which shortens the muscle by exciting a dynamic stretch reflex, resulting in leg moving forward What is this telling us? Information about the sensitivity of the stretch reflex –Index of the facilitation of the gamma efferent neurons If there is a delay in the muscle jerk response this gives us information that these sensory neurons are either damaged or somehow depressed If increased jerk response, then there is increased excitation in the system, increased tone in internuncial pool Flexion Reflex and the Crossed Extensor Reflex Flexor Reflex vs Cross Extensor Reflex Flexor Reflex Crossed Extensor Reflex Painful stimulus causes Painful stimulus elicits a flexor muscles to become flexor reflex in affected excited to allow the limb limb and an extensor to withdrawal from the reflex in the opposite limb stimulus. Extensor reflex begins 0.2 Neural pathway passes - 0.5 seconds after the through interneuron pool painful stimulus through 3-4 neurons Serves to push body away before reaching the from the stimulus, also to anterior motor neurons shift weight to the opposite limb Another example of reflexes in action Simultaneous excitation of agonist muscle and inhibition of antagonistic muscles GOLGI TENDON ORGAN Golgi Tendon Organ  Golgi Tendon Organ: Responds to Tension Located at junction between tendon and end of muscle fiber – Arranged in series, whereas muscle spindles are found parallel to the muscle fibers When tension becomes too great, muscle fibers become inhibited – Generate a relaxation reflex ~10-15 muscle fibers are are connected to each GTO How are Signals Transmitted to the CNS? Signals from the GTO are relayed to the CNS via type Ib nerve fibers (~16 microns diameter) – Free nerve endings intertwined with collagen fibers – Upon contraction, collagen fibers apply pressure to the free nerve endings stimulating the type Ib afferent fibers Sensory information that enters into the spinal cord will excite an inhibitory interneuron which will inhibit the anterior alpha motor neuron – Muscle contraction will be reduced/stopped type Ib nerve fibers  Golgi Tendon Organ and the Lengthening Reaction When the GTO is stimulated, the reflex is always inhibitory to prevent the development of excess tension Goal is to prevent muscle damage Lengthening reaction: If the tension generated by a muscle is too great the spinal cord will cause instantaneous relaxation of the entire muscle to prevent damage Illustration of the Golgi Tendon Reflex (type Ib)