9/27/16 – W5D1H4 Cell Physiology:Muscle Physiology

Learning Objectives: 1. Explain the mechanism by which muscle contracts, outlining how the sliding of actin filaments in sarcomeres is driven by ATP-dependent chemo-mechanical cycling of myosin motor proteins. 2. Explain excitation-contraction (EC) coupling and relaxation in skeletal muscle by identifying the roles of the t-tubules, the calcium channels (Cav1.1 [L-type] and Ryanodine receptor [RyR]), the thin filament regulators (troponin and tropomyosin), and the ATP-dependent SERCA pumps in these processes. 3. Understand the differences in EC coupling in skeletal, cardiac, and smooth muscle. Describe the two- stage phospho-regulatory cascade that initiates contraction in smooth muscle. 4. Compare the twitch contractions of slow/type 1 and fast/type 2 skeletal muscle fibers and explain the molecular bases for the differences in twitch behavior. 5. Outline the shift in energy sources in a working skeletal muscle as it goes from rest to extended periods of activity. Compare the energy needs in different muscle types. 6. Define muscle mechanics concepts including contractility, preload, afterload, isotonic and isometric contractions. Compare the mechanics in different muscle types. 7. Explain how smooth, graded contractions of a skeletal muscle are produced by changes in stimulus intensity and by the size principle of motor unit recruitment. 8. Contrast the innervation and location of single-unit vs. multi-unit smooth muscle types.

From organ to molecule and back again LO #1. Explain the mechanism by which muscle contracts, outlining how the sliding of actin filaments in sarcomeres is driven by ATP-dependent chemo-mechanical cycling of myosin motor proteins. (continued) From organ to molecule and back again

During muscle contraction: From your syllabus: During muscle contraction: a. The myosin heads (bound by ADP-Pi) bind to the actin filaments b. Next, the myosin heads undergo a conformational change called a “power stroke” to pull the thin filaments a short distance past the thick filament (ADP released) c. Then the links between actin and myosin break (requires ATP) d. Then the myosin head returns to its original conformation and the process is repeated, as the muscle is contracting. e. ATP is required to break the actin-myosin links to allow another power stroke. Project this slide and briefly review it with students.

LO #2. Explain excitation-contraction (EC) coupling and relaxation in skeletal muscle by identifying the roles of the t-tubules, the calcium channels (Cav1.1 [L-type] and Ryanodine receptor [RyR]), the thin filament regulators (troponin and tropomyosin), and the ATP-dependent SERCA pumps in these processes. 1 Somatic motor neuron releases ACh at neuromuscular junction (1 impulse = ~60 vesicles of ACh) End plate: Net entry of Na+ through nACh receptor channel initiates first an endplate potential (+30-40mV), and then a muscle action potential is produced when nearby voltage-gated channels activated Action potential travels deep into muscle fiber along t-tubules. T-tubles form triad with SR. Voltage change in t-tubules triggers conformational change in Ca2+ channels (DHP receptors & RyRs) to increase intracellular Ca2+ Ca2+ binding to regulatory protein troponin C in thin filaments which displaces tropomyosin to expose myosin binding sites on actin, allowing contraction to begin & myosin binding initiates the cross bridge cycle to generate force. See the SYLLABUS for more detail!! 2 3 4 Ryanodine receptor 6 SERCA Project this slide and briefly review it with students. Note that the 6 steps are expanded upon in the syllabus!!!!!! Ryanodine receptor ATP 5 VIDEO

Cardiac muscle has similar EC coupling as skeletal muscle. LO #3. Understand the differences in EC coupling in skeletal, cardiac, and smooth muscle. Describe the two- stage phospho-regulatory cascade that initiates contraction in smooth muscle. Cardiac muscle has similar EC coupling as skeletal muscle. Some Important Differences: Cardiac muscle has pacemaker cells that generate current; current can pass between cells through gap junctions Cardiac action potential is broad because of Ca2+ influx. Diads not triads in cardiac L-type Ca2+ channels are NOT coupled to ryanodine Ca2+ release channels as they are in skeletal muscle. Cardiac muscle requires Ca2+ induced Ca2+ release (CICR) from SR. Different isoforms of troponin present in cardiac muscle ANS controls the rate of contraction (parasympathetic & sympathetic) and contractility (sympathetic only). During recovery, most Ca2+ is recovered by the SR, but also extruded by exchanger & Ca2+ pump

EC coupling in cardiac muscle from your syllabus: Please note that this diagram “shows” a triad, but we’ll call it creative license to show both excitation and recovery on one slide. Project this slide and briefly review it with students.

LO #3. Understand the differences in EC coupling in skeletal, cardiac, and smooth muscle. Describe the two-stage phospho-regulatory cascade that initiates contraction in smooth muscle.

Important points in smooth muscle EC coupling. Action potential (AP) / nervous system not required; all that is necessary is increase in Ca2+ through a variety of mechanisms Ca2+ influx in myocyte can lead to calcium-induced calcium release (CICR) Ca2+ binds to calmodulin (CaM) Smooth muscle lacks troponin; for actin and myosin to interact, the light chain of myosin must be activated by myosin light chain kinase (MLCK), which is activated by CaM Active MLCK phosphorylates light chain myosin heads and increases myosin ATPase activity Active myosin crossbridges slide across actin to create muscle tension (no sarcomeres; cell contracts like “corkscrew”) Relaxation occurs when Ca2+ levels renormalize via Ca2+ ATPases & Na+/Ca2+ on membrane, SERCAs Crossbridge cycling continues as long as ATP is present; relaxation is dependent on myosin phosphatase, which dephosphorylates MLCK to deactivate it. Relaxation of smooth muscle in some vasculature can also occur through a nitric oxide (NO)-mediated pathway

LO #4. Compare the twitch contractions of slow/type I and fast/type II skeletal muscle fibers and explain the molecular bases for the differences in twitch behavior. Three types of skeletal muscle fibers: Type I (slow), Type IIa (fast oxidative), and Type IIb (fast glycolytic). Fiber types are defined based on: velocity, myosin ATPase isoform, and biochemical profile.

A twitch has three phases: latent, contraction & relaxation periods LO #4. Compare the twitch contractions of slow/type 1 and fast/type 2 skeletal muscle fibers and explain the molecular bases for the differences in twitch behavior. A twitch contraction is when tension is generated in a motor unit in response to a single stimulus (like an AP). A twitch has three phases: latent, contraction & relaxation periods Latent period is not variable, but contraction & relaxation are determined by the fiber types in any given muscle.

Four sources of energy (ATP): LO #5. Outline the shift in energy sources in a working skeletal muscle as it goes from rest to extended periods of activity. Compare the energy needs in different muscle types. Four sources of energy (ATP): Stored ATP (about 5 mmol/kg muscle; only about 3-5 s of max power before stores are depleted, but they NEVER do get depleted!) Phosphocreatine (about 25 to 30 mmol/kg muscle; about 30 s max power until depleted) Anaerobic glycolysis (peaks in as early as 30 s but can result in [Lac]blood of 15-20 mmol/L). Aerobic respiration (limited at the muscle by mitochondrial capacity)

Energy sources and ATP utilization LO #5. Outline the shift in energy sources in a working skeletal muscle as it goes from rest to extended periods of activity. Compare the energy needs in different muscle types. (continued) Energy sources and ATP utilization Short high intensity activities like weight lifting or sprinting rely on ATP and phosphocreatine stores. Burst-like activities like tennis rely on glycolysis and respiration. Endurance activities depend on respiration using both glucose and fatty acids. As long as a muscle has oxygen (and it always does unless blood flow or PO2 is restricted) it will form ATP by aerobic pathways. But if the exercise demands are great, the cells will resort to anaerobic pathways for ATP. Energy sources for cardiac and smooth muscles are generally aerobic. The heart primarily uses fatty acids (60-80 %) as energy sources. Limited anaerobic pathways (2 % ATP derived from glycolysis) means that the heart requires high amounts of O2 and if it becomes limited, irreversible hypoxic muscle damage ensues (myocardial infarction). Smooth muscle has slow ATPase activity so the ATP depletion is not usually an issue. Project this slide and briefly review it with students.

LO #5. Outline the shift in energy sources in a working skeletal muscle as it goes from rest to extended periods of activity. Compare the energy needs in different muscle types. (continued) Contractility is the ability to shorten forcibly when adequately stimulated. Most skeletal muscles exhibit some amount of tension, or muscle tone even if they are not consciously being contracted. A muscle is typically stretched to some length (preload) such that the overlap between actin filaments and myosin heads is optimized; too little or too much preload will lead to less contraction in skeletal muscle because fewer crossbridges can form. This is referred to as the length-tension relationship. Skeletal and Cardiac muscle have very different length-tension curves. Innumerable crossbridge cycles occur during muscle contraction, but the amount of force they generate is finite. The force resisting further shortening after the muscle is stimulated to contract is called the afterload. In skeletal muscle, the load is the weight of the object being moved. IN cardiac muscle, afterload in mean arterial blood pressure.

LO #6. Define muscle mechanics concepts including contractility, preload, afterload, isotonic and isometric contractions. Compare the mechanics in different muscle types. Two types of muscle contractions: Isotonic: occur when a muscle shortens but maintains a constant tension – not common in normal use. Isometric: when the afterload is too heavy to lift and the muscle cannot shorten even though crossbridge cycling continues to generate tension. Cardiac muscle mechanics: Myocardial excitation always involves all fibers, and so all fibers are involved in generating force as a unit. In cardiac, there is no option for recruiting motor units, so if the force is to be changed, the ANS will regulate Ca2+ permeability. Contractility, preload, and afterload should wait until the cardiovascular class in the next block! They are the major determinants of stroke volume! Contractility considers the force of contraction at a given cell length and is changed by changes in calcium availability. Preload involves stretching the myocytes and is changed by ventricular filling. Afterload involved the degree of pressure in the aorta which is affected by MAP. Smooth muscle mechanics: Smooth muscle can contract rapidly, and then completely relax, but in other cases, smooth muscle can maintain a low level of active tension for long periods without cyclic contraction and relaxation.

Temporal summation in muscle: Increased frequency of stimulation generates more force; tetanus results when maximum force is produced. Project this slide and briefly review it with students.

Spatial Summation of Motor Units Motor units are recruited by size: as motor units with larger and larger muscle fibers ae excited, contractile strength increases (size principle). Size principle allows for force increases to occur in small steps. Rarely are all motor units recruited; typically they are recruited asynchronously and “take turns” to delay fatigue. Project this slide and briefly review it with students.

LO #8: Contrast the innervation and location of single-unit vs LO #8: Contrast the innervation and location of single-unit vs. multi-unit smooth muscle types. Smooth muscle has two main functional types: Multiunit, tonic: Individual myocytes function independently and allows for fine control. capable of maintaining sustained contractions. Found in e.g. large airways, ciliary & iris muscle of eye, sphincters, vasculature, arrector pili (the small muscles that raise hairs, and goose-bumps, on skin). Visceral, phasic, single-unit, unitary: Gap junctions between neighboring cells allow for current to spread across the muscle cells, so they act together as a syncytium. Some smooth muscle cells contain pacemaker cells but autonomic neurons do synapse on a few of the cells in the muscle to help regulate contraction. Phasic smooth muscle contracts transiently when stimulated. Found in e.g. GI tract and urogenital walls However, most smooth muscles are a blend of phasic and tonic, which allows them to respond to a range of stimuli. Project this slide and briefly review it with students.