Neuromuscular Physiology Lecture 1 and Summary

LIFE 311 Notes Lecture 1 /2 Types of Muscle Skeletal Muscle = Fat contraction Smooth Muscle = Slow contraction Skeletal Muscle Fibres You get these network of tubules, which allow the signals to go deep into the cells and allows the whole muscle to contract. It also hold an internal store of calcium ions. This is found close to the T tubules. Myosin is shown in blue actin is in red . A sarcomere is the smallest unit of contraction you can get. neurone Skeletal muscle is involved in the somatic reflex arc. This is the fast response we have when we come across a painful stimulus. This goes from the stimulus to a motor neurone and then straight to the effector. Neuromuscular Junction As the signal comes down, there is an influx of calcium. This causes the vesicles to move and fuse to the synaptic outer membrane. This then causes the release of acetylcholine. Which then binds to the receptors on the muscle which causes a release of calcium in to the muscle. For a contraction to occur you need energy: ATP and Ca+ The role of ATP Atp is responsible for an array of vital functioning. It transports macromolecules in and out of the cell and through the cell membrane . It is also an extracellular and intracellular signalling molecule. In ATP the charges mainly oppose each other so it is an unstable molecule. When the phosphate group is remove energy I s released. Hydrolysis of ATP To remove the phosphate group you need a hydrolytic enzyme. In this case actin is the hydrolytic enzyme which cleaves the phosphate of the ATP. The Potential energy in ATP is released when the terminal high energy bond is broken by a hydrolytic enzyme. The end products of the hydrolysis is ADP and Pi + energy Single Cross bridge cycle 1. Rise of intracellular Ca2+ 2. Binding of Ca2+ to troponin moves tropomyosin away from myosin binding sites on actin. (at this point we have the energy and the binding sites exposed because tropomyosin is removed) 3. This causes the formation of a cross-bride between the actin and myosin 4. There is then a power stroke which releases the actin and the myosin moves along 5. The cross bride then detaches which also requires ATP. If the ATP isn't there: No ATP = no detachment = rigor (when someone dies there is rigormortis, and this is because of the cross bridges remaining attached. 6. The myosin head is reactivated by the release of energy from ATP 7. The energised myosin head is then repositioned to restart the cycle ATP is playing a dual role in CB cycling In muscle cells the energy molecule ATP plays an important role in: 1. Energising the power stroke of the myosin cross bridge 2. Disconnecting the myosin cross bridge from the binding site on actin at conclusion of the power stroke 3. Energising the calcium ion pumps : a. Ca2+ pump of the SR b. CA2+ pump of the PM Cross bridge cycling in smooth muscle The contraction (thick and thin) filaments are connected to dense bodies. Therefore the areas with dense bodies are squeezed together (contracted) 1. Cytotoxic Calcium is released into the muscle 2. Calcium bind to calmodulin in the cytosol 3. Calcium calmodulin complex binds to myosin light-chain kinase 4. Myosin light-chain kinase uses ATP to phosphorylate myosin cross bridges 5. Phosphorylated cross bridges binds to action filaments 6. Cross-bridge cycle produces tension and shortening. If you have a highly active phosphate myosinase you wont get contraction Low vs High concentrations of ATP • Decrease in ATP level reduces the CB cycling rate by increasing the time spent by the CB is attached configuration. Therefore more force is produced by individual CB (paradoxically less ATP = more force) So where odes ATP come from: • Concentration of ATP is relatively low at rest enough for 8 twitches • Backup energy source = phosphocreatine (Pcr) • Amount is limited and is rapidly depleted. PCR will help rev up your muscles to maximum, but will only last 10s or so • Another ATP is coming from respiration. This is the metabolism glucose. The first step is glycolysis which converts the glucose in pyruvic acid (pyruvate) This is the only sep that takes up energy, the rest generate ATP. • Ideally we want aerobic condition where pyruvate is converted into acetyl coA. This enters the citric acid cycle (krebs cycle) and then you get 3NADH2 and 1FADH2. Lecture 3: Introduction to smooth muscles – a varied tissue Structure: • Muscle means it has to be able to produce motor force – movement. • It uses the same molecular motor proteins – actin and myosin • Smooth means no striations when you look down the microscope Many of the myocytes can synthesise their own eactinextracellular matrix (this is normally done by fibroblasts) Thin and thick filaments: contraction Thin filaments and their proteins Actin and tropomyosin are main constituents 1of thin filament, no troponin in smooth muscle. Caldesmon and calponin. Major thin filament associated proteins Potent inhibitors of actin-activation of myosin ATP’ase They slow rate-limiting release of products of ATP hydrolysis (ADP and Pi) from actomyosin When phosphorylated, their inhibition of myosin ATP’ase is removed Calmodulin: Ca binding protein. A regulatory subunit of several enzymes, including myosin light chain kinase General Structure Smooth muscle cells (SMC) are spindle-shape ~ 20-500um ~ 2-10um diameter. They have a large are/volume ratio Extracellular matrix Smooth muscle myocytes are embedded in connective tissue. Some SM e.g. uterine and arterial, synthesise collagen, elastin and proteoglycans and have a well developed ER (endoplasmic reticulum) Plasticity: The cells can chase their characteristics in response to certain stimuli e.g. in culture can loose contractile ability and become secretory-like. Growth of SMC can be stimulated by stretch e.g. GI vessels, uterus – the growth in pregnancy s mainly by hypertrophy regeneration : SMs have varying capacity for self-renewal. Blood vessels regenerate –but also growth can be inappropriate e.g. after ballon angioplasty Plasma Membrane • The plasma membrane is the site of receptors, ion channels, pumps and exchangers • Contrains caveolae (a form of lipid rafts) – invaginations of membrane contrains Muscle means it has to be able to produce motor force (movement) it uses the same molecular motor proteins - actin and myosin. - Smooth means no striations Smooth muscle cells are usually spindle shape cells. Usually 20-500u moon ~2-10um diameter Smooth muscle cells are strange as they can create their own extracellular metric, which is usually made by fibroblasts. Myosin ATP’ase activity One ATP’ase site in each myosin head. The difference in ATPase activity of smooth muscle and striated muscle is due to the different effect of actin • In striated muscle, myosin is activated >100x on addition of actin • In smooth muscle activation by actin is NEGLIGIBLE Because activation of smooth muscle by actin requires phosphate of regulatory high chain (lc). Which produces conformational change in myosin, allowing actin to activate its ATP’ase Thin and thick filaments Actin and tropomyosin are main proteins on thin filament NO TROPONIN IN SMOOTH MUSCLE Caldesmon and Calponin are major thin filament associated protein Potent inhibitors of actin-activation of myosin ATP’ase • They slow rate-limiting release of products of ATP hydrolysis - (ADP+Pi) from actomyosin • When phosphorylated, their inhibition of myosin ATP’ase is removed • This will increase force Calmodulin: Ca binding protein • A regulatory subunit of several enzymes, including Myosin light chain kinase (MLCK) To transmit the tension generated by cross-bridge interaction, smooth muscle uses dense bodies (analogous to Z lines) Upon activation myosin undergoes cyclic interaction with actin. Relative sliding of thick and thin filament a.b. little direct evidence for this in smooth muscle Driven by ATP hydrolysis Contraction Mechanics - Basics In smooth muscle the rate limiting step in cross bridge cycle is Pi (inorganic phosphate) release The cross-bridge has a longer life in smooth muscle than striated (economical) > economy of force production in smooth muscle i.e. lot of tension time/ATP • Economy = force x time/ATP EFFICIENCY IS LOWER IN SMOOTH MUSCLE The pathway to force production in smooth muscle. Muscle contraction in smooth muscle is a balance of enzymatic activity between MLCK and MLCP. THIS IS A KEY DIFFERENCE IN SMOOTH MUSLCE.