Muscle Physiology Types of muscle Contraction of skeletal muscle Control of contractility and mechanical properties Energetics of skeletal muscle Comparison of cardiac and skeletal muscle Comparison of smooth and skeletal muscle Skeletal Muscle Large, long, multinucleate cells. Multiunit control (ie each fibre can be activated independently http://www.meddean.luc.edu/lumen/MedEd/Histo/HistoImages/hl3A-46.jpg Smooth Muscle Medium sized, small diameter fibres. Single nucleus. Multiunit: pilomotor. Mixed: blood vessel. Single Unit (i.e. all fibres contract together): intestinal muscle Cardiac Muscle Short diameter, medium length, branching cells. Single Unit Skeletal muscle; light microscope Striations caused by ordered arangement of actin and myosin fibres Skeletal muscle: em Actin Myosin Z-disc Sarcoplasmic reticulum The sarcoplasmic reticulum acts as Ca2+ store http://www.bris.ac.uk/Depts/Physiology/ugteach/ugindex/m1_index/nm_tut3/page2.htm Triad Structure An action potential from the outside of the cell travels down The t-tubules. It is transmitted to a calcium release channel on the SR. This opens, releasing Ca2+ into the cytoplasm Actin, troponin and tropomyosin http://www.cryst.bbk.ac.uk/PPS95/course/10_interactions/actin_myosin3.html Muscle contraction 1) Myosin binds ATP and hydrolyses it to ADP and Pi 2) Myosin binds to actin 3) Myosin undergoes a conformational change in which the actin is pulled forward 4) ADP is released, ATP binds and myosin dissociates from actin Normally actin filaments are covered with troponin- tropomyosin complexes, which prevents myosin binding. In the presence of calcium (released from the sarcoplasmic reticulum), the troponin allows the myosin to bind. http://www.cryst.bbk.ac.uk/PPS95/course/10_interactions/a_m_contract.gif Control of contractility (1) Recruit more motor units Motor Unit 1 Muscle fibres Nerves Motor unit 2 Control of contractility (2) Twitch v tetanus Single Summation Tetanus Twitch Force Time Action potential is much shorter than the contraction, so it is possible to re-excite the muscle fibre before the previous contraction has finished. Types of contraction • Isometric contraction: muscle remains at the same length but produces tension (pushing, supporting). • Isotonic contraction: muscle remains at the same tension but shortens (movement). Force-length curve Energetics • ATP is present at low concentrations and is hydrolysed to give ADP and Pi • Large stores of creatinine phosphate (CP); ADP+CP→ATP+Creatinine • Glycogen breakdown creates more ATP which can be used directly in contraction or to rephosphorylate creatinine. Glycogen breakdown is stimulated by increased [ADP]. It may be oxidative or anaerobic, in which case lactic acid is produced and the muscle goes into oxygen debt. When enough oxygen is available the lactate can be oxidised to give more ATP. • Red (slow twitch) muscles have very high ATP generating capabilities and contain myoglobin; can produce sustained contractions. White (fast twitch) muscles produce powerful short contractions. Cardiac Muscle Main differences from skeletal muscle 1) Source of Ca2+ for contraction (extracellular) 2) Single unit muscle Control of Contractile Force • Calcium for contraction enters during the action potential which is as long as the contraction. The subsequent refractory period occurs when the muscle is at rest. Thus it is not possible to produce a tetanus (important for a pump!). • Can get facilitation with increased frequency of stimulation. Role of nerves • Gap junctions ensure the spread of Ca2+ from cell to cell so that the muscle behaves as a single unit. Contraction is initiated spontaneously. Nerves act to increase or decrease calcium entry. Smooth Muscle Differences from skeletal muscle 1) Source of Ca2+ (extracellular) 2) Single unit muscle (sometimes) 3) Control of contraction There is no troponin or tropomyosin. Myosin has extra subunits, myosin light chains (MLC), that prevent it from interacting with actin. Calcium activates an enzyme, myosin light chain kinase (MLCK) which phosphorylates the MLC. This allows myosin to interact with actin. 4) Structure No striations. Structure The less regular oranisation of actin and myosin means that smooth muscle cells "round up" when contracting, producing a much greater degree of shortening. Due to the irregular arrangement of the actin and myosin, the muscle has a much broader working range over which the maximum tension can be developed. Stress-relaxation • When a muscle is stretched, it initially has high tension. Over time, the cross bridges and filaments re-arrange to reduce this tension. Consequently smooth muscle can distend without exerting unacceptable force. Energetics • There is a very low rate of cross-bridge cycling, so there is low ATP use. Consequently muscles are adapted for long, slow contractures. Force-Length curve Heart muscle fibres are usually overcompacted. Stretching results in a greater force of contraction (Frank-Starling mechanism).
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