2Properties of Muscle Contractility Excitability Extensibility Ability of a muscle to shorten with forceExcitabilityCapacity of muscle to respond to a stimulusExtensibilityMuscle can be stretched to its normal resting length and beyond to a limited degreeElasticityAbility of muscle to recoil to original resting length after stretched
3Muscle Tissue Types Skeletal Smooth Cardiac Attached to bones Nuclei multiple and peripherally locatedStriated, Voluntary and involuntary (reflexes)SmoothWalls of hollow organs, blood vessels, eye, glands, skinSingle nucleus centrally locatedNot striated, involuntaryCardiacHeartStriations, involuntary, intercalated disks
4Four types: skeletal, cardiac, smooth and myoepithelial cells Morphology of MuscleFour types: skeletal, cardiac, smooth and myoepithelial cells7
5Long multinucleated cells that respond only to motor-nerve signals, which cause Ca release from sarcoplasmic reticulum and activation of actin-myosin interaction.Shorter mononucleated cells linked to each other by intercalated disks that contain many gap junctions. Capable of independent, spontaneous contraction, with electrical depolarization transmitted from cell to cell through gap junctions.Spindle-shaped mono-nucleated cells. Contraction influenced by hormones and autonomic nerves. Contraction governed through myosin light chain kinase.
6Skeletal muscle 40% of adult body weight 50% of child’s body weight Muscle contains:75% water20% protein5% organic and inorganic compoundsFunctions:MovementMaintenance of posture
9Structure of Thick Filaments Myosin - 2 heavy chains, 4 light chains Heavy chains kDLight chains - 2 pairs of different 20 kD chainsThe "heads" of heavy chains have ATPase activity and hydrolysis here drives contractionLight chains are homologous to calmodulin11
14Sliding filament model of muscle contraction "Crossbridges" form between myosin and actin, with myosin pulling actin into "H zone" and shortening distance between Z disks.
15Length-tension curve for skeletal muscle Full overlap between thick and thin filamentsDecreasing overlap limits maximum tensionActin poking through M line; myosin bumping into Z disk.No overlap(Muscles are not naturally stretched to this point)Contraction range with normal skeletal movements
20Dihydropyridine Receptor In t-tubules of heart and skeletal muscleNifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubulesIn heart, DHP receptor is a voltage-gated Ca2+ channelIn skeletal muscle, DHP receptor is apparently a voltage-sensing protein and probably undergoes voltage-dependent conformational changes20
21The "foot structure" in terminal cisternae of SR Ryanodine ReceptorThe "foot structure" in terminal cisternae of SRFoot structure is a Ca2+ channel of unusual designConformation change or Ca2+ -channel activity of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels21
22Ca2+ Controls Contraction Release of Ca2+ from the SR triggers contractionReuptake of Ca2+ into SR relaxes muscleSo how is calcium released in response to nerve impulses?Answer has come from studies of antagonist molecules that block Ca2+ channel activity19
33Time is required for maximal twitch force to develop, because some shortening of sarcomeres must occur to stretch elastic elements of muscle before force can be transmitted through tendons.By the time this maximal force is developed, [Ca2+] and number of active crossbridges have greatly decreased, so an individual twitch reaches much less than the maximum force the muscle can develop.
34Generate ATP* Mitochondria generate ~32 ATP from one glucose (slow, but efficient).* Glycolysis generates 2 ATP from one glucose (fast, but inefficient; lactate accumulates).* Creatine kinase reaction: (fastest)ADP + creatine-P ATP + creatine
35Fatigue: * Central — involving central nervous system may involve such factors as dehydration, osmolarity,low blood sugar, and may precede physiologicalfatigue of actual muscles.* Peripheral — in or near musclesaccumulation of lactate and pH, especially infast-twitch fibers inorganic phosphate — may increasingly inhibitcleavage of ATP in the crossbridge cycle or inthe sequestering of Ca2+.
36Incomplete tetanus Muscle fibers partially relax between contraction There is time for Ca 2+ to be recycled through the SR between action potentials
37Complete tetanus No relaxation between contractions Action potentials come sp close together that Ca 2+ does not get re-sequestered in the SR
38A skeletal muscle twitch lasts far longer than the refractory period of the stimulating action potential, so many additional stimuli are possible during the twitch, leading to summation of tension and even tetanus.
39In cardiac muscle, the action potential — and therefore the refractory period — lasts almost as long as the complete muscle contraction, so no tetanus, or even summation, is possible. Sequential contractions are at the same tension, though gradual increases and decreases occur with autonomic nervous system input.