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Properties of Muscle Contractility Excitability Extensibility

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Presentation on theme: "Properties of Muscle Contractility Excitability Extensibility"— Presentation transcript:


2 Properties of Muscle Contractility Excitability Extensibility
Ability of a muscle to shorten with force Excitability Capacity of muscle to respond to a stimulus Extensibility Muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity Ability of muscle to recoil to original resting length after stretched

3 Muscle Tissue Types Skeletal Smooth Cardiac Attached to bones
Nuclei multiple and peripherally located Striated, Voluntary and involuntary (reflexes) Smooth Walls of hollow organs, blood vessels, eye, glands, skin Single nucleus centrally located Not striated, involuntary Cardiac Heart Striations, involuntary, intercalated disks

4 Four types: skeletal, cardiac, smooth and myoepithelial cells
Morphology of Muscle Four types: skeletal, cardiac, smooth and myoepithelial cells 7

5 Long 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.

6 Skeletal muscle 40% of adult body weight 50% of child’s body weight
Muscle contains: 75% water 20% protein 5% organic and inorganic compounds Functions: Movement Maintenance of posture



9 Structure of Thick Filaments Myosin - 2 heavy chains, 4 light chains
Heavy chains kD Light chains - 2 pairs of different 20 kD chains The "heads" of heavy chains have ATPase activity and hydrolysis here drives contraction Light chains are homologous to calmodulin 11

10 RyR = ryanodine receptor Ca2+ channel
DHPR RyR = ryanodine receptor Ca2+ channel DHPR = dihydro-pyridine receptor



13 Mechanism of muscle contraction

14 Sliding filament model of muscle contraction
"Crossbridges" form between myosin and actin, with myosin pulling actin into "H zone" and shortening distance between Z disks.

15 Length-tension curve for skeletal muscle
Full overlap between thick and thin filaments Decreasing overlap limits maximum tension Actin 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



18 Molecular mechanisms of crossbridge action

19 T-tubules are NOT positioned at M lines.

20 Dihydropyridine Receptor
In t-tubules of heart and skeletal muscle Nifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubules In heart, DHP receptor is a voltage-gated Ca2+ channel In skeletal muscle, DHP receptor is apparently a voltage-sensing protein and probably undergoes voltage-dependent conformational changes 20

21 The "foot structure" in terminal cisternae of SR
Ryanodine Receptor The "foot structure" in terminal cisternae of SR Foot structure is a Ca2+ channel of unusual design Conformation change or Ca2+ -channel activity of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels 21

22 Ca2+ Controls Contraction
Release of Ca2+ from the SR triggers contraction Reuptake of Ca2+ into SR relaxes muscle So how is calcium released in response to nerve impulses? Answer has come from studies of antagonist molecules that block Ca2+ channel activity 19

23 This gap is actually only ~10 nm.


25 Function of Neuromuscular Junction

26 end-plate potential (EPP)
acetate + choline Na+ – – – – – ~ +40mV ~ -15mV - + end-plate potential (EPP) ~ -15 mV


28 Summation of skeletal muscle tension; tetanus

29 Contractile force can also be regulated through activation of more, or fewer, motor units.


31 Muscle contraction Types Isometric or static Isotonic
Constant muscle length Increased tension Isotonic Constant muscle tension Constant movement


33 Time 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.

34 Generate 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

35 Fatigue: * Central — involving central nervous system
may involve such factors as dehydration, osmolarity, low blood sugar, and may precede physiological fatigue of actual muscles. * Peripheral — in or near muscles accumulation of lactate and pH, especially in fast-twitch fibers  inorganic phosphate — may increasingly inhibit cleavage of ATP in the crossbridge cycle or in the sequestering of Ca2+.

36 Incomplete tetanus Muscle fibers partially relax between contraction
There is time for Ca 2+ to be recycled through the SR between action potentials

37 Complete tetanus No relaxation between contractions
Action potentials come sp close together that Ca 2+ does not get re-sequestered in the SR

38 A 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.

39 In 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.

40 Cardiac Muscle Figure 12.37 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.

41 Treppe Graded response Occurs in muscle rested for prolonged period
Each subsequent contraction is stronger than previous until all equal after few stimuli


43 Slow and Fast Fibers:

44 Slow and Fast Fibers:

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