Sliding filament mechanisms ofcontraction The sliding filament theory of contraction states that during contraction, the actin filaments slide past the myosin filaments so that they overlap to a greater degree.
Events leading to musclecontraction Cross bridge attachment The working stroke Cross bridge detachment Return of myosin head to high energy position
Regulation of contraction The neuromuscular junction is formed from the axon of each motor neuron as each divides profusely as it enters the muscle. The synaptic cleft separates the axonal ending from the muscle, and is filled with a gel-like substance.
Within axonal ending are synaptic vesicles,which are small membranous sacscontaining a neurotransmitter calledacetylcholine.The motor end plate is the trough-like partof the muscle fiber’s sarcolemma that helpsform the neuromuscular junction, whichprovide surface area for the ACh receptors.
Depolarization is an event that occurs whenthere is a change in membrane potentialsuch that the muscle cell interior becomesslightly less negative.The action potential is a large transientdepolarization event that is conducted alongthe membrane of a muscle cell or a nervefiber.
Repolarization is movement of themembrane potential to the initial restingstate.Refractory period is the period duringwhich an excitable cell is not responsive toa threshold stimulus.
All-or-none response refers to the fact thatmuscle fibers contract to the full extent oftheir ability or not at all.Acetylcholinesterase is an enzyme presentat the neuromuscular junction that preventscontinued muscle contraction in the absenceof additional stimulation.
Excitation-contraction coupling is asequence of events by which transmissionof an action potential along the sarcolemmaleads to the sliding of myofilaments.
Contraction of a skeletal muscle
The motor unit A motor unit is a motor neuron and all the muscle cells it stimulates.
The muscle twitch anddevelopment of muscle tension A myogram is a graphic recording of mechanical contractile activity produced by an apparatus that measures muscle contraction. A muscle twitch is the response of a muscle to a single brief threshold stimulus.
Phases of a muscle twitch Latent period is the period of time between stimulation and the onset of muscle contraction. Period of contraction is the time from the onset of shortening to the peak of tension development. Period of relaxation is initiated by reentry of Ca+2 into the SR.
Graded muscle responses A graded response is a variation in the degree of muscle contraction by changing the frequency or strength of the stimulus. Wave summation occurs if two identical stimuli are delivered to a muscle in rapid succession, and the second twitch is stronger than the first.
Tetanus is a sustained muscle contractionresulting from high-frequency stimulation.Recruitment is achieved by deliveringshocks of increasing voltage to the muscle,increasing the number of muscle fibersassociated with the contraction.
Threshold stimulus is the weakest stimuluscapable of producing a response in airritable tissue.Maximal stimulus is the strongest stimulusthat produces increased contractile force.
Treppe: The staircase effect Treppe refers to the increasing force of muscle contractions as they occur later in response to stimuli of the same strength.
Muscle tone Muscle tone refers to a sustained partial contraction of a muscle in response to stretch receptor inputs.
Isotonic and isometriccontractions Muscle tension is the force exerted by a contracting muscle on some object. Load is the weight exerted by the object on the muscle.
Isotonic contraction is a contraction inwhich muscle tension remains constant andthe muscle shortens.Concentric contractions are isotoniccontractions in which the muscle shortensand does work.
Eccentric contractions are isotoniccontractions in which the muscle contractsas it lengthens.Isometric contractions cause the tension toincrease, but the muscle neither shortensnor lengthens.
Providing energy for contraction Creatine phosphate is a compound that serves as an alternative energy source for muscle tissue. Creatine kinase is an enzyme that catalyzes the transfer of phosphate from phosphocreatine to ADP, forming creatine and ATP.
Aerobic endurance is the length of time amuscle can continue to contract usingaerobic pathways.The anaerobic threshold is the point atwhich muscle metabolism converts toanaerobic glycolysis.
Muscle fatigue Muscle fatigue is a state of physiological inability to contract. Contractures are states of continuous contraction.
Oxygen debt Oxygen debt is the volume of oxygen required after exercise to oxidize the lactic acid formed during exercise.
Force, velocity, and duration ofmuscle contraction
Force of contraction Series-elastic elements refers to all the non- contractile structures of muscles. Internal tension is generated by the myofibrils, and stretches the series-elastic elements.
External tension is transferred from theseries-elastic elements to the load.Length-tension relationship refers to theoptimal resting length for muscle fibers atwhich they can generate maximum force,which is when a muscle is slightly stretchedand the filaments barely overlap.
Velocity and duration ofcontraction Slow oxidative fibers contract slowly and depend on plentiful oxygen delivery. Fast oxidative fibers contract quickly and depend on plentiful oxygen delivery. Fast glycolytic fibers contract quickly and depend on plentiful glycogen reserves, but do not use oxygen.
Effect of exercise on muscles
Adaptations to exercise Aerobic exercise results in greater synthesis of myoglobin, and increased capillaries and mitochondria surrounding the muscle fibers. Resistance exercise results in muscle hypertrophy, and occurs under anaerobic conditions.
Arrangement and microscopicstructure of smooth muscle fibers
Peristalsis is the alternating contraction andrelaxation of opposing layers of smoothmuscles to mix substances in the alimentarycanal.
Bulbous swellings called varicositiesrelease neurotransmitter into a widesynaptic cleft in junctions in the smoothmuscle called diffuse junctions.The plasma membrane of the smoothmuscle fiber has multiple pouch-likeinfoldings called caveoli.
Dense bodies attach to non contractilefilaments that resist tension.Dense bands act as anchoring points forgroups of thin filaments, and correspond tothe Z discs of the skeletal muscles.
Contraction of smooth muscle
Special features of smoothmuscle contraction Stress-relaxation response, in which stretching of the smooth muscle moves substances along an internal tract, allows a hollow organ to fill or expand slowly to accommodate a greater volume without promoting contractions to expel their contents.