Lecture 25 Flashcards
What are the three muscle types and why do we need them?
Cardiac: pumping blood around the body, (supplying nutrients to cells).
Smooth muscle: to control the movement of fluids E.g blood, urine, digestion.
Skeletal muscle: To move, maintain posture and generate heat.
What is the structure of skeletal muscle?
Muscle is composed of muscle fascicles which are wrapped up in a layer of connective tissue (epineurium). These fascicles are seperated by the perimysium connective tissue (which contains blood vessels). The fascicles are composed of myocytes which are seperated by the endomysium layer. These myocytes contain predominantly myofibrils which are composed of individual units known as sarcomeres. The sarcomeres are formed from the myofilaments actin and myosin.
What structures are contained within myocytes and what are their functions?
The sarcolemma is the plasma membrane of the muscle cell, it seperates the myocyte from the exterior environment, the sarcoplasm is the cytoplasm of the muscle cell.
The sarcoplasmic reticulum acts as the muscle cells smooth endoplasmic reticulum, stores calcium ions to allow contraction to occur when stimulated.
T tubules are folded in sections of the sarcolemma, these connect with the sarcoplasmic reticulum on both sides to form a triad.
Also contrains mitochondria for energy production and myofibrils to allow for contraction.
What does the thin filament consist of? How about the thick filament?
The thin filament consists of actin, tropomyosin and troponin. The thick filament consists of a myosin shaft and myosin heads.
What is a motor unit?
A motor neuron and all the muscle fibers it innervates.
What are the three steps involved in muscle contraction?
Excitation, contraction and relaxation.
What steps occur during excitation of a skeletal muscle fiber?
An action potential reaches the end of the motor neuron axon, this causes Ca2+ to enter the cell and release vesicle containing acetyl choline into the synaptic cleft, this stimulates Ach receptors on the muscle fiber, leading to depolarisation of the sarcolemma and causing another action potential, as this travels over the T tubules Ca2+ is released from the sarcoplasmic reticulum.
What steps occur during contraction of a skeletal muscle fiber?
Ca2+ binds to troponin, this causes the tropomyosin to move off the myosin binding sites on actin and this allows the pre charged myosin heads to react with actin, (forming a cross bridge). The energised myosin heads than pull the actin forward in a power stroke. To allow another power stroke actin must bind to recharge the myosin heads.
What steps occur during relaxation of a skeletal muscle fiber?
The action potential stops and hence the Ca2+ is no longer released into the sarcoplasm and Ca2+ re uptake occurs. Also the Ca2+ no longer easily binds with troponin. ATP must then bind to the myosin to uncouple the cross bridge, returning the muscle to its resting state.
Which stage in muscle contraction requires the most ATP?
Relaxation as it requires that the cross bridges are released by ATP.
