Muscle Physiology Flashcards
What is the primary muscle of the body?
Skeletal muscle makes up 40% of muscle in the body. It is innervated by somatic motor neurones only and generates movement and heat through its contraction.
What is the structure of skeletal muscle?
Single unit of muscle fibre is the fasiculus arranged in a sarcomere unit. Between the fasiculus is the connective tissue called endomysium. There are capillaries which vascularise the muscle. Enclosing the muscle is a deep fascia called the epimysium.
What is the epimysium?
Thick connective tissue which surrounds the entire muscle tissue
What is the perimysium?
Connective tissue surrounding muscle bundles
What is the endomysium?
Connective tissue between individual muscle fibres in the fasiculus
What is the fasiculus?
A bundle of muscle fibres
What are satellite cells?
Multipotent progenitor cells in the muscle fibre which lie between the sarcolemma and the basement membrane with cell-matrix interactions and cell -cell interactions. They give rise to new muscle, immune cells and fibroblasts in the muscle.
How does new muscle form?
Satellite cells remain in a quiescent state until other satellite cells release growth factors to stimulate differentiation. It forms a multinucleate myocyte which undergoes fission to form a myotube that matures into a muscle fibre.
What are myocytes?
Single cell unit of the muscles which is multinucleate.
What are myotubes?
Intermediate in the formation of muscle fibres from the fission of myocytes to form a synctium.
What is the structure of the sarcomere?
M band is the centre of the sarcomere containing myomesin protein. This is in the H band region containing myosin only. A band contains actin and myosin. I band contains actin only. Actin is attached to the a-actinin protein which makes up the Z line. Myosin attaches to the Z line via the titin protein which stabilises it.
What is the composition of the Z line?
A-actinin protein which allows direct attachment to actin and indirect attachment to myosin via titin protein.
What is titin?
Protein which allows myosin to attach to the Z line and stabilises it.
What is a sliding filament theory?
Change in sarcomere zones during muscle contraction. I band and H band decrease; A band stays the same
What is the thick filament?
Myosin which consists of 6 subunits; 4 light polypeptide chains which regulate the myosin head and 2 heavy polypeptide chains. Myosin heads contain ATPase and binds to actin.
What is the thin filament?
Actin which consists of 2 polypeptide chains composed of a-globular protein. It has a tropomyosin regulatory complex which blocks the active site and a troponin complex that regulates tropomyosin.
What happens if ATP levels in the muscle is low?
Actin-myosin cross bridge does not break and muscle remains in a relaxed state and muscle contraction does not occur again.
What is lusitropy?
Rate of muscle relaxation.
What determines tension of a muscle fibre?
Initial length of the muscle at rest
What is the length-tension curve?
Describes the non-linear relationship between muscle length at rest and muscle tension achieved during contraction.
How does increased length affect tension?
Reduced tension beyond optimal length; Less cross bridges form because of the distance between myosin and actin.
How does decreased length affect tension?
Reduced tension below optimal length; Not enough space for filaments to slide over each other for contraction.
What is the neuromuscular junction?
Motor end plate; synapse between the somatic motor neuron in contact with the terminal cisternae of the sarcoplasmic reticulum
How does neuromuscular transmission occur in the neuron?
Pre-synaptic neuron receives action potential via its dendrites which causes voltage gated Ca2+ channels to open and Ca2 to leave and induce exocytosis of acetlycholine vesicles to bind to nicotinic receptors on the sarcoplasmic reticulum
How does neuromuscular transmission occur in the muscle?
Action potential reaches the t-tubules and causes opening of DHPR L-type calcium channels which open and release Ca2+. It is mechanically coupled to ryanodine receptor Ca2+ release channels that are triggered to open.
What is a miniature end plate potential?
Binding of a single neurotransmitter of acetylcholine generates a small change in the membrane potential.
What is the end plate potential?
Summation of the minituare end plate potentials of multiple acetlycholines which generates an action potential and triggers opening of voltage gated Na+ channels.
What are the terminal cisternae?
Specialised regions in the sarcoplasmic reticulum which store calcium and release in response to t-tubules.
What are the T tubules and sarcoplasmic reticulum?
T-tubules are invaginations in the sarcoplasmic reticulum which maintain calcium store in the terminal cisternae of the sarcoplasmic reticulum via the channel DHPR.
What is the triad of the sarcoplasmic reticulum?
Both the terminal cisternae and the t-tubules.
What is DHPR?
L-type calcium channel which opens in response to depolarisation in the T tubules.
What is the Ca2+ release channels?
Ryanoidine receptor which is mechanically coupled to the L-type DHPR channel to release calcium.
What is a ryanodine receptor?
Intracellular calcium channel.
What is excitation-contraction coupling?
Action potential reaches the t-tubules and causes opening of DHPR L-type calcium channels which open. It is mechanically coupled to ryanodine receptor Ca2+ release channels that are triggered to open.
What is calreticulin?
A protein used by Ca2+ to sequester Ca2+ into stores in the sarcoplasmic reticulum.
What is rigor mortis?
Stiffness after death when Ca2+ stores in the body are released by the sarcoplasmic reticulum which binds to troponin C complex and initiates cross bridge formation. ATP synthesis cannot occur so cross bridge remains present in locked position until autolysis triggers release of proteolytic lysosomal enzymes to induce muscle relaxation.
Why do muscles need energy?
To form cross bridge, to sequester ATP for muscle relaxation and to break the cross bridge
What are the sources of energy in muscle?
Glucose, fatty acids during exercise or starvation and rarely proteins
What is the source of ATP in the muscle?
Creatine phosphate, aerobic respiration and anaerobic respiration.
What are the ranked order of ATP sources?
Endurance of energy: Aerobic respiration > anaerobic respiration > creatine phosphate
Release of energy: Creatine phosphate > anaerobic respiration > aerobic respiration
What is the creatine phosphate pathway?
Creatine is phosphorylated to form creatine phosphate in contracting muscles undergoing exercise, catalysed by creatine kinase for quick source of ATP
Where is creatine synthesised?
Liver, pancreas and kidneys
What is the role of creatine kinase?
Phosphorylation of creatine to provide fast ATP source for muscles
What is muscle twitch?
Cycle of single contraction-relaxation phase in muscle fibres
What are the stages of muscle twitch?
Lag phase when Ca2+ binds to troponin C
Contraction when sarcomere has cross bridge form
Relaxation when elastic elements induce relaxation in sarcomere by releasing tension
What are the types of muscle twitch fibres?
Type 1-slow twitch oxidative which fatigues slowly and has the smallest fibre diameter and low levels of myosin ATPase
Type 2a fast twitch- oxidative-glycolytic which fatigues quicker and has an intermediate fibre diameter and medium levels of myosin ATPase
Type 2b fast twitch-oxidative which fatigues quickly and has a large fibre diameter for conduction speed and large fibre diameter and high levels of myosin ATPase
How does stimulus affect muscle twitch?
Stimuli from the somatic motor neuron increases the forcefulness of contraction, then the rate of contraction.
What is stimuli summation?
Stimuli from somatic motor neuron reduces intervals between relaxation to increase forcefullness of contraction.
What is unfused tetanus?
High stimuli increases the rate of contraction to the extent there is only partial relaxation before the muscle contracts again
What is fused tetanus?
Maximum stimuli leads to no relaxation between contraction so muscle remains contracted in a steady state
What is a motor unit?
Individual somatic motor neuron and the muscle fibres it innervates
What is Henneman’s size principle?
Stimuli affects the type and number of motor units recruited for muscle contraction
How does stimulation affect Henneman’s size principle?
Greater the stimuli, the more motor units recruited and larger motor units with a faster muscle twitch
What happens at maximum stimulation?
Larger motor units with fast twitch glycolytic muscle fibres are recruited
What is asynchrous recruitment?
To prevent fatigue, some muscles will contract while simultaneously some muscles relax
What is isotonic contraction?
At maximum load, muscle will shorten as it contracts
What is isometric contraction?
At maximum load, there is no change in muscle length as it contracts
What is the load velocity relationship?
Non-linear as load increases velocity decreases
What is Vmax?
Greatest muscle velocity when there is no load
What is maximum load?
Zero muscle velocity when it contracts at maximum load
What is the lever and fulcrum system?
Musculoskeletal system which consists of the load as the weight, the effort being the muscle action between and the fulcrum which is the joint.
What are the advantages of the lever and fulcrum system?
Optimum length-tension relationship and distance and speed to move load
What are the disadvantages of the lever and
fulcrum system?
More force is required to move load
