Skeletal muscles Flashcards
What are the 3 types of muscle and where are they found?
- cardiac muscle - found in the heart
- smooth muscle - found in the walls of blood vessels and the gut
- skeletal muscle - attached to the bone and acts under voluntary, conscious control
What does antagonistic pairs mean?
Whilst one muscle contracts (agonist) the other relaxes (antagonist)
What is the gross structure of skeletal muscle?
the muscle is divided into bundles of muscle fibres (cells). Each muscle fibre consists of sarcoplasm, sarcoplasmic reticulum, mitochondria, sarcolemma, transverse T tubules, nucleus and myofibrils
What is the sarcolemma?
- cell membrane of a muscle fibre - like an axon membrane because action potentials can pass along to cause contraction
What do transverse T tubules do?
Folded sarcolemma which stick into the sarcoplasm - this helps to spread impulses throughout the sarcoplasm to reach all parts of the muscle, to allow for simultaneous contraction
What is the role of the sarcoplasmic reticulum?
stores and releases calcium ions involve in muscle contraction
What are myofibrils?
bundles of protein filaments, consisting of actin and myosin, which cause contraction
What are thin and thick filaments?
actin = thin filament, lighter
myosin = thick filament, darker
What is the ultrastructure of skeletal muscle?
- light bands are called I bands because they appear lighter as they consist of only thin filaments (actin)
- dark bands are called A bands because the thick and thin filaments overlap in this region (actin and myosin)
- at the centre of each A band is the H zone (consisting of just myosin), with an M line at the centre
- the centre of each I band is called the Z line
What is a sarcomere?
the distance between adjacent Z lines - when the muscle contracts, the sarcomeres shorten
What happens to the distance between Z lines when the muscle contracts?
decreases
What happens to the width of the I band when the muscle contracts?
decreases
What happens to the width of the A band when the muscle contracts?
stays the same
What happens to the length of the myosin filaments when the muscle contracts?
stays the same
What is the siding filament theory?
- when the muscle contracts, the sarcomeres become smaller
- however the filaments do not change in length
- instead they slide past each other (overlap)
- so actin filaments slide between myosin filaments and the zone of overlap is larger
What is the difference between slow and fast twitch muscle fibres?
- slow-twice fibres contract more slowly and provide less powerful contractions over a longer period
- they are adapted to endurance work
- fast-twitch fibres contract more rapidly and produce powerful contractions for a shorter period
- adapted to intense exercise such as weightlifting
How are slow-twitch muscle fibres adapted?
- large stores of myoglobin - higher affinity for oxygen at lower partial pressures
- rich supply of blood vessels - for glucose and oxygen
- numerous mitochondria - aerobic respiration produces more ATP
How are fast-twitch muscle fibres adapted?
- thicker and more numerous myosin filaments
- high concentration of enzymes involves in anaerobic respiration
- high concentration of phosphocreatine to rapidly generate ATP from ADP
- supply of glycogen
What is a neuromuscular junction?
Where a motor neurone meets a skeletal muscle fibre
How are impulses transmitted across a neuromuscular junction?
- an action potential causes calcium ions to be released, vesicles fuse with the membrane causing AcH to diffuse across the synaptic cleft
- binds to complementary receptors on Na+ channels in sarcolemma, Na+ ions enter and membrane is depolarised
- causes actin and myosin bridge cycle
What are the similarities between neuromuscular junctions and synapses?
- both have neurotransmitters transported by diffusion
- both have receptors, that on binding with a neurotransmitter cause an influx of sodium ions
- both use a sodium potassium pump to repolarise the axon
- both use enzymes to break down the neurotransmitter
What are the differences between neuromuscular junctions and synapses?
- neuromuscular junctions are only excitatory, whereas cholinergic synapses can be excitatory or inhibitory
- neuromuscular junctions link neurones to muscles, whereas cholinergic synapses can link neurones to neurones, or neurones to other effectors
- neuromuscular junctions only involve motor neurones, whereas cholinergic synapses can involve motor, sensory or intermediate neurones
- the action potential ends at a neuromuscular junction, whereas at a cholinergic synapse a new action potential may be produced
- at neuromuscular junctions acetylcholine binds to receptors on the sarcolemma of the muscle fibre, whereas at cholinergic synapses acetylcholine binds to receptors on the membrane of the postsynaptic neurone
What is the evidence for the sliding filament theory?
- I band narrows
- Z lines move closer together
- sarcomere shortens
- H zone narrows
How is the muscle stimulated during contraction?
- action potential reaches many neuromuscular junctions, causing calcium ion channels to open and calcium ions diffuse into the synaptic knob
- calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft
- acetylcholine diffuses across the synaptic cleft and binds with receptors on the sarcolemma, causing an influx of sodium ions and depolarisation
How does muscle contraction occur?
- tropomyosin molecule prevents myosin head from attaching to binding sites on the actin molecule
- action potential travels through T tubules and causes calcium ions to be released from the sarcoplasmic reticulum and diffuse into myofibrils
- calcium ions bind to troponin, which causes the tropomyosin to pull away from binding sites on actin, exposing them
- myosin heads attach to the exposed binding sites on actin, forming an actinomyosin cross-bridge
- once attached, the myosin heads change their angle, pulling along the actin filament “power stroke” and ADP is released
- an ATP molecule attaches to the myosin head, causing it to detach from the actin filament
- ATP is then hydrolysed by ATP hydrolase which provides energy for the myosin head to bend to its original position
- myosin head attaches to a binding site further along the actin filament and the cycle repeats
How does the muscle relax again?
- calcium ions are actively transported back into the endoplasmic reticulum using energy from ATP hydrolysis
- tropomyosin blocks binding sites on actin
- myosin heads are unable to bind and contraction ceases
What is energy needed for during muscle contraction?
- movement of myosin heads
- reabsorption of calcium ions into endoplasmic reticulum
How is phosphocreatine used to supply energy during muscle contraction?
- during very high intensity exercise
- phosphocreatine is broken down into creatine and inorganic phosphate at rest
- during exercise, ADP will combine with the inorganic phosphate to form ATP
- phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed
