|Name three types of muscles and state where they are found:
|Cardiac muscle - found exclusively in the heart Smooth muscle - found in the walls of the blood vessels and the gut Skeletal muscles - attached to bond
|Describe the macroscopic structure of skeletal muscles
|Muscles are made of many tiny muscle fibres called myofibrils; Many myofibrils are grouped into a single muscle fibre; Several muscle fibres are grouped into a bundle which group to form a muscle
|What are myofibrils?
|Tiny muscle fibres; Which share a nuclei and a cytoplasm, known as the sarcoplasm
|Why are muscle fibres not made up of individual cells?
|Individual cells would be inefficient; Gaps between cells would be points of weakness
|What two proteins/filanments are myofibrils made of?
|Actin and myosin
|Thin proteins/filaments; Consit of two strands twisted around one another
|Thicker; Consist of long rod-shaped fibres with bulbous heads which project to the side
|What is the isotropic (I) band?
|Also known as the I or light band; Where the actin and myosin filaments do not overlap
|What are the anisotropic (A) bands?
|Also known as the A or dark bands; Where the actin and myosin filaments overlap
|Describe the following structures: a) H-zone b) Z-line c) Sarcomere
|a) Light region at the centre of the A (dark) band b) Centre of the I (light) band c) distance between adjacent z-lines
|What are the two types of skeletal muscles?
|Fast twitch and slow twitch
|What is the function of slow twitch muscles?
|Contract slowly and less powerfully; For endurance
|How are the slow-twitch muscles adapted for their function?
|Adapted for aerobic respiration: Large store of myoglobin (red molecule which stores oxygen); Large supply of glycogen (source of metabolic energy); Rich supply of blood vessels (to deliver oxygen and glucose); Many mitochondria
|What is the function of fast-twitch muscles?
|Contract rapidly and powerfully; Used for intense exercise
|How are fast-twitch muscles adapted for their function?
|Thicker and more numerous myosin filaments (allow for powerful contractions); High concentration of enzymes involved in anaerobic respiration; Stores of phosphocreatine (rapidly generates ATP from ADP in anaerobic conditions)
|What is a neuromuscular junction?
|Point where a motor neurone meets a skeletal muscle fibre; Many junctions along a muscle fibre
|What is a motor unit, and what do they allow?
|All muscles fibres supplied by a single motor neurone are known as a motor unit; They allow control over the force of a contraction; Powerful contraction stimulate many motor units
|How does acetylcholine transmit an impulse across a neuromuscular junction?
|A nerve impulse causes the synaptic vesicles to fuse with the presynaptic membrane releasing acetylcholine; Acetylcholine diffuses across the synaptic cleft to the postsynaptic neurone; This alters the membrane's permeability to sodium (Na+) ions; Na+ diffuses into muscle depolarising the membrane
|How is acetylcholine 'recycled' and why?
|Acetylcholine is broken down by acetylcholinesterase; The resulting ethanoic acid and choline diffuses back across the synaptic cleft into the presynaptic neurone where it is recombined into acetylcholine; This ensures the muscle isn't overstimulated
|What is the evidence for sliding filament theory?
|Theory = actin and myosin filaments slide over each other during contraction; If correct in a contracted muscles there will be more overlapping filaments; I band will become narrower; Z lines are more close together (sarcomere shortens); H zone becomes narrower
|Explain how the A band helps support sliding filament theory:
|A (dark) band remains the same width as its length is determined by the length of the myosin filaments; Thus discounting the theory that contraction is due to filaments shortening
|What two proteins is the myosin filament made of?
|Fibrous protein arranged into filaments; Globular protein formed into two bulbous structures (the heads)
|Describe the structure of actin
|Globular protein arranged into long chains that are twisted into a helical strand
|Describe the structure of tropomyosin
|Forms long thin threads that are woven around the actin filaments
|Summarise sliding filament theory:
|Impulse stimulated tropomyosin to move, exposing the myosin binding sites; Bulbous heads of the myosin filament form cross bridges with a binding site on the actin filament; Filaments flex in unison pulling the actin filament along the myosin filament; Cross-bridge breaks (using ATP) and heads return to their original position; Heads can now re-attach further along the actin filament
|How is a muscle stimulated?
|An action potential reaches many neuromuscular junctions simultaneously; Ca2+ (calcium) ion channels open so Ca2+ diffuses into the synaptic knob; Ca2+ ions cause synaptic vesicles to fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft; Diffuses to, and alter the permeability of the postsynaptic neurone to Na+, Na+ diffuse into and depolarise the membrane
|How does an action potential cause calcium ions (Ca2+) to leave the sarcoplasmic reticulum?
|Action potential travel into fibres through a system of t-tubules that branch throughout the sarcoplasm; T-tubules in contact with the sarcoplasmic reticulum which has absorbed Ca2+; Action potential opens Ca2+ ion channels on the sarcoplasmic reticulum so Ca2+ flood into the sarcoplasm down a diffusion gradient
|How do calcium ions (Ca2+) in the sarcoplasm allow cross-bridges to be formed?
|Ca2+ cause the tropomyosin molecules, that were blocking the binding site on the actin filament, to pull away; ADP molecules attached to the myosin heads means they can bind to the actin filaments, forming a cross-bridge
|How does the cross-bridge cause the actin filaments to move?
|Once attached to the actin filaments the myosin heads change their angle; Pulling the actin filament along; Releasing a molecule of ADP
|How does the cross-bridge break, allowing the myosin heads to return to their original position?
|ATP molecule attaches to the myosin head, causing it to become detached from the actin filament; Calcium ions activate ATPase which hydrolyses ATP to ADP providing energy for the myosin heads to return to their original position
|How do muscles relax?
|When nervous stimulation ceases Ca2+ ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP; This reabsorption of Ca2+ ions allows tropomyosin to block the myosin binding sites on the actin filaments again; Myosin is now unable to bind to the actin filaments so contraction ceases