Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
Muscles act as antagonistic pairs against an incompressible skeleton
Muscles work in antagonistic pairs
One muscle contracts (agonist) and pulls on bone/produces force
One muscle relaxes (antagonist)
Attached to bones by tendons
Ligaments attaches one bone to the other
Skeletal muscle is incompressible so muscle can transmit force to bone (act as levers)
Advantage of skeletal muscles acting as antagonistic pairs against an incompressible skeleton
The advantage of skeletal muscles being arranged in antagonistic pairs are that muscles can only contract/pull.
The 2nd muscle is required to reverse the movement caused by 1st
This helps maintain posture as there is contraction of both muscles
Gross and microscopic structure of skeletal muscle
Muscle made up of bundles of muscle fibres (muscle cells) packaged together
Muscle cells contain a cell membrane, which is called the sarcolemma
Sarcolemma folds inwards into the muscle cells cytoplasm, which is called sarcoplasm. These folds are called transverse tubules and they help to spread electrical impulses throughout the sarcoplasm.
Sarcoplasmic reticulum in sarcoplasm stores and releases calcium ions
Myofibrils are made up of two proteins, actin and myosin
Shared nuclei and are multinucleate
Lots of endoplasmic reticulum
Ultrastructure of a myofibril
Myofibrils are made up of many sarcomeres which are made up of partly overlapping myosin and actin filaments (proteins)
The ends of each sarcomere are marked with a Z-line
Middle marked with M-line
Around the M-line is the H-zone which only contains myosin
Myosin filaments are thicker than thinner actin filaments
This causes a banding pattern to be seen (in a relaxed myofibril) under an electron microscope:
I-bands are light bands containing only thick actin filaments
A-bands are dark bands containing thick myosin filaments and some overlapping actin
Muscle contraction
Myosin heads slide actin past/along myosin causing the sarcomere to contract (myofilaments themselves dont contract)
Simultaneous contraction of lots of sarcomeres causes myofibrils and muscle fibres to contract
When sarcomeres contract (shorten)…
H zones shorter
I band shorter
A band same
Z lines closer
Sarcomeres return to their original length as the muscle relaxes
Myosin filament heads
Globular heads that are hinged, enabling back and forth movement
Each myosin head has a binding site for actin and a binding site for ATP
Actin filament binding sites
Actin filaments have binding sites for myosin heads called actin-myosin binding sites
Tropomyosin role
Another protein called tropomyosin is found between actin filaments which helps myofilaments move past eachother
In a resting muscle the actin-myosin binding site is blocked by tropomyosin
Myofilaments can no longer slide past each other because myosin heads cant bind to the actin-myosin binding site on the actin filaments
The sliding filament theory of muscle (myofibril) contraction
Action potential from a motor neurone stimulates a muscle cell
Depolarisation spreads down the T-tubules causing the release of calcium ions from the sarcoplasmic reticulum which diffuses through the sarcoplasm into the myofibril
Calcium ions bind to tropomyosin, causing it to move as it changes shape, exposing the myosin binding site on the actin filament
Hydrolysis of ATP by ATP hydrolase (which is activated by calcium ions) releases energy
So myosin heads (with ADP attached) attach to binding sites forming an actin-myosin cross bridge (requires ATP)
Energy released from ATP causes myosin head to bend, pulling actin filament
ATP binds to myosin head, causing it to detach from the actin binding site, breaking the cross bridge
Myosin heads to move back into original position (requires ATP)
Myosin reattaches to different binding site further along actin filament, forming a new cross bridge
Cycle will continue as long as calcium ions are present
Calcium ions leave
When the muscle stops being stimulated, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (requires ATP)
This causes tropomyosin to move back, blocking the actin-myosin binding sites again
Muscles not contracted because no myosin heads are attached to actin filaments (no cross bridges)
Sarcomere lengthened as actin filaments slide back into relaxed position
Structure, location and general properties of slow and fast skeletal muscle fibres
(Slow twitch)
Specialised for slow, sustained contractions (endurance)
Endurance activities e.g. maintaining posture, long distance running
Located in muscles that give posture and in leg muscles of long distance runners, for example
Aerobic respiration produces ATP (oxidative phosphorylation) to release energy slowly
High levels of myoglobin (red coloured protein that stores oxygen) makes them a reddish colour and it stores large amount of oxygen in muscle for aerobic respiration
Many mitochondria (site of aerobic respiration) present so high rate of aerobic respiration
Many capillaries present so short diffusion pathway/large surface area, so there is a high concentration of oxygen supply/ glucose (little/no glycogen/myoglobin) for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue
Structure, location and general properties of slow and fast skeletal muscle fibres
(Fast twitch)
Specialised for producing rapid, intense contractions of short duration
Short bursts of speed and power e.g. sprinting
Located in the legs of sprinters, for example
Anaerobic respiration produces ATP to release energy quickly
Low levels of myoglobin makes them a whitish colour since anaerobic respiration doesn’t need oxygen
Lots of glycogen which is hydrolysed to lots of glucose and used during glycolysis (anaerobic respiration) which is inefficient, yielding only 2 ATP per glucose molecule
Higher concentration of enzymes involved in anaerobic respiration (in cytoplasm) which means a high rate of
anaerobic respiration
Store phosphocreatine which rapidly generates ATP from ADP by providing phosphate
Muscles can get fatigued quickly because of high amounts of lactate
Role of phosphocreatine (PCr) in muscle contraction
Phosphocreatine (PCr) stored inside cells
Rapidly makes ATP by phosphorylating ADP (adding phosphate group from PCr)
PCr runs out after a few seconds so it’s used in short bursts of vigorous exercise e.g. tennis serve
Anaerobic (doesnt need oxygen) and alactic (doesnt form any lactate) system
