Muscles - B8 Flashcards
What are the three types of muscle?
-smooth muscle - contracts without conscious control. Found in the walls of internal organs e.g. stomach, intestine, and blood vessels
-cardiac muscle -contracts without conscious control but only found in the heart
-skeletal muscle -the type used to move
What is the role of skeletal muscle?
-attached to the bone by tendons
-ligaments attach bone to other bones
-pairs of skeletal muscles contract and relax to move the bones at joints
-bones are incompressible so they act as levers
What are antagonistic pairs?
-muscles that work together to move a bone are called antagonistic pairs
-the contracting muscle is called the agonist and the relaxing muscle is called the antagonist
-muscles only ever pull
What is the structure of skeletal muscle?
-made up of large bundles of long cells, called muscle fibres
-the cell membrane of muscle fibres cells is called the sarcolemma
-bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm
-these folds are called transverse tubules and they help spread electrical impulses through the sarcoplasm so they reach all parts of the muscle fibre
-a network of internal membranes called sarcoplasmic reticulum runs through the sarcoplasm
-SR stores and releases Ca2+ that are needed for muscle contraction
-many mitochondria to provide ATP for contraction
-multinucleate have lots of long cylindrical organelles called myofibrils - contains proteins specialised for contraction
What are myofibrils?
-contain bundles of thick and thin myofilaments that move past each other to make muscles contract
-thick=myosin protein, thin=actin protein
-dark bands contain thick myosin filaments and some overlapping thin actin filaments, called A-bands
-light bands only contain thin actin filaments, called I-bands
What is the structure of myofibrils?
-made up of many short units called sarcomeres
-the ends of each sarcomere are marked with Z-lines
-middle of the sarcomere is an M-line -the middle of the myosin filaments
-around the M-line is the H-zone - only myosin is present
What is the sliding filament theory?
-the myosin and actin filaments slide over each other to make the sarcomeres contract - the myofibrils themselves don’t contract
-the simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract
-sarcomeres return to their original length as the muscles relax.
What is the structure of myosin filaments?
-myosin filaments have globular heads that are hinged, so they can move back and forth
-each myosin head has a binding site for ATP
What is the structure of actin filaments?
-actin have binding sites for myosin heads, called actin-myosin binding sites
-another protein called tropomyosin is found between actin filaments
What happens when muscles are at rest?
-for myosin and actin filaments to slide past each other, the myosin head needs to bind to the actin-myosin binding site on the actin filament
-at rest the myosin-actin binding site is blocked by tropomyosin
-this means myofilaments can’t slide past each other because the myosin heads can’t bind to the actin filaments
Describe the process of muscle contraction.
-action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma
-depolarisation spreads down the T-tubules to the sarcoplasmic reticulum
-SR releases stored Ca2+ into the sarcoplasm
-influx of Ca2+ into the sarcoplasm triggers muscle contraction
-Ca2+ bind to a protein(troponin) attached to tropomyosin, causing the protein to change shape
-this pulls the attached tropomyosin out of the actin myosin binding site on the actin filaments
-exposing the binding site, allowing the myosin head to bind
-the bond formed when a myosin head binds to an actin filament is called an actinomyosin cross bridge
-Ca2+ also activates the enzyme ATP hydrolase, which hydrolyses ATP, to provide energy needed for muscle contraction
the energy released from ATP causes the myosin head to bend which pulls the actin filament along in a stroke action, called the power stroke
How is the Actin-Myosin Cross bridge broken?
-another ATP molecule provides energy to break the actinomyosin cross bridge
-the myosin head detaches from the actin filament after it has moved
-ATP is used to recock the myosin head and reattaches to a different binding site further along the actin filament
-a new actin-myosin cross bridge is formed and the cycle is repeated
How does the muscle return to the rest state?
-when muscle stops being stimulated, ca2+ leave their binding sites and are moved by active transport back into the SR(also requires ATP)
-this causes tropomyosin to move and block the actin-myosin binding sites again
-during this stage the actin filaments can slide back to their relaxed position- lengthening the sarcomere
What are the uses of ATP in muscle contraction?
-myosin head power stroke
-breaking the actin myosin cross bridge(detaching the head)
-hydrolysis ATP returns the head to the start position/recocks the head
-reabsorbing Ca2+ back into the sarcoplasmic reticulum by active transport
What are the uses of Ca2+ in muscle control?
-binds troponin, changing troponins shape. Pulling tropomyosin away from the actin myosin binding site. So myosin head can now bind to actin, forming a crossbridge. Allowing the myosin head to power stroke and slide the actin towards the M-line
-activates ATPase/hydrolase(ATP->ADP+Pi)
What is myoglobin?
Red pigment, similar to haemoglobin
-acts as a store of oxygen in muscles
-speeds up the absorption of oxygen from the capillaries
What are the differences between fast and slow muscle fibres?
-f:short contraction-relaxation cycle, s:long contraction-relaxation cycle
-f:fewer capillaries, s:denser network of capillaries
-f:ATP supplied mostly from anaerobic respiration, s: ATP supplied mostly from aerobic respiration
-f:fewer, smaller mitochondria present , s: many, large mitochondria present
-f: large store of calcium ions in the sarcoplasmic reticulum, s: small store of calcium ions in the SR
-f: large amounts of glycogen and phosphocreatine present, s: small amounts of glycogen present
-f: faster rate of ATP hydrolysis in myosin heads, s: slower rate of ATP hydrolysis in myosin heads
-f: fatigues rapidly due to greater lactate formation, s:fatigues more slowly due to reduced lactate formation
What are the three ways of obtaining energy for muscle contraction?
1)aerobic respiration
2)anaerobic respiration
3)ATP-phosphocreatine(PCr) system
How is aerobic respiration used in muscle contraction?
-most ATP generated oxidative phosphorylation in mitochondria
-only works when there’s enough oxygen available so good for exercise that’s over a long period of time of low intensity
How is anaerobic respiration used in muscle contraction?
-ATP generated in glycolysis
-end product is pyruvate, then converted to lactate by lactate fermentation
-lactate can quickly build up in muscle and cause fatigue
-good for short periods of hard exercise
How is ATP-phosphocreatine (PCr) system used in muscle contraction?
-ATP formed by phosphorylation ADP
-done by adding a phosphate group taken from PCr( PCr provides phosphates to make ATP)
-PCr is stored inside cells and the ATP-PCr system generates ATP quickly
-PCr runs out after a few seconds-used for short bursts of vigorous exercise
-PCr system is anaerobic and alactic
