L13. Muscles Flashcards
(19 cards)
Three types of muscle tissue
SMOOTH MUSCLE- in the walls of organs, moves the contents of organs
CARDIAC MUSCLE- contractile component of the heart
SKELETAL MUSCLE- attaches and moves the skeleton
Characteristics of muscle tissue
RESPONSIVENESS- when stimulated, muscle cells will respond with electrical changes across the plasma membrane
CONDUCTIVITY- stimulation of a muscle fibre will trigger a wave of excitation which travels along the fibre
CONTRACTILITY- ability to shorten when stimulated
EXTENSIBILITY- ability to stretch out again between contractions
ELASTICITY- ability to recoil to original length when tension is released
Skeletal muscle
- voluntary
- appears striated (striped) under microscope
- muscle cells/ fibres are long, thin and cylindrical
- multinucleated
- moves the skeleton
- produces heat
- attaches to bone via tendon
Function of skeletal muscle
- produces movement (shortens > levers bones > movement)
- maintain posture
- stabilises joints
- generates heat
- protects viscera
- storage of glycogen
- guards body entrances and exits
Smooth muscle
- involuntary
- lacks striation
- muscle fibres are relatively short
- single nucleated
- “fusiform” or spindle shaped
- form layers in the walls of hollow organs (digestive, respiratory, urinary tracts, blood vessels, uterus, etc)
Function of smooth muscle
- moves contents of organs*
- churning and mixing action of the stomach
- propulsion of food along the GIT
- contraction of the uterus during childbirth
- vasodilation/ constriction of blood vessels, etc
Cardiac muscle
- specialised muscle tissue that has properties of both smooth muscle and skeletal muscle
- involuntary
- striated and branched
- cells called myocytes; short and thick
- single nucleated
- localised to the heart
Single skeletal muscle organisation
MUSCLE FIBRE (CELL)- bunch of myofibrils wrapped in endomysium
MUSCLE FASCICLE- bunch of muscle fibres wrapped in perimysium
MUSCLE- bunch of fascicles wrapped in epimysium
Components of muscle cell
- myofibrils
- sarcolemma (plasma membrane)
- sarcoplasm (cytoplasm)
- sarcoplasmic reticulum (sacs containing calcium)
- Transverse tubules
Myofibrils
- each muscle fibre contains myofibrils arranged lengthways
- each myofibril contains myofilaments:
> myosin (thick)
> actin (thin)
> titin (elastic)
Sarcomere
- repeating unit that occurs the length of a myofibril
- the patterning gives striated appearance in skeletal muscle
- a sarcomere extends from one z-line to the next z-line
- actin and myosin overlap each other in the sacomere
- titan don’t allow overstretch
Transverse tubules
- narrow tubes which extend from the surface of the sarcolemma deep into the sarcoplasm
- conduct action potentials
Sarcoplasmic reticulum
- similar to endoplasmic reticulum
- membranous sacs filled with calcium
- surrounds myofibrils
- bound to T-tubules
4 steps in muscle contraction
- excitation
- excitation/ contraction coupling
- contraction
- relaxation
Neuromuscular junction
- where the nerve meets the muscle (synaptic cleft)
- SYNAPTIC KNOB (axon terminal) of neuron meets with the motor end plate of the muscle
- synaptic knobs release neurotransmitter (ACh)
- ACh diffuses across the synaptic cleft
- ACh receptors located in junctional folds on motor end plate bind with ACh
- Impulse is passed onto the muscle, stimulating it to contract
- ACh is then broken down by AChE, stops impulses
Excitation
- nerve impulses arrives
- causing calcium channels to open
- calcium moves into the neuron causing the vesicles to fuse with the plasma membrane to release neurotransmitter
- neurotransmiter binds to receptor
- receptors open, allowing more sodium in and potassium out
- restart another action potential on the muscle
Excitation/ Contraction Coupling
- The AP spreads from the motor end plate to all over the muscle fibre
- When the AP reaches a T-tubule, it continues down into the sarcoplasm, reaching the sarcoplasmic reticulum
- this triggers the sarcoplasmic reticulum to release calcium
- calcium release exposes the binding site on actin
- this allows myosin heads to bind to actin
Contraction
- myosin binds to an ATP molecule and hydrolyses it to ADP > release energy which is used to “spring load” the myosin head
- The “loaded” myosin heads then bind to the exposed actin filaments, forming cross-bridges
- myosin pulls along the actin, releasing the ADP- “power stroke”
- myosin will bind to another ATP molecule, detach from actin, “re-load” (recovery stroke) the head, attach to new active site on actin and repeat the cycle> sarcomere shorten= contraction
Relaxation
- Nerve impulses stop arriving at the NMJ so no more ACh is released by the synaptic knob
- ACh dissociates with its receptors and is broken down by AChE
- calcium in the cytosol is now actively pumped back into the SR for storage
- calcium dissociates from actin
- binding site is no longer exposed so no more cross bridge with myosin
- relaxation occurs due to elastic recoil as well as action of the antagonist muscle
