6: Skeletal Muscle Flashcards
skeletal muscle functions
movement
maintaining posture
support soft tissues
guard entrances and exits
maintain body temperature
connective tissue organisation
three layers of connective tissue support each muscle:
epimysium = layer of collagen fibres that surrounds entire muscle
perimysium = divides skeletal muscle into compartments, contains blood vessels and nerves
endomysium = within fascicle, surrounds each skeletal muscle fibre, has capillaries and nerve fibres
fascicles = bundle of muscle fibres
sarcolemma and transverse tubules
sarcolemma = plasma membrane of a muscle fibre which surrounds the sarcoplasm
network of transverse tubules with openings on the sarcolemma form passageways through the muscle fibre
myofibrils
inside each muscle fibre are hundreds of cylinder shaped structures called myofibrils
myofibrils are bundles of myofilaments (protein filaments made of actin and myosin)
actin molecules found in thin filaments
myosin molecules found in thick filaments
myofibrils can actively shorten creating a muscle fibre contraction, as they’re attached to the sarcolemma at each end the whole cell contracts
sarcoplasmic reticulum
SR is a specialised form of smooth endoplasmic reticulum
forms a tubular network around each myofibril and is tightly bound to T tubules
expanded chambers of the SR called terminal cisternae lie on either side of T tubules
muscle contraction occurs when stored calcium ions in terminal cisternae are released into cytosol of sarcoplasm
sarcoemeres
myofilaments (thin and thick filaments) that make up myofibrils are organised into repeating functional units called sarcomeres
each myofibril has 10,000 sarcomeres arranged end to end
interactions between thick and thin filaments of sarcomeres = contraction
striated appearance = sarcomeres lying side by side
thick filaments lie in centre of sarcomere, thin filaments at either side are attached to interconnecting proteins that make the Z line (boundary of each sarcomere)
A band = dArk area containing thick filaments
I band = lIght area in between two successive A bands (includes Z line)
M line = proteins that connect central portions of each thick filament to its neighbours
H band = includes M line and only thick filament
thin filaments
consists of twisted strand of actin molecules
each actin molecule has an active site that can interact with myosin
when resting, the active sites are covered by strands of the protein tropomyosin
the tropomyosin stands held in position by molecules of troponin
thick filaments
composed of myosin molecules, each with a tail and a globular head
during a contraction, the myosin heads attach to the actin molecules but only when the troponin changes position, moving the tropomyosin to uncover the active sites
calcium is the “key” that “unlocks” the active sites and starts a contraction
calcium ions from the terminal cisternae of the SR, binds to troponin causing it to change shape, swinging the tropomyosin away from the active site
structural proteins
titin: acts like molecular spring to allow sarcomeres to have passive elasticity, keeps thin and thick filaments aligned
nebulin: provides structural stability to sarcomere, particularly at Z line
desmin: acts architecturally to aid structure of thin filaments
sliding filaments and cross bridge
thin filaments slide toward centre of sarcomere, alongside the stationary thick filaments - sliding filament theory
when myosin heads interact with thin filaments during a contraction, they are called cross bridges
a cross bridge covalently binds to an active site, pivoting the head of myosin toward the centre of the sarcomere
this pulls the thin filament in that direction towards the centre
cross-bridge then detaches and returns to original position, ready to bind again and repeat cycle
length tension relationship
tension = muscle cells contracting, pulling on collagen fibres to produce an active force
when muscle is at its shortest length = not able to generate tension because filaments are overlapped
muscle at mid length = able to produce tension because filaments no longer butting against Z line
muscle at longest length = harder to develop tension because there is less overlap between myosin and actin
amount of tension produced determined by:
- the frequency of muscle fibre stimulation
- the number of muscle fibres activated
number of muscle fibres activated
motor unit = single motor neuron and all the muscle fibres it controls
more precise movement comes from one motor neuron controlling less muscle fibres
muscle fibres of each motor unit are intermingled with those of other motor units
activation of more and more motor units is called recruitment and results in a smooth steady increase in muscular tension
peak tension = all motor units in muscle are contracting but doesn’t last long as available energy is used fast
long duration contraction = motor units activated on a rotating basis so some are resting while other contract, maximal tension not produced
force velocity relationship
the faster the rate of the muscle shortening, the less force produced
because, faster movement means less time for myosin heads to attach to active site
force during shortening < isometric force
force during lengthening > isometric force
type I muscle fibres
suited to endurance activites
fatigue resistant
produce lower force and power
more myoglobin = more oxygen carrying capabilities = red colour
lots of mitochondria
type IIa muscle fibres
in between I and IIx
pink appearance
type IIx muscle fibres
for anaerobic activities
great amounts of power and force
fatigue quickly
large cross section
low mitochondria
white colour
transmission of nerve cells
when a nerve is stimulated, membrane potential increases from -70 to 30
at rest more sodium (positively charged) outside the cell
action potential stimulates opening of sodium channels to let sodium enter axon
exposed part of axon between myelin sheaths depolarises and sodium gates close
potassium gates open
potassium leave decreasing the charge = repolarisation
potassium gates close
neuromuscular junction
neuromuscular junction is where a motor neuron meets a muscle fibre
axon branches within perimysium
each branch ends at an expanded axon terminal
acetylcholine is a neurotransmitter (chemical) released by a neuron to communicate
synaptic cleft = narrow space separating the axon terminal from sarcolemma
motor end plate = pocket formed around motor neuron by sarcolemma
synaptic cleft and motor end plate contain enzyme acetylcholinesterase which breaks down ACh
stimulation at neuromuscular junction
action potential arrives
axon terminal depolarised
calcium influx into axon terminal
encourages release of ACh into synaptic cleft
ACh interacts with sodium ion channels on post synaptic membrane
sodium ion channels open and sodium influx causes depolarisation
charge of muscle cell membrane increases, voltage gated calcium channels open causing influx of calcium
increases charge, stimulating calcium release from SR
action potential ceases, ACh re-enters neuron as choline and acetic acid
efferent vs afferent
efferent neurons = neurons that send impulses from the central nervous system to your limbs and organs
afferent neurons = neurons that carry nerve impulses from the sensory receptors or sense organs towards the central nervous system
Golgi tendon organ
regulates tension
it sense tension in the tendon when the muscle contracts
when excessively large forces are generated feedback from GTO causes activation of muscle to decrease
message is sent via afferent neuron to a reflex action that relays it back to efferent neuron for an inhibitory effect
acts as protective mechanism
muscle spindles
maintain muscle length
highly specialised encapsulated muscle fibres (intrafusal) positioned parallel to normal muscle fibres (extrafusal)
sensitive to changes in muscle length
afferent neuron wraps around muscle spindle
efferent neuron causes muscle spindles to contract to maintain tension in middle of fibres
hence if muscle is stretching rapidly, vigorous contraction is causes to prevent overstretching
