Skeletal muscle and Force Generation Mechanism Flashcards
what are the three types of muscle
skeletal, smooth, cardiac
what is the classification of skeletal muscle
striated, voluntary
what is the sarcolemma
the cell membrane that encloses each muscle cell
what is the endomysium
the connective tissue that wraps each individual muscle fibre
what is the perimysium
connective tissue that wraps bundles of muscle fibres
what is the epimysium
connective tissue that wraps the whole muscle.
what is the fascia
a layer of thickened connective tissue that covers the entire muscle
what are the layers of musscle tissue
sarcolemma
endomysium
perimysium
epimysium
fascia
what happens when a nerve impulse reaches the neuromuscular junction
causes chemical transmitter (acetylcholine) to be released
Triggers muscle contraction
what is skeletal muscle supplied by
the efferent arm of the somatic nervous system
where are lower motor neuron cell bodies
in the ventral horn of the spinal cord
what is the motor end plate
The specialized postsynaptic region of a muscle cell. The motor endplate is immediately across the synaptic cleft from the presynaptic axon terminal
Excitatory signal sent from the terminal button to the motor end
plate using ACh via ________
the nicotinic receptor
what is muscle fibre composed of
myofibrils which are composed of sarcomeres which is composed of filaments
what causes striations
thick and thin filaments which run parallel
what is in the H zone of a sarcomere
think filaments (myosin) only
what is in the A band of a sarcomere
thick filamants and overlapping thin filaments (actin)
what is in the I band of a sarcomere
thin filaments only
name the proteins of a sarcomere
Contractile Proteins e.g. actin and myosin
Structural Proteins e.g. titin, dystrophin
Regulatory Proteins e.g. troponin-Complex, tropomysoin
how is a thin myofilament formed
- Small actin molecules (G-actin) link together to form long, twisted strands called F-actin.
- Tropomyosin wraps around actin, blocking binding sites, while troponin attaches to tropomyosin and controls muscle contraction.
- Together, these proteins form the thin myofilament, which plays a key role in muscle contraction by interacting with thick myofilaments (myosin).
describe the formation of a thick myofilament
- Many myosin proteins come together. Each myosin has a long tail and a head that sticks out.
- The tails of the myosin molecules bundle together in the center, while the heads extend outward.
- The myosin heads are arranged at both ends of the filament, allowing them to interact with thin filaments during muscle contraction.
what is in the head of a myosin
actin-binding site
Nucleotide binding site for ATP and ATPase
what is the function of titin
Anchors thick filaments between M-line and Z-line
Provides structural support and elasticity - helps prevent sarcomere popping and Z-line streaming
what is Excitation-contraction coupling
how muscle contractions are turned on and off
what is Muscle cell metabolism
how muscle cells provide ATP to drive the crossbridge cycle
what happens to a Thin Myofilament when muscle is contracting
when calcium ions bind to troponin, this causes troponin to change shape.
tropomyosin moves away from mysoin binding sites on actin, allowing myosin head to bind to actin. forming crossbridge
what is a tropomyosin
Spans 7 G-actin monomers
Overlaps binding sites on actin for myosin
what is a troponin complex
1 per tropomyosin
Complex of three proteins (T,I & C)
(regulates skeletal muscle contraction)
what is the steric blocking model
At rest
– myosin binding sites on actin blocked by tropomyosin
what is the function of troponin C in a Troponin Complex
binds to calcium ions to produce a conformational change in Troponin I.
what is the function of troponin I in a Troponin Complex
binds to actin to hold the troponin-tropomyosin complex in place.
what is the function of troponin T in a Troponin Complex
binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex.
how are mysoin heads activated
when ATP binds to myosin head and is hydrolized to ADP and a phospate. the energy from the hydrolysis, activates the mysoin head
head must be activated for crossbridge to form
what are the four steps of the cross bridge cycle
- cross bridge formation
- the power stroke
- cross bridge detachment
- Reactivation of myosin head
what is step 2 of the cross bridge cycle (the power stroke)
ADP is released from mysoin head and the activated myosin head pivots, sliding the thin myofilament towards the centre of the sacromere
describe stage 3 of the cross bridge cycle (cross bridge detachment)
when another ATP binds to the myosin head, the link between the myosin head and actin weakens, and the myosin head detaches
what is the importance of the cross bridge cycle
causes thin myofilaments to be pulled toward each other and the sarcomere shortens. this shortening causes the whole muscle to contract
sarcomere shortens by pulling in of the Z line at each end
when does the cross bridge cycle end (muscle stops contracting)
when calcium ions are actively transported back to the sarcoplasmic reticulum.
troponin returns to its original shpe allowing tropomyosin covers binding site again
what happens within a sarcomere during contraction
- A band stays same length
- I band shortens
- H zone shortens
how is calcium rleased from thesacrcoplasmic reticulum
action potential runs deep into muscle fibre vis T tubles, activating voltage-gated calcium channels ( Dihydropyridine receptors), releasing the calcium
where is ATP used by muscles
- Crossbridge cycle
* Splitting of ATP by myosin (for power stroke)
* Binding of fresh ATP to myosin to cause dissociation - Active transport of calcium back into sarcoplasmic reticulum
* Relaxation
what are the 3 pathways of sources for ATP
Creathine Phosphate
Glycolysis
Oxidative Phosphorylation
what is the creatine phosphate system
Creatine phosphate + ADP <——> (creatine kinase) Creatine + ATP
what is Non-Oxidative Glycolysis
glucose is broken down into 2 lactic acids and 2 ATP
what is a twitch contraction
Contraction produced in a muscle fiber in
response to a Single Action Potential
what are the phases of a muscle twitch
- latent period
- contraction phase
- relaxation phase
what is the latent period of a muscle twitch
Time from action potential in muscle
cell to onset of contraction
what is the contraction phase of a mucsle twitch
Crossbridge cycles take place
repeatedly
what is the relaxation phase of a muscle twitch
Tension is decreasing back to zero
(longer than contraction phase)
Calcium reuptake
what is a isotonic contraction
tension remains the same, whilst the muscle’s length changes.
what is an isometric contraction
contractions in which there is no change in the length of the muscle.
what are the differences between an isotonic ans isometric contraction
Isometric Twitch Contraction
– Length constant
– Contractile elements generating tension
– Muscle does NOT shorten, load not lifted
- can occur daily
Isotonic Twitch Contraction
– Constant tension
– Load is lifted as muscle shortens
– rarer
Increases in frequency of action potentials in muscle fibers increases tension in two ways _____ and _______
treppe and summation and tetanus
what is treppe
Treppe occurs at a frequency of muscle stimulation where independent, identical twitches follow one another closely such that the peak tension rises in a stepwise fashion with each twitch, until eventually it reaches a constant level
what is a cause of treppe
Increase cytosolic calcium
what are causes of summation and tetanus
- Amount of tension developed depends on amount of calcium bound to troponin
- Crossbridge cycles max out
- Maximum tetanic contraction
Small motor units are recruited _____
first
Large motor units are recruited _____
last
Force-generating capacity depends on _____
number of crossbridges in each sarcomere and geometrical arrangement of sarcomeres
More crossbridges/sarcomere → more force
More sarcomeres in PARALLEL→ more force
What is tetanus
When a muscle is stimulated so rapidly that it does not have time to relax between stimuli, it reaches a sustained, maximal contraction.
what is summation
When a muscle is stimulated repetitively such that additional action potentials arrive before twitches can be completed, the twitches become superimposed on one another, yielding a force greater than that of a single twitch; this process is called summation.
Summation happens whenever twitches occur at a frequency
such that calcium cannot be removed from the cytosol as rapidly as it is released from the SR. Calcium removal is necessary for relaxation; thus the muscle fiber cannot relax completely between twitches.
