Muscle Physiology Flashcards
What are contractile systems?
Biological mechanism that utilize chemical energy from the metabolism of food in the form of ATP hydrolysis to perform useful work
What are the 3 kinds of muscle cells?
Skeletal
Smooth
Cardiac
What is the primary function of the 3 different kinds of cell muscles?
Skeletal - voluntary movement
Smooth - walls of hollow sister, blood vessels and various ducts
Cardiac - creating the heart muscle
What is a whole muscle made out of?
Fascicles
What is a fascicle?
Each fascicle contains bundles of muscle cells/fibres
What is within each muscle cell?
Myofibrils
-long cylindrical structures
What are myofibrils made of?
Myofilaments
Why are myofilaments important?
They contain the actual proteins responsible for muscle contraction
What are the 2 kinds of myofilaments?
Thin
Thick
What happens when thin and thick myofilaments interact?
Muscles contract
What is the pattern of muscle cells?
Striated
What is the pattern of muscle cells due to?
The arrangement of thin and thick myofilaments
What does the number of muscle cells depend on in one muscle?
Size of whole muscle
-larger the whole muscle, the greater number of muscle cells
How long are muscle cells?
vary from few mm to 12cm
-depending on the size of the whole muscle
Why are skeletal muscle cells unique?
They contain many nuclei unlike most cells in the body where they contain only 1 nucleus
They contain mitochondria to make ATP for muscle contraction
What is the biggest and smallest muscle cell?
Biggest= Sarturius 12cm from hip to let
Smallest= Ear
What happens to the muscle when you work out?
The size of the whole muscle is changing, not the number of muscle cells in the muscle
What is surrounding muscle cells?
Sacrolemma (membrane)
-sarcoplasma on the inside
What are the Transverse tubules?
Are a continuation of the sarcolemma that travels down into the muscle cell and wrap around each myofibril
What is the function of T Tubules?
They allow for the action potential to travel from the surface of the sarcolemma down inside the muscles cell where the contractile proteins are located
What is the sarcoplasmic reticulum?
Specialized organelle between the T tubules and wraps around the myofibrils
What is stored in the SR?
SR is a hallow structure that holds Ca++ in the muscle cell
Does the SR come into contact with the T Tubules?
NO
What is the lateral sac?
The chubby ends pop the SR that are closest to the T tubules
What is the function of the lateral sac?
Ca++ is released to trigger the muscle contraction
What are myofibrils made of?
A number of different proteins
-mix of contractile proteins and structural proteins
Where are contractile proteins found?
In both thin and thick myofilaments
-but each has a different contractile protein
What are the 3 contractile proteins that make up thin myofilaments?
G Actin
Tropomyosin
Troponin
What is G Actin?
Golubular protein all linked together to form 2 strands of a helical chain
Each G actin has a myosin binding site
What is tropomyosin?
Rod snapped protein composed of 2 chains wrapped together in a supercoil
Lies lengthwise on actin
Where is tropomyosin located when relaxed?
Is situated so that it partially covers the myosin binding sites on the actin molecule
What is troponin?
Holds the tropomyosin molecule over the myosin binding site on the actin when the muscle is relaxed
How does Ca++ affect tropomyosin?
Ca++ pulls tropomyosin off the actin binding site to allow the head of the thick filament to bind
What are thick myofilaments composed of?
Many myosin molecules
-myosin molecules itself is 2 long polypeptide chains making and alpha helical tail and a globular head
Do all myosin molecules point in the same direction?
No, heads point away from the centre of the thick myofilament
What does each thick myofilament head have?
Binding site for actin and ATPhase
What does ATPase do?
Breaks down ATP to ADP + Pi and pleases energy for contraction
How are thin myofilament joined together?
Z line
What is a sarcomere?
The section between z lines
-the smallest functional unit of the muscle cell
What is a cross bridge and when does it form?
Forms when a myosin head attaches t and actin molecule.
When this occurs the power stroke is initiated and contraction will occur
For the power stroke, which direction do the myosin heads move?
They contract towards the centre of the sarcomere
-moving the 2 attached filaments towards the centre
What is titin and what does it do?
Holds the thick myofilament in place
- makes sure the muscle doesn’t over stretch
- helps return muscle to original resting length
What is the sliding filament theory?
When myosin binds to actin a cross bridge is formed
Myosin head changes shape and a power stroke occurs
Thin and thick myofilaments: actin slides past myosin
What happens to the length of sarcomere during contraction?
The whole sarcomere as a whole shortens
- Thick myofilament doesn’t change length
- Thin myofilament doesn’t change length
What is the neruomuscular junction (NMJ)?
Point of contact between the motor nerve and the muscle cell/fibre
What is the ratio of motor nerve to muscle cell?
Each nerve can touch more than one muscle cell but a single muscle cell and only have one motor nerve
At the NMJ what is contained within the vesicles in the neuron?
Acetylcholine neurotransmitter (Ach)
What is the region directly underneath the axon terminal?
Muscle end plate
What will 1 action potential on the motor nerve alway produce?
Will always produce one action potential on the muscle cell membrane
Know the 9 Muscular Junction events
- Action potential depolarizes the neuron axon terminal
- Depolarization causes Ca++ voltage gated channels to open
- Ca++ flows into the axon terminal and causes vesicles to dock and release acetylcholine neurotransmitter
- 2x Ach binds to the chemically gated ion channels
- Na+ flows into the muscle end plate, little K+ leaves
- Depolarization of the muscle end plate triggers the EPP (end plate potential)
- EPP depolarization causes the Na+/K+ voltage gated channels to open
- Action potential is generated
- Acetylcholine is degraded in the synapse into choline and acetic acid by Acetylcholinesterase.
- the parts are taken back up by the axon terminal to be recycled
What is excitation-contraction coupling?
Process where an action potential on the sarcolemma of the muscle cell/fibre leads to the release of Ca++ from the sarcoplasmic reticulum, cross bridge activity and contraction
What stops the muscle contraction?
We need to stop Ach
Which areas are the t tubules in contact with?
T tubules are a continuation of the sarcolemma and also wrap around the myofibrils
What does the continuation of the t tubules into the deepest parts means?
That the action potential; can reach the deepest pars in the cell
What triggers the release of the Ca++ from the SR?
The action potential triggers the release of Ca++ from the lateral sac of the SR
What are the 10 events that are involved in excitation-contraction coupling (ECC)?
- Action potential is generated at the endplate of the muscle cell
- Action potential propagates over the sarcolemma and down the t-tube
- Voltage sensors on the t-tube detects the action potential and changes the shape of the sensor
- Voltage sensor opens Ca+ release channels on the lateral sac of the SR (Ca++ is released from the SR)
- Ca++ binds to troponin causing the tropomyosin to roll off the myosin binding sites
- Myosin attaches to actin (cross bridge formed) and power stroke occurs
- Actin (thin myofilaments) slide over the myosin (thick myofliaments) and muscle contracts (sarcomeres shorten)
- Ca++ is actively pumped back into the SR by Ca++ ATPase to stop the contraction
- When Ca++ is removed, tropomyosin covers myosin binding sites
- Muscle relaxes
What happens to ATP when it is hydrolyzed on the myosin head?
Breaks down to ADP and Pi which both remain on the myosin head and release a lot of energy
Where does the energy from breaking the ATP go?
Energy is transferred to myosin
-produces a high-energy form of myosin
What is special about high energy myosin?
It has a high affinity for actin
What does the breakdown of the ATP do to the myosin head?
Positions the myosin head so that it is ready to attach actin
Why cant the myosin head attach actin right away after ATP has been broken down?
Myosin binding site is still being covered by tropomyosin . The energized myosin is waiting for the action potential to initiate excitation contraction coupling (ECC) to move the tropomyosin
What happens to the myosin head when the action potential occurs?
Ca++ is released from the SR and binds to troponin which pulls tropomyosin off the myosin binding site
What happens when the myosin head finally binds to actin binding site?
Triggers the release of Pi and stored energy
When myosin is bound to the actin binding site, energy is released. What does this energy do?
Discharge of energy fuels the cross bridge movement and a muscle contraction occurs
What happens after the muscle contraction?
Remaining ADP is released from the myosin head
-myosin head its still attached to actin
What happens when a new molecule of ATP binds to the myosin head?
Triggers the release of myosin from the actin, breaking the cross bridge
-cycle repeats
What are the 5 stages of the ATP-Actin-Myosin cycle?
- ATP is attached to the myosin head. Myosin is not attached to actin
- ATP is hydrolyzed to ADP+Pi and is still attached to the myosin head. energy from the break transfers to the head and is repositioned to bind actin
- ECC occurs. AP releases Ca++ from SR, Ca++ binds troponin. Tropomyosin rolls off myosin binding sites. Myosin attaches to a tin forming the cross bridge. Pi is released to trigger the power stroke. Muscle contraction occurs
- ADP is released after power stroke. Cross bridge still formed
- Myosin still attached to form cross bridge. New ATP binds to myosin, breaking the cross bridge and cycle repeats
What does the ATP-Actin-Myosin cycle need in order to keep going?
ATP and Ca++
What is rigor mortis?
Muscle become very stiff after death
Begins 4 hours after death and it complete in 12 hours.
Slowly disappears over the next 24-48 hours
What is the cause of rigor mortis?
No oxygen, no ATP produced, no ATP then Ca++ cant be pumped into the SR, cross bridges stay formed. No ATP actin myosin cant dissociate. Muscles stay permanently fused until muscle begin to decompose
What is one motor unit?
Motor neuron and all muscle cells it comes into contact with.
What is motor unit size dependent on?
of muscle cells it comes into contact with
What will one AP on a motor neuron do?
Cause all the muscles it is in contact with to contract
What is a muscle twitch?
Muscle contraction in réponse to 1 action potential on the motor nerve
What is the ECC and relaxation dependent on?
The release and uptake of the Ca++ back into the SR
-defines the rate for both ECC and relocation of a muscle cell
How long is one twitch?
2-3 msec
Are you able to change the force of contraction in your muscles?
Yes, can adjust to the weight being lifted
What are the 2 ways in which muscles can alter the force of contraction?
Motor unit recruitment
Summation of twitch contractions
What is Motor unit recruitment?
When the recruitment of motor units is increased the force of a muscle can increase as well
-as more units are activated, mire muscle cells will contract and the overall contractile force of the muscle increases as a whole
What is summation of twitch contractions?
As Ap frequency is increased, each muscle twitch has less time ti relax before the next one occurs. twitches start to stack up on one another causing greater tension (more forceful contraction)
What happens to twitches at medium-high AP frequency?
Twitch swill summate but there will still be a little time to relax before the next twitch begins
What is the name for medium-high AP frequency?
An unfused titanic contraction (unfused tetanus)
What happens at very high AP frequencies?
There is little or not relaxation between twitches. All twitches will summate to produce a large smooth, sustained contraction.
What is the name for very high AP frequencies?
Complete tetanus
What happens when the muscle contraction is sustained for a long period of time?
Muscle will begin to fatigue and the force of contraction will begin to decrease
Can the max tension increase?
Yes, when you work out more, the max tension can increase
