Muscles: Skeletal Muscle part 2 Flashcards
Isotonic Contraction
Shortening, Tension constant, Velocity variable
Isometric Contraction
No shortening, Length constant, Tension variable
When is maximum active force generated in an isometric contraction?
When there is the greatest amount of actin and myosin overlap to produce the greatest number of cross-bridges
What is the optimal length for the greatest amount of tension/force
2-2.2μm
What does a length greater/smaller than the optimal length do to the amount of tension/force generated?
The amount of tension generated decreases
What occurs in lengths <2.0 µm that results in reduced force?
At lengths <2.0 µm filaments collide and interfere with each other reducing force developed
What occurs in lengths >2.2 µm that results in reduced force?
At lengths >2.2 µm active forces decline as the extent of overlap between filaments reduces, reducing the number of cross bridges
Equation for total tension
Total tension = active + passive force
Elastic components of muscle
Elastin
What is passive force
The connective tissue that resists stretch at a certain length (acts against the stretch)
What is active tension dependent on?
Sacromere length (actin and myosin overlap)
When is passive tension NOT generated
When there is no contraction occurring
What occurs a muscle is stimulated through a contraction
There is a “twitch” - peak in length v. stress graph due to active force and cross bridge cycle
What happens as active force decreases due to a muscle being stretched out?
Passive tension increases
What is Excitation-contraction coupling
a sequence of events that converts APs in a muscle fibre to a contraction
What is a motor unit?
a motor neuron and all the muscle fibers it innervates (nerve touches at neuromuscular junction)
What is the first stage of excitation contraction coupling?
ACh released into neuromuscular junction
AP travels down motor neuron. At axon terminal Ca2+ channels open, Ca2+ enters axon terminal → vesicles containing ACh to fuse with terminal membrane, releasing ACh into neuromuscular junction (synaptic cleft), a specialized synapse
What is the second stage of excitation contraction coupling
Activation of ACh receptors
Binding of ACh to receptors on muscle end plate → opening of ligand (ACh) gated ion channels. Opening of channels allows movement of mainly Na+ into muscle cell making it less -ve (end plate potential). Effects of Ach = short lasting as acetylcholinesterase (enzyme) rapidly breaks down Ach
What is the third stage of excitation contraction coupling
A Muscle Action Potential is triggered
If sufficient ligand gated channels are opened the end plate potential reaches threshold. Voltage gated Na+ channels open and an AP is triggered. AP is then propagated along the sarcolemma into the T tubule system
What is the fourth stage of excitation contraction coupling
Calcium is released from the SR
The AP is conducted down the t-tubules coming in close contact with the SR → voltage gated Ca2+ channels in the SR opening (Ca channel directly connected to voltage sensor). Ca2+ is released into the cytosol
What is the fith stage of exciatation contraction coupling
Ca2+ binds with troponin
When [Ca2+] reach a critical threshold the myosin binding sites on the actin filament are exposed allowing the cross-bridge cycle to occur
What occurs after Ca2+ binds to troponin
The cross bridge cycle
Cross-bridge formation, Power stroke, Detachment, Energization of myosin head
How does excitation-contraction coupling end
Contraction ends when Ca2+ levels decrease
Ca is actively pumped back into SR via Ca2+-ATPase pumps. Troponin moves tropomyosin back covering myosin binding site. Muscle “twitch” is complete
The 4 types of channels involved in excitation contraction coupling
- ACh ligand channel that allows Na+ to enter postsynaptic membrane
- Na+ voltage gated channels which allow Na+ influx to cause an AP
- Ca2+ voltage gated channels which release Ca2+ into the cytosol to cause a contraction
- Ca2+ ATPase which pumps Ca2+ back into the SR
What is the function of creatine phosphate
For brief periods (<15s) creatine phosphate can act as an ATP “store”
What type of metabolism is the use of creatine phosphate
Anaerobic (no O2)
How long can creatine phosphate be used for
<15s
Creatine phosphate + ADP = ?
creatine + ATP
What is anaerobic glycolysis?
Short intense exercise: fast, inefficient metabolism
How long is anaerobic the dominant system of muscle metabolsim
10-30s of maximal effort
What builds up during anaerobic metabolism and what does this cause?
lactate and H+ limits duration to max 120s
Is oxygen needed in anaerobic metabolism?
No
What is aerobic metabolism
Efficient, but slow metabolsim ∴ good blood supply. Important for postural muscles and endurance exercise. Using aerobic respiration produces lots of ATP.
Does aerobic metabolism require oxygen
Yes
Maximum wattage of aerobic respiration
300W
What is the Vo2 max
Aerobic exercise capacity (a breaking point) where you cannot exercise anymore
What type of muscle fibre is type 1
slow oxidative
What type of muscle fibre is type 2
fast glycolytic
Max ATPase rate of type 1 muscle fibres
slow
Max ATPase rate of type 2 muscle fibres
fast
SR pumping capacity of type 1 muscle fibres
moderate
SR pumping capacity of type 2 muscle fibres
high
Diameter of type 1 muscle fibres
small
Diameter of type 2 muscle fibres
large
Mitochondira/myoglobin/blood supply of type 1 muscle fibres
high
Mitochondira/myoglobin/blood supply of type 2 muscle fibres
low
Glycolytic capacity of type 1 muscle fibres
moderate
Glycolytic capacity of type 2 muscle fibres
high
Primary ATP pathway of type 1 muscle fibres
aerobic
Primary ATP pathway of type 2 muscle fibres
anerobic glycolysis
What are type 1 motor units
(“slow twitch”): Units with neurons innervating slow efficient aerobic cells (maintaining posture, walking) innervates type 1 muscle fibres
What are type 2 motor units
(“fast twitch”): Units with the neurons innervating the large fibres that fatigue rapidly but develop large forces (jumping, weight lifting) innervates type 2 muscles fibres
What is the regulation of force dependent on?
- Increase no. of AP down one motor unit (twitches merge = force increase but same number of cells involved) Rate of stimulation of individual motor units
- No. or motor units recruited (depends on what muscle is needed for changes the amount of force needing to be developed → no. of muscle fibres recruited)
When are large anaerobic fibres recruited?
Last
When are small aerobic fibres recruited?
First
What type of fibre is the most fatigue RESISTANT
type 1 (small aerobic)
What type of muscle fibre uses the least amount of voltage
type 1
What happens as more motor units are recruited/used
the tension increases
How many twitchs does a single stimulus cause?
A single twitch: A single stimulus is delivered causing the muscle to contract and relax
How long is the twitch compared to the length of the AP in skeletal muscle
Twitch is multiple times the length of an AP (voltage change)
Complete Tetanus meaning (in relation to stimuli)
There is no relaxation of the muscle fibers between stimuli
When occurs when there is a low stimulation frequency (i.e. temporal summation)?
The twitch turns into an unfused (incomplete) tetanus: If another stimulus is applied before the muscle relaxes completely, then more tension results. This is temporal (or wave) summation and results in unfused (or incomplete) tetanus
What occurs when there is a high stimulation frequency?
The twitch turns into a fused (complete) tetanus: At higher stimulus frequencies, there is no relaxation at all between stimuli.
What does an increased frequency of stimuli cause?
Increased frequency = Temporal Summation
