Lecture 31. Muscle 2 Flashcards
When does muscle develop maximum tension?
when all myosin is aligned with actin filament and every myosin head can attach to actin
when does muscle develop no tension?
when the muscle is stretched out so that myosin and actin filaments do not overlap and myosin heads cannot bind to actin
OR
When the muscle is shortened so that all myosin and actin filaments are jammed together and it cannot generate tension( myosin pulls actin in opposite direction)
Why when the muscle is shortened it is harder to develop tension?
Myosin and actin overlap to the point, where myosin starts interacting with the wrong actin filament.
Cross-bridge cycle is happening but it is pulling in the wrong direction-> ineffective
Length-Tension relationship
During isometric muscle contraction
At the level of the sarcomere the maximum active force (tension
developed) is dependent on the degree of actin and myosin overlap
What is the optimum range for sarcomeres?
The maximal force between 2.0 – 2.2 µm
• A lengths >2.2 µm active forces decline as the extent of overlap between
filaments reduce, reducing the number of cross-bridges
• At lengths <2.0 µm filaments collide and interfere with each other reducing
force developed
What is the optimum range for sarcomeres?
The maximal force between 2.0 – 2.2 µm
• A lengths >2.2 µm active forces decline as the extent of overlap between
filaments reduce, reducing the number of cross-bridges
• At lengths <2.0 µm filaments collide and interfere with each other reducing
force developed
Elastic components of muscle
- Internal( eg titin)
- Externals( elastin in CT)
Passive vs active force in muscle
As muscle is stretched the connective
elements resist the stretch = passive force. No energy required
Active force- due to muscle contraction
Total tension is:
Total tension is the sum of the active tension
dependent on the sarcomere length and the passive tension
As the muscle is getting stretched passive tension contributes more to the total tension than active tension
When the muscle is stretched out what type of tension is it?
passive
Excitation contraction coupling
-Alpha motor neuron transmits a signal to the muscle fiber
Motor unit
A motor unit consists of a motor neuron and all the
muscle fibers it innervates.
One motor neuron can innervate multiple muscle cells at the same time
-> One AP in a nerve will cause contraction in more than 1 muscle fiber
Steps of muscle cell excitation
- Ach released into the neuromuscular junction
AP travels down the motoneuron
The AP at the synapse causes the opening of V-gated Ca2+ channels
Ca2+ causes the neurotransmitter vesicles to merge with the membrane, causing them to release Acetylcholine in the synaptic cleft - Activation of Ach receptors
Binding of Ach to the receptors in the muscle causes opening of ligand-gated ion (Na+) channels-> Na+ moves into the cell-> depolarization of end-plate potential - A muscle AP is triggered. If the threshold is reached -> V-gated Na+ channels open and AP is triggered. AP is propagated along sarcolemma into T-tubule
What is an area of high concentration of V-gate Ca2+ channels
In the synapse of a pre-synaptic neuron
When does the change from electrical signal to chemical signal happen?
when the neurotransmitter is released into the synapse
What channels propagate AP?
V-gated Na+ channels
why are the effects of Ach short-lasting?
It is quickly broken down
What breaks down Acetylcholine?
enzyme acetylcholine esterase
AP in skeletal muscle
- Slow depolarization at the start mostly due to the ligand-gated Na+ channels
- Once the threshold is reached-V-gated Na+ channels open-AP
- V-gated Na+ channels close, V-gated K+ channels open- repolarization
How quick is AP in muscle cell?
1-2 msec
Steps of muscle contraction
- AP spreads along the sarcolemma and down T-tubule, triggering the opening of V-gated Ca2+ channels in SR→ release Ca2+
- Ca2+ binds to troponin, moving tropomyosin of the binding site
- Cross-bridge cycle
- Ca2+ removed→ contraction ends
Where are Calcium channels and their receptors located?
Ca2+ channels- Sarcoplasmic reticulum SR
Receptors for channels(Voltage sensors)- t-tubules
How is Ca2+ removed after contraction?
Calcium is actively pumped back into the sarcoplasmic reticulum via Ca2+-ATPase pumps
Sources of ATP for muscle metabolism
- Creatine phosphate
- Anaerobic glucolysis
- Aerobic metabolism
Creatine phosphate
a spare phosphate in the muscle, that can bind to ADP to make ATP
How quickly is creatine phosphate used up?
15 sec
What is creatine phosphate good for?
Energy source
Good For quick jump, power lifting, quick and powerful movements
Does creatine phosphate require oxygen for ATP production?
NO. Anaerobic
How long is anaerobic glycolysis effective for?
Dominant system from about 10-30 sec of max effort
Can go up to max 120 sec(2 min)
Is anaerobic glycolysis effective?
only 2 ATP per glucose→ not very effective
What if the negative effect of anaerobic glycolysis?
Built-up of metabolites that change the ph and stop some processes that require a steady ph from happening
This limits its duration
What would anaerobic metabolism be good for?
Sprinting
Aerobic metabolism
- Can get energy from multiple sources, not just glucose
- Long lasting
- Efficient. 32 ATP per glucose
BUT does not generate a lot of power- max 300 W
Requires oxygen
Aerobic metabolism
- Can get energy from multiple sources, not just glucose
- Long lasting
- Efficient. 32 ATP per glucose
BUT does not generate a lot of power- max 300 W
Requires oxygen
VO2 max
measures the max oxygen consumption
More oxygen is used as the workload is increased
At the max point- no more Oxygen consumed, aerobic-to anaerobic
How can muscle types vary among people?
- genetics and lifestyle
- Some people have more Type 1 or 2 muscle fibers
the ratio of 2 varies among individuals
Type 1 and 2 muscle fibers
Type 1- slow twitch, rely on aerobic metabolism. More mitochondria and blood supply. Look redder
Type 2- fast-twitch. Anaerobic( eg weight lifting). Bigger. Higher SR pumping capacity
Types of motor units
Type 1 (“slow twitch”):
• Units with neurons innervating the slow efficient aerobic cells (maintaining posture, walking)
Type 2 (“fast twitch”):
• Units with the neurons innervating the large fibres that fatigue rapidly but develop large forces (jumping, weight lifting)
How can force be regulated?
- Rate of stimulation of individual motor units
- The number of motor units recruited
How long is the muscle contraction?
50-200 msec
~100 times longer than AP
tetanus
merging of twitches into a bigger signal
Frequency modulation
Temporal summation
Increasing the number of AP and thus muscle twitches leads to tetanus and an increased contraction strength
-longer and more powerful contraction
tetanus is caused by a bacteria( different from tetanus in contraction)
- leads to a tetanic contraction in the whole body
- need to give a relaxant and ventilate them
- toxin produced by the bacteria leads to continuous AP down the motor neurons
→ lack of inhibition
-
Regulating force by muscle fiber recruitment
- typically start with small fibers( aerobic metabolism) and recruit more and more to generate more force( anaerobic)
- changing voltage in one muscle fiber will not generate more force, but recruiting more fibers at the same time will
- the first ones to be recruited are the most sensitive, the last ones are the lest sensitive
- the bigger the voltage the more AP in different muscle cells the more muscle fibers are involved
