Lecture 5 - Muscle Performance Flashcards
When an action potential travels across the sarcolemma, and down it’s invagination in between the myocytes called the T-tubules, what sensor becomes triggered? And how does this result in cross bridge formation?
The action potential travels down the T-tubule, changing the membrane potential of the surrounding membrane. This triggers the voltage sensor in the T-tubule, and once this sensor is activated it will open intracellular Ca2+ channels. This lets Ca2+ leave the sarcoplasmic reticulum into the surrounding intracellular space to increase cytosolic Ca2+
Ca2+ binds to troponin, and this changes the shape of tropomyosin to expose the binding sites on actin. The myosin head then extends and binds with the action to form a cross-bridge
What happens to the voltage sensor when the membrane potential surrounding it is unable to repolarise?
The voltage sensor continues to keep the Ca2+ channels open, allowing the Ca2+ to continuously flow from the sarcoplasmic reticulum, thus maintaining cytosolic Ca2+ level, and this maintains cross bridge formation. This results in continued muscle tension, causing rigid paralysis
What is unique to myocyte membranes which assists repolarisation alongside K+ leaving the cell down its conc. gradient?
Cl- ions are expressed on the myocte membrane, and so during repolarisation these will open to allow an influx of Cl- to repolarise the cell back to RMP
What occurs during isometric contraction?
Muscle contraction with length held constant
What occurs during isotonic contraction?
Muscle contraction with force held constant
What occurs during eccentric contraction?
Eccentric contraction is when the force exerted on a muscle is greater than the force exerted by that muscle (causes muscle to lengthen)
What occurs during concentric contraction?
Concentric contraction is where the force exerted by a muscle is more than the force exerted on the muscle, causing the muscle to shorten
Describe the Force - Length relationship for muscle
The amount of force generated depends the length of the muscles.
When the muscle is maximally short the muscle can’t generate much force, but as the muscle lengthens the force generated increases to a maximum level, and then it decreases back down.
What causes there to be passive tension?
The passive tension is just the opposing tension force when we pull on it - its due to the elastic elements within muscle
The passive tension reaches a maximum when the muscle is at it’s longest due to it almost being torn
Describe the force-velocity relationship
The force - velocity relationship for muscle is a non-linear relationship
The higher the velocity of muscle contraction, the smaller the force that can be generated. The slower the muscle, the more force that can be generated.
Higher velocities means less force can be produced since there is less time for force exertion by myosin heads. The slower the velocity, the more time there is for force exertion by myosin heads.
What are two relationships for muscle performance that occur at the same time?
Force-length relationship and the force-velocity relationship
Which muscle structure provides information about muscle length and velocity?
The muscle spindle provides information about velocity and length. The length and velocity of muscle will determine the amount of force generated by a signal from the brain
Which two muscle regulators control muscle force?
Muscle force is regulated by:
- Changing the number of units active (Recruitment)
- Changing the rate of activity in each unit (rate modualtion)
What are some properites of motor units?
Motor units can innervate either white or red fibres, and the motor unit size can vary from ~6 fibres to greater than 2000 fibres.
When an action potenial is sent out from a motor unit, all of the muscle fibres it innervates will all depolarise at once, therefore the max force generated depends on the size of the motor unit.
The size of the motorneuron cell bodie depends on how many fibres are in the motor unit.
What kind of movements are motor units with 6 muscle fibres used for?
Fine movement, whereas motor units with many more fibres (>2000) are used for large movements that require alot of force
How does rate modulation regulate the muscle force generated?
Rate modulation is the frequency of action potentials in the motor neuron, and this controls how many release events there are of Ach, and this determines the frequency of post synaptic action potenial generation in the membrane, which controls the level of intracellular concentration. The higher the freqeuncy, the higher the Ca2+ concentration, the more force generated due to more corss-bridges forming
The higher the level of Ca2+, the more oppurtunity there is for actin and myosin to interact
Describe the size principle of motor unit recruitment
The recruitment of motor units is orderly, where the small motor units are recruited first, and then as more force is required larger and larger motor units are recruited
By us recruiting small motor units first, this allows for fine graded control of small forces.
Why are motor units recruited in an orderly fashion?
Smaller motor units require a smaller stimulus to reach threshold to depolarise their entire cell body membrane to generate an action potential in their axon.
Whereas for larger motor neurons they have a larger cell body and lose more current thorugh leak and capacitance, and thus require a larger stimulus to reach threshold potential to depolarsie its membrane to generate an action potential in the axon.
