Muscle Physiology 2 Flashcards
What is a muscle twitch?
Contraction produced in a muscle fiber in response to a single action potential
Twitch is an all or nothing event for given muscle fiber at rest
-twitch can be defined for a muscle fiber, motor unit, or whole muscle level
What is preload?
The load on a muscle in a relaxed state, I.e, before it contracts
Applying preload to the muscle results in:
-stretching of the muscles
-generation of passive tension in the muscle
What is afterload?
The load of the muscle works against during contraction
Contractile elements= sarcomere
Series elastic elements= connective tissue, tendons
Force exerted by contracting muscle= tension
Force opposing contraction (such as weight to be moved)= load
What is isotonic contraction?
- constant tension
- when tension > load
- load is lifted as muscle shortens
What is isometric contraction?
- length constant
- contractile elements contract, generating tension
- when load > tension
- stretches series elastic elements
- muscle does NOT shorten, load not lifted
What is the function of the isometric contraction?
These create force without moving a load
- series elastic elements
- sarcomere shorten while elastic elements stretch, resulting in little change in overall length
What is the function of the isotonic contraction?
- Create force and move a load
- Concentric action is a shortening action
- Eccentric action is a lengthening action
- There are 4 phases of the isotonic contraction
What are the 4 phases of isotonic contraction?
Phase 1 isometric contraction
Phase 2 isotonic contraction
Phase 3 isotonic relaxation
Phase 4 isometric relaxation
What is the function of series elastic elements in muscles?
Elastic elements allow isometric contractions:
-In an isometric contraction, sarcomeres shorten, generating force, but elastic elements stretch, allowing muscle length to remain the same.
-In isotonic contractions, sarcomeres shorten more but, because elastic elements are already stretched, the muscles shorten
Summarize the mechanics of normal muscle activity
Some purely isometric contractions occur
- Purely isotonic contractions are rare
- Even if load is constant, isometric precedes isotonic phase of contraction (remember the phases of isotonic contraction)
Load generally not constant
-load changes with the change of limb position
What is passive tension force ?
The elastic component of skeletal muscle creates a resistance to stretch. The force of this resistance is measured as passive tension.
Preload inversely proportional to length inversely proportional to passive tension
Note: passive tension is force
How does active tension affect sarcomere length?
Sarcomere length and the amount of myosin-actin filament overlap on the active tension developed by a contracting muscle fiber
What is caused by an increase in preload?
Increase Fmax:
- shifts curve to the right
- increase Crossbridge overlap
- increases maximum weight that can be moved, max force generated
What is caused by increased velocity?
Same weight/after load can be moved faster
No change Vmax:
-ATPase. Activity not allowed
What is the function of Myosin-ATPase?
Hydrolysis of ATP- step to energize myosin head group
- Activity determine the rate of myosin head resetting in Crossbridge cycling (how fast it spins)
- Vmax is determined by myosin ATPase activity—> how fast Crossbridge cycle spins
- Increased myosin ATPase activity also increases ATP utilization during contraction
Explain the functioning of the type 1 muscle fibers
Slow, oxidative fibers
Generally, this is smaller (less powerful) muscle, utilized long term (endurance muscle), e.g. postural muscle. Major characteristics:
- lower myosin ATPase activity (slower muscle)
- Contains a large amount of myoglobin pigment
- High capacity for aerobic Metabolism
- Slow twitch, fatigue resistant motor units
Describe type 2 muscle fibers
Fast, glycolytic
Generally, this is the large (powerful) muscle that is utilized short-term, e.g. ocular muscles, leg muscles of a sprinter.
Major characteristics:
-High Myosin ATPase activity(fast muscle)
- low myoglobin
- high capacity for anaerobic glycolysis.
- Vast twitch fibers, which have rapid conduction rate, and which are adapted to fast, short lived forces
How do skeletal muscles regulate their force?
Graded muscle contractions
- Temporal summation: tension produced by each fiber (due to high frequency stimulation)
- Spatial summation (recruitment): number of fibers contracting
What determines the tension produced by individual muscle fibers?
Number of active crossbridges that bind to actin
-More crossbridges that bind —> more force
Factors affecting force of individual muscle fiber
- frequency of stimulation
- fiber diameter
- changes in fiber length (already covered before)
What does the basis of summation represent?
Increases in frequency of action potential in muscle fibers increases tension
What are the events at each phase of tension-timr(msec) graph?
- action potential (two msec) -action potential causes Ca2+ release
- contraction (20-100msec)- release of Ca2+ evokes muscle contraction. Muscle membrane has completely depolarized before relaxation of the muscle
- contractions can overlap and sum
What causes tetanus?
Amount of tension developed depends on amount of calcium bound to troponin
- At high frequencies, release exceeds re-uptake
- Calcium increases in cytosol
- Eventually saturates system
- All troponin has calcium bound to it
- Crossbridge cycling maxed out
- Maximum tetanic contraction
What causes tetanus?
A neurological disorder caused by the toxin produced in the bacterium clostridium tetani.
The toxin that is released blocks the release of inhibitory transmitters, y-aminobutyric Acid (GABA)
Diminished inhibition leads to increase in the resting firing rate of motor rate
-Increased repetitive stimulation of the motor neuron, the calcium released from the SR remains bound to troponin and extends the time for cross-bridge cycling, resulting in muscles that do not relax
What are the symptoms of tetanus?
“Lockjaw” and the neck, shoulder, back, upper and lower extremities spasms. Generalized spasms may jeopardize. Breathing
How can tetanus be managed?
Not a comprehensive management
Antitoxin- neutralize circulating and unbound toxin
Diazepam (GABA agonist)- to control spasm
What are the factors affecting whole muscle contraction
Spatial summation /motor unit recruitment
Force developed by the whole muscle depends on:
- number of muscle fibers contracting
- more muscle fibers contracting, greater force
More fibers contracting —> greater tension
- recruit motor units
- Activate motor neuron activates all muscle fibers in the motor unit
- Increases in tension occur in steps proportional to size of motor unit
What is the motor unit size principle?
Order of motor unit recruitment related to size of motor units
