Muscle Physiology Flashcards
What are the 4 mechanics of skeletal muscle contraction?
- twitch
- force
- length
- velocity
What is a twitch?
contraction produced in a muscle fibre in response to a single AP
What is the all-or-none principle?
- the principle that the strength by which a nerve or muscle fiber responds to a stimulus is independent of the strength of the stimulus
- If that stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response; otherwise, there is no response
A twitch can happen for a ______, _______ or ______.
- muscle fibre
- motor unit
- whole muscle level
What are the 3 phases of a twitch?
- latent period
- contraction phase
- relaxation phase
What happens during the latent period?
- AP in cell –> onset of contraction (2-3 ms)
- excitation-contraction coupling
What happens during the contraction phase?
- 10-100 msec, tension development peaks
- crossbridge cycling
What happens during the relaxation phase?
- peak - end of contraction (longest)
- Ca2+ reuptake, crossbridges decrease
What is the cause of summation and tetanus?
- tension depends on Ca2+ bound to troponin
- high frequencies = release exceeds reuptake
- saturation
In summation, action potential lasts ___ msec.
2 msec.
In summation, how long does contraction last?
10-200 msec.
In summation, contractions can ____ and _____.
overlap and sum
In summation, high frequencies = ______ exceeds _____.
release exceeds reuptake
In summation, Ca2+ increases in cytosol –>
saturation
Describe saturation in summation.
- all troponin has Ca2+ bound to it
- cross bridge cycling maxed out
- maximum tetanic contraction
What are the 2 types of twitches?
- isometric
- isotonic
Contractile elements =
sacromeres
Series elastic elements =
connective tissue, tendons
Force exerted by contracting muscle =
tension
Force opposing contraction (ie. weight to be moved) =
load
In isometric twitch contraction, the length is _______.
constant
In isometric twitch contraction, _____ _____ contract, generating ______.
- contractile elements
- tension
Isometric twitch contraction occurs when:
load > tension
What happens when load > tension in isometric twitch contraction?
- stretches series of elastic elements
- muscle does not shorten, load not lifted
Isotonic twitch contraction is _____ tension.
constant
Isotonic twitch contraction occurs when:
tension > load
What happens when tension > load in isotonic twitch contraction?
load is lifted as muscle shortens
In normal muscle activity, some purely _______ contractions occur.
- isometric
- postural
In normal muscle activity, purely ______ contractions are rare.
isotonic
In normal muscle activity, load generally _____ constant.
not
In normal muscle activity, load changes as ____ ____ changes.
limb position
Graded muscle contractions depend on 2 factors:
- tension produced by each fibre
- number of fibres contracting
What consists of tension produced by each fibre?
- number of active cross bridges that bind to actin
- more crossbridges that bind –> more force
Force generating capacity =
inherent ability of muscle to generate force
Force generating capacity depends on:
- the number of crossbridges in each sarcomere
- the genetic arrangement of sarcomeres
More crossbridges/sarcomere =
more force
More sarcomeres in parallel =
more force
Number of thick and thin filaments/area =
constant
Is fibre diameter constant?
no, it varies
Large diameter =
more filaments = more force
Length of fibre at the _____ of contraction affects:
- onset
- force generated
What is optimal length?
- resting length of muscle at which the fibre can develop the greatest amount of tension
- due to maximum overlap of thick filament crossbridges and thin filaments
Most muscles are at _____ length.
optimal
If a muscle is at greater than optimum length …
decrease crossbridge overlap
If muscle is at less than optimum length…
- thin filaments overlap each other
- Z lines contact thick filaments
More fibres contracting =
greater tension
Recruitment:
stimulating more muscle fibres to contract
Recruitment occurs at level of…
motor unit (MU)
Activation of the motor neuron activates:
all muscle fibres in the motor unit
Increases in tension occur in steps proportional to…
the size of the motor unit
What motor units are used in muscles for delicate movements?
small motor units
What motor units are used in muscles for strength?
large motor units
Number of motor units varies in _____ ______.
different muscles
Small fibre diameter in motor units are used for what movements?
weaker
Large fibre diameter in motor units are used for what movements?
stronger
Small motor units =
- small fibres
- small motor neuron cell bodies
- small axon diameters
Large motor units =
- large fibres
- large motor neuron cell bodies
- large axon diameters
The size principle:
order of muscle unit recruitment related to the size of muscle units
The size principle says ____ excited first, _____ last.
- small
- large
Why are large motor units excited last?
- difficult to depolarize to threshold
- greater synaptic input
When muscle contracts isotonically, _____ period of shortening _____ with increasing load.
- latent
- increases
When muscle contracts isotonically, duration of shortening with _______ load.
- decreases
- increasing
When muscle contracts isotonically, velocity of shortening ______ with _____ load.
- decreases
- increasing
When muscle contracts isotonically, the rate of change of the distance is _____.
shortened
Velocity of shortening reaches 0 when…
load > or equal to max tension
Velocity of shortening is greatest when…
no load on muscle
As the angle of pennation increases, there is…
greater force development (not power)
Greater mechanical advantage with a _____ angle of insertion (_____ to force arm) = _____ force development, but a _____ in the rate at which that force is developed (power)
- greater
- closer
- greater
- reduction
Potassium is thought to contribute to fatigue due to…
- increased efflux out of the cell into the tissue space (Na+/K+ pump activity)
- impairs membrane excitability
