Skeletal muscle physiology Flashcards
what is a twitch
mechanical response to 1 single AP
what is latent period
Time from AP initiation to cross-bridge formation, start of contraction (~1-3 msec)
What is the contraction time/?
beginning of contraction to beginning of relaxation (peak tension)
when active sites are exposed until they begin to be covered up
when Ca is high enough to keep active sites exposed
/what is relaxation time
Peak tension to complete relaxation (~50msec)
- ↓ tension as active sites are covered
- time for Ca2+ sequestering into the SR***
(taken back up into SR or pumped out into ECF)
total force generated = ?
sum of forces independently produced by many cycling cross-bridges
number of simultaneously cycling cross-brdiges can vary with?
Initial length of muscle fiber (length tension–> isometric action)
Pattern or frequency of muscle fiber stimulation
(twitch summation, multiple-fiber or multiple motor unit summation)
what are the two types of muscle action?
Isometric contractions
Isotonic contractions
what is isometric contraction?
SAME LENGTH (muscle is not shortening but tension is being produced)
stimulation can increase in tension, but no shortening)
force production is equal to resistance
NO MOVEMENT
Isotonic contraction
Muscle contraction occurs at a constant load (SAME TONE)
Not really a constant force (dynamic movement)
Length changes during production of tension: muscle length, joint angle & leverage changes with ROM amount of force production also changes through the ROM
2 phases
what are the two phases of isotonic contraction?
concentric = muscle shortens as tension is produced (bicep curl)
eccentric = muscle lengthens as tension is produced (lowering a bucket)
what is the length tension relationship?
same length of a give fiber
how much force can be produced at a given starting length of a fiber
***Muscle length influences tension development by determining region of overlap between actin & myosin
what is passive tension
tension measured prior to muscle contraction
Passive tension increases as a fiber is progressively lengthened (stretched) because muscle becomes stiffer as it is distended
active tension
total tension minus passive tension
how much energy is actually being formed as a result of contraction
When muscle is stimulated to contract at any fixed length (isometric), cross-bridge cycling produces active tension in addition to passive tension
when is active tension maximal?
near 100 percent of normal muscle length
what does increase in fiber length cause
ends of actin filaments are pulled away from eachother
When length is increased > 150% of resting sarcomere length, ends of actin are pulled beyond myosin filaments
No interaction/overlap occurs between actin and myosin and no development of active tension
what happens when fiber length is decreased?
Actin and myosin increase overlap
Ends of actin filaments are pushed toward each other
Tension can develop, depending on degree of overlap
can’t produce a meaningful contraction
tension is decreased b/c opposing actin filaments slide over one another and can hit against opposing Z disks
sarcomere length is near normal resting length .. means what?.
Maximal overlap between actin and myosin filaments and maximal active tension
total tension =
passive plus active tension
Force velocity relationship?
having to do with isotonic contractions
(shortening velocity decreases as load increases)
LIGHTER LOADS CAN BE LIFTED FASTER
what is maximum velocity determined by?
by the maximum velocity of myosin ATPase enzyme (Vmax varies with fiber type)
slide 14-15
look at
what is work?
load x displacement
measurable mechanical work: only when muscle displaces a load
in order to do work you have to move a load a distance so isometric contraction is not doing that
what is power?
work/time
rate work is performed
Zero at maximum load
zero at zero load
what is frequency (twitch) summation
Tension developed by a single fiber depends on stimulation frequency
Repetitive stimulation: ↑ tension
Contractile responses (twitches) can be summed if APs fire rapidly –> no fiber relaxation between stimuli due to sustained elevation of Ca2+
tetanus is what
twitches merge into a smooth, sustained, maximal contraction
DON”T confuse with tetanus (excitability cause by toxin) or tetany (spasms caused by increased excitability associated with hypocalcemia)
low stimulation frequencies?
tension developed falls to resting level between individual twitches
what is the duration of each twitch compared with duration of AP
twitch is LONG
high stimulation frequency?
ual twitches occur so close in time they fuse, causing muscle tension to remain at a steady plateau
slide 19
what is multiple fiber summation
?
motor pool
all of the motor neurons who supply any muslce fiber that comprises a given muscle (biceps for example)
group of all motor neurons innervating a single muscle
whole muscle tension depends on?
size of muscle
number of motor units recruited
size of each motor unit being recruited
muscles performed refined, delicate movements have …
few muscle fibers per motor unit
muscles performing stronger, coarser movements have …
a large number of fibers per motor unit
what is Multiple-fiber/Multiple Motor Unit Summation (MMUS):
Excitation of additional motor neuron cell bodies recruits the fibers of the motor unit, adding them to the contractile pool
this is how CNS controls force production for specific situation (By the number of individual fibers that it stimulates)
what is another benefit of multiple fiber summation?
DELAY FATIGUE- asynchronous recruitment
Delays or prevents muscle fatigue during submaximal contractions
mechanism allowing whole-muscle force development to be relatively constant
Asynchronous activation of individual motor units by the CNS so that some units develop tension while others relax
contractile strength can vary with?
number of active alpha motor neurons in pool
frequency of firing of each alpha motor neuron
Electromyography?
gross measure of electrical activity as it is projected to skin surface!!
records sum of all electrical activity (AP’s of all activated fibers in all activated motor units)
Henneman’s size principal?
size of cell body dictates excitability
give same input; threshold reached sooner in a smaller motor neuron
which type of alpha motor neurons are most easily recruited?
SMALLEST - reach threshold quicker
how can you differentiate between central and peripheral fatigue on an EMG?
experiencing central fatigue–> decreases EMG, output and excitation is decreasing
peripheral fatigue–> ability to produce force is tapering off and off, but still able to maximally recruit all the motor units possible , so still have electrical activity
what determines degree of use and what does degree of use influence?
motor unit excitability determines degree of use
degree of use influences fiber type
small motor units are recruited with MINIMAL neuronal stimulation meaning what?
A given excitatory stimulus will generate a larger EPSP in motor neurons with smaller cell bodies
as neuronal stimulation intensifies, larger motor neurons are recrutied
slow twitch motor units? (I) type I
Small amount of force, prolonged period of time
smaller motor neurons, supplying a fewer amount of muscle fibers for a long period of time
smal cell diameter
fast conduction velocity
high excitability
fast twitch fatigue resistant FR type II
produce moderate amount of force but can’t do this as long as slow twitch
fast twitch fatiguable FF type II
large amount of force produced
only for brief period of time
what is the order of recruitment of motor units?
I–> FR–> FF
fast twitch fibers in general characteristics
large cell diameter
very fast conduction velocity
low excitability
propteries of muscle cells of type I motor units
few number of fibers
moderate fiber diameter
low force of unit
oxidative metabolic profile
moderate contraction velocity
low fatigability
properties of muscle cells of type II motor units
many number of fibers
large fiber diameter
high force of unit
glycolytic metabolic profile
fast contraction velocity
high fatigability
watch videos
only can watch on campus
muscle fatigue is what?
Inability to maintain desired power output, decline in force production & shortening velocity
why does a decline is maximal force happen?
results from reduction in number of active cross-bridges
Lower rates of force production and relaxation: due to impaired release and reuptake of Ca2+ from the sarcoplasmic reticulum (SR)
why have muscle fatigue?
protective role
Allows contractions to occur at lower rates/forces while preventing extreme changes that could cause damage
is muscle fatigue reversible?
yes with rest
various factors contribute to fatigue
Motivation, physical fitness, nutritional status, and the type of motor units (i.e., fibers) recruited with respect to the intensity and duration of activity
what is central fatigue
anything that is going on within the CNS
from brain–> motor neuron cell bodies in ventral horn of spinal cord
possible involvement of:
- -Altered input from muscle sensory nerve fibers
- -Reduced excitatory input to motor control centers of the brain & SC
- –Resulting in altered excitability of α - and γ - motor neurons
CONTRIBUTIONS of these factors vary by individual and type of activity
slide 35
what is peripheral fatigue
Motor neuron axon –> NMJ –> Fiber
Possible involvement of:
Impaired initiation/propagation of muscle APs
Impaired release/handling of [Ca2+ ]i
Depletion of energy metabolism substrates
Accumulation of metabolic byproducts
Peripheral fatigue and the time required for recovery varies with recruitment pattern of motor units & metabolic properties of muscle fiber type
3 sources of ATP in skeletal muscle
Anarobic sources:
Creatinine phosphate
Glycolysis
Aerobic source:
oxidative phosphorylation
slide 37
what are the different characteristics/functions of skeletal muscle fibers?
strength, speed and fatigability
slide 40
look at
3 major human skeletla muscle fiber types
slow oxidative type I
fast oxidative type IIA
fast glycolytic Type IIX fibers
classificationof muscle fibers by….
Pathway for ATP synthesis (oxidative versus glycolytic)
Rate of ATP hydrolysis (mATPase isoform)
Contractile Velocity (fast vs. slow)
slow twitch fibers
greater oxygen transport capability as oxidative metabolism as primary source of ATP
small cross sectional area
can act for longer periods of time
“red fibers” due to myoglobin content
slide 42
type IIA
fatigue resistant
oxidative metabolism
similar to Type I in myoglobin content (red) and metabolic machinery
similar number of mitochondria vs. slow-twitch fibers
abundant glycogen
Ensures adequate ATP generation to compensate for the increased rate of ATP hydrolysis for rapid contraction
type IIX
fatigable (couch potato fibers)
high proportion in untrained individuals
rely on glycolysis
few mitochondria (white fibers)
low concentrations of myoglobin and oxidative enzymes
high glycolytic enzyme content
can you train to change from type I to type II fibers
NO
but what you can do with training is switch between fast fiber population
so cannot go from slow to fast
but fast type Iix to fast type IIa
see slide 44 for review of types of fibers
do it
why do slow twitch fibers tetanize at lower stimulation frequencies ?
due to longer duration of contraction
why do fast twitch fibers develop larger maximal force ?
due to greater twitch tension and large motor units
don’t maintain that level of tension that long
how can you identify fiber types based on ATPase enzyme
staining intensity (having to do with pH) gives you an idea of what type of ATPase enzyme
don’t need to know
what does peripheral fatigue involve?
ATP depletion
lactic acid accumulation
glycogen depletion
(not going to be assessed on it)
slide 48-50
Factors influencing tension development in skeletal muscle ***
Number of fibers contracting
- motor units recruited
- muscle fibers per motor unit
- size of muscle
Tension developed by each contracting fiber
- frequency of stimulation (twitch summation and tetanus)
- fiber length at onset of contraction
- extent of fatigue
- thickness of fiber
slide 51 ***
changes that can occur with training (fiber adaptations)
skeletal muscle plasticity
response to changes in functional demands include:
- fiber size (RIPPED)
- fiber type composition (change between type II fibers X and A)
- mitochondrial content
- capillary density
- oxidative enzymes
what is proprioception
detailed information sensed about location in space
2 main purposes of proprioceptors
- Identification of external objects
2. Accurately guide movement
what are the muscle proprioceptors?
1 Muscle spindles
2 Golgi tendon organs
what do muscle spindles do and what are they?
detect changes in muscle length and rate of stretch
1> send proprioceptive information about muscle –> CNS
2> respond to muscle stretching
these are INTRAFUSAL muscle fibers aligned in PARALLEL with force generating extrafusal fibers
(this is why they can give info about length)
what do golgi tendon organs do and what is their position?
detect muscle tension in muscle tendon (near MTJ)
these are aligned in SERIES with extrafusal fibers (so as the tension is developed and pulled, they can detect differences in muscle force)
when does firing rate of sensory fibers of muscle spindle fibers increase?
when the muscle is stretched
what is the difference between primary and secondary endings
primary sensitive to change in length
secondary sensitive to static length
why does the spindle need to have motor innervation
b/c the intrafusal fiber must also be contracted to match the extrafusal fibers
For continued maintenance of spindle sensitivity, γ-motor neurons cause intrafusal fibers to contract in parallel with extrafusal fibers
Sensory response of the spindle depends on BOTH the length of the whole muscle AND on the contractile state of the intrafusal fiber itself
what is the monosynaptic reflex
involves 1 synapse
When skeletal m. is abruptly stretched, a rapid, reflexive contraction of the same muscle can occur
Contraction increases muscle tension and opposes the initial stretch
Stretch reflex is especially strong in extensor muscles
1 alpha motor neuron that supplies the muscle is stimulated
slide 63 on relfexes
look at
reciprocal innervation
slide 64
golgi tendon organ receptor
protective of muscle where stimulus is detected to be too much tension
slide 66
Autogenic inhibition
