Lecture 9.4 main points Flashcards
what is the functional unit of the nervous system
neuron
how does the functional unit of the nervous system communicate
neurons communicate via the release of neurotransmitters
what are the signal relaying portions of neurons called and what structure are they often bundled in
nerves, bundles of axons
what is muscle connected to
at least one motor nerve
what is a motor nerve
contains axons to a few or hundreds of motor neurons
are motor axons a singular extension or can they branch
can branch many times before ending at nerve terminals
define motor unit
a motor neuron and all (four to several hundred) muscle fibers it innervates (controls)
how are motor units arranged for fine control
produce small, finely controlled movements. small motor units can contain as few as 10-20 muscle fibers
examples of fine control motor units
eye muscles, larynx muscles
how are motor units arranged for strength control
produce large, strong movements. Can have as many as 2000 muscle fibers in a motor unit
example of strength control motor unit
gastrocnemius
how are muscle fibers of a motor unit distributed in a whole muscle
spread throughout the muscle
how does the activation of a single motor unit affect the contraction of the whole muscle
typically causes a weak contraction of that muscle
do all motor units activate at the same time or do they activate asynchronously
only the muscle fibers of the specific motor unit will contract when activated by the motor unit.
ASYNCHRONOUSLY
why is it important for motor units to activate asynchronously
helps prevent, or decrease, overall muscle fatigue
describe a motor units response to a single AP in its motor neuron
results in an AP in the muscle fibers connected through NMJs with that neuron, produces a short period of motor unit’s muscle fiber contraction that generates tension
different strength and duration of twitches in whole muscle can be observed, why does this happen
due to variations in metabolic properties ad enzymes between muscle fiber types, different whole muscles have differing ratios of muscle fiber types
what is the graphical representation of a lab - measured muscle contraction called
myogram
three phases of muscle twitch
latent period, period of contraction, period of relaxation
latent period of muscle twitch
events of excitation- contraction coupling. AP must reach threshold, no visible shortening or tension of the muscle during this period
period of contraction
sarcomere shortening from cross bridge formation and power stroke produce visible tension (external tension)
period of relaxation
Ca reentry into the SR, tension declines to zero
differentiate between speed of contraction and relaxation
muscle contracts faster than it relaxes
what does a “graded response” mean in reference to muscle contraction
varying strength of contraction for different demands
why is a graded response important in muscle contraction
required for proper control of skeletal movement
how is the muscle response graded?
- changing frequency of stimulation
- changing strength of stimulation
is AP strength affected by increasing stimulation strength
no
if a stronger stimulus is applied, what happens in a graded muscle reponse
more motor units are activate more muscle fibers but the AP strength does not change
what happens to the muscle cell once threshold is met
threshold stimuli produce twitch
what does the “all or none” law of muscle contraction mean
is it 100% true? if not what can affect twitch strength?
contracts to its maximum or not at all
not 100% true
twitch strength depends on Ca concentration, previous stretch of the muscle, temperature, pH, and hydration
list different time required for muscle AP and contraction
AP in muscle cell occurs much faster than actual contraction
2 milliseconds (AP) vs up to 100 milliseconds (contraction)
why is the difference in time important between muscle AP and contraction
a second action potential may be initiated during the period of mechanical activity of the muscle fiber
propertied of low frequency (10 stimuli/ sec) muscle stimulation, why is peak tension the same
each stimulation produces an identical twitch response, max tension produced from each twitch remains equal
peak tension is the same for every one bc the sarcoplasmic calcium goes back to resting levels
properties of moderate frequency (10-20stimuli/ sec) muscle stimulation
each twitch has time to recover but develops more tension than the one before (treppe phenomenon)
treppe phenomenon
not enough time between stimuli for sarcoplasmic calcium to return to full resting levels, therefore calcium is not completely put back into the SR
an increase in sarcoplasmic calcium concentration with each successive Ap stimulation results in longer unblocking and more cross bridge formation and power strokes resulting in increase in tension produced (to a certain point)
wave (temporal) summation
increased stimulus frequency (muscle does not completely relax between AP stimuli) –> second contraction of greater force
additional Ca release from the SR with the second stimulus stimulates re -unblocking of the myosin binding sites and more shortening before full relaxation is obtained
continued temporal stimulus frequency results in unfused incomplete tetanus
produces sustained, but quivering, contraction that increase in successive twitch maximum tension
during wave (temporal) summation how is another contraction elicited during relaxation and how does that affect peak tension
the increase stimulus frequency does not allow the muscle cell to completely relax resulting in the second contraction of greater force
the peak tension increases with each contraction to a certain point
what is it called if we continue to temporal stimulation frequency? how does the muscle contraction appear
unfused (incomplete) tetanus
sustained, quivering contraction
how is fused (complete) tetany achieved
if sustained stimuli are given quickly enough then muscle reaches maximal tension, no muscle relaxation due to consistently high sarcoplasmic Ca from continued release through SR
what type of tension is achieved through fused (complete) tetany
maximum tension
through fused tetany, what will it eventually lead to is successive stimulation is continued and how does this affect tension
eventually leads to muscle fatigue as metabolites accumulate and ionic imbalances form, this leads to a decrease in tension towards zero due to inability for muscle to contract
compare tetanus vs twitch for
AP
effect on sarcoplasmic reticulum
time of unblocked thin filaments
amount of cross bridges formed
effects on elastic elements and transfer of tension
Twitch - single AP release enough calcium to saturate troponin C and allow all of the myosin binding sites to become available, but the binding of the myosin head takes time and during that time sarcoplasmic Ca is being pumped back into the SR by ATPases and SERCA pumps, therefore the sarcoplasmic CA begins to derease allowing for re-blocking of the myosin binding sites and before many of the binding sites are even formed
Tetanus - successive AP each release calcium from the SR before much of the calcium from the previous AP can be pumped back into the SR, this results in persistent elevation of sarcoplasmic Ca concentration, keeping myosin binding sites unblocked, more cross bridge formation and power strokes
another cause for difference between twitch and tetanus due to elasticity of muscle tendons and protein titin
the elasticity means that these must stretch/ compress before tension is produced from the contractile unit being transferred (like a bungee cord). Because a single twitch is so brief, the cross bridge activity is already declining before force has been fully transferred through the elastic structure
during tetanic stimulation - this is less of a factor due to the long duration of cross bridge activity and force generation
what is recruitment and why is it important
multiple motor unit summation, controls force of contraction of a whole muscle. during recruitment muscle contracts more vigorously as stimulus strength increases above threshold
it is used to increase the force of a muscle contraction by activating additional motor units of that msucle
what does it mean to have a sub-threshold stimulus
a stimulus too weak to illicit activation of any motor units in a muscle ( no observable contractions or tension)
what does it mean to have a threshold stimulus
stimulus strength that causes first observable muscle contraction (activation of motor unit) and generation of tension
what does it mean to have maximal stimulus
strongest stimulus that increases contractile force of whole muscle to its maximum (all possible motor units are turned on)
once threshold is met, what does an increase in applied stimulus voltage mean for motor unit activation and tension generated
more motor units are activated therefore increasing the tension generated
why does tension increase with additional motor unit activation
more active motor units results in more muscle fibers shortening and pulling therefore more force generated
why is there no additional tension generated when the stimulus voltage is increased above maximal
because the maximum amount of motor units have been excited despite an increase in voltage
what does it mean when it is stated that recruitment words on the size principle
motor units with the smallest muscle fibers are recruited first, if more force is required motor units with more and larger fibers are recruited, and if maximal force is required the largest motor units are activated
are there differences in the principles of contraction for single muscle cells vs whole muscle ?
no, contraction produces muscle tension, so however much force is needed determines what is used
force and duration of contraction vary in response to stimuli or different frequencies and intensities
what is muscle tension
the force exerted on load or object to be moved
muscle tension is produced by contraction
does contraction always lead to a shortening of a muscle
no, isometric contraction does not shorten the muscle but still places tension on the muscle
define isometric contraction
no shortening, muscle tension increases but does not exceed load (same length)
define isotonic contraction
muscle shortens because muscle tension exceeds load (same tension, or tone, once load exceeded)
during isotonic contraction, do the thin filaments move with respect to the thick?
thin filaments slide
when is an isotonic contraction concentric
when the muscle shortens and does work
when is an isotonic contraction eccentric
the muscle generates force as it lengthens
the negative rep
during isometric contraction do the thin filaments move with respect to the thick?
no, cross bridges do form but do not move the actin filaments
what does it mean for a muscle to have tone?
constant, slightly contracted state of all muscles due to spinal reflexes.
Groups of motor units are alternatively activated in response to input from stretch receptors in muscles
keeps muscles firms, healthy and ready to respond
