lecture 4: skeletal muscle contraction Flashcards
What are the 2 types of filaments involved in contraction of skeletal muscles
actin and myelin
What are skeletal muscles and what are they composed of?
the muscles that surround the skeleton (bones)
are composed of numerous muscle fibers (or muscle cells)
what are muscle fibers and what are they innervated by
are excitable cells: generate and propagate action potentials
are innervated by nerve fibers
(to cause muscle contraction)
TRUE or false: One nerve fiber usually innervates multiple muscle fibers
TRUE
What is it called when one nerve motor innervates multiple msucle fibers?
MOTOR UNIT
Which disease is characterized by the degeneration of alpha motoneurons in the spinal cord and brain stem?
Amyotrophic lateral sclerosis
What does each muscle cell fiber contain?
Each muscle fiber (cell) contains several hundred to several thousand Myofibrils (tiny tubes which contain myson and actin)
What are the 2 contractile elements of a myofibril?
myofilaments (proteins):
Thick filament: Myosin
Thin filament: Actin
Which contractile element is thick
myosin
Which contractile unit is thin
actin
what is the sarcolemma ?
Sarcolemma is the membrane of the muscle fiber that surrounds myofibrils
explain sarcomeres
Sarcomeres are the smallest contractile unit of muscle fibers (2 micrometers during contraction)
Where the contraction occurs
They are aligned end-to-end in myofibrils. Separated by Z discs
What causes the actin filaments to slide inward on the myosin
Forces generated by interaction between the two filaments: Power strokes
During muscle contraction, the actin filaments are pulled by the myosin filament inward or outward toward the center of the sarcomere
INWARD
What is myosin composed of and be able to label it
Myosin: Thick filament
Composed of the body (stem) and cross-bridges (rowers) (head & arm)
What are actin filaments composed of ?
Actin: Actin filaments are composed of Actin, Tropomyosion, and Troponin
Tropomyosin molecule: hides active sites (long filament during relaxion)
Troponin molecule: moves tropomyosin from active sites
What is the function of the tropomysin molecule?
Tropomyosin molecule: hides active sites (long filament during relaxion)
What is the function of troponin
Troponin molecule: moves tropomyosin from active sites
What is the interaction between actin and myosin in the relaxed state>
NO INTERACTION
In the relaxed state
Active sites on the actin filament are covered by tropomyosin
What is the interaction between actin and myosin in the muscle contraction state?
Ca2+ is released and Ca2+ ions bind on troponin
Troponin undergoes a conformational (i.e. shape) change, moving away the tropomyosin and uncovering the active sites
Myosin cross-bridges (heads) are attracted to the active sites of the actin
What are the 4 steps of cross bridge cycling
1) ATP binding
2) uncovering of active sites
3) power stroke
4) detachment
explain the first step of the cross bridge cycling
ATP binds on the head of the myosin and splits into ADP & Pi (stored energy),
The activated head extends perpendicularly toward the actin filament (not attached yet)
ATP STORED AND AVAILABLE
explain the 2nd step of the cross bridge cycling
Ca2+ causes the Troponin to move Tropomyosin and uncovering the active sites of the Actin.
Myosin head attaches to the active site of the actin filament.
Active sites are uncovered
explain the 3rd step of the cross bridge cycling (powerstroke)
Liberation of the stored energy (ADP & Pi) forces the head to pivot toward the arm and thus pulling the actin filament and sliding it toward the center line, Power Stroke.
explain the 4th step of the cross bridge cycling
A new ATP attaches to the myosin head and causes detachment of the head from the actin
recovering the active sites
what is. a muscle twitch
A muscle twitch (contraction) is the tension developed in response to one nerve stimulation
explain sumamtion of muscle twitches
Summation means the adding together of individual muscle twitches to increase the intensity of overall muscle contraction
What are the 2 ways that summation occurs
1) multiple fiber summation
2) frequency summation
explain multiple fiber summation
Increasing # of motor units contracting at the same time
recruitment of muscle
explain frquency summation
Increasing the frequency of stimulation of one motor unit
explain the size principle and what type of summation doe it relate to
Size Principle: Depending on the intensity of the stimulation, motor units are recruited in an orderly fashion according to their size:
Smallest motor units are recruited first (for weak stimulation)
Largest motor units are recruited last (for strong stimulation)
MUSCLE FIBER SUMMATION
small motor units are recruited first or last and what type of stiumlation occurs
first and weak
What does the size principle allow?
It allows the graduation of muscle force from small steps (weak contraction) to great steps (strong contraction)
EXplain incomplete tetanus
As the frequency of the signal increases, individual twitches summate (no recovery form previous contactions)
explain tetanization
If the frequency reaches a critical level, the summated twitches fuse together to form one continuous contraction, TETANIZATION
Contraction reaches a Max level, no response to further stimulations
Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is sustained (myosin & actin stay attached).
be able to explain frequency summation
LOOK IN SLIDES
Do small muscles that require precision have high or low innervation ration?
Low innervation ration (ie. one nerve innervates only 2-3 muscle fibers)
Do Large muscles that do not require precision (e.g. gastrocnemius) have a low or high innervation ration
-High innervation ratio (ie. one nerve innervates 1000-2000 muscle fibers)
What are the 2 types of muscle fibre classifications
1) slow fibers (type 1, red muscle)
2) fast fibers (type 2a, 2b, white msucle)
Explain slow fibers
Smaller than fast fibers
Innervated by smaller nerve fibers
Have a more extensive blood vessel system and more capillaries to supply extra amounts of O2
Have more mitochondria to support high levels of oxidative metabolism
explain fast fibers
Larger for great strength of contraction
An extensive sarcoplasmic reticulum is present for rapid release of Ca2+ to initiate contraction
Large amounts of glycolytic enzymes are present for rapid release of energy (do not last as long)
Less extensive blood supply and fewer mitochondria vs. slow fibers
True or false: The mixture of our slow and fast fibers is determined by training?
faslse its genetically determined
In most muscles used for movement:
what is the average percentage of slow and fast fibers
50% slow fibers
50% fast fiber
know the graph for Speed and Strength of Contraction of Muscle Fibers
check slides
Can Exercise Training Change Muscle Fiber Types?
yes but very limited changes
-Fibers can change within their own type—IIa can convert to IIb and vice versa
what are the unknown factors when it comes to muscle fibers (3)
unknown if:
1) muscle types are more malleable early in life
2) certain muslces like the biceps, are more adept at changing fiber types than others
3) muscle fibers can truly change between type 1 and 2 and how long it takes to do so
Why do you think soleus & gastrocnemius muscles have a greater % of slow twitch vs. fast twich fibers?
ANSWER:To sustain prolonged periods of standing and walking
What is another name for the neuromuscular junction
MOTOR END PLATE
What is The released neurotransmitter in muscular contraction ??
acetylcholine
Explain the neuromusclar junction
Contact between nerve terminals (branching from motor neuron axon) and muscle fiber plasma membrane (Sarcolemma)
Equivalent of chemical synapse in the CNS
Nerve terminal invaginates into the surface of the muscle fiber
explain the steps at the neuromuscular junction
- AP arrives at the presynaptic terminal
- Local depolarization opens voltage-gated Ca2+ channels and Ca2+ ions flow into terminal
- Ca2+ ions trigger the fusion of synaptic vesicles with the pre-synaptic membrane
- Release of transmitter onto the synaptic cleft (exocytosis)
What does the muscle fiber membrane contrain
Muscle fiber membrane has acetylcholine receptors that contain cation channels (Na+, K+).
explain what happens at the neuromuscular junction
Acetylcholine (ACh) molecules attach to binding sites in the muscle fiber membrane
Cation channels open: (Positive ions (Na+, K+, Ca++) flow through channels BUT far more Na+ ions flow inside 1 Na+ channel can transmit: (15 000 to 30 000 Na+ ions 1 ms!))
Entry of Na+ ions creates an end plate potential (equivalent of EPSP).
End plate potential initiates action potential (Threshold approx. -40 mV)
What is the neuromuscular. junction equivalent to in the CNS
gthe chemical synapse
be able to label the sarcolemma, sarcoplasmic reticulum, t tubles, myofibirls, terminal cisternea
docs
Where is the SR location and what does it contain
Surrounds myofibrils (as a net)
Contains Ca2+ (necessary for muscle contraction process)
Contains voltage-sensitive Ca2+ receptors(diffusion of Ca2+ into the sarcoplasm)
Contains a Ca2+ pump to remove the Ca2+ from the sarcoplasm
Where is the T tubles located and what are the fucntion
Begin at cell membrane (sarcolemma) and penetrate all the way from one side of the membrane to the opposite side
Run transverse to myofibrils
For easy propagation of Aps inside the muscle fiber (rich with Na+/K+ channels)
how many tranverse tubules per sarcomere?
2 transverse tubules per sarcomere
Where is the Ca2+ stored and what does it do
Terminal cisternae
increasing the capacity of SR to release Ca2+ rapidly
be able to explain the excitation cotnraction couple grpahic
slides
be able to describe MS
Antibodies block/destroy Ach receptors
Symptoms: muscle weakness, that develops progressively across the day (worse towards the end of the day)
Affects facial muscles (ex: eyelid drooping), limb muscles and respiratory muscles etc.
Immunosuppressant treatments
Symptomatic treatment: Acetylcholinesterase inhibitors
What is another name for Rigor Mortis aand what is it and how long does it last
postmordem rigidity
Sustained muscle contraction after death
Occurs within the first day (3-12 hours), and decreases gradually (depending on the ambient temperature: cold slows down)
Why does Rigor Mortis occur?
LACK OF ATP
Ca2+ pump cannot transport Ca2+ back into the sarcoplasmic reticulum, thus Ca2+ keeps the actin active sites continuously exposed
Myosin head cannot detach from actin (needs ATP)
be able to recognize the tension graph vs lengthof sarcomere
graph
What point on the tension/lengh graph has the least tension and why
Point D: Actin filament has pulled all the way out to the end of myosin filament
what points on the graph have the most tension and why
Point C: Sarcomere shortens and actin filament begins to overlap myosin.
Tension ↑ until sarcomere reaches 2.2 micrometers (μm)
Point B: sarcomere maintains full tension (length = 2.0 μm)
explain point A on the tension/length graph
The end of actin filaments overlap each other. As the sarcomere length ↓ from 2.0 μm down to 1.65 μm, the strength of contraction ↓ rapidly
explain passive, active and total tension
Passive tension: Tension/force generated in absence of energy (no ATP) when muscle fibers are pulled apart
(ex: stretching fingers after stroke, MS)
Active tension: Tension/force when muscle is stimulated to contract
Due to overlapping of actin and myosin heads
Involves energy (ATP)
Total tension: Summation of passsive and active tension
be able to reconize passive, active and total tension on a graph
slides
what are the passive structures that elongate in passive tension
TITIN (inside the muscle)
COLLAGEN (surrounding muscle cell
explain. Titin:
Protein believed to be the greatest contributor to passive force throughout normal ranges of motion
Connects the Z disk to the M line in the sarcomere
Limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle
true or false: titin is believed to be the greatest contributor to passive force throughout normal ranges of motion
.true
what determines tension in the muscle
Interaction between myosin heads and actin filaments determines the tension
what is the resting length of a muscle
the length at rest from which a muscle develops MAXIMUM isometric tension
what is muscle tension dependent on?
Muscle tension is dependent on muscle length
When a muscle becomes more flexible (as a result of stretching), what do you think happen to the passive tension curve?
moves to the right (longer length becfore tension occurs)
What is maximum strenght of xontraction of a muscle
Maximum strength of contraction of a muscle operating at a normal length averages 3.5 Kg/cm2 (sq. cm) of muscle
Does occular movement muscle contraction have to be slow or fast and why
Must be extremely rapid to
maintain fixation of the eyes on specific objects to provide accuracy of vision
Does gastrocnemius movement muscle contraction have to be slow or fast and why
Gastrocnemius must contract moderatly rapidly to provide enough velocity of limb movement for running and jumping
Does soleus muscle contraction have to be slow or fast and why
Soleus is concerned mainly with slow contraction for continual, long-term support of the body against gravity
what is muscle hypertrophy
increase in total muscle mas
How does muscular mypertrohy happen?
Results from an ↑ in number of actin and myosin filaments in each muscle fiber, causing enlargement of the individual muscle fibers
true or false: Only a few strong contractions each day are required to cause significant hypertrophy within 2-4 weeks
false: 6-8 weeks
what is the effect of training? (2)
1) Number of actin and myosin filaments in myofibrils can increase by as much as 50%!
2) Myofibrils can split to form new myofibrils and ↑ fiber & muscle size
