Contraction and Excitation of Skeletal Muscle Flashcards
What are the two types of filaments involved in the contraction of skeletal muscles?
Actin & myosin
What are skeletal muscles?
are the muscles that surround the skeleton (bones)
composed of numerous muscle fibers
What are muscle fibers?
they are excitable cells: generate and propagate action potentials
are innervated by nerve fibers (motor neurons) to cause muscle contraction
Which disease is characterized by the degeneration of alpha motor neurons in the spinal cord and brain stem?
Amyotropic lateral sclerosis (ALS)
What are myofibrils?
They compose the muscle fiber
What is sarcolema?
It is the membrane of the muscle fiber that surrounds myofibrils
What are myofibrils composed of?
Two contractile elements called myofilaments (proteins)
thick filament: Myosin
Thin filament: Actin
What are sarcomeres?
Where the contraction takes place. they are the smallest contractile unit of the muscle fibers, they are aligned end-to-end in myofibrils and separated by z-discs
What causes the actin filaments to slide inward among the myosin filaments?
Forces generated by interactions between the two filaments (power stroke)
What happens during muscle contraction?
Actin filaments are pulled together by the myosin filaments inward towards the center of the sarcomere
Describe Myosin
Thick filament composed of the body and cross-bridges (head and arm)
Describe Myosin
Thick filament composed of the body and cross-bridges (head and arm)
Describe actin.
thin filament,
composed of actin, tropomyosin, and troponin
What does tropomyosin do?
Hides active sites during relaxation
What does troponin do?
Moves tropomyosin from active sites during contraction
What happens during muscle contraction?
Ca++ ions are released and bind on troponin. Troponin undergoes a conformational 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 steps of the cross-bridge cycle?
1) ATP binds on the head of the myosin and splits into ADP+P (stored energy), the activated head extends perpendicularly toward the actin filament
2) Ca++ causes the troponin to move tropomyosin and uncovers the active sites, myosin head attaches to the active sites
3) Liberation of the stored energy forces the head to pivot towards the arm and thus pulling the actin filament and sliding it towards the center line (Power Stroke)
4) A new ATP attaches to the myosin head and causes it to detach from the actin filament
What is a muscle twitch?
The tension developed in response to one nerve stimulation
What does Summation mean?
It means the adding together of individual muscle twitches to increase the intensity of overall muscle contraction
What are the two ways in which summation occurs?
Multiple fiber summation: increasing the # of motor units contracting at the same time
Frequency summation: increasing the frequency of the stimulation of one motor unit
Which motor units are recruted first?
The smallest motor units
What is tetanization?
if the frequency of a signal reaches a critical level, the summated twitchs fuse together to form one continuous contraction.
Contraction reaches a max level, no response to further stimulation
What are motor units?
one nerve fiber innervating multiple muscle fibers
What muscles require a low innervation ratio (one nerve innervates 2-3 muscle fibers)?
Small muscles that require precision
What muscle require a high innervation ratio (one nerve innervates 1000-2000 muscle fibers)?
Large muscle that do not require precision
What are slow muscle fibers?
(Type 1, red muscle)
smaller than fast fibers, innervated by smaller nerve fibers.
Have a more extensive blood vessel system (more capillaries to supply extra amounts of O2)
Have more mitochondria to support high levels of oxidative metabolism
What are fast muscle fibers?
(type 2, white muscle)
Larger for great strength of contraction
an extensive sarcoplasmic reticulum is present for rapid release of Ca++ to initiate contraction
Large amounts of glycolytic enzymes are present for very rapid release of energy
Less extensive blood supply and fewer mitochondria vs. slow fibers
Can exercise training change muscle fiber types?
Yes, but changes are very limited
fibers can change within their own type
What are the three unknown factors related to muscle fibers?
- muscle types are more malleable early in life
- certain muscles like the biceps, are more adept at changing fiber types than others
- muscle fibers can truly change between type 1 and type 2 and how long it take to do so
Why do the soleus and gastrocnemius have a greater percentage of slow twitch vs. fast twitch fibers?
To sustain prolonged periods of standing & walking
What is the neuromuscular junction?
Contact between nerve terminals and muscle fibers plasma membrane (sarcolemma)
Equivalent of chemical synapse in CNS
Nerve terminal invaginates into the surface of the muscle fiber
The released neurotransmitter is acetylcholine
What is the sarcoplasmic reticulum?
Surrounds myofibrils ( as a net)
contains Ca++ (necessary for muscle contraction)
Contains voltage-sensitive Ca++ receptors
Contains a Ca++ pump to remove the Ca++ from sarcoplasm
What are T-tubules?
begin at cell membrane and penetrate all the way from one side of the membrane to the other
run trnasverse to myofibrils
Are in contact with the sarcoplasmic reticulum
for easy propagation of APs inside the muscle fiber (rich in sodium/potassium pumps)
What is Myasthenia Gravis?
It is a neuromuscular disease cause by an auto-immune response; antibodies block or destroy Ach receptors
What are the symptoms of Myasthenia Gravis?
Muscle weakness that develops progressively across the day (worse towards the end of the day
What is acetylcholinesterase?
enzyme that catalyzes the breakdown of acetylcholine
What is rigor mortis (also known as postmortem rigidity)
body rigidity due to sustained muscle contraction after death, occurs within the first day and decreases gradually (depends on temperature, cold slows down the process)
What are the mechanisms behind rigor mortis?
Lack of ATP, Ca++ pump cannot transport Ca2+ back into the sarcoplasmic reticulum, thus Ca2+ keeps the actin active sites continuously exposed
Myosin head cannot detach from actin filaments since it needs ATP
At what degree is the strongest tension developed by a contracted muscle?
at 90 degrees
What is passive tension?
force generated when there is no energy when muscle fibers are pulled apart
What are the two passive structures that elongated elastically?
Titin (inside muscle cell)
Collagen ( surrounding muscle cells)
What is titin?
Protein believed to be the greatest contributor to passive force throughout normal ranges of motion
connects the z discs to the m-line in the sarcomere
Limits the range of motion of the sarcomere in tension thus contributing to passive stiffness in muscle
What is Active tension?
tension when muscle is stimulated to contract
Due to overlapping of actin & myosin heads
Involves energy
What is resting length?
The length at rest from which a muscle develops maximum isometric tension
What is total tension?
Summation of both passive and active tensions
What is muscle tension dependent on?
It is dependent on muscle length, the more flexible a person is the longer a muscle can be stretched
What does the speed of muscle contraction relates to?
It relates to the function of the muscle. ex.: ocular movement, contracts really fast to maintain fixation of eye on specific objects to provide accuracy of movement
What is muscle hypertrophy?
Increase in total muscle mass
Muscle hypertrophy results from…
an increase in the number of actin and myosin filaments in each muscle fibers, causing the enlargement of the muscle fibers
What is the training effect?
Number of actin & myosin filaments in myofibrils can increase by as much as 50%
Myofibrils can split to form new myofibrils & increase fiber and muscle size
