Section 4 - Skeletal muscle contraction Flashcards
Can you name the two types of filament involved in the contraction of skeletal muscles?
Actin and myosin (their binding allows contraction)
True or false? One nerve fiber usually innervates only one muscle fiber
FALSE ( multiple muscle fibers)
What is the definition of skeletal muscles?
Muscles that surround the skeleton + are composed of numerous muscle fibers/cells
What are excitable cells who generate and propagate action potentials + are innervated by nerve fibers to cause muscle contraction?
Muscle fibers
Which disease is characterized by the degeneration of
alpha motoneurons in the spinal cord and brain stem?
A. Multiple sclerosis
B. Parkinson’s disease
C. Muscular dystrophy
D. Alzheimer’s disease
E. Amyotrophic lateral sclerosis)
E. Amyotrophic lateral sclerosis (ice bucket challenge)
What is the name of the nerve fibers that cause muscle contraction?
Motor neurons
Define what is a sarcolemma:
The membrane of the muscle fiber that surrounds myofibrils
Each myofibril is composed of two contractile elements called “myofilaments”, what is the thick one and the thin one called?
Thick = myosin Thin = actin
Where does the contraction of a muscle takes place?
In the sarcomeres
What is the smallest contractile unit of muscle fibers? What separates them?
Sarcomeres are separated by Z discs
T or F : What causes the actin filaments to slide inward
among the myosin filaments is the forces generated by the interaction between the two filaments
TRUE
How do we call the binding of actin and myosin?
Power stroke
T or F : The actin filaments are composed of a body and cross-bridges
False, actin filaments are composed of actin, tropomyosin and troponin
What is the role of the tropomyosin molecule in the actin filament?
Hide/covers the active sites during relaxed state
What molecule moves the tropomyosin from active sites?
Troponin
What does Ca2+ do during a muscle contraction? explain
Ca2+ is released and Ca2+ ions bind on troponin, which makes a change in the molecule to let the tropomyosin move away = uncovering the active sites
What happens to the myosin filaments when the active sites are uncovered?
The myosin cross bridges (heads) are attracted to the active site of the actin (see p.11 image)
Tension developed in responses to one nerve stimulation
Muscle twitch (contraction)
Describe the first summation process : multiple fiber summation
Increasing the number of motor units contracting at the same time
Describe the second summation process : frequency summation
Increasing the frequency of stimulation of one motor unit
T or F : Largest motor units are recruited first for strong stimulation
False, smallest motor units are recruited first for weak stimulation
If the frequency reaches a critical level, the summated twitches fuse together to form one continuous contraction : ???
Tetanization
Ca2+ reaches a max level = no response to further stimulations
Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is sustained (actin and myosin stay attached)
T or F : when the frequency of the signal increases, individual twitches summate and there is no recovery from previous contractions
TRUE
Does small muscles that require precision have a high innervation ratio?
No, one nerve innervates 2-3 muscle fibers = low innervation
Which muscles have a high innervation ratio? (one fiber innervates 1000-2000 muscle fibers)?
Large muscles that do not require precision (ex: gastrocnemius + quad)
What are the two types of muscle fibers classification?
Slow fibers = type 1
Fast fibers = type 2
T or F : slow fibers (type 1) are used in long distance running because they have a more extensive blood vessel system + more mitochondria
True
What makes the rapid release of Ca2+ in the fast fibers (type 2)?
An extensive sarcoplasmic reticulum (more active sites open for binding)
Which type of muscle fibers has the best anaerobic capacity?
Type 2B
Which type of muscle fibers has the best aerobic capacity?
Type 1
Can Exercise Training Change Muscle Fiber Types?
A. Yes
B. Yes but very limited changes
C. No
B. Yes, but very limited changes
Ex: Type 1 stays type 1 but type 2A can become type 2B
Why do you think soleus & gastrocnemius muscles have a greater % of slow twitch vs. fast twitch fibers?
To be well adapted to sustain prolonged periods of standing and walking
Scenario see p.28
1, 2A, 2B
What is the other name for neuromuscular junction?
Motor end plate
What is the released neurotransmitter at the neuromuscular junction?
Acetylcholine
The motor end plate makes contact between ? and ?
Nerve terminals from the motor neuron AND sarcolemma of the myofibril
Process at neuromuscular junction see p.31 + 32 + 34
Steps 1-4 to understand
T or F : muscle fiber membrane has acetylcholine receptors that contain anion channels
False, cation channels (Na+, K+)
What is the name of the structure that :
- 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
Sarcoplasmic reticulum
What is the principle use of transverse tubules?
Easy propagation of the action potential inside the muscle fiber (rich with Na+/K+ channels)
Explain the disease of myasthenia gravis
Auto-immune response: anti bodies block or destroy the acetylcholine receptors that affects facial muscles, limb muscles and respiratory muscles.
What is the treatment for myasthenia gravis?
Immunosuppressant treatments : acetylcholinesterase inhibitors
** acetylcholinesterase is an enzyme that catalyzes the breakdown of acetylcholine
How can you explain rigor mortis (or postmortem rigidity)?
Due to sustained muscle contraction after death, because the myosin head cannot detach from actin (needs ATP)
See p.40 for the tension diagram
Understand point A,B,C,D
T or F: passive tension is when muscle fibers are pulled apart, in absence of energy (no ATP)
TRUE
Describe active tension
When muscle is stimulated to contract (overlapping of actin and myosin filaments)
What is the summation of passive and active tension?
Total tension
Name the two passive structures that elongate elastically
Titin and collagen
What is the role of titin?
Limits the range of motion of the sarcomere in tension, thus
contributing to the passive stiffness of muscle.
What is the length at rest which a muscle develops MAXIMUM isometric tension?
Resting length (p.47)
T or F : the more flexible a person, the longer a muscle can be stretched
TRUE + see graph p.50
What is the maximal strength of contraction of a muscle operating at a normal length?
3.5 Kg/cm2 of muscle
What is the name of this movement: Must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision
Ocular movement
What is the name of the muscle providing this movement : must contract moderatly rapidly to provide enough velocity of limb movement for running and jumping
Gastrocnemius
What is the name of the muscle responsible for this movement : concerned mainly with slow contraction for continual, long term support of the body against gravity
Soleus
How do we call an increase in total muscle mass?
Muscle hypertrophy
T of F : muscle hypertrophy results from an increase in number of actin/myosin fiber, causing enlargement of the individual muscle fibers
TRUE
How long does it takes to see results of hypertrophy if a few contractions each day are done?
6 to 10 weeks
