what is the levels of organization of the muscle again and the three connective tissue layers?
Levels Of Organization of the Muscle
Muscle → fascicle → muscle fiber (myofiber) → myofibril → sarcomere → myofilaments
Connective tissue layers:
Epimysium → wraps entire muscle
Perimysium → wraps fascicle
Epimysium → individual fiber (myofiber)
what is a contractile cell and what does it contain?
Muscle fiber = contractile cell and contains
Myofibrils
Sarcoplasm
Sarcolemma
Mitochondria
Nuclei
what chnges during contraction (like the h and i bands etc)
- H band gets smaller
- I band gets smaller
- Zone of overlap increases
- Z line moves closer
- A band stays the same width
Filaments dont shorten → they slide
what is the sarcolemma and t-tubules? what do t-tubules ensure?
Sarcolemma is:
The plasma membrane of muscle fiber and carries electrical signal
Transverse Tubules (T-Tubules) are:
- extension of the sarcolemma
- penetrate deep into muscle fiber
- surround sarcomere
- allow rapid spread of the action potential as they form passageways through muscle fiber and entire sarcomere
T-Tubules ensure that the entire fiber contracts at once
what is triad?
triad is af unctional unit which consists of one T-tubule in between two terminal cisternae (the sarcoplasmic reticulum)
what is the sarcoplasmic reticulum and its function
Sarcoplasmic Reticulum
This is specialized Smooth ER
Stores calcium (Ca2+)
When its stimulated, it releases Ca and Ca enables contraction (calcium is the trigger)
what does a sarcomere contain
Sarcomere proteins
The sarcomere contains:
Thick filaments (myosin)
Thin filaments (actin)
Stabilizing proteins which are in charge of stabilizing position of the thick and thin filaments
Proteins that regulate interaction between thin and thick filaments
what does the thin filament (actin) contain?
- actin (G-actin forms F-actin)
- tropomyosin (blocks myosin binding site at rest)
- troponin (3 active sites, TnC, TnI, TnT)
- nebulin (stabilizer)
Regulatory role:
Tropomysin blocks binding sites
Troponin binds to Ca (TnC) to bind to tropomyosin and expose myosin binding sites (TnT)
what does myosin contain?
myosin contains:
1. titin → connected to Z line to stabilize myosin and extends out to M line, also connects myosin and is a molecular spring/elastic recoil protein as well as a stabilizer
2. Tail
3. Hinge
4. Free head which binds to myosin binding site on actin to allow for a cross bridge interaction to occur
what is the difference between titin and actinin and nebulin?
Titin is a spring that is connected to Z line and stabalize myosin also extending out the M line
Actinin is a protein part of the Z line and anchors thin filaments/actin
nebulin is like a “molecular ruler” that regulates the length of thin filaments with the sarcomere
what is neuromuscular junction
Definition → neuromuscular junction is the synaptic connection between a motor neuron and a muscle fiber (skeletal, smooth, cardiac) (point of contact between a nerve ending and the muscle fiber)
what is a motor unit and motor neuron and what does it control
Motor unit = 1 motor neuron and all muscle fibers it controls / combination of a motor neuron and the muscle fibers it controls
A motor neuron is a nerve that transmits an impulse to a muscle (from CNS to body)
causing a contraction
Important:
Small motor unit = fine control
Large muscle unit = stronger contraction
A muscle will ONLY contract if:
A muscle will ONLY contract if an applied stimulus reaches a certain level of intensity which is the threshold stimulus → the minimal level of stimulation required to cause a muscle fiber to contract. once stimulated by threshold, a muscle fiber will contract completely (all or none)
do different motor units have different thresholds? explain.
A whole muscle can produce graded forces because different motor units have different thresholds
different motor units responding to different threshold stimuli allow a muscle as a whole to excecute contractions of graded force
(soDifferent motor units have different thresholds so they can be recruited gradually, meaning more motor units can be activated and added together to produce a stronger muscle contraction.)
why is NMJ important
- site of transmission of action potential from nerve to muscle
- understand pathophysiology of diseases
- site of action for many drugs
how does the transmission of an action potential from the nerve to muscle occur
Signaling begins when a neuronal action potential travels along the axon of a motor neuron to terminate at the NMJ
what are the three action potential stages
Action potential stages
Depolarization
Na+ voltage gated channels open, it rushes in, and membrane goes from -55 mV (threshold) to +30
Repolarization
Na channels close, K voltage gated channels open, K exits the cell
Hyperpolarization
Slight overshoot, membrane briefly more negative than rest
Then: returns to resting potential due to sodium potassium pumps
explain the propogation of a nerve impulse
Propagation of nerve impulse
1. Depolarization in one region where Na channels open and creates a wave of depolarization
- This causes a wave of depolarization along the cell axon and causes nerve impulse
There is a refractory period since once it enters hyperpolarization those Na channels cannot reopen so it creates a wave
