Lesson 2 - Mechanisms of contraction and relaxation Flashcards
What is a refractory period?
Refractory periods refer to periods when sodium channels are closed.
What are the three phases of refractory periods?
- Absolute refractory period – period wherein no impulses, regardless of strength,
will depolarize the cell - Relative refractory period – period wherein in impulses stronger than normal will
depolarize the cell - Supernormal refractory period – period wherein the slightest impulse will
depolarize the cell
What is afterdepolarization?
Also called re-excitation, afterdepolarization refers to abnormal depolarization of cardiac muscle cells. Such events can interrupt phases 2, 3, or 4 of the action potential.
Afterdepolarizations are caused by alterations in potassium channel function and
overloading of calcium in the heart. This usually occurs during the refractory period, which
makes patients vulnerable to lethal arrhythmias.
What are the two kinds of afterdepolarization?
- Early afterdepolarizations occur during the refractory period when the membrane
potential falls to about -20 mV. This can be due to low potassium levels in the blood,
bradycardia, or drug toxicity. - Delayed afterdepolarizations occur during the refractory period when the
membrane potential has returned to resting state. This can be caused by premature
beats, increased calcium levels in the blood, increased adrenaline levels, or drug
toxicity.
What are the four contractile proteins?
- actin
- myosin
- troponin
- tropomyosin
Describe Actin.
Sometimes referred to as the thin myofilament, actin is smaller than myosin and is ovoid
and globular in form. Actin is a double-stranded string of beads, which are wound around
one another.
Describe Myosin.
Sometimes referred to as the thick myofilament, myosin has a long helical tail and two
globular heads. The myosin heads have the ability to form cross-bridges with actin. This
interaction results in muscle contraction.
Describe Troponin.
Troponin is a globular protein with 3 subunits:
- TnC – a calcium binding subunit
- TnI – subunit that inhibits muscle contraction
- TnT – subunit that connects the troponin complex to tropomyosin and actin
Describe Tropomyosin.
Tropomyosin is a double-stranded and helical-coiled protein. Tropomyosin binds to actin, covering its myosin binding sites. This interaction prevents myosin from freely binding with actin.
Describe the sarcolemma and its function in muscle cells.
The sarcolemma is the thin membrane that encloses the muscle cell, consisting of a plasma membrane that helps maintain the cell’s integrity and facilitates communication with the external environment.
Explain the role of transverse tubules (T-tubules) in muscle contraction.
Transverse tubules (T-tubules) are deep invaginations of the sarcolemma that cross myofibrils, allowing electrical impulses to quickly enter the muscle cell, which is essential for initiating muscle contraction.
Define the function of the sarcoplasmic reticulum in muscle cells.
The sarcoplasmic reticulum is an internal membrane system in muscle cells that stores and releases calcium ions, which are crucial for muscle contraction.
How do terminal cisternae contribute to muscle function?
Terminal cisternae are enlarged areas of the sarcoplasmic reticulum that surround T-tubules and serve as reservoirs for calcium ions, releasing them when depolarized to facilitate muscle contraction.
What is the role of mitochondria in muscle cells?
Mitochondria are membrane-enclosed organelles in muscle cells that generate energy in the form of adenosine triphosphate (ATP), which is essential for various cellular processes, including muscle contraction.
How is energy released from ATP?
Energy is released upon
hydrolysis (broken down by water) of these bonds. ATP is thus converted into adenosine
diphosphate (ADP) when energy is released, and further converted into adenine
monophosphate (AMP).
What is phosphorylation?
Phosphorylation is the process in which AMP is converted back to ADP and then ATP. Phosphorylation is carried out by means of dehydration synthesis, which is the opposite of hydrolysis. ATPase is the enzyme that catalyzes the hydrolysis of ATP to
ADP, while ATP-synthase is the enzyme that catalyzes the phosphorylation of ATP.
What is excitation-contraction coupling?
The process in which electrical activity is converted into
mechanical activity in the form of muscle contraction.
Describe the mechanism of contraction.
- Depolarization of the sarcolemma initiates the sequential depolarization of the T-tubules, terminal cisternae, and then sarcoplasmic reticulum respectively. Depolarization of the terminal cisternae causes it to release calcium ions.
- calcium ions cause the
sarcoplasmic reticulum to release even more calcium ions into the sarcoplasm surrounding
the myofibrils, a phenomenon known as calcium-induced calcium release - released calcium ions bind to TnC subunits of troponin complexes. This causes the troponin complex to move tropomyosin away from the binding sites of the actin filaments, allowing the myosin heads to bind freely with actin
- binding of myosin to actin causes the activation of ATPase in the myosin heads and the production of energy in the form of ATP. This causes the myosin head to pull the actin filaments toward the center of the sarcomere, resulting in the shortening of the I-bands. The shortening of the I-bands results in the shortening of the entire muscle fiber which causes muscle contraction
Describe the mechanism of relaxation.
When stimulation ends, calcium ions detach themselves from the TnC subunits of troponin
complexes. Some calcium ions are pumped back into the sarcoplasmic reticulum, while
some remain in the sarcoplasm. This detachment of calcium from the troponin complex
allows tropomyosin to reposition itself over the myosin binding site of actin. Myosin can no longer bind with actin, so contraction ends and muscles relax.
