Physio Flashcards
Discuss motor units and what comprises them
Each skeletal muscle fiber receives neural input from a motor neuron via an NMJ. A single motor neuron and the muscle fiber it attaches to are known as a motor unit.
Large motor units in large muscle groups execute coarse movements (like the quadriceps) and small motor units control fine movements, like extraocular muscles.
Discuss an action potential going from the neuron to the myocyte and what happens
In the synaptic cleft, the action potential that has propagated along the neuron is transferred to the myocyte, or muscle cell. The NT Acetylcholine is released from the axon button. The myocyte’s postsynaptic membrane, the motor end plate, has specialized nicotinic ACh receptors, which are transmembrane cation channels (Na+ and K+) that open when bound by ACh.
Activation of these ligand-gated channels results in increased local cation flux, causing membrane depolarization that is propagated to the nearby transverse tubule system.
Excess ACh is hydrolyzed by acetylcholinesterase which resides on the postsynaptic or postjunctional membrane, into acetate and choline.
Choline gets reabsorbed by the presynaptic membrane via Na+-coupled transport to make more ACh
Discuss the levels of organization from small to large for muscle
Myofilament Sarcomere Myofibril Muscle Cells Muscle Fiber Muscle Fasciculus Whole Muscle
So now we have some energy down at the level of the T-Tubule System. What happens?
Myofibrils are the functional components of contraction. The T-tubule system, a system of plasma membrane invaginations, allow the AP to propagate deep into the cytoplasm, facilitating Ca2+ release from the SR. The increase in intracellular Ca triggers excitation contraction coupling amoung longitudinal intracellular contractile proteins in the sarcomere. Repeating units of sarcomeres comprise myofibrils within a single multinucleate myocyte.
Discuss the thin and thick filaments of the myofibril
Thick filaments contain a large heavy protein, myosin, which is made of both heavy and light chains. The light chains contain actin-binding sites and an ATP cleavage site.
Thin filaments have three parts:
- Actin: Bound by myosin, it contributes to cross-bridge formation that allows for movement of myosin filaments and change in the myofibril length
- Tropomyosin: At rest, this protein occupies potential myosin-binding sites on the actin protein, preventing contraction
- Troponin: Ca released from the SR binds here to induce a conformational change that consequently moves tropomyosin, freeing actin’s myosin binding sites for contraction.
Once tropomyosin uncovers actin’s myosin-binding sites what happens?
Once tropomyosin uncovers actin’s myosin-binding sites, actin binds myosin light chains, creating cross-bridges. The myosin light chains pivot, and the myosin heavy chain slides along the actin filament. This is known as a twitch, and develops the tension to generate force during a muscle contraction.
To return the pivoted/flexed myosin light chains to their original state requires the cleavage of ATP to ADP + Pi. Once regenerated, the myosin light chain binds a new molecule of ATP for future cross-bridge coupling.
The process of contracting continues as long as cytoplasmic Ca levels remains high. How do we control this?
Ca-ATPase functions to ensure that Ca gets taken back up into the SR to reduce cytoplasmic Ca.
Once the Ca levels are low enough, troponin returns to the original state and tropomyosin again blocks the myosin-binding sites on actin.
If a muscle fiber gets stimulated repeatedly without time for Ca to reaccumulate in the SR, the sustained high cytoplasmic Ca levels lead to sustained contraction, or tetanus.
Discuss the structure of a sarcomere
Spans the two Z lines. The A band (big myosin block) within this Z line does not change size with contraction, but the I band (space between the Z line and A band) and the H band (space between Actin filaments running parallel to the A band on either side) both shrink as the actin filaments use the A band to come closer to eachother.
What makes smooth muscle different from skeletal muscle?
- Myofilaments not organized into sarcomeres and thus are not striated
- Innervation is primarily via the ANS, not the Somatic System
- Different excitation contraction coupling system
How does excitation contraction coupling differ with smooth muscle?
Smooth muscle lacks troponin. Instead, the protein calmodulin acts as the cross-bridging gatekeeper.
Cascade begins with an AP like normal, which opens the Ca channels and an increase in intracellular Ca like normal.
Calmodulin then binds Ca and activates MLCK, myosin light chain kinase, which phosphorylates myosin.
Activated myosin is able to bind and release actin, repeatedly forming and breaking cross-bridges. Like skeletal muscle, each cycle consumes one molecule of ATP.
What happens in smooth muscle when the Ca concentration goes down?
When the Ca concentration decreases (again due to Ca-ATPase), and myosin is dephosphorylated via MLCP (Myosin light chain phosphorylase) the dephosphorylated form of myosin can still interact with actin via latch-bridges, which are residual attachments that allow for the maintence oftonic tension in smooth muscle without consuming energy. That way, tonic contracture can be maintained without ATP.
When combined with gap junctions, this allows smooth muscle to produce coordinated tonic contractions necessary for aiding digestion, maintaining BP, voiding urine and accomplishing labor and delivery.
What is bulk flow?
The net difference in pressure between two compartments drives the solution through the dividing membrane (both solutes and solvent)
What, generically, are the ion levels inside and outside of a cell? What about resting permeability?
Potassium and big anions are usually inside and sodium and chloride outside.
In resting state, cells are highly permeable to Potassium and Chloride, have low permeability to Na and have zero permeability to A- (big anions)
Which way do ions move across the membrane?
An ion will move across the membrane in a direction to move the membrane potential (Em) toward the equilibrium potential (Ex) for that ion.
For example, Sodium has a potential of +66, it really wants to be +66. But all of the sodium is outside, so in the cell, the potential is low. Sodium thus moves in to restore the balance.
Chloride tends to stay where it is because it’s happy at -90, and Potassium may leave a little to go from its -90 to -97 but not that much.
What two receptor types do we use on the postsynaptic membrane?
Ionotropic - Directly opens or closes a channiel in the postsynaptic membrane
Metabotropic - Releases a messenger, usually a G protein, to activate enzymes, gene transcription, or secondary channels in the membrane
