Problems: Across Modules

Write your answer first, then open the solution. Assignment key items show the points the grader awards; chart problems use the course figures; variants change the numbers or flip the premise; check-yourself items answer the end-of-module review questions.

Variant¶

Exercise 1 (Variant · 9 pts)

Compare how skeletal, cardiac, and smooth muscle raise intracellular calcium, name each tissue’s calcium sensor, and say which filament it regulates.

Solution to Exercise 1

Skeletal: DHPR mechanically opens RyR; intracellular source only; troponin C; thin filament.

Cardiac: phase 2 influx opens RyR by CICR; extracellular and intracellular, 20/80; troponin C; thin filament.

Smooth: CICR through voltage-gated channels, or IP₃ through GPCRs; extracellular and intracellular; calmodulin; thick filament. (M4 slides 46–47)

Exercise 2 (Variant · 6 pts)

Hyperkalemia makes V_m less negative. Using Module 2, explain why that raises the risk of cardiac arrhythmia, and why the effect is dominated by potassium rather than sodium.

Solution to Exercise 2

Higher outside K⁺ shrinks K⁺'s gradient, so less K⁺ leaves, E_K becomes less negative, and V_m follows. The gap to threshold shrinks, so the membrane is more excitable and fires at inappropriate times: arrhythmias.

K⁺ dominates because resting conductance is overwhelmingly K⁺ leak. In chord conductance K⁺'s weight is 0.70 against Na⁺'s 0.025, so a change in E_K moves V_m far more. (M2 slides 22–23)

Exercise 3 (Variant · 6 pts)

Summation appears three times in Modules 2 and 3. Name each, what is being summed, and what the endpoint is.

Solution to Exercise 3

Spatial summation (M2): graded potentials from several synapses at once, aiming at threshold at the axon hillock.

Temporal summation (M2): graded potentials from one synapse firing rapidly, aiming at threshold.

Frequency summation (M3): whole twitch contractions piggybacking, climbing toward unfused and then fused tetanus.

Action potentials themselves do not sum in neurons, because refractory periods prevent it, but smooth muscle action potentials can (M4 slide 11).

Exercise 4 (Variant · 6 pts)

Map skeletal muscle relaxation onto the five-step template, then explain why cardiac relaxation looks like skeletal and cardiac calcium removal looks like smooth.

Solution to Exercise 4

Steps 4–5: stimulus ends and ACh is broken down → SERCA lowers free Ca²⁺ → Ca²⁺ leaves troponin C → tropomyosin covers actin → no cross-bridges → relaxation.

Cardiac relaxation mirrors skeletal because both use troponin and tropomyosin. Cardiac calcium removal mirrors smooth because both use SERCA plus NCX and PMCA to push calcium out of the cell, since both also bring calcium in from outside. (M4 slides 10, 31, 35–36)

Physiology I

Problems: Module 4