5H · Across Modules 1–4
Short-answer questions reward connecting modules. These are the threads that run through all four.
Calcium, module by module¶
Where | What calcium does | How it gets there |
|---|---|---|
Module 1 | Huge inward gradient, relied on heavily in the heart | Pumps establish it |
Neuron terminal | Triggers exocytosis of neurotransmitter | Voltage-gated Ca2+ channels |
Skeletal muscle | Binds troponin C | DHPR mechanically opens RyR. Intracellular only |
Cardiac muscle | Binds troponin C | Influx opens RyR, CICR. Extracellular and intracellular |
Smooth muscle | Binds calmodulin, activates MLCK | CICR and IP3. Extracellular and intracellular |
Action potentials by tissue¶
Tissue | Shape | Duration | Depolarizing ion |
|---|---|---|---|
Neuron | Spike | ~1 ms | Na+ |
Skeletal muscle | Spike | ~2 ms | Na+ |
Cardiac muscle | Plateau | ~200 ms | Na+ upstroke, Ca2+ plateau |
Smooth muscle | Variable: spikes, graded, slow waves | variable | Ca2+ |
Receptors and speed¶
Ligand-gated ion channels act fast, in few steps: neuron to neuron, and nicotinic receptors at the neuromuscular junction. G protein-coupled receptors act slower, through second messengers: cardiac muscle, smooth muscle, glands, adipose tissue. (M1 slide 16, M2 slide 44)
Pumps that keep appearing¶
Na+-K+ pump: in every cell (M1), electrogenic and a modest resting contributor (M2), one of muscle’s three ATP sinks (M3), phase 4 of the cardiac action potential (M4). SERCA: introduced in M1, drives skeletal relaxation (M3), the majority of calcium removal in smooth and cardiac muscle (M4), braked by phospholamban (M4).
Temperature¶
Higher temperature speeds diffusion (M1 simulation), speeds axon conduction velocity (M2, cat neurons up to 5× baseline at 40°C), and cooling lengthens the absolute refractory period (Assignment 2).
Cross-module questions¶
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)
Solution to Exercise 2
Higher outside K+ shrinks K+'s gradient, so less K+ leaves, EK becomes less negative, and Vm 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 EK moves Vm far more. (M2 slides 22–23)
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).
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)