Neurobiologie und Sinnesphysiologie / Neurobiology and sensory physiology
Neurobiology items ask you to trace a signal from stimulus to response: ion distributions, the action potential, synaptic transmission, and the transduction step in a sense organ. Many tasks use toxins or ion-concentration changes as experimental probes, so link each manipulation to a specific channel or pump.
Resting potential and action potential
- Resting potential around −70 mV: the Na+/K+-ATPase maintains gradients, and K+ leak channels make the membrane more permeable to K+, so the inside is negative.
- Depolarisation to threshold opens voltage-gated Na+ channels; Na+ influx causes the steep upstroke. Na+ channels inactivate and voltage-gated K+ channels open, giving repolarisation and a brief hyperpolarisation before the resting value is restored.
- All-or-nothing: stimulus strength is coded as frequency, not amplitude. The refractory period limits maximum frequency. Myelination allows saltatory conduction between nodes of Ranvier, raising speed.
Synapses and signal transduction
- Chemical synapse: arrival of the action potential opens voltage-gated Ca2+ channels, Ca2+ influx triggers exocytosis of neurotransmitter, which binds postsynaptic receptors.
- Excitatory (e.g. Na+ influx, EPSP) and inhibitory (e.g. Cl− influx, IPSP) inputs sum spatially and temporally; threshold at the axon hillock decides whether an action potential fires.
- Signal transduction often runs through G-protein-coupled receptors and second messengers (cAMP), amplifying a small extracellular signal.
Senses
- In the retina, rods contain rhodopsin; light isomerises retinal, activates transducin, lowers cGMP, closes Na+ channels, and the cell hyperpolarises. Rods are sensitive, cones provide colour and acuity; the signal is processed by bipolar and ganglion cells.
Worked example
Toxin X blocks voltage-gated Na+ channels; toxin Y blocks voltage-gated K+ channels. Reasoning: with X, no Na+ influx is possible, so no action potential can be generated at any stimulus strength - the membrane stays near resting potential. With Y, depolarisation still occurs, but repolarisation is slowed, so the action potential is broadened and the refractory period lengthened, reducing the maximum firing frequency.
Traps that cost marks
Saying the Na+/K+ pump generates the action potential; drawing the spike with reversed ion movements; confusing EPSPs with action potentials (EPSPs are graded); claiming myelin speeds conduction by carrying the current inside the axon; forgetting that photoreceptors hyperpolarise in response to light.