Nervous System and Sensory Organs
Core concepts
Know neuron structure, resting potential (Na⁺/K⁺ pump, K⁺ leak), action potential phases (depolarization by Na⁺ influx, repolarization by K⁺ efflux, refractory period), and synaptic transmission (neurotransmitters, excitatory vs. inhibitory synapses). Understand the organization of the nervous system (CNS/PNS, somatic/autonomic, sympathetic vs. parasympathetic) and reflex arcs. For sensory organs, know the eye (photoreceptors, rods vs. cones, accommodation) and the ear (mechanoreceptors, cochlea, balance).
Traps in questions
- Depolarization is caused by Na⁺ entering the neuron; repolarization by K⁺ leaving.
- Myelin speeds conduction via saltatory conduction; it does not change the action potential amplitude.
- The refractory period ensures one-way propagation.
- Rods = dim light, no color; cones = color and acuity, concentrated in the fovea.
- Sympathetic = fight-or-flight; parasympathetic = rest-and-digest - do not swap.
Worked example / rule of thumb
A reflex arc: receptor → sensory neuron → (synapse) → interneuron in spinal cord → motor neuron → effector muscle. Rule of thumb: follow the ion, then follow the signal. For any synapse question, name the neurotransmitter, the receptor type, and whether the postsynaptic effect is excitatory or inhibitory.