Neurophysiology Study Guide

📖 Core Concepts Resting potential – the baseline voltage across a neuronal membrane when the cell is idle (≈ ‑70 mV); set by ion gradients and membrane permeability. Action potential – a rapid, all‑or‑none depolarization that travels down an axon; generated when membrane voltage exceeds the threshold. Synaptic transmission – release of neurotransmitter from the presynaptic terminal → binding to receptors on the postsynaptic cell, producing a graded postsynaptic potential. Neuroplasticity – the nervous system’s capacity to remodel its structure or function in response to experience, learning, or injury. Levels of organization – Molecular (ion channels, receptors) → Cellular (neurons, glia) → Circuit (synaptic networks) → System (brain regions, behavior). 📌 Must Remember Bell–Magendie law – motor fibers exit the spinal cord ventrally; sensory fibers enter dorsally. Helmholtz (1849) measured nerve‑impulse speed ≈ 20 m s⁻¹ in frog nerves. Bernstein (1902) membrane hypothesis → resting & action potentials arise from changes in ion permeability; resting potential given by the Nernst equation: $$E = \frac{RT}{zF}\ln\frac{[{\text{outside}}]}{[{\text{inside}}]}$$ Lapicque (1907) threshold concept – an action potential fires only when depolarization reaches a critical voltage. EEG alpha rhythm (8–12 Hz) discovered by Hans Berger (1924). Fiber diameter → conduction velocity (Erlanger & Gasser, 1944). Optogenetics = light‑activated proteins (e.g., Channelrhodopsin) → precise control of defined neuronal populations. 🔄 Key Processes Generation of an action potential Resting → depolarization → threshold reached → Na⁺ channels open → rapid upstroke → K⁺ channels open → repolarization → hyperpolarization → return to resting. Synaptic transmission (chemical) Action potential arrives → Ca²⁺ influx → vesicle fusion → neurotransmitter release → receptor activation (ionotropic or metabotropic) → postsynaptic potential. Optogenetic activation Gene delivery → expression of light‑sensitive opsin → illumination → opsin opens → depolarizing (or hyperpolarizing) current → controlled firing. Neuroplasticity after training Intensive practice → increased cortical grey‑matter thickness (≈ 3 months) → structural remodeling → regression if training stops. 🔍 Key Comparisons EEG vs. MEG – EEG records electric potentials on scalp; MEG records magnetic fields generated by the same currents, offering better spatial resolution for superficial cortical sources. Optogenetics vs. Chemogenetics – Optogenetics: light‑triggered, millisecond precision; Chemogenetics: synthetic‑drug‑activated receptors, slower (minutes‑hours) but no light needed. Resting potential vs. Action potential – Resting: stable, negative, set by ion gradients; Action: rapid, transient, positive overshoot, requires threshold crossing. Single‑cell recording vs. Local field potentials – Single‑cell: spikes from one neuron; LFPs: summed synaptic activity from a population near the electrode. ⚠️ Common Misunderstandings “All neurons fire the same way.” – Firing patterns (regular spiking, bursting, fast‑spiking) depend on ion channel complement. “Neuroplasticity only occurs in childhood.” – Adult brain retains plastic capacity; changes may be subtler and require stronger stimuli. “EEG directly measures neuronal firing.” – EEG reflects summed postsynaptic potentials, not action potentials themselves. “Optogenetics works without a gene delivery step.” – Opsins must be expressed via viral vectors or transgenic lines first. 🧠 Mental Models / Intuition “Voltage‑gate = gate‑keeper.” – Think of ion channels as doors that open only when voltage (or ligand) reaches a specific setting; crossing the threshold flips the main door (Na⁺) wide open. “Synapse as a bridge.” – Presynaptic neuron sends a messenger across the bridge (synaptic cleft); the bridge’s width (receptor density) and traffic (neurotransmitter amount) determine how much gets across. “Plasticity as remodeling a city.” – Repeated use (training) builds new roads (synapses) and widens existing ones (grey‑matter thickness); disuse leads to demolition. 🚩 Exceptions & Edge Cases Myelinated vs. unmyelinated axons – Myelin dramatically speeds conduction; unmyelinated fibers rely solely on diameter. Neurotransmitter release can be facilitated or depressed by previous activity (short‑term plasticity), altering synaptic strength transiently. Optogenetic inhibition – Not all opsins are excitatory; halorhodopsin (Cl⁻ pump) hyperpolarizes cells. 📍 When to Use Which Diagnosing epilepsy – Choose EEG for rapid bedside screening; add MEG if spatial localization of epileptiform foci is needed. Mapping functional circuits in research – Use optogenetics for millisecond‑scale causality; switch to chemogenetics when chronic, less invasive manipulation is sufficient. Assessing metabolic activity – fMRI for hemodynamic changes; PET when specific radioligand binding (e.g., dopamine receptors) is required. Studying single‑neuron dynamics – Intracellular patch‑clamp or single‑cell extracellular recording; for population dynamics, record local field potentials or Neuropixels probes. 👀 Patterns to Recognize “Speed ∝ diameter.” – Larger axons = faster conduction; look for this cue in questions about latency. “Threshold + all‑or‑none → action potential.” – Any stimulus that meets threshold will generate a full spike, regardless of strength beyond threshold. “Training → grey‑matter increase → regression after cessation.” – Pattern appears in neuroplasticity questions. “Alpha rhythm appears with eyes closed, attenuates with eye opening.” – Classic EEG pattern clue. 🗂️ Exam Traps Distractor: “Action potentials are caused by potassium influx.” – Wrong; initial depolarization is Na⁺ influx, K⁺ mainly repolarizes. Distractor: “MEG records electrical potentials.” – MEG records magnetic fields, not potentials. Distractor: “Neuroplasticity only involves synaptic strength changes.” – It also includes structural changes (dendritic branching, grey‑matter thickness). Distractor: “All optogenetic tools excite neurons.” – Some (e.g., halorhodopsin, ArchT) inhibit. Distractor: “Resting potential equals the equilibrium potential of Na⁺.” – Resting potential is usually close to the K⁺ equilibrium potential, not Na⁺. --- Use this guide for quick recall; focus on the bolded keywords and the step‑by‑step sequences before the exam.