Neuron Study Guide

📖 Core Concepts Neuron – an excitable cell that fires action potentials to transmit information. Action potential – an all‑or‑none, rapid depolarization of the membrane when voltage‑gated Na⁺ channels open. Synapse – a specialized junction where a presynaptic axon terminal releases neurotransmitter onto a postsynaptic membrane (usually a dendrite). Resting membrane potential – the steady voltage across the neuronal membrane (‑70 mV, “a little less than one‑tenth of a volt”) created by ion pumps and leak channels. Law of Dynamic Polarization – signals are received on dendrites/soma and travel away from the soma along the axon. Neurotransmitter classification – excitatory (e.g., glutamate), inhibitory (e.g., GABA), modulatory (e.g., dopamine, acetylcholine). 📌 Must Remember Neuron polarity types – unipolar (sensory only), bipolar (sensory; retina, olfactory), multipolar (most CNS neurons). Functional direction – afferent = sensory → CNS; efferent = motor → effectors. Action potential threshold – once membrane voltage exceeds a critical value, Na⁺ channels open → full spike. Myelination – myelin → saltatory conduction; nodes of Ranvier concentrate voltage‑gated Na⁺/K⁺ channels. Average connectivity – 86 billion neurons, 7,000 synapses per neuron. Excitatory vs inhibitory receptors – excitatory ↑ firing rate, inhibitory ↓ firing rate. Fast‑spiking interneurons – can sustain very high firing rates; often GABAergic. 🔄 Key Processes Action potential initiation Depolarization → membrane > threshold → voltage‑gated Na⁺ channels open → rapid influx of Na⁺ → spike. Propagation Local current depolarizes adjacent membrane segments → sequential opening of Na⁺ channels → wave travels down axon. Synaptic transmission AP arrives at axon terminal → opens voltage‑gated Ca²⁺ channels → Ca²⁺ influx → vesicle fusion → neurotransmitter release → diffusion across cleft → receptor activation. Myelin‑mediated conduction AP jumps from node to node (saltatory) → conduction speed ↑, energy demand ↓. Frequency coding Stronger stimulus → higher firing frequency (not larger spikes). 🔍 Key Comparisons Unipolar vs Pseudounipolar – both have a single process; pseudounipolar’s single process functions as both axon and dendrite. Sensory vs Motor neurons – sensory: transmit peripheral → CNS; motor: CNS → muscle/gland. Glutamatergic vs GABAergic – glutamate = excitatory (ionotropic ↑ Na⁺/Ca²⁺), GABA = inhibitory (Cl⁻ influx hyperpolarizes). Tonic vs Phasic receptors – tonic: fire continuously while stimulus present; phasic: fire only at stimulus onset. Electrical vs Chemical synapse – electrical: direct ion flow via gap junctions; chemical: neurotransmitter release, slower, modulatable. ⚠️ Common Misunderstandings “All‑or‑none” applies to each spike, not to the total number of spikes – a neuron can fire multiple full‑amplitude spikes. Dendrites only receive inputs – they can also release neurotransmitter (output site) in some circuits. Myelinated axons always conduct faster – very thin unmyelinated fibers can still transmit adequately for short distances. “Excitatory” always means depolarization – excitatory metabotropic receptors can have more complex intracellular effects. 🧠 Mental Models / Intuition “Electrical highway” analogy – axon = highway, nodes = toll stations (where the car (AP) can speed up). “Bucket of charge” – resting potential = water level in a bucket; opening Na⁺ channels lets charge flood in (spike). “Lock‑and‑key” for synapses – voltage‑gated Ca²⁺ channels are the lock opened by the arriving AP (key). 🚩 Exceptions & Edge Cases Dendritic output – dendrites can form axon‑like presynaptic sites. Axonal input – axons may receive synapses (e.g., collateral inputs). Non‑spiking neurons – some sensory/interneurons use graded potentials instead of full spikes. Electrical synapse prevalence – more common than once thought, especially in early development. 📍 When to Use Which Identify neuron type → If cell body in peripheral ganglion & single process → unipolar sensory. Choose synaptic model → For fast, precise timing → chemical synapse with ionotropic receptors; for synchronization → electrical synapse. Predict firing pattern → High‑frequency >100 Hz → likely fast‑spiking interneuron; burst‑spiking → phasic neuron. Assess conduction speed → Long‑range pathway → look for myelination; short‑range → unmyelinated may suffice. 👀 Patterns to Recognize “Node‑of‑Ranvier–gap” pattern in myelinated axons → rapid depolarization spikes at each node. “Burst‑spike → calcium influx → vesicle release” = standard chemical synapse workflow. “Excitatory → Na⁺/Ca²⁺ influx → depolarization” versus “Inhibitory → Cl⁻ influx → hyperpolarization” in postsynaptic potentials. 🗂️ Exam Traps Choosing “all‑or‑none” for sub‑threshold stimuli – a sub‑threshold depolarization does not produce a partial spike. Assuming every neuron is multipolar – sensory neurons in ganglia are often unipolar/pseudounipolar. Confusing tonic vs phasic receptors – tonic = sustained firing; phasic = brief onset firing. Attributing speed solely to diameter – myelination, not just thickness, is the dominant factor for fast conduction. Thinking inhibitory always hyperpolarizes – shunting inhibition can be “neutral” (increase conductance without large voltage change).