Nervous system Study Guide

📖 Core Concepts Nervous system – network that detects changes, transmits signals, and coordinates body actions; works with the endocrine system for responses. Central vs. Peripheral – CNS = brain + spinal cord; PNS = nerves (bundles of axons) linking CNS to the rest of the body. Somatic vs. Autonomic – Somatic controls skeletal muscle & sensory input; Autonomic regulates internal organs (sympathetic = “fight‑or‑flight”, parasympathetic = “rest‑and‑digest”, enteric = gut). Neuron – excitable cell that sends electrochemical action potentials down an axon; communicates via synapses (chemical or electrical). Glial cells – support neurons (myelin by oligodendrocytes/Schwann cells, immune surveillance by microglia, metabolic support by astrocytes). Myelin – fatty sheath that speeds action‑potential propagation (up to >100 m/s). Grey vs. White matter – Grey = neuronal cell bodies; White = myelinated axon tracts. Reflex arc – simple circuit: sensory receptor → afferent neuron → (interneuron) → efferent neuron → muscle/gland. Central pattern generator (CPG) – neural network that creates rhythmic output (e.g., walking) without external timing cues. Mirror neurons – fire when performing an action and when observing the same action; proposed role in imitation, theory of mind, language. Neural development – neural plate → neural tube (CNS); neural crest (PNS). BMP inhibition (by Noggin/Chordin) drives ectoderm to become neural tissue. Blood‑brain barrier (BBB) – tight endothelial junctions + meninges protect CNS from most blood‑borne substances. --- 📌 Must Remember CNS components: brain, spinal cord, meninges (dura mater outermost). PNS components: cranial nerves (exit brain), spinal nerves (exit cord), somatic & autonomic branches. Myelin makers: Oligodendrocytes (CNS) vs. Schwann cells (PNS). Primary neurotransmitters: Glutamate = excitatory, GABA = inhibitory. Dale’s Principle: a neuron releases the same neurotransmitter(s) at all its synapses. Long‑term potentiation (LTP): NMDA‑receptor‑mediated Ca²⁺ influx → more AMPA receptors → stronger synapse → memory. Fastest nerve conduction: >100 m/s (myelinated peripheral fibers). Reflex hierarchy: simple monosynaptic (e.g., stretch reflex) → polysynaptic (withdrawal) → modulated by descending pathways. Circadian clock: suprachiasmatic nucleus drives 24 h rhythms via clock‑gene expression. Peripheral nerve regeneration: possible after transection, but long distances = months; CNS regeneration is limited by scar tissue. --- 🔄 Key Processes Action Potential Generation Resting membrane ‑70 mV → depolarization → Na⁺ influx → threshold → rapid upstroke → K⁺ efflux → repolarization → refractory period. Synaptic Transmission (chemical) AP reaches presynaptic terminal → Ca²⁺ channels open → vesicle fusion → neurotransmitter release → bind postsynaptic receptors → ion channel opening → EPSP/IPSP. Reflex Arc (withdrawal) Heat receptor → sensory AP → dorsal horn → excitatory interneuron → motor neuron → flexor muscle contraction. Central Pattern Generation Intrinsic pacemaker neurons (voltage‑gated channels) → rhythmic burst firing → coordinated motor output (e.g., locomotion). Neurulation Ectoderm → neural plate → neural groove → neural tube closure → neural crest cells migrate → peripheral structures. BMP Inhibition for Neural Induction BMP‑4 active → epidermal fate; Noggin/Chordin bind BMP‑4 → inhibition → ectoderm becomes neural plate. --- 🔍 Key Comparisons CNS vs. PNS – brain/spinal cord vs. nerves; protected by skull/vertebrae & meninges vs. no bony protection. Somatic vs. Autonomic – voluntary skeletal muscle control vs. involuntary organ regulation. Sympathetic vs. Parasympathetic – emergency “fight‑or‑flight” (↑ heart rate, dilated pupils) vs. “rest‑and‑digest” (↓ heart rate, increased digestion). Oligodendrocyte vs. Schwann cell – single CNS oligodendrocyte myelinates multiple axons; each Schwann cell myelinates one peripheral axon. Excitatory (glutamate) vs. Inhibitory (GABA) synapses – depolarizing vs. hyperpolarizing postsynaptic potentials. Monosynaptic vs. Polysynaptic reflex – one synapse (stretch) vs. multiple interneurons (withdrawal). --- ⚠️ Common Misunderstandings “All nerves are the same.” Nerves differ: sensory (afferent), motor (efferent), and autonomic subdivisions have distinct functions. “Myelin is only in the CNS.” Schwann cells myelinate peripheral axons; oligodendrocytes do the same in the CNS. “Mirror neurons prove empathy.” Evidence for high‑level functions (theory of mind, language) is still debated. “The BBB blocks all drugs.” Some small, lipophilic molecules cross; transporters can also shuttle specific substances. “All reflexes are immutable.” Descending pathways can enhance or suppress reflex strength. --- 🧠 Mental Models / Intuition “Cable and Switch” – Think of a neuron as a cable (axon) that carries a binary signal (AP) and a switch (synapse) that can turn on/off downstream circuits based on neurotransmitter type. “Highway vs. Local Roads” – White matter = high‑speed highways (myelinated tracts); grey matter = local streets (cell bodies, processing centers). “Fire Alarm System” – Reflex arcs are like a fire alarm: sensor (pain receptor) → central panel (spinal cord) → sprinkler (muscle) – fast, automatic, but can be overridden by higher control (brain). --- 🚩 Exceptions & Edge Cases Basal ganglia – called “ganglia” but located in the brain (grey matter). Peripheral nerve transection – can regenerate, unlike most CNS axons where scar tissue blocks regrowth. Enteric nervous system – operates semi‑independently from CNS, often called the “second brain.” Glial-to-neuron ratio – roughly 1:1 in the human brain, not vastly more glia as sometimes claimed. --- 📍 When to Use Which Identify a structure: Grey matter → look for neuronal cell bodies → nuclei (CNS) or ganglia (PNS). White matter → look for myelinated axon tracts → pathways (e.g., corticospinal tract). Choose a nervous system subdivision for a symptom: Motor weakness → motor (efferent) nerve → check somatic vs. autonomic depending on muscle type. Abdominal pain with no skin changes → consider autonomic/enteric pathways. Select a reflex analysis: Simple, quick response → likely monosynaptic. Multi‑joint, adaptable response → polysynaptic with interneurons. Determine developmental origin: Structures derived from neural crest → peripheral ganglia, adrenal medulla, melanocytes. Structures from neural tube → brain & spinal cord. --- 👀 Patterns to Recognize “Axon → Myelin → Faster Conduction” – whenever myelin is mentioned, expect higher conduction speed. “Sensory → Dorsal root ganglion → Spinal cord → Interneuron → Motor” – classic reflex pathway pattern. “Excitatory → Glutamate” / “Inhibitory → GABA” – neurotransmitter ↔ effect pairing. “BMP inhibition → Neural fate” – whenever Noggin/Chordin appear, think neural induction. “Scar tissue → No CNS regeneration” – pattern in spinal cord injury discussions. --- 🗂️ Exam Traps “All autonomic fibers are unmyelinated.” – False; many pre‑ganglionic autonomic fibers are myelinated. “The BBB is formed by the meninges.” – Incorrect; BBB is created by tight endothelial cells of cerebral capillaries, not the meninges. “Mirror neurons are proven to cause language.” – Over‑interpretation; current data are correlational, not causal. “Peripheral nerves contain neuronal cell bodies.” – Misleading; cell bodies reside in CNS or ganglia, not within the nerve trunk. “Spinal cord injury always leads to permanent loss.” – Not always; swelling may cause temporary deficits, while complete fiber loss is permanent. ---