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Field notes · chapter 42

The Nervous System

pp. 924–952 · Raven Part VII

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41 points
  1. Big picture: sensory receptors detect a stimulus, the nervous system processes it, motor effectors (muscles, glands) respond. Watch Nervous System on YouTube, from Amoeba Sisters
  2. CNS = brain + spinal cord — the command center, made mostly of interneurons and neuroglia. PNS = all the sensory and motor neurons outside it. Watch Nervous System on YouTube, from Amoeba Sisters
  3. Three functional neuron types: sensory (afferent) = receptor → CNS. Motor (efferent) = CNS → effector. Interneuron (association) = lives entirely inside the CNS and links the other two. Memory hook: Afferent Arrives, Efferent Exits. Watch Nervous System on YouTube, from Amoeba Sisters
  4. PNS motor side splits two ways: somatic = voluntary, to skeletal muscle. Autonomic = involuntary, to smooth muscle, cardiac muscle, and glands. Watch Peripheral Nervous System: Crash Course Anatomy & Physiology #12 on YouTube, from CrashCourse Watch Autonomic Nervous System: Crash Course Anatomy & Physiology #13 on YouTube, from CrashCourse
  5. Autonomic splits again: sympathetic = "fight or flight," parasympathetic = "rest and repose." They are antagonistic — they counterbalance each other on the same organs. Watch Autonomic Nervous System: Crash Course Anatomy & Physiology #13 on YouTube, from CrashCourse
  6. Neuron parts: dendrites receive input, the cell body holds the nucleus and integrates that input, and exactly one axon carries the impulse away. An axon may branch, but there is never more than one. Watch The Nervous System, Part 1: Crash Course Anatomy & Physiology #8 on YouTube, from CrashCourse
  7. Glial cells (neuroglia) are about 1/10 the size of neurons but roughly 10× more numerous. Five to know: Schwann cells, oligodendrocytes, astrocytes, microglia, ependymal cells. Watch The Nervous System, Part 1: Crash Course Anatomy & Physiology #8 on YouTube, from CrashCourse
  8. Myelin: Schwann cells myelinate in the PNS (one cell per segment); oligodendrocytes myelinate in the CNS (one cell can wrap several axons). Watch 2-Minute Neuroscience: Myelin on YouTube, from Neuroscientifically Challenged
  9. Nodes of Ranvier are gaps in the myelin every 1–2 µm. Action potentials fire only at nodes, so the impulse appears to jump — saltatory conduction (Latin saltare, to jump). Watch 2-Minute Neuroscience: Myelin on YouTube, from Neuroscientifically Challenged
  10. In the CNS, white matter = myelinated axons; gray matter = unmyelinated cell bodies and dendrites. In the PNS, a bundle of myelinated axons is a nerve. Watch The Nervous System, Part 1: Crash Course Anatomy & Physiology #8 on YouTube, from CrashCourse
  11. Astrocytes build the blood–brain barrier with capillaries and mop up excess extracellular K⁺. Microglia are the CNS macrophages. Ependymal cells are ciliated and make cerebrospinal fluid. Watch The Nervous System, Part 1: Crash Course Anatomy & Physiology #8 on YouTube, from CrashCourse
  12. Resting membrane potential in vertebrate neurons runs -40 to -90 mV; the standard example value is -70 mV. Negative sign = inside negative relative to outside. Watch Neurology | Resting Membrane, Graded, Action Potentials on YouTube, from Ninja Nerd
  13. Three causes of the resting potential: (1) the Na⁺/K⁺ pump moves 3 Na⁺ out for every 2 K⁺ in; (2) the membrane has more K⁺ leakage channels, so K⁺ diffuses out; (3) big negative proteins, nucleic acids, and organic phosphates are trapped inside. Watch Neurology | Resting Membrane, Graded, Action Potentials on YouTube, from Ninja Nerd
  14. The pump is not the main voltage source — the K⁺ concentration gradient it builds is. K⁺ leaking out leaves negative counter-ions behind, and the resulting electrical pull inward balances the outward diffusion at the equilibrium potential. Watch Neurology | Resting Membrane, Graded, Action Potentials on YouTube, from Ninja Nerd
  15. Nernst equation for K⁺: E_K = 58 mV · log([K⁺]out / [K⁺]in). Equilibrium potentials: K⁺ = -90 mV, Na⁺ = +60 mV, Cl⁻ = -70 mV. Resting sits at -70, not -90, because a little Na⁺ leaks in. Watch Neurology | Resting Membrane, Graded, Action Potentials on YouTube, from Ninja Nerd
  16. Two channel classes matter. Leakage channels are always open and set the resting potential. Gated channels open on cue: ligand-gated (chemical) make graded potentials; voltage-gated make action potentials. Watch Neurology | Resting Membrane, Graded, Action Potentials on YouTube, from Ninja Nerd
  17. Graded potentials are small, proportional to stimulus strength, they fade with distance, and they sum. Action potentials are large, fixed-size, all-or-none, and they do not sum. Watch Graded Potentials, EPSPs, IPSPs, & Summation on YouTube, from Catalyst University
  18. Action potential, phase by phase: rest -70 mV → depolarize to threshold -55 mV → voltage-gated Na⁺ activation gates open, Na⁺ rushes IN (rising phase) → peak +35 to +40 mV → Na⁺ inactivation gates close while voltage-gated K⁺ gates open, K⁺ flows OUT (falling phase / repolarization) → K⁺ channels close slowly, giving a brief undershoot (hyperpolarization) → resting state restored. Whole thing takes about 1 millisecond. Watch The Nervous System, Part 2 - Action! Potential!: Crash Course Anatomy & Physiology #9 on YouTube, from CrashCourse Watch 2-Minute Neuroscience: Action Potential on YouTube, from Neuroscientifically Challenged
  19. All-or-none: below threshold nothing happens; at or above threshold you get a full-size spike. Stimulus intensity is coded by the frequency of action potentials, not their size. Watch 2-Minute Neuroscience: Action Potential on YouTube, from Neuroscientifically Challenged
  20. Absolute refractory period = Na⁺ inactivation gates are shut, so no stimulus can fire the cell. Relative refractory period follows — a strong stimulus can fire it, but the spike is smaller. This is why the impulse cannot run backward. Watch The Nervous System, Part 2 - Action! Potential!: Crash Course Anatomy & Physiology #9 on YouTube, from CrashCourse
  21. One action potential moves very few ions. The Na⁺/K⁺ ATPase pump is not required for a single firing — it is required to maintain the gradients over repeated firing. Classic exam question.
  22. Two ways evolution sped up conduction: bigger axon diameter (less resistance — the invertebrate solution, e.g. the squid giant axon, 500 µm, 25 m/s) and myelination (the vertebrate solution — a 20 µm myelinated human motor axon hits 120 m/s). Watch 2-Minute Neuroscience: Myelin on YouTube, from Neuroscientifically Challenged
  23. Electrical synapses = gap junctions, direct cytoplasm-to-cytoplasm, extremely fast, common in invertebrates. Chemical synapses = the vast majority in vertebrates; a synaptic cleft separates the cells. Watch The Nervous System, Part 3 - Synapses!: Crash Course Anatomy & Physiology #10 on YouTube, from CrashCourse
  24. Synaptic transmission: action potential arrives → voltage-gated Ca²⁺ channels open → Ca²⁺ influx → synaptic vesicles fuse and release neurotransmitter by exocytosis → transmitter crosses the cleft → binds ligand-gated receptors → graded potential in the postsynaptic membrane. Higher firing frequency = more vesicles released. Watch 2-Minute Neuroscience: Neurotransmitter Release on YouTube, from Neuroscientifically Challenged Watch 2-Minute Neuroscience: Synaptic Transmission on YouTube, from Neuroscientifically Challenged
  25. Transmitter must be cleared fast, three ways: enzymatic digestion, reuptake by the presynaptic neuron, or uptake by glial cells. Acetylcholinesterase (AChE) — one of the fastest enzymes known — clears ACh. Nerve gas and parathion inhibit AChE, causing spastic paralysis. Watch The Nervous System, Part 3 - Synapses!: Crash Course Anatomy & Physiology #10 on YouTube, from CrashCourse
  26. Key transmitters: ACh (neuromuscular junction), glutamate (main excitatory CNS transmitter), glycine and GABA (inhibitory, they open Cl⁻ channels), dopamine (movement; loss → Parkinson, excess → schizophrenia), norepinephrine (sympathetic synapses), serotonin (sleep and mood; SSRIs block its reuptake), substance P (pain), enkephalins/endorphins (block pain — the endogenous opiates), nitric oxide (a gas, made from arginine on demand, cannot be stored in vesicles). Watch The Nervous System, Part 3 - Synapses!: Crash Course Anatomy & Physiology #10 on YouTube, from CrashCourse
  27. EPSP = depolarization, pushes the cell toward threshold. IPSP = hyperpolarization (e.g. GABA driving the membrane from -70 to -85 mV), pushes it away. Adding them all up at the cell body is synaptic integration. Watch Graded Potentials, EPSPs, IPSPs, & Summation on YouTube, from Catalyst University
  28. Spatial summation = EPSPs from many different synapses arriving at once. Temporal summation = one synapse firing repeatedly within about 15 ms (the life of an EPSP). Many shovels vs one fast shovel. Watch Graded Potentials, EPSPs, IPSPs, & Summation on YouTube, from Catalyst University
  29. Vertebrate brain has three primary divisions, present since jawless fish 500 mya: hindbrain (rhombencephalon — cerebellum, pons, medulla), midbrain (mesencephalon — optic tectum), forebrain (prosencephalon — diencephalon + telencephalon/cerebrum). Watch Central Nervous System: Crash Course Anatomy & Physiology #11 on YouTube, from CrashCourse
  30. Cerebral cortex = a few-mm gray-matter sheet with over 10 billion neurons (~10% of all brain neurons); folding triples its surface area. Association cortex is ~95% of the human cortical surface. Watch Central Nervous System: Crash Course Anatomy & Physiology #11 on YouTube, from CrashCourse
  31. Cortex landmarks: primary motor cortex on the rear edge of the frontal lobe, primary somatosensory cortex just behind the central sulcus on the front edge of the parietal lobe. Cortical area matches fineness of control, not body-part size — hands, lips, tongue get huge maps. Watch Central Nervous System: Crash Course Anatomy & Physiology #11 on YouTube, from CrashCourse
  32. Each hemisphere handles the contralateral (opposite) side of the body — which is why a stroke on the left paralyzes the right.
  33. Broca’s area (frontal) = motor output of speech. Wernicke’s area (parietal) = comprehension; damage gives fluent but meaningless "word salad." Left hemisphere is language-dominant in 90% of right-handers and about two-thirds of left-handers.
  34. Spinal cord: gray matter inside (interneuron, motor neuron, and glial cell bodies), white matter outside (sensory axons dorsally, motor axons ventrally). Note this is the opposite arrangement from the cerebrum.
  35. Reflex = a fast, involuntary response handled by the spinal cord without waiting for the brain. The knee-jerk is monosynaptic — sensory neuron straight onto motor neuron, no interneuron. A cutaneous reflex adds one or more interneurons. Watch Reflex action (& reflex arc) | Neural Control & Coordination | Class 11 | Biology | Khan Academy on YouTube, from Khan Academy India - English
  36. Spinal nerve roots: dorsal root = sensory, with cell bodies in the dorsal root ganglion outside the cord. Ventral root = motor, with cell bodies inside the cord. Watch Peripheral Nervous System: Crash Course Anatomy & Physiology #12 on YouTube, from CrashCourse
  37. Every autonomic pathway uses two neurons: a preganglionic neuron (CNS cell body, always releases ACh) and a postganglionic neuron (ganglion cell body → effector). Somatic pathways use only one neuron and only ACh. Watch Autonomic Nervous System: Crash Course Anatomy & Physiology #13 on YouTube, from CrashCourse
  38. Sympathetic: preganglionic neurons leave the thoracic and lumbar cord, synapse in the sympathetic chain, and postganglionic neurons release norepinephrine. Some preganglionic axons run straight to the adrenal medulla, which dumps epinephrine into the blood.
  39. Parasympathetic: preganglionic neurons come from the brain and sacral cord (many in the vagus nerve), run to ganglia near or inside the target organ, and postganglionic neurons release ACh. There is no parasympathetic chain.
  40. ACh can excite or inhibit depending on the receptor: nicotinic receptors on skeletal muscle open as Na⁺ channels (fast depolarization); muscarinic receptors on the heart use a G protein to open K⁺ channels, hyperpolarizing and slowing the heart.
  41. 12 pairs of cranial nerves (I–XII) arise from the underside of the brain. The vagus (X) is the odd one — it leaves the head entirely and innervates heart, lungs, and abdominal organs. Watch Peripheral Nervous System: Crash Course Anatomy & Physiology #12 on YouTube, from CrashCourse