Here are 20 interesting facts about the cerebral cortex, spinal cord, and neurons, drawn from authoritative neuroscience and physiology textbooks:

1. Over 80 billion neurons The total cerebral cortex contains an estimated over 80 billion neurons — a number that dwarfs many comparisons in biology. — Guyton and Hall Textbook of Medical Physiology

2. Six distinct layers The cortex is organized into six layers (I–VI), each with unique cell types and functions. Layer I has almost no neurons; Layer IV is where incoming sensory signals first arrive; Layers V and VI send output signals to the spinal cord and thalamus respectively. — Guyton and Hall

3. Sensory signals enter at layer IV, motor output leaves from layer V Incoming sensory signals terminate in layer IV, while the giant pyramidal (Betz) cells of layer V send the longest axons in the body — all the way down to the spinal cord. — Guyton and Hall

4. Layers II and III talk to the opposite hemisphere Neurons in layers II and III send axons through the corpus callosum to mirror areas in the contralateral cortex — enabling the two hemispheres to coordinate. — Guyton and Hall

5. Cortex is organized in vertical columns The somatosensory cortex is arranged in vertical columns 0.3–0.5 mm wide, each containing ~10,000 neurons responding to one specific sensory modality from one body spot (e.g., joint stretch, hair touch, deep pressure). — Guyton and Hall

6. Three neuron types dominate The cortex is built from three main cell types: granular (stellate) cells — mostly interneurons using glutamate or GABA; pyramidal cells — the primary output neurons; and fusiform cells. — Guyton and Hall

7. Granular cells control intracortical processing Granular neurons have short axons and act as local circuit processors. Sensory and association areas are densely packed with them, reflecting intense intracortical analysis of incoming signals. — Guyton and Hall

8. Horizontal fibers dominate the internal wiring The cortex contains a striking number of horizontal fibers linking adjacent cortical areas, plus vertical fibers projecting down to subcortical structures and the spinal cord. — Guyton and Hall

9. Layer VI feeds back to the thalamus Axons from layer VI project back to the thalamus in enormous numbers, allowing the cortex to actively regulate which sensory signals are amplified or suppressed at the thalamic "relay station." — Guyton and Hall

10. Neurons born last end up deepest — then reverse During cortical development, newborn neurons migrate outward using glial scaffolds. Critically, layers form in an inside-out sequence — later-born neurons leap past earlier ones to occupy more superficial layers. — Eric Kandel, Principles of Neural Science, 6th Ed.

11. Gray matter core, white matter shell The spinal cord has central gray matter (cell bodies and their processes) surrounded by white matter (myelinated ascending and descending fiber tracts) — the inverse of cerebral cortex organization. — Rosen's Emergency Medicine

12. One artery supplies the front two-thirds A single anterior spinal artery (arising from the vertebral arteries) supplies the anterior two-thirds of the cord. Damage to it causes the distinctive anterior cord syndrome — paralysis and loss of pain/temperature with preserved vibration sense. — Rosen's Emergency Medicine

13. The artery of Adamkiewicz is critical The largest segmental reinforcing vessel, the artery of Adamkiewicz, typically originates from the aorta between T8 and L4 and is the main blood supply to the thoracolumbar cord. Aortic surgery risks cutting it off. — Rosen's Emergency Medicine

14. Crossing patterns are opposite for sensory tracts In the dorsal (posterior) columns, sacral fibers lie medially and cervical fibers laterally. In the lateral spinothalamic tract, it's reversed — sacral fibers lateral, cervical medial. This somatotopic arrangement helps localize spinal lesions clinically. — Rosen's Emergency Medicine

15. Pain and temperature cross immediately; touch crosses much higher Spinothalamic tract fibers carrying pain and temperature cross at their entry level in the cord → contralateral deficit below. Dorsal column fibers carrying vibration and proprioception cross only in the medulla → ipsilateral deficit below a cord lesion (Brown-Séquard pattern). — Rosen's Emergency Medicine

16. Tracts are named by origin → destination A simple naming convention: the spinothalamic tract originates in the spinal cord and terminates in the thalamus; the corticospinal tract originates in cortex and descends to the spinal cord. The name reads like a postal address. — Rosen's Emergency Medicine

17. A neuron's axon can be over a meter long A single lower motor neuron can have an axon extending from the lumbar spinal cord to the foot — over 1 meter — making neurons the longest cells in the human body. — Eric Kandel, Principles of Neural Science

18. The action potential is all-or-nothing Once a neuron reaches its threshold voltage, it fires a full action potential regardless of stimulus strength. Intensity is encoded not by the size of the spike but by the frequency of firing. — Neuroscience: Exploring the Brain, 5th Ed.

19. Spatial and temporal summation determine firing Whether an action potential is generated depends on summation: multiple synapses firing simultaneously (spatial) or the same synapse firing in rapid succession (temporal). A single EPSP is rarely enough — neurons are natural coincidence detectors. — Ganong's Review of Medical Physiology

20. Hebb's rule: neurons that fire together, wire together Synaptic strengthening occurs when a presynaptic neuron repeatedly and successfully contributes to firing a postsynaptic neuron. This Hebbian plasticity — "neurons that fire together, wire together" — is the cellular basis of learning and memory. — Neuroscience: Exploring the Brain, 5th Ed.

These facts span structural anatomy, functional organization, blood supply, sensory pathways, and cellular physiology — covering the breadth of what makes the nervous system remarkable.