The Command Center and Its Armor
The Central Nervous System (CNS) — the brain and spinal cord — is the body's command center, and it is guarded accordingly. Three protective membranes called the meninges wrap around it: the tough outer dura mater, the web-like middle arachnoid mater, and the delicate inner pia mater, which hugs the neural tissue directly. Between the arachnoid and pia lies the subarachnoid space, filled with cerebrospinal fluid (CSF) — produced in the brain's ventricles, it cushions the CNS against impact and helps maintain a stable chemical environment. The blood-brain barrier, formed by tight junctions in the capillary walls, further filters what can pass from the bloodstream into brain tissue, admitting small lipid-soluble molecules (like alcohol or nicotine) far more readily than most drugs or pathogens.
Structurally, the spinal cord inverts the brain's layout: in the brain, grey matter (cell bodies) forms the outer cortex around an inner core of white matter (myelinated axon tracts); in the spinal cord, white matter is on the outside and an H-shaped column of grey matter sits at the core.
The spinal cord also runs its own fast, local circuits called reflex arcs, which react to danger without waiting for the brain. A withdrawal reflex, for instance, routes the signal from a sensory (afferent) neuron, through an interneuron in the spinal cord, straight to a motor (efferent) neuron — the hand pulls back from the heat before pain even reaches conscious awareness.
Common pitfall: assuming a spinal reflex still involves the brain "just very quickly." It does not need to — the entire arc can complete inside the spinal cord, which is precisely why it is faster than any voluntary response.