The CNS and Its Armor
The Central Nervous System (CNS), comprising the brain and spinal cord, is guarded by three meninges: the tough dura mater, web-like arachnoid mater, and delicate pia mater.
The subarachnoid space holds cerebrospinal fluid (CSF), which cushions the CNS and stabilizes its chemical environment. Meanwhile, the blood-brain barrier uses tight capillary junctions to filter blood contents, letting small lipid-soluble molecules (like alcohol) pass much easier than pathogens.
| Region | Grey Matter Layout | White Matter Layout |
|---|---|---|
| Brain | Outer cortex | Inner core |
| Spinal Cord | H-shaped core | Outer tracts |
Spinal Reflexes
The spinal cord runs fast, local reflex arcs to react to danger before the brain even gets involved.
A withdrawal reflex routes a signal from a sensory (afferent) neuron, through a spinal interneuron, straight to a motor (efferent) neuron to pull away instantly.
Common pitfall: Assuming a spinal reflex still involves the brain "just very quickly." It does not. The entire arc completes inside the spinal cord, making it faster than any voluntary response.