Neurological System

The nervous system links the brain and spinal cord with the cranial nerves, spinal nerves, and autonomic pathways. Neurons maintain electrical gradients with the sodium-potassium pump; voltage-gated channels then create and propagate action potentials, and synapses convert those signals into chemical or electrical communication. Myelin speeds conduction by allowing impulses to “jump” between nodes along the axon. The skull limits expansion. Brain tissue, cerebrospinal fluid (CSF), and intracranial blood share a fixed space, described by the Monro-Kellie doctrine. A small increase in one component may initially be compensated by shifting CSF and venous blood. Once that reserve is exhausted, intracranial pressure (ICP) rises, cerebral perfusion pressure falls, and brain tissue becomes ischemic. Cerebral perfusion pressure is calculated as mean arterial pressure minus ICP. That relationship explains why a patient can deteriorate even when the original injury has stopped expanding. Edema, impaired venous drainage, hypercapnia, fever, seizures, and hypotension can all increase secondary brain injury. A headache alone is nonspecific; a new decline in consciousness with vomiting, pupillary asymmetry, posturing, or a Cushing pattern is not. The blood-brain barrier limits movement...