Central and Peripheral Nervous Systems

The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS) consists of the rest of the neuronal pathways outside the brain and spinal cord. Functionally, the brain is organized in two main sections: the cerebral cortex and the subcortical areas; the latter includes the basal ganglia, hippocampus, internal capsule, brainstem, and cerebellum. The spinal cord extends from the foramen magnum to the L1-L2 level in adults. It consists of neuronal tracts that carry signals between the brain and peripheral nervous system. Cerebral blood flow (CBF) fluctuates minimally with cerebral perfusion pressure (CPP) due to corresponding changes in cerebral vascular resistance (CVR) attributed to cerebral autoregulation. CPP and CVR are influenced by pH, PaCO₂, PaO₂, and cerebral metabolic rate for O₂ (CMRO₂). CPP below the lower limit of cerebral autoregulation can lead to cerebral ischemia. Anesthetic medications can affect CBF and CMRO₂ although no anesthetic drugs have definitively been shown to be neuroprotective in humans. During neurosurgical procedures, evoked potentials are the most commonly used neurophysiological monitors to detect ischemia or tissue injury in the central nervous system.