Chapter 14: Structure and Function of the Neurologic System

The nervous system is structurally divided into the central nervous system, comprising the brain and spinal cord, and the peripheral nervous system, which includes all neural structures outside the CNS. Functionally, the peripheral nervous system separates into the somatic division controlling voluntary skeletal muscle activity and the autonomic division managing involuntary organ regulation through sympathetic and parasympathetic branches. Neurons, the primary signaling cells, transmit information through a soma, dendrites, and axons, with conduction velocity enhanced by myelin wrapping from oligodendrocytes in the CNS and Schwann cells in the PNS. Neuroglia including astrocytes, microglia, ependymal cells, and satellite cells provide structural scaffolding, metabolic support, immune surveillance, and cerebrospinal fluid production. Nerve injury initiates degenerative and regenerative processes, with the PNS demonstrating greater recovery capacity than the CNS. Neural signaling occurs through action potentials propagating along axons and synaptic transmission mediated by neurotransmitters such as acetylcholine, catecholamines, serotonin, gamma-aminobutyric acid, glutamate, and endogenous opioids. Postsynaptic potentials summate to determine whether a neuron fires, while neuroplasticity allows lifelong reorganization of synaptic networks following learning or injury. The brain architecture includes the cerebrum with specialized cortical regions controlling motor and sensory functions, the basal ganglia modulating movement, the limbic system processing emotion and memory, and brainstem structures including the midbrain, pons, and medulla directing vital functions. The spinal cord contains ascending sensory tracts and descending motor pathways, integrating reflex responses at segmental levels. Protective mechanisms encompass the meninges, cerebrospinal fluid circulation, the vertebral column, and the blood-brain barrier selectively regulating molecular passage. Cerebral perfusion occurs through carotid and vertebral arterial systems with collateral circulation via the circle of Willis, while spinal cord perfusion derives from radicular arteries. The peripheral nervous system comprises spinal nerves organized into dermatomes and twelve cranial nerves subserving sensory and motor functions including vision, hearing, and autonomic control. Sympathetic activation triggers catecholamine release producing fight-or-flight responses, whereas parasympathetic signaling through acetylcholine promotes rest-and-digest physiology. Age-related changes including neuronal loss, myelin degeneration, neurofibrillary accumulation, and altered neurotransmitter levels explain functional declines in elderly individuals.