Chapter 3: Nervous System Overview & Fundamentals

The nervous system is structurally divided into the central nervous system (CNS), which includes the brain, spinal cord, and retina, and the peripheral nervous system (PNS), consisting of cranial and spinal nerves, specialized sensory organs, and the peripheral autonomic nervous system (ANS). The ANS is further subdivided into sympathetic and parasympathetic components, primarily responsible for internal environmental control. Neurons, the fundamental signaling cells, encode and transmit information via electrical action potentials along their elongated axons (efferent signals) and communicate with target cells through specialized junctions called synapses, releasing neurotransmitters. Neurons are categorized by structure, such as multipolar, bipolar (often special sensory), and pseudounipolar (general sensory) types, and possess complex dendritic trees (afferent structures) which receive synaptic input, often on small projections called dendritic spines. Intracellular material is transported via axoplasmic flow, consisting of slow bulk movement of cytoskeletal components and faster, bidirectional movement of vesicles and mitochondria powered by molecular motors like kinesin and dynein along microtubules. Communication at chemical synapses involves the release of neurotransmitters, which bind to postsynaptic receptors, classified as either ionotropic (direct channel opening) or metabotropic (G-protein coupled). Key neurotransmitters include the major excitatory amino acid glutamate and the major inhibitory amino acids GABA and glycine, alongside classic transmitters like acetylcholine (ACh) and various monoamines and neuropeptides. Non-neuronal support cells, known as neuroglia, include CNS macroglia (astrocytes and oligodendrocytes) and microglia, and PNS glia (Schwann cells and satellite cells). Astrocytes are crucial for maintaining the metabolic environment, converting glucose to lactate for neurons, and establishing the blood–brain barrier through specialized perivascular end-feet. Oligodendrocytes in the CNS and Schwann cells in the PNS form the lipid-rich myelin sheath, which wraps around axons with a diameter (greater than) 2 micrometers to enable rapid saltatory conduction where the action potential jumps between exposed Nodes of Ranvier. In contrast to the CNS, the PNS glial environment facilitates axonal regrowth following injury. Peripheral nerves are organized into fasciculi protected by connective tissue sheaths—the outer epineurium, the multilayered perineurium (forming the blood–nerve barrier), and the inner endoneurium. Sensory information is collected by diverse receptor types, including encapsulated endings like Pacinian corpuscles (vibration), Meissner’s corpuscles (light touch), and the essential proprioceptors: Golgi tendon organs (muscle tension) and neuromuscular spindles (muscle length/velocity). Efferent skeletal muscle control occurs at the neuromuscular junction, a specialized cholinergic synapse ensuring a one-to-one transmission ratio, while autonomic terminals are typically varicose and diffuse.