Chapter 6: Neuronal Signaling and the Structure of the Nervous System

Neurons comprise three functionally distinct regions: dendrites that receive incoming signals, the soma that integrates information, and the axon that propagates electrical messages to downstream targets. Glial cells including astrocytes, oligodendrocytes, Schwann cells, microglia, and ependymal cells provide essential support through metabolic nourishment, myelin formation, immune surveillance, and maintenance of the extracellular ionic environment. Myelin sheaths dramatically accelerate signal transmission through saltatory conduction, allowing action potentials to skip between nodes of Ranvier. The chapter establishes how resting membrane potential emerges from asymmetric ion distributions maintained by the sodium-potassium pump and selective membrane permeability, with the Nernst and Goldman-Hodgkin-Katz equations quantifying these electrochemical gradients. Graded potentials representing excitatory and inhibitory postsynaptic currents integrate at the axon hillock to determine whether threshold for action potential initiation is reached. Action potentials result from sequential opening of voltage-gated sodium and potassium channels, producing characteristic phases of depolarization, repolarization, and after-hyperpolarization, while absolute and relative refractory periods ensure unidirectional propagation and constrain firing frequency. Synaptic transmission occurs through two mechanisms: chemical synapses in which calcium influx triggers vesicular neurotransmitter release and binding to postsynaptic receptors, and electrical synapses utilizing gap junction coupling for rapid synchronized activity. Major neurotransmitter systems including acetylcholine, glutamate, gamma-aminobutyric acid, dopamine, serotonin, and norepinephrine mediate diverse signaling functions and neuromodulatory effects throughout the nervous system. The nervous system structurally divides into the central nervous system comprising the brain and spinal cord, and the peripheral nervous system subdivided into sensory afferent pathways and motor efferent pathways including somatic and autonomic divisions. Reflex arcs exemplify how neural circuits directly link sensory input to motor output without requiring conscious processing. Multiple sclerosis serves as a clinical illustration of how demyelination disrupts action potential conduction, demonstrating the critical importance of myelin integrity for normal neurological function.