Chapter 4: Excitable Tissue: Nerve

Excitable Tissue: Nerve begins by defining the morphological components of the neuron, including the metabolic soma, the receptive dendrites, and the impulse-conducting axon, while classifying neurons based on structure (unipolar, bipolar, and multipolar) and function. A significant portion of the text is dedicated to membrane physiology, explaining how the resting membrane potential is established and maintained around negative 70 millivolts through ionic concentration gradients, selective membrane permeability, and the electrogenic Sodium-Potassium ATPase pump. The chapter thoroughly describes the electrochemical events of the action potential, detailing the role of voltage-gated sodium channels in depolarization and the subsequent repolarization driven by potassium efflux, alongside the concepts of threshold potential, the all-or-none law, and absolute versus relative refractory periods. It contrasts continuous conduction in unmyelinated fibers with the rapid saltatory conduction seen in myelinated axons, where electrical impulses jump between Nodes of Ranvier. The text further categorizes nerve fibers (Groups A, B, and C) based on diameter and conduction velocity, noting their differential sensitivities to hypoxia, pressure, and local anesthetics, which function by blocking sodium channels. Essential cellular transport mechanisms are outlined, specifically orthograde and retrograde axonal transport facilitated by the molecular motors kinesin and dynein. The vital role of neuroglia is also examined, differentiating between the scavenging microglia and macroglia, which include astrocytes involved in the blood-brain barrier, as well as myelin-producing oligodendrocytes in the CNS and Schwann cells in the PNS. Finally, the chapter addresses the clinical implications of electrolyte imbalances like hyperkalemia and hypokalemia on neuronal excitability, the pathophysiology of demyelinating diseases such as Multiple Sclerosis and Guillain-Barre syndrome, and the importance of neurotrophins like Nerve Growth Factor (NGF) and BDNF in neuronal survival and regeneration via Trk receptors.