Chapter 8: Neurons: Cellular and Network Properties

A significant portion of the chapter details the structural and functional classification of neurons—multipolar, pseudounipolar, bipolar, and anaxonic—and the critical role of axonal transport in moving organelles and proteins. It places equal emphasis on the essential support provided by glial cells, distinguishing between those in the PNS (Schwann cells and satellite cells) and the CNS (oligodendrocytes, astrocytes, microglia, and ependymal cells), specifically noting how myelin sheaths formed by Schwann cells and oligodendrocytes insulate axons to accelerate signal transmission. The biophysics of electrical signaling is explored through the concepts of resting membrane potential, determined by electrochemical gradients and selective membrane permeability to ions like potassium, sodium, and chloride. The chapter explains how the Nernst equation calculates the equilibrium potential for a single ion, while the Goldman-Hodgkin-Katz (GHK) equation accounts for the contributions of multiple ions to the membrane potential. Neural communication is categorized into graded potentials, which are variable-strength signals used for short-distance integration, and action potentials, which are all-or-none phenomena capable of traveling long distances without diminishing. The text provides a step-by-step mechanism of the action potential, describing how voltage-gated sodium channels initiate depolarization followed by potassium channel-mediated repolarization and hyperpolarization, all regulated by absolute and relative refractory periods. Factors influencing conduction velocity, such as axon diameter and membrane resistance, are discussed alongside the mechanism of saltatory conduction at the nodes of Ranvier, with clinical references to demyelinating diseases like Guillain-Barre syndrome and multiple sclerosis. Synaptic transmission is analyzed, contrasting rapid electrical synapses via gap junctions with chemical synapses that rely on calcium-dependent exocytosis of neurocrine molecules. The chapter catalogs major neurotransmitters—acetylcholine, amines (norepinephrine, dopamine, serotonin), amino acids (glutamate, GABA, glycine), peptides, purines, and gases—and their interaction with ionotropic (fast) and metabotropic (G protein-coupled) receptors. Finally, the text addresses neural integration and synaptic plasticity, explaining concepts such as divergence, convergence, spatial and temporal summation, presynaptic inhibition, and the mechanisms of long-term potentiation (LTP) and depression (LTD) involving NMDA and AMPA receptors, which are foundational to learning and memory.