Chapter 37: Neurons, Synapses, and Signaling

Neurons are specialized cells with distinct anatomical regions: dendrites receive incoming signals, cell bodies process information, and axons transmit signals to other cells via synaptic terminals where neurotransmitters are released. Glial cells provide essential support by supplying nutrients, insulating axons with myelin, and maintaining the extracellular environment. The nervous system processes information through three integrated stages involving sensory neurons detecting stimuli, interneurons relaying signals within the central nervous system, and motor neurons triggering responses in muscles and glands. Neuronal signaling depends critically on ion concentration gradients established by sodium-potassium pumps and maintained by selective membrane permeability. The resting membrane potential of approximately minus 70 millivolts reflects the combined electrochemical driving forces on potassium and sodium ions, mathematically described by the Nernst equation. When stimuli depolarize the membrane toward threshold voltage, voltage-gated sodium channels open rapidly, initiating action potentials that follow an all-or-none principle characterized by distinct phases including rapid depolarization, repolarization, and an undershoot phase followed by a refractory period during which new action potentials cannot be generated. Action potentials propagate along axons through sequential opening of voltage-gated channels in adjacent membrane regions, with propagation speed enhanced by increased axon diameter or insulating myelin sheaths that enable saltatory conduction at unmyelinated gaps called nodes of Ranvier. At chemical synapses, arriving action potentials trigger calcium ion influx that causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. Neurotransmitters bind postsynaptic receptors, producing either excitatory postsynaptic potentials that depolarize the membrane or inhibitory postsynaptic potentials that hyperpolarize it. Temporal and spatial summation of these potentials at the axon hillock determines whether threshold is reached and new action potentials are generated. Neurotransmitters function through ionotropic receptors that directly gate ion channels or metabotropic receptors coupled to intracellular signaling cascades producing slower, longer-lasting effects. Major neurotransmitter classes include acetylcholine with varied effects depending on receptor type and location, amino acids such as glutamate and GABA serving excitatory and inhibitory roles respectively, biogenic amines including dopamine and serotonin regulating mood and cognition, neuropeptides modulating pain and reward, and gaseous messengers like nitric oxide affecting smooth muscle function. Disruptions in neuronal signaling underlie neurological and psychiatric conditions including epilepsy from channel mutations, Parkinson's disease involving dopamine depletion, depression from serotonin imbalances, and addiction affecting reward pathways. Pharmacological agents and toxins exploit neurotransmitter systems, demonstrating the clinical importance of understanding synaptic mechanisms.