Chapter 7: Sensory Physiology

Two synapse types enable different communication patterns: electrical synapses use gap junctions to allow direct ionic flow between neurons, enabling rapid synchronized firing, while chemical synapses employ neurotransmitters to cross the synaptic cleft, providing greater signaling complexity and modulatory control. Chemical synaptic transmission occurs through a precise sequence where presynaptic depolarization opens voltage-gated calcium channels, causing synaptic vesicles to fuse with the membrane and release neurotransmitters that bind postsynaptic receptors, generating excitatory or inhibitory graded potentials. Neurotransmitter inactivation through enzymatic breakdown, membrane reuptake, or diffusion terminates signals with precise timing. The chapter surveys diverse chemical messengers including acetylcholine, biogenic amines such as dopamine and serotonin, amino acid transmitters like glutamate and GABA, neuropeptides, and gaseous molecules like nitric oxide, each with distinct physiological roles. Postsynaptic neurons integrate multiple inputs through spatial and temporal summation of excitatory and inhibitory potentials, determining whether threshold for action potential generation is reached. Presynaptic mechanisms including facilitation and inhibition modulate transmitter release strength, while synaptic plasticity mechanisms such as long-term potentiation and long-term depression alter connection strength to enable learning and memory formation. Neuronal circuits organize into reflex arcs and networks where synaptic divergence distributes signals widely and convergence allows integrative processing. Neuromodulatory systems employing dopamine and serotonin regulate behavior and motivation throughout the brain. Clinical examples demonstrate disease consequences: botulism toxins block acetylcholine release, myasthenia gravis destroys acetylcholine receptors through autoimmune attack, and Parkinson's disease results from dopamine neuron loss, illustrating how synaptic disruption compromises nervous system function and behavior.