After studying this chapter, you should be able to:
Describe the main morphologic features of synapses.
Describe fast and slow excitatory and inhibitory postsynaptic potentials, outline the ionic fluxes that underlie them, and explain how the potentials interact to generate action potentials.
Define and give examples of direct inhibition, indirect inhibition, presynaptic inhibition, and postsynaptic inhibition.
Describe the neuromuscular junction, and explain how action potentials in the motor neuron at the junction lead to contraction of the skeletal muscle.
Define denervation hypersensitivity.
Impulses are transmitted over chemical or electrical synapses linking one neuron (presynaptic cell) with another neuron, muscle, or gland (postsynaptic cell). At chemical synapses, an impulse in the presynaptic axon causes secretion of a chemical that diffuses across the 30-nm-wide (approximately) synaptic cleft and binds to receptors on the surface of the postsynaptic cell. This triggers events that open or close channels in the membrane of the postsynaptic cell, mediating excitation or inhibition. At electrical synapses, the membranes of the presynaptic and postsynaptic neurons are close together, and gap junctions form low-resistance bridges through which ions pass with relative ease from one neuron to the next. Because most synaptic transmission is chemical, this chapter discusses chemical transmission.
STRUCTURE & FUNCTION OF SYNAPSES
A presynaptic axonal terminal can synapse on a cell body, dendrite, dendritic spine, or an axon to form axosomatic, axodendritic, and axoaxonic synapses, respectively (Figure 6–1). The thickening on the postsynaptic membrane is called a postsynaptic density. There are three kinds of membrane-enclosed vesicles in the presynaptic nerve terminal: small, clear synaptic vesicles that contain acetylcholine, glycine, GABA, or glutamate; small vesicles with a dense core that contain catecholamines; and large vesicles with a dense core that contain neuropeptides. The vesicles and the chemicals they contain are synthesized in the neuronal cell body and transported along the axon to nerve terminals by fast axoplasmic transport. The large dense-core vesicles release their neuropeptide contents by exocytosis from all parts of the terminal. The small vesicles are located near the synaptic cleft and fuse to the membrane, discharging their contents very rapidly into the cleft at areas of membrane thickening called active zones (Figure 6–1).
Axodendritic, axoaxonal, and axosomatic synapses. Many presynaptic neurons terminate on dendritic spines, as shown at the top, but some also end directly on the shafts of dendrites. Note the presence of clear and granulated synaptic vesicles in endings and clustering of clear vesicles at active zones.
In response to an action potential in the nerve terminal, Ca2+ enters and the transmitter is released close to the postsynaptic receptors. A response can occur in the postsynaptic neuron after a synaptic delay. The delay is due to the time it takes for the neurotransmitter to ...