After studying this chapter, you should be able to:
Describe the major location and functional components of a neuron-to-neuron synapse.
Contrast the ionic fluxes responsible for the fast and slow excitatory and inhibitory postsynaptic potentials.
Compare and contrast the terms temporal summation and spatial summation and their role in action potential generation in a postsynaptic neuron.
Explain postsynaptic inhibition, presynaptic inhibition, and presynaptic facilitation.
Identify the components of the neuromuscular junction and the sequence of events that leads to a propagated action potential in the skeletal muscle fiber.
Explain how autonomic neurons communicate with their effector organs at a neuroeffector junction.
Define denervation hypersensitivity.
Describe some pathologies associated with dysfunction at the neuromuscular junction.
The “all-or-none” type of conduction seen in axons and skeletal muscle has been discussed in Chapters 4 and 5. Impulses are transmitted from one neuron to another at a synapse. This is the region where the axon or some other portion of one neuron (presynaptic neuron) terminates on the dendrites, soma, or axon of another neuron (postsynaptic neuron). Neuron-to-neuron communication occurs across either a chemical or an electrical synapse. Because most synaptic transmission is chemical, consideration in this chapter is primarily related to chemical neurotransmission.
When a neuron terminates on a muscle, the connection is properly called a neuromuscular junction rather than a synapse. Transmission from a nerve to a muscle resembles chemical synaptic transmission from one neuron to another. The contacts between autonomic neurons and smooth and cardiac muscle or glands are less specialized than those between a neuron and skeletal muscle, and transmission in these locations is a more diffuse process. The region where the neuron communicates with the effector organ is called the neuroeffector junction. These forms of transmission are also considered in this chapter.
SYNAPTIC TRANSMISSION: FUNCTIONAL ANATOMY
The anatomic structure of synapses varies considerably in the different parts of the mammalian nervous system. The ends of the presynaptic fibers are generally enlarged to form terminal boutons or synaptic knobs (Figure 6–1). In the cerebral and cerebellar cortex, endings are commonly located on dendrites (axodendritic synapse) and frequently on dendritic spines, which are small knobs projecting from dendrites (Figure 6–2). Some presynaptic nerves terminate on the soma (axosomatic synapse) or axons (axoaxonal synapses) of postsynaptic neurons. On average, each neuron divides to form over 2000 synaptic endings; thus, communication between neurons is very complex. Synapses are dynamic structures, increasing and decreasing in complexity and number with use and experience.
Electron micrograph of synaptic knob (S) ending on the shaft of a dendrite (D) in the central nervous system. P, postsynaptic density; M, mitochondrion (×56,000).
Axodendritic, axoaxonal, and axosomatic synapses. Many presynaptic neurons ...