Synaptic knobs

Active reuptake of the neurotransmitter into the synaptic knob of the presynaptic neuron for reuse or enzymatic destruction... 

Synapses between the autonomic postganglionic neuron and effector tissue — the neuroeffector junction — differ greatly from the neuron-to-neuron synapses discussed previously in Chapter 5 (see Table 9.1). The postganglionic fibers in the ANS do not terminate in a single swelling like the synaptic knob, nor do they synapse directly with the cells of a tissue. Instead, the axon terminals branch and contain multiple swellings called varicosities that lie across the surface of the tissue. When the neuron is stimulated, these varicosities release neurotransmitter over a large surface area of the effector tissue. This diffuse release of the neurotransmitter affects many tissue cells simultaneously. Furthermore, cardiac muscle and most smooth muscle have gap junctions between cells. These specialized intercellular communications... 

The ends of the fine nerve fibers are thickened to form the synaptic knobs, which make synaptic contacts with dendrites on cell bodies of other neurons. 

The synaptic knob is filled with memhrane-hounded vesicles containing a neurotransmitter. 

Figure 1. Electron photomicrograph a synapse (56,000X) illustrating the synaptic knob (S) as it ends on the shaft of a dendrite (D) in the central nervous system. P, postsynaptic density M, mitochondrion. (From Review of Medical Physiology, 22nd Edition, The McGraw-Hill Co., 2005)...

At the axon terminus, neurotransmitters are released into the synaptic gap. Through aptic gaps, a typical neuron may intercormect with thousands and tens of thousands of other neurons. Axon terminals have knob-like swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a apse. Under very high magnification, a very tiny space, the aptic clefl or gap (about one millionth of an irKh, or mm), can be delected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). 

When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers, anesthetics, nicotine, and caffeine target the synapse and can cause an alteration of impulse transmission. 

The axon ari.ses from a thickened area of the cell body called the axon hillock. Its membrane is mainly composed of lipids and proteins and is known as the axitlentnttt. Many of the axons of the CNS and PNS ate partly covered from near the axon hillock to the. synaptic knob by a sheath of myelin (myelinated axons), but some axons do not have this type of covering (unmyelinated axons). The myelin sheath of PNS myelinated axons is not continuous but is broken at about I-mm intervals to expose the axulenima to the extracellular fluid. These exposed areas, which are about I /[Pg.679]

A nerve consi.sts of myelinated or unmyelinated nerve fibers (Fig. 20-5a). These nerve fibers consist of chains" of neurons. The junction between adjacent neurons in the chain consists of the synaptic knob of the transmitting neuron separated by a gap of about 30 to 50 nm from either the dendrite, axon hilhx k. or cell body of the other neuron. This... 

Figure 6.1 A diagrammatic representation of the structure of the neuron showing the cell body with dendrites, nucleus, mitochondria, and the axon terminating in synaptic knobs with mitochondria and vesicles. 

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