Feedback pathways

The excitatory transmitter released from these cells is, in most instances, glutamate. Local circuit neurons are typically smaller than projection neurons, and their axons arborize in the immediate vicinity of the cell body. Most of these neurons are inhibitory, and they release either GABA or glycine. They synapse primarily on the cell body of the projection neurons but can also synapse on the dendrites of projection neurons as well as with each other. Two common types of pathways for these neurons (Figure 21-6A) include recurrent feedback pathways and feed-forward pathways. A special class of local circuit neurons in the spinal cord forms axoaxonic synapses on the terminals of sensory axons (Figure 21-6B). In some sensory pathways such as the retina and olfactory bulb, local circuit neurons may actually lack an axon and release neurotransmitter from dendritic synapses in a graded fashion in the absence of action potentials. 

Kamermans M, Spekreijse H (1999) The feedback pathway from horizontal cells to cones. A mini review with a look ahead. Vision Res 39 2449-2468. 

One such phenomenon is endotoxin tolerance. It is known that LPS stimulation is associated with a prompt response (NF-/cB translocation to the nucleus and cytokine production) followed by a refractory state, wherein a second challenge is far less effective at provoking such a response (97,98). Cross-tolerance has been observed when a primary stimulus with lipopeptides is used in place of LPS (99). Although some have attributed tolerance to the production of antiinflammatory cytokines such as TGF/3 and/or IL-10 (100), it is more widely held that tolerance reflects the activation of a feedback pathway within cells, causing paralysis of the LPS response. One example of tolerance at the cellular level involves the production of NF-kB p50 homodimers, which can bind to diverse promoters within the cell and prevent activation by p50/p65 heterodimers (101). Other levels of blockade are also possible and are currently under investigation. 

Figure 50-12 Key elements in water homeostasis. So/id //nes indicate osmotically stimulated pathways, and dashed lines Indicate volume-stimulated pathways.The dotted lines indicate negative feedback pathways. Abbreviations ANfl atrial natriuretic peptide AVP, arginine vasopressin CNS, central nervous system C/- extracellular fluid OPR, oropharyngeal reflex. (From Reeves W,AndreoliT.The posterior pituitary and water metabolism. /n Wj7son JD, Foster DW, eds. Williams textbook of endocrinology, 8tb ed. Philadelpbia WB Sounders Co, 1992 312.)...

Figure 14.2 Principles of signal transduction. An environmental signal is first received by interaction with a cellular component, most often a cell-surface receptor. The information that the signal has arrived is then converted into other chemical forms, or transduced. The transduction process often comprises many steps. The signal is often amplified before evoking a response. Feedback pathways regulate the entire signaling process.

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