Third-order neuron

As discussed, the first-order neuron is the afferent neuron that transmits impulses from a peripheral receptor toward the CNS. Its cell body is located in the dorsal root ganglion. This neuron synapses with the second-order neuron whose cell body is located in the dorsal horn of the spinal cord or in the medulla of the brainstem. The second-order neuron travels upward and synapses with the third-order neuron, whose cell body is located in the thalamus. Limited processing of sensory information takes place in the thalamus. Finally, the third-order neuron travels upward and terminates in the somatosensory cortex where more complex, cortical processing begins. 

The cell bodies of third-order sensory neurons are located in the thalamus. These neurons transmit the pain signal to the somatosensory cortex. The function of this region of the brain is to localize and perceive the intensity of the painful stimulus. Further transmission of the signal to the association areas of the cerebral cortex is important for the perception and meaningfulness of the painful stimulus. 

Figure 8-1 The oculosympathetic pathway. Note its origin in the hypothalamus and its course through the brainstem and cervical spinal cord (central or first-order neuron), the upper thorax and lower neck (pregangUonic or second-order neuron), and upper neck, middle cranial fossa, cavernous sinus, and orbit as it finally reaches Muller s muscle of the lid and the iris dilator muscle (postganglionic or third-order neuron), (a. = artery n. = nerve.) (Reprinted with permission from Glaser JS.The pupils and accommodation. In Duane TD, Jaeger EA, eds. Clinical ophthalmology. Hagerstown, MD Harper Row, 1987.)...

A number of putative third-order neurons have been identified in the lateral hom (Jefferis et al. 2007 Tanaka et al. 2004) however, their roles in mediating odour-driven behaviours are hard to predict, because their dendritic fields overlap with many classes of projection neurons and because they target different brain areas. Establishing a complete neuronal circuit diagram of such behavioural programmes will require the identification of the as yet unknown downstream premotor and motor neurons as well as of biologically meaningful behavioural modules . 

Substance P, calcitonin-gene-related peptide (CGRP), and neurokinin A have been identified by immunocytochemistry in primary sensory neurones and in cutaneous sensory nerve terminals and these are often in close proximity to mast cells [54, 56], For example, in Lewis rats that were infected with the parasitic larvae of Nippostrongylis brasiliensis in order to induce a proliferation of mucosal mast cells, nearly two-thirds of the lamina propria mast cells were in intimate contact with peptidergic nerves containing SP and CGRP, while an additional 20% of the mast cells were within 2 gm [54], All three peptides are... 

A primary reason for the above disparities is the critically important structural organization of the chromophores when found in-vivo. These relationships make a major (several orders of magnitude) difference in the absorbance of the material and also lead to anisotropic absorption. These relationships have not been maintained by the chemists. A second reason relates particularly to the L-channel. The chromophore of that channel exhibits a more intimate relationship with the electronic portion of the photoreceptor neuron than do the S- and M-channel chromophores. As a result, the L-channel exhibits an effective absorption characteristic very different from that observed by the chemist. This characteristic also accounts for the loss of red response in the mesopic and scotopic regions. These relationships have not been emulated in the environment of the chemist. Failure to emulate these conditions leads to extraneous absorption spectra for the L-channel chromophore. A third reason is due to the frequent chemical reactions occurring in the chemists solutions that he may not be aware of. It has been rare in the past for the chemist to document the pH of the solutions he has measured. This is a particular problem as mentioned in a later section [Section 5.5.12], The chromophores of vision are members of the "indicator class of chemicals. Their spectral characteristics are intimately related to the pH of their environment. They are also complex organics. Their spectral characteristics are a function of the organic solvent used. They are also subject to chemical attack. This mechanism has been documented by Wald, et. al. and more recently by Ma, et. al. 

Figure 6 Kohonen ANN represented as a block. Neurons are drawn as columns. The weights (circles) are ordered in levels. The weights in the weight level handling the third input variable xj are shaded. The top-map is a table or matrix of entries where labels of objects which excite the corresponding neurons are stored...

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