Projection tracts

Distinguish among the three types of tracts in the CNS projection tracts, association tracts, and commissural tracts... 

Following the development of the motor program, neurons originating in the multimodal motor association areas transmit impulses by way of association tracts to neurons of the primary motor cortex. The primary motor cortex is located in the precentral gyrus, which is the most posterior region of the frontal lobe adjacent to the multimodal motor association areas (see Figure 6.3) this area initiates voluntary contractions of specific skeletal muscles. Neurons whose cell bodies reside here transmit impulses by way of descending projection tracts to the spinal cord, where they innervate the alpha motor neurons (which innervate skeletal muscles). 

Detailed Development Plan, Vol. 1 and 2, Oil Shale Lease Tract C-b, C-b Shale Oil Project, 1976. 

The nigrostriatal tract is one of the four main dopaminergic pathways in the central nervous system. About 75% of the dopamine in the brain occurs in the nigrostriatal pathway with its cell bodies in the substantia nigra, whose axons project in the corpus striatum. Degeneration of the dopaminergic neurons in the nigrostriatal system results in Parkinsons disease. 

The termination pattern exhibited by A -fibres is entirely different from that of large AjS-fibres. A -fibres travel extensively in Lissauer s tract, overlying the dorsal horn and their terminals form a plexus at the surface of the spinal cord A(5-fibres from high-threshold mechanoreceptors distributed to laminae I, II outer and V. Projections also appear to terminate on the contralateral side, in lamina V. A(5-fibre innervations from deep tissues (muscles and joint) have been shown to terminate exclusively in lamina I, or in laminae IV and V. 

Figure 22.3 Possible links in the induction of circadian rhythm between daylight, the suprachiasmatic nucleus and melatonin release from the pineal gland. Some fibres in the optic nerve, projecting from the eye to the lateral geniculate nucleus (LGN) in the thalamus, innervate the suprachiasmatic nucleus (SCN) in the anterior hypothalamus, via the retinohypothalamic tract (RHT). Others project to the SCN from the LGN in the geniculohypothalamic tract (GHT). The release of melatonin into the circulation from the pineal gland (PG) is maximal at night and appears to be controlled partly by noradrenaline released from sympathetic nerves originating in the superior cervical ganglion (SCG). Melatonin receptors are found in the SCN, the removal of which dampens melatonin secretion...

Central/Tertiary structures The fish olfactory bulb is a fourlayered structure much as in higher vertebrates. Within the 2nd layer, the first synapse for olfactory input is on the dendrites of the mitral cells (MC). About 1000 ORN axons converge on one MC, a ratio similar to mammals. The MC output, from cells at various levels, leads into several glomeruli and receives (inhibitory) input from granule cells. The latter also innervate a distinct cell type in the MC layer of teleosts — the ruffed cells (RC), with which they have reciprocal synapses [Fig. 2.18(a)] both relay cells send ascending fibres to forebrain centres (Kosaka and Hama, 1982). The RC are unlike the MC since they are not stimulated by the ORNs directly. Their interactions (Chap. 5) may contribute to the processing of pheromonal stimuli (Zippel, 2000). The main bulbar pathways project to several nuclei in the forebrain via two ipsilateral tracts, the lateral and medial [Fig. 2.18(b)], the latter mediates sexual behaviour and the former probably other behaviours (Hara,... 

Fig. 2.20 Efferent pathways into bulb showing (a) cholinergic (ACh) fibres projecting to MOB from basal forebrain nuclei. AON = ant. olfactory nucleus, OT = olfactory tract, DB = diagonal band nuc. (from Davis et al., 1978). (b) Nor-Adrenalin input to AOB, via MFB pathway from brain stem centres (nuclei A1-2, A6) (from Keveme, 1971).

The majority of DDC-expressing cells in the brain lobes are dopamine cells. Most of these dopamine cells have axons that project into a common axonal fiber extending anteriomedially within the brain lobe and then separating into finer fibers that cross between the lobes. The dopamine cells occur in small clusters of two to six cells, which suggests that these cells might share common lineages. The serotonin cells within the lobes are also found in pairs, and each pair projects axons into closely associated tracts. The pathways of the serotonin tracts often parallel those of the dopamine cells but are distinct (Lundell and Hirsh, 1994). 

Stimulation of a nociceptor in the periphery of the body elicits action potentials in the first-order neuron, which transmits the signal to the second-order neuron in the dorsal horn of the spinal cord. From the spinal cord, the signal is transmitted to several regions of the brain. The most prominent ascending nociceptive pathway is the spinothalamic tract. Axons of the second-order sensory neurons project to the contralateral (opposite) side of the spinal cord and ascend in the white matter, terminating in the thalamus (see Figure 8.1). The thalamus contributes to the basic sensation or awareness of pain only it cannot determine the source of the painful stimulus. 

A major side-effect of morphine is respiratory depression. Opiates are believed to cause this effect via actions in brainstem nuclei, fi receptor immunoreactivity and mRNA were detected in neurons of the nucleus of the solitary tract, nucleus ambiguous, and parabrachial nucleus. mRNA was detected in the bed nucleus of the stria terminalis which projects to the nucleus of the solitary tract, fi receptor immunoreactivity is found in the nucleus of the solitary tract and dorsal rhizotomy reduced receptor immunoreactivity in the nucleus suggesting a presynaptic localization of the receptor. 

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