Somatosensory pathways

The cerebral hemisphere and diencephalon have a more complex organization than that of the brainstem and spinal cord (Martin, 1989). The thalamus relays information from subcortical structures to the cerebral cortex via two different functional classes of nuclei namely, those that are for relay and those that are for diffuse projection. Three of the four anatomical divisions of the thalamus serve relay functions (anterior, medial, and lateral nuclei) and one is a diffuse projection nuclei (intralaminar). Thalamic neurons send the axons to the cerebral cortex via the internal capsule, as do cortical neurons that project to subcortical sites. There are two major somatosensory pathways the dorsal column of the medial lemniscal system, which mediates tactile, and vibration, and the anterolateral system, which mediates pain and temperature sense. 

Neurophysiological evaluation of 40 patients with beta-thalassemia major showed abnormal findings in brain-stem-evoked potentials auditory (25%), visual (15%), and somatosensory (7.5%) some had abnormal nerve conduction velocity (25%) and 15% had involvement of multiple neural pathways (39). Subclinical involvement of the auditory pathway was statistically associated with a higher mean daily dose of deferoxamine and a longer duration of treatment. Abnormalities of the somatosensory pathways were related to old age, a long duration of deferoxamine use, and low serum copper concentrations. Multiple neural pathway involvement was related to the duration of treatment. However, deferoxamine is only partly responsible for the subclinical abnormalities of neural pathways often found in patients with beta-thalassemia major. 

Neurons at the origin of several ascending somatosensory pathways have been examined for the presence of glutamatergic inputs. Westlund et al. (1992) investigated inputs to three intracellularly labeled spinothalamic tract neurons in the deep dorsal horn. Of the... 

The spinocervical tract terminates before reaching the brainstem and will therefore be included in this section. Other somatosensory pathways will be included in the sections below. 

Broman J (1994) Neurotransmitters in subcortical somatosensory pathways. Anat Embryol (Berl) 77(9 181-214. 

Examples of fiiis type include all the different evoked responses (auditory, somatosensory, visual, etc.) and event-related potentials recorded in response to controlled stimuli administered to the body (or any biological system in general). These signals usually reveal functional characteristics of specific pathways in the body. For instance, evoked resptmses to peripheral somatosensory stimu-laticHi reveal the performance of the somatosensory pathway leading to sensory cortex. A segment of cmtical somatosensory evoked potential is shown in Fig. 18.1fi that was obtained after averaging 1(X) stimulus-response pairs. Evoked responses or event-related potentials are usually superimposed... 

These ascending sensory pathways cross from one side of the CNS to the other so that sensory input from the left side of the body is transmitted to the somatosensory cortex of the right cerebral hemisphere and visa versa. Therefore, damage to this region of cortex in a given hemisphere results in... 

Figure 8.1 The pain pathway. The pain signal is transmitted to several regions of the brain, including the thalamus reticular formation hypothalamus limbic system and somatosensory cortex. Each region carries out a specific aspect of the response to pain.

Due to its relevance to an understanding of movement disorders, the motor circuit has received the most attention. This circuit is centered on somatosensory, motor and premotor cortices, which send projections to the motor portions of striatum. The connections between the striatum and the basal ganglia output nuclei (GPi/SNr) are organized into direct and indirect pathways [1]. The direct pathway is a monosynaptic projection between striatum and GPi/ SNr, while the indirect pathway is a polysynaptic connection that involves intercalated neurons in GPe and STN. Some striatofugal neurons may also collateralize more extensively, reaching GPe, GPi/SNr and STN. Other motor -related inputs to striatum and STN arise from the intralaminar thalamic nuclei, i.e. the centromedian and parafascicular nuclei (CM/Pf). 

Evoked potentials with latencies below 100 ms are used clinically for functional testing of the optic, auditory and various somatosensory nerve pathways they are not influenced by factors such as motivation and tiredness, including drug-induced changes of alertness, and are considered to be uninteresting for psychological questions. 

Putative sites of action of opioid analgesics (darker color). On the left, sites of action on the pain transmission pathway from the periphery to the higher centers are shown. A Direct action of opioids on inflamed peripheral tissues. B Inhibition occurs in the spinal cord. C Possible site of action in the thalamus. Different thalamic regions project to somatosensory (SS) or limbic (L) cortex. Parabrachial nuclei (medulla/pons) projects to the amygdala. On the right, actions of opioids on pain-modulating neurons in the midbrain (D) and medulla (E) indirectly control pain transmission pathways. 

In the normal state, the putamen receives afferents from the motor and somatosensory cortical areas and communicates with the GPi/SNr through a direct inhibitory pathway and though a multisynaptic (GPe, STN) indirect pathway. In PD, dopamine deficiency leads to increased inhibitory activity from the putamen onto the GPe and disinhibition of the STN. STN hyperactivity by virtue of its gluta-matergic action produces excessive excitation of the GPi/SNr neurons, which overinhibit the thalamocortical and brain stem motor centers. 

Recent B(a)P studies have established reductions in learning and memory correlates both in rodent and humans (Grova et al, 2007 Wormley et al, 2004 Widhohn et al, 2003 Gilbert et al, 2000 Hack et al, 1991 Perrera et al, 2003, 2006 Landrigan et al, 2004). We decided to use the rat cortex as a model of primary somatosensory (SI) cortex (Figure 17.6). Because of the unique organization of the rodent whisker to cortex pathway, and the aheady... 

Fig. 7. Schematic drawing of a transverse section through the forebrain depicting pathways likely to use glutamate as a neurotransmitter. I = principal subcortical afferents to the thalamus from. somatosensory relay nuclei and the spinal cord (a), cerebellar nuclei (h). and retina (c) 2 = intrinsic neurons and retinal inputs to the hypothalamus 3 = thalamocortical inputs 4 = corticothalamic inputs 5 = cortical inputs to the basal ganglia and other areas in the brainstem and spinal cord 6 = associational and commi.ssural connections in the cerebral cortex. For further details, see Sections 3.5-3.9.

Ascending pathways to thalamus limbic system and somatosensory cortex... 

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