Porth's Essentials of Pathophysiology, 4e

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Somatosensory Function, Pain, and Headache

C h a p t e r 3 5

Somatosensory and other areas of the cerebral cortex

Primary somatosensory cortex

Thalamic neuron 3

Thalamus

Medial lemniscus

Reticular formation

Medulla

2

1

Dorsal root ganglion

Dorsal column neuron

Paleospinothalamic tract

Neospinothalamic tract

Dorsal root ganglion

A

B

Spinal cord

Spinal cord

FIGURE 35-4. (A) Rapid-transmitting discriminative (dorsal column–medial lemniscal) pathway carrying axons mediating tactile sensation and proprioception. (B) Neospinothalamic and paleospinothalamic subdivisions of the anterolateral sensory pathway.The neurons of anterolateral pathways cross within the same segment as the cell body and ascend in the contralateral side of the spinal cord.The neospinothalamic tract travels mainly to thalamic nuclei that have third-order fibers projecting to the somatosensory cortex.The paleospinothalamic tract sends collaterals to the reticular formation and other structures, from which further fibers project to the thalamus.

pathway is functional but the association area has been damaged, the person can correctly describe the object but does not recognize it as a screwdriver. This deficit is called astereognosis. Anterolateral Pathway. The anterolateral pathway (anterior and lateral spinothalamic tracts) consists of bilateral, multisynaptic, slow-conducting tracts (see Chapter 34, Fig. 34-11). These tracts provide for trans- mission of sensory information such as pain, thermal sensations, crude touch, and pressure that does not require discrete localization of signal source or fine dis- crimination of intensity. The fibers of the anterolateral pathway originate in the dorsal horns at the level of the segmental nerve, where the dorsal root ganglion neu- rons enter the spinal cord. These spinal neurons cross in the anterior commissure, within a few segments of origin, to the anterolateral column on the opposite side of the cord, and then ascend upward toward the brain. The anterolateral tract fibers synapse with sev- eral nuclei in the thalamus, but en route they give off numerous branches that travel to the reticular activat- ing system of the brain stem. These projections provide the basis for increased wakefulness or awareness after

strong somatosensory stimulation and for the general- ized startle reaction that occurs with sudden and intense stimuli. They also stimulate autonomic nervous system responses, such as a rise in heart rate and blood pres- sure, dilation of the pupils, and the pale, moist skin that results from constriction of the cutaneous blood vessels and activation of the sweat glands. There are two subdivisions in the anterolateral path- way: the neospinothalamic and the paleospinothalamic tracts are important in pain perception 1 (see Fig. 35-4B). The neospinothalamic tract consists of a sequence of at least three neurons with long axons. It provides for relatively rapid transmission of sensory information to the thalamus. The paleospinothalamic tract , which is phylogenically older, consists of bilateral, multisynaptic, slow-conducting tracts that transmit sensory signals that do not require discrete localization or discrimination of fine gradations in intensity. This slower-conducting pathway also projects into the intralaminar nuclei of the thalamus, which have close connections with the limbic cortical systems. This circuitry gives touch its affective or emotional aspects, such as the particular unpleasant- ness of heavy pressure and the peculiar pleasantness of the tickling and gentle rubbing of the skin.