Porth's Essentials of Pathophysiology, 4e

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Nervous System

U N I T 1 0

C2

C3

C2 C3

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C4

T2 T3 T4 T5 T6 T7 T8 T9

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T2 T3 T4 T5 T6 T7 T8 T9

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T10 T11 T12

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L1 L2 L3

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T10 T11 T12 L1

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C7 8

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Coccyx

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FIGURE 35-2. Cutaneous distribution of spinal nerves (dermatomes). (From Barr M. The Human Nervous System. NewYork, NY: Harper & Row; 1993.)

S1

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L5

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S1

two-pathway system has several advantages. It adds richness to the sensory input by allowing sensory infor- mation to be handled in two different ways, and it ensures that if one pathway is damaged, the other still can provide input. Discriminative Pathway. The discriminative pathway is used for the rapid transmission of sensory informa- tion, such as discriminative touch. It uses only three types of neurons to transmit information from a sensory receptor to the somatosensory cortex on the opposite side of the brain: (1) the dorsal root ganglion neurons,

which project their central axons to the dorsal horn in the spinal cord; (2) the dorsal column nuclei, which send their axons upward and across the midline of the medulla where they form a band of white matter called the medial lemniscus that passes upward to the thala- mus; and (3) the thalamic neurons, which project to the primary sensory cortex (Fig. 35-4A). As the fibers in the medial lemniscus ascend through the brain stem, they are joined by sensory fibers from the trigeminal nerve that supplies the head and face. Sensory information arriving at the sensory cortex by this route can be dis- cretely localized and discriminated in terms of intensity. One of the distinct features of the discriminative path- way is that it relays precise information regarding spatial orientation. This is the only pathway taken by the sensa- tions of muscle and joint movement, vibration, and deli- cate discriminative touch, which is required to correctly differentiate the location of touch on the skin at two neighboring points (i.e., two-point discrimination). One of the important functions of the discriminative pathway is to integrate the input from multiple receptors. The sense of shape and size of an object in the absence of visu- alization, called stereognosis, is based on precise afferent information from muscle, tendon, and joint receptors. For example, a screwdriver is perceived as being differ- ent from a knife in terms of its texture (tactile sensibility) and shape based on the relative position of the fingers as they move over the object. This complex interpretive perception requires that both the discriminative pathway and the somatosensory association area of the cerebral cortex are functioning properly. If the discriminative

Central processes

Dorsal root ganglia

Peripheral processes

Dermatomes

FIGURE 35-3. The dermatomes formed by the peripheral processes of adjacent spinal nerves overlap on the body surface.The central processes of these fibers also overlap in their spinal distribution.