TWO SYSTEMS: A BRIEF ANATOMICAL REVIEW
A proper sensory examination requires knowledge of the skin areas supplied by individual dorsal roots (dermatomes) and peripheral nerves (Figure 5–1) as well as awareness of the ascending sensory pathways of the spinal cord, brain stem, and forebrain (Figures 5–2 and 5–3).
The distribution of dermatomes in the spinal cord and brain stem. A dermatome is the area of skin and deeper tissues innervated by a single dorsal root or branch of the trigeminal nerve. The dermatomes of the 31 pairs of dorsal root nerves are projected onto the surface of the body and labeled by the foramen through which each nerve enters the spinal cord. The 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and single coccygeal roots are numbered rostrocaudally for each division of the vertebral column. The facial skin, cornea, scalp, dura, and intraoral regions are innervated by the ophthalmic (I), maxillary (II), and mandibular (III) divisions of the trigeminal nerve (cranial nerve V). Level C1 has no dorsal root, only a ventral (or motor) root. Dermatome maps provide an important diagnostic tool for localizing the site of injury to the spinal cord and dorsal roots. However, the boundaries of the dermatomes are less distinct than shown here because the axons comprising a dorsal root originate from several different peripheral nerves, and each peripheral nerve contributes fibers to several adjacent dorsal roots. (Reproduced with permission from Kandel ER, Koester JD, Mack SH, Siegelbaum SA. 2021. Principles of Neural Science, 6th ed. New York: McGraw Hill.)
Diagram of the major ascending somatic sensory systems. The dorsal column–medial lemniscus system, mediating proprioception and discriminative tactile sensation, decussates after its first synapse in the dorsal column nuclei of the medulla. The anterolateral system, mediating pain and temperature sensation, and, to a lesser degree, tactile sensation, decussates after its first synapse in the dorsal horn of the spinal cord. S-I, primary sensory cortex; S-II, secondary sensory cortex. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM. 1991. Principles of Neural Science, 3rd ed. Norwalk, CT: Appleton & Lange.)
Somatotopic organization of the dorsal columns and the anterolateral system of the spinal cord. In the anterolateral system, fibers representing the leg are closest to the surface of the cord. (Reproduced with permission from Martin JH. 1996. Neuroanatomy Text and Atlas, 2nd ed. Stamford, CT: Appleton & Lange.)
There are two major sensory systems, each of which has first-order neurons in the dorsal root ganglia. The dorsal column–medial lemniscus system mediates touch (superficial, deep, and vibratory) through a variety of encapsulated mechanoreceptors and mediates proprioception (static and dynamic) through ...