Shingles, a common disease, demonstrates the dermatomal organization of the dorsal roots. It is caused by the reactivation of the herpes zoster virus, which typically causes chickenpox during the initial infection. The virus infects dorsal root and cranial nerve ganglion cells during the initial infection, where it can remain dormant for years to decades. When the virus reactivates, the cells of the specific dorsal root ganglion become infected, and the virus travels along the peripheral axon branches. This results in a painful or itchy rash that is confined to one side of the body in a dermatomal distribution or to the distribution of a cranial nerve.

The fibers within the dorsal roots are not randomly distributed. Instead, the large myelinated primary afferent fibers occupy a medial position, while the small myelinated and unmyelinated fibers are located more laterally. The large afferent fibers in the medial position enter the dorsal column and bifurcate into branches directed rostrally and caudally. These branches give off collaterals that terminate in neighboring segments. The rostral branch also ascends to the medulla as part of the dorsal column-medial lemniscus pathway. The axonal branches that terminate locally in the spinal cord gray matter transmit sensory information to neurons in the dorsal horn and contribute to the afferent limb of reflex pathways.

The innervation of the face follows a similar pattern to the body and is primarily provided by the trigeminal nerve. Neurons in the trigeminal ganglion pass through the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve to innervate specific regions of the face. These fibers carry tactile information, as well as pain and temperature information. The trigeminal nerve is also responsible for innervating the teeth, oral and nasal cavities, and the cranial dura mater. The trigeminal nuclear complex consists of four main divisions, three of which are sensory. These sensory divisions include the mesencephalic, chief sensory, and descending trigeminal nuclei. The mesencephalic nucleus contains first-order neurons, similar to the dorsal root ganglion, while the latter two divisions consist of second-order neurons. The fourth division is the motor nucleus of the trigeminal nerve, which projects motor neurons to the skeletal muscles of the head.
Acommondiseasethatillustratesthedermatomalorganizationofthedorsalrootsisshingles. Shinglesistheresultofreactivationoftheherpeszostervirus,whichtypicallycauseschickenpoxduringtheinitialinfection.Duringtheinitialinfectionthevirusinfectsdorsalroot(andcranialnerve)ganglioncells,whereitcanremainlatentforyearstodecades.Whenthevirusreactivates,thecellsofthatparticulardorsalrootganglionbecomeinfected,andthevirustravelsalongtheperipheralaxonbranchesandgivesrisetoapainfuloritchyrashthatisconfinedtoonesideofthebody(endsatthemidline)inadermatomal,belt-likedistributionortothedistributionofacranialnerve.
Within the dorsal roots, fibers are not randomly distributed. Rather the large myelinated primary afferent fibers assume a medial position in the dorsal root, whereas the small myelinated and unmyelinated fibers are more lateral. The large medially placed afferent fibers enter the dorsal column, where they bifurcate into rostrally and caudally directed branches. These branches give off collaterals that terminate in several neighboring segments. The rostral branch also ascends to the medulla as part of the dorsal column–medial lemniscus pathway. The axonal branches that terminate locally in the spinal cord gray matter transmit sensory information to neurons in the dorsal horn and also provide the afferent limb of reflex pathways (see
Chapter 9).
Innervation of the Face
The arrangement of primary afferent fibers that supply the face is comparable to that of fibers that supply the body and is provided for primarily by fibers of the trigeminal nerve. Peripheral processes of neurons in the trigeminal ganglion (also called the gasserian or semilunar ganglion) pass through the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve to innervate dermatome-like regions of the face. These fibers carry both tactile information and pain and temperature information. The trigeminal nerve •Fig. 7.4 A,Dermatomesrepresentedonadrawingofapersonassumingaquadrupedalposition.NotenerveC1generallyhaslittleornosensorycomponent,andtheunlabeledportionoftheheadandthefaceareinnervatedbysensoryfibersofthecranialnerves,primarilythetrigeminalnerve.B,SagittalviewofthespinalcordshowingtheoriginofnervescorrespondingtoeachofthedermatomesshowninA.
Thetrigeminalnuclearcomplexconsistsoffourmaindivisions,threeofwhicharesensory.Thethreesensorydivisions(fromrostraltocaudal)arethemesencephalic, chief (ormain)sensory, anddescending (orspinal)trigeminal nuclei. Thelattertwoaretypicalsensorynucleiinthatthecellbodiescontainedinthemaresecond-orderneurons.Themesencephalicnucleusactuallycontainsfirst-orderneuronsandthusisanalogoustoadorsalrootganglion.Thefourthdivisionofthetrigeminalcomplexisthemotornucleusofthetrigeminalnerve,whosemotorneuronsprojecttoskeletalmusclesoftheheadviathetrigeminalnerve(see
Fig.4.6D–E also innervates the teeth, the oral and nasal cavities, and the cranial dura mater.