Mediolateral and dorsoventral projection patterns of cutaneous afferents within transverse planes of the mouse spinal dorsal horn

Abstract

The central projection patterns of cutaneous afferents from the forelimb and shoulder of mice were studied in the spinal dorsal horn after intracutaneous injection of AlexaFluor 488-conjugated and/or 594-conjugated cholera toxin subunit B (CTB). Based on their dermatomes, the following eight skin regions are thought to be innervated by spinal nerves from the sixth to eighth cervical spinal nerve roots: the dorsal surface of the shoulder, brachium, proximal forearm, distal forearm, hand, palmar surface of the second and third digits, and palm. The termination areas of afferents from the dorsal surface of the shoulder and forearm were narrow, distributed in a dorsoventral direction, and aligned in order from lateral to medial within the sixth to eighth cervical dorsal horns. By contrast, the termination areas of the palmar surface of the second and third digits largely overlapped. We also injected CTB into the dorsal surface of the hindlimb and pelvic regions. Skin regions there are thought to be innervated by nerves from the third to fifth lumbar spinal nerve roots. The observed projection patterns in the lumbar dorsal horn were similar to the cervical patterns. Injection of a mixture of CTB and wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), which are thought to label Aβ and Aδ/C fibers, respectively, showed segregated termination areas of CTB-and WGA-HRP-labeled afferents. Moreover, alignment of the termination areas was in the dorsoventral direction. These results suggest there is fine somatotopic (mediolateral axis) and modality-specific (dorsoventral axis) organization within the spinal dorsal horn.

Comparison of Sensory and Motor Innervation Between the Acupoints LR3 and LR8 in the Rat With Regional Anatomy and Neural Tract Tracing

“…The transganglionic axon terminals associated with LR3 locate more medially in the spinal dorsal horn than that of LR8, and the motor neurons associated with LR3 are situated more laterally on the spinal ventral horn than that of LR8. These segmental and regional distribution is accorded with the “somatotopic arrangement” in the CNS of rats, which has been well defined in the previous studies ( Swett and Woolf, 1985 ; Maslany et al, 1991 , 1992 ; Takahashi et al, 2010 ; Odagaki et al, 2019 ). Increasing pieces of evidence from the human have proved that “somatotopic arrangement” does not only exist on the cerebral cortices but is also present in the spinal cord ( Standring, 2008 ; Stroman et al, 2012 ; Willoughby et al, 2021 ).…”

Differential innervation within a transverse plane of spinal gray matter by sensorimotor cortices, with special reference to the somatosensory cortices

“…A similar relationship was observed between axon terminals from S1c and cutaneous afferents from the trunk (Figures d and ). We also observed the somatotopic organization of the cutaneous afferents in the spinal dorsal horn: afferents from the digits and palm projected more medially than those from the rest of the forepaw, and those from the forearm and brachium terminated between the “forepaw” and “trunk” areas in the dorsal horn (Odagaki, Kameda, Hayashi, & Sakurai, in press). Together with those observations, our results in this study suggest that the projection areas of S1r axons overlap those of afferents from the digits, palm, and dorsal surface of hand, while the projection areas of S1c axons overlap those of afferents from the brachium and trunk region.…”

Innervated Properties of Acupuncture Points LI 4 and LR 3 in the Rat: Neural Pathway Tracing with Cholera Toxin Subunit B

“…to the CNS. [17][18][19][20][21] Their successive neural connections with the corresponding targets in the spinal cord, thalamus, and cerebral cortex was not determined in the current study. In 2016 and 2018, results from functional magnetic resonance imaging (fMRI) studies indicated that acupuncture stimulation at LR 3 can specifically activate or deactivate the visual, motor, and sensory cortices, etc., while stimulating both LI 4 and LR 3 can modulate activities of cognition-related brain regions in patients with Alzheimer's disease.…”

“…to the spinal dorsal horn and brainstem, as well as motor efferents from the spinal ventral horn. [17][18][19][20][21] As a comparison, innervation on acupuncture points located on the head and face is more complicated. 6,11 For example, GB 14, ST 2, and ST 6 establish their sensory connections with the trigeminal ganglion, cervical DRG, spinal trigeminal nucleus, and cervical spinal dorsal horn via trigeminal and spinal sensory pathways, as well as making motor connections with the facial nucleus, motor trigeminal nucleus, and cervical spinal ventral horn via trigeminal, facial and spinal motor pathways.…”