Sensory nerve fibres and receptors in equine distal forelimbs and their potential roles in locomotion

Abstract

Summary Sensory innervation of the equine distal forelimb was examined using several different anatomical techniques with each revealing a different morphological component of the sensory apparatus in the foot. Electron microscopic examination of the medial palmar nerve (N. digitalis communis II) revealed predominantly unmyelinated nerve fibres with a ratio of unmyelinated/ myelinated nerves at the level of the metacarpophalangeal joint of 3.67:1. Sensory peptides (substance P, neurokinin A, calcitonin gene‐related peptide and peptide histidine isoleucine) were observed immunocytochemically to innervate much of the equine foot, including the dermis of the dorsal wall and sole, the suspensory ligaments (ligamenta sesamoidea collateralia) (CSL) and the distal sesamoidean impar ligament (ligamentum sesamoideum distale impar), the synovial cavities and bones. These sensory peptides were usually associated with the adventia of the arterial vessels rather than the muscularis layer in contrast to neuropeptide Y. None or only scant immunoreactivity was present in the venules and capillary beds. Lamellated corpuscles in the loose connective tissues proximal to CSL appeared to be innervated by myelinated nerves. These different morphological components of sensory nerves innervating the equine digit suggest that these nerves are functionally complex in that during neurotransmission they have effects both within the spinal cord and within the peripheral tissues.

Histopathology in horses with chronic palmar foot pain and age‐matched controls. Part 1: Navicular bone and related structures

“…Although changes in the DSIL were generally not severe, the importance of any degenerative change in this ligament should not be underestimated because the DSIL contains an extensive network of sensory nerve fibres (Bowker et al 1995;Van Wulfen et al 2002). Therefore, it is possible that relatively low-grade degenerative changes within the ligament might cause significant pain.…”

Prevalence and clinical significance of increasing head height asymmetry as a measure of forelimb lameness in horses when trotting in a straight line after palmar digital nerve block

“…A concentration of nociceptors that send proprioceptive information to the central nervous system and brain is found in the palmar frog and proximal navicular area. [19][20][21] Clusters of lamellated corpuscles proximal to the ligaments supporting the navicular bone suggest that this region is a critical site for the perception of different proprioceptive stimuli by the horse during movement. 20 These structures are anaesthetized by a PDN block.…”

“…[19][20][21] Clusters of lamellated corpuscles proximal to the ligaments supporting the navicular bone suggest that this region is a critical site for the perception of different proprioceptive stimuli by the horse during movement. 20 These structures are anaesthetized by a PDN block. A likely explanation for an increase in lameness after a PDN block may be related to reducing proprioception in a normal part of the limb, such that the task of weight bearing must now be borne more in the painful limb structures outside the area blocked.…”

The Equine Hoof: Laminitis, Progenitor (Stem) Cells, and Therapy Development

“…21 While the epidermis is nonsensitive and avascular, the dermis has arteries, veins, nerves, and capillaries in a dense matrix of tough connective tissue. 22,23 Both epidermal and dermal lamellae have frond-like shapes, primary lamellae, that have smaller frond-like structures, secondary lamellae, on their surface. The junction between the epidermal and dermal components of the lamellae is a complex basement membrane to which epithelial basal cells are attached via hemidesmosomes (Figure 2) or electron-dense adhesion plaques, and the dermis is connected via insertion of anchoring fibrils composed of proteins such as collagen and laminin.…”