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Nerve Injury: Anatomy and Definitions J EQUINE VETERINARY EDUCATION / AE / january 2011 17 Clinical Commentaryeve_156 17..18 Nerve injury: Anatomy and definitions J. Peroni University of Georgia, Large Animal Medicine, 501 DW Brooks Drive College of Veterinary Medicine, College of Veterinary Medicine, Athens, USA. Nerve injury: anatomy and definitions Schwann cells. There are points along the axon called nodes of Ranvier at which the myelin sheath is Although the neuropathy in the horse in the report by discontinued. These nodes are sites at which ions are easily Ljungvall and Jonsson (2010) in this issue is an interesting exchanged between the nerve and the extracellular and unique cause of lameness, nerve trauma is generally matrix, a process impeded along the rest of the axon by uncommon in horses. Knowledge of the anatomy of the the insulating property of myelin sheath. Depolarisation of peripheral nerve fibre may be helpful in understanding the axon occurring at the nodes of Ranvier creates the nerve injury and restoration of nerve function. action potential responsible for the so-called saltatory (jump) neural conduction. Anatomy Nerves can be damaged as a result of trauma and lacerations and during invasive surgical procedures in which Most peripheral nerves are a collection of myelinated and the landmarks are obscured by abnormal tissues (Colohan unmyelinated neural fibres (axons) enclosed within a highly et al. 1996; Cornwall and Radomisli 2000; Trumble 2000). elastic connective tissue sheath called the epineurium. Equine surgeons electively transect the palmar digital nerves Bundles of nerve fibres contained within the epineurium in an attempt to eliminate pain arising from conditions are often referred to as fasciculi, which are comprised of localised within the caudal portion of the hoof that otherwise smaller neural fibres called funiculi. Funiculi are contained are untreatable. Palmar digital neurectomy is generally within a tubular connective tissue sheath called the regarded as an uncomplicated procedure except for the perineurium. occasional development of painful neuromas at the site of Nerve fibres within each funiculus support the nerve transection (Jackman et al. 1993). In addition to painful endoneurium, a connective tissue that has a septal neuromas, iatrogenic nerve damage also may lead to arrangement extending from the innermost layer of the peripheral neuropathies. The most common neuropathy in perineurium. The endoneurium supports a network of horses is facial nerve paralysis, which is both unsightly and a capillary vessels responsible for neural blood supply. cause for owner dissatisfaction. Capillaries originate from surrounding soft tissues, perforate the perineurium and form a network that runs parallel to Nerve injuries defined the direction of the nerve fibres. From the central nervous system, nerves divide into In the human medical literature, nerve injuries have been branches with frequent communications with branches of classified based on the components of the nerve that have adjacent nerves. These communications form what is been injured, their relative degree of function and the rate of known as a plexus. Even though a plexus is a complex recovery after injury (Kuntz et al. 1996; Bonnard et al. 1999; arrangement of nerves, individual nerve fibres are thought Horowitz Stephen 2005). Nerve injury may result in prolonged to run an independent course from the peripheral tissues neuropathic pain, known as allodynia, in which case a to the central nervous system. In fact, occasional acute decrease in the pain threshold results in pain being perceived angled connections between funiculi have been after application of a stimulus that normally does not result in demonstrated without individual fibres ever coalescing pain. In its mildest form, nerve injury is known as neurapraxia in with each other. which there is no disruption of the nerve or its neural sheath. Axons thicker than 1 micron are myelinated. Myelin is a Compressive trauma resulting form blunt force may spiral sheet of cell membrane that wraps around the axon temporarily disrupt nerve conduction and result in loss of and is produced in the peripheral nervous system by function although recovery will usually be complete and does not require regeneration of the nerve. An example of Corresponding author email: [email protected] neurapraxia is the radial nerve dysfunction evident when a © 2010 EVJ Ltd 18 EQUINE VETERINARY EDUCATION / AE / january 2011 laterally recumbent horse recovers from general a nerve injury that resulted in neuroma formation and anaesthesia. This loss of function is due to muscle compression subsequent prolonged neuropathic pain (allodynia). and should be reversible within a time period that depends on the degree and duration of the compression. References A more severe form of nerve injury is that defined by the term axonotmesis. This injury occurs when the axon Bonnard, C., Anastakis, D.J., van Melle, G. and Narakas, A.O. (1999) is damaged, while the neural connective tissue sheath Isolated and combined lesions of the axillary nerve. A review of 146 (endoneurium), the epineurium, Schwann cell tubes and cases. J. Bone Joint Surg. 81, 212-217. Colohan, A.R., Pitts, L.H. and Rosegay, H. (1996) Injury to the peripheral other supporting structures remain intact. Because the nerves. In: Trauma, 3rd edn., Eds: D.V. Feliciano, E.E. Moore and K.L. internal structures of the nerve are not damaged, proximal Mattox, Appleton & Lange, Stamford, CT. pp 853-862. axonal regeneration may occur and result in reinnervation of Cornwall, R. and Radomisli, T.E. (2000) Nerve injury in traumatic tissues normally innervated by that nerve. Peripheral nerve dislocation of the hip. Clin. Orthop. 377, 84-91. Horowitz, S.H. (2005) Venipuncture-induced nerve injury, a review. J. regeneration may occur at rate of 2 mm/day, although the Neuropathic Pain Symptom Palliation 1, 109-114. quality of the function regained after restoration of nerve Huang, J.H., Cullen, D.K., Browne, K.D., Groff, R., Zhang, J. and Pfister, structure depends on the integrity of the neural sheath and B.J. (2009) Long-term survival and integration of transplanted the distance between the site of injury and the target tissue. In engineered nervous tissue constructs promotes peripheral nerve regeneration. Tissue Eng. Part A 15, 1677-1685. fact, the axon may regain anatomical integrity in the Jackman, B.R., Baxter, G.M., Doran, R.E., Allen, D. and Parks, A.H. (1993) absence of restoration of the myelin sheath, thereby resulting Palmar digital neurectomy in horses. 57 cases (1984-1990). Vet. Surg. in the ineffective transmission of electrical impulses. 22, 285-288. Neurotmesis is the most severe grade of peripheral nerve Kuntz, C., Blake, L., Britz, G., Filler, A., Hayes, C.E. and Goodkin, R. (1996) Magnetic resonance neurography of peripheral nerve lesions in the injury and occurs with total nerve rupture. Recovery through lower extremity. Neurosurg. 39, 750-756. axonal regeneration cannot occur and neurosurgical Ljungvall, K. and Jonsson, L. (2010) Traumatic neuroma of the medial techniques aimed at restoring nerve continuity are required palmar nerve as a possible cause of a severe gait abnormality in a to restore function (Huang et al. 2009). Warmblood riding horse. Equine Vet. Educ. 23, 14-16. Trumble, T.E. (2000) Peripheral nerve injury: pathophysiology and repair. It appears that, although not definitively proven, the In: Trauma, 4th edn., Eds: D.V. Feliciano, E.E. Moore and K.L. Mattox, horse in the accompanying report may have suffered from McGraw-Hill, New York. pp 1047-1055. Continued from page 10 Maddison, J.E. (1992) Hepatic encephalopathy. Current concepts of Peek, S.F., Divers, T.J. and Jackson, C.J. (1997) Hyperammonaemia the pathogenesis. J. vet. intern. Med. 6, 341-353. associated with encephalopathy and abdominal pain without Mechtcheriakov, S., Graziadei, I.W., Kugener, A., Schuster, I., Mueller, J., evidence of liver disease in four mature horses. Equine vet. J. 29, Hinterhuber, H., Vogel, W. and Marksteiner, J. (2006) Motor 70-74. dysfunction in patients with liver cirrhosis: Impairment of handwriting. Reed, S., Bayly, W. and Traub, J. (1981) Ataxia and paresis in horses. I. J. Neurol. 253, 349-356. Differential diagnosis. Comp. cont. Educ. pract. 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