Pain in Birds: The Anatomical and Physiological Basis

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Key points

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    Three types of nociceptors have been identified in the avian peripheral nervous system: high-threshold mechanothermal nociceptors, mechanical nociceptors, and thermal nociceptors.

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    The C and Aδ nociceptor axons are located outside of the spinal cord at the dorsal root ganglia, where they bifurcate into peripheral and central branches, terminating at the nociceptors and the dorsal horn of the spinal cord respectively.

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    The spinothalamic tract is the most significant sensory pathway for transmission

Peripheral nervous system

Noxious stimuli, events that damage or threaten damage to tissues,1 are initially detected and encoded via nociceptors present in the avian peripheral nervous system (PNS). Largely, nociceptors are nonselective, gated cation channels, opening or closing in response to temperature, chemical ligands, or mechanical shearing forces.6 When activated, nociceptors lead to a local depolarization of the terminal to initiate a conducted action potential. The nociceptors are associated with primary

Central nervous system

A notable relevant difference in the avian spinal cord is its lack of cauda equina, with the avian spinal cord running the entire length of the vertebral canal.6, 32 The vertebrae of birds are generally fused and their spinal nerves travel through the intervertebral foramina in a lateral direction (contrasting their caudal direction of travel in mammals). Most spinal nerves share numbers with the vertebrae just caudal to where they exit the spinal cord, although the last cervical spinal nerve

Peripheral and central sensitization

Pain is not only a nociceptive event. Peripheral sensitization is an increased response to an afferent stimulus from the nociceptors.81 Sensitization of the mammalian PNS occurs once tissue damage results in lowering of pH and release of inflammatory mediators, 2 events that C fibers are sensitive to.82 Inflammatory mediators released during peripheral sensitization include H+, K+, and prostaglandins (from damaged cells); bradykinin (from plasma); serotonin (from platelets); histamine (from

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