Sensory nervous system, defined

Association of Pain, neuropathic pain is defined as pain initiated or caused by a primary lesion, dysfunction in the nervous system". Neuropathy can be divided broadly into peripheral and central neuropathic pain, depending on whether the primary lesion or dysfunction is situated in the peripheral or central nervous system. In the periphery, neuropathic pain can result from disease or inflammatory states that affect peripheral nerves (e.g. diabetes mellitus, herpes zoster, HIV) or alternatively due to neuroma formation (amputation, nerve transection), nerve compression (e.g. tumours, entrapment) or other injuries (e.g. nerve crush, trauma). Central pain syndromes, on the other hand, result from alterations in different regions of the brain or the spinal cord. Examples include tumour or trauma affecting particular CNS structures (e.g. brainstem and thalamus) or spinal cord injury. Both the symptoms and origins of neuropathic pain are extremely diverse. Due to this variability, neuropathic pain syndromes are often difficult to treat. Some of the clinical symptoms associated with this condition include spontaneous pain, tactile allodynia (touch-evoked pain), hyperalgesia (enhanced responses to a painful stimulus) and sensory deficits. 

Local application of concentrated sodium channel blockers can provide complete pain relief through nerve conduction block (local anesthetics). This approach to pain relief is limited to a few applications involving short-term treatment of acute pain, since sodium channels are also vital to conduction in the heart, CNS, skeletal muscle, and non-nociceptive sensory neurons. However, some types of chronic pain signaling appear to be sensitive to sodium channel blockers at concentrations that do not cause conduction block. In particular, neuropathic pain, defined as chronic pain resulting from a primary lesion or dysfunction of the peripheral nervous system by the International Association for the Study of Pain (IASP) [58], is thought to originate from aberrant signaling in the nervous system and can be ameliorated by sodium channel blockers. 

The PNS is defined as that part of the nervous system external to the brain and spinal cord (Figure 30.1). As such, the PNS includes the cranial nerves, dorsal and ventral spinal roots, spinal nerves and their branches, and ganglia. The primary function of the PNS is to convey sensory and motor information, including informa-... 

The International Association for the Study of Pain (lASP) has defined pain as an unpleasant sensory and emotional experience that is evoked by actual or potential noxious (i.e. tissue-damaging) stimuli or by tissue injury. Normally, pain is the subjective result of nociception. Nociception is the encoding and processing of noxious stimuli in the nervous system. It can be objectively measured with various techniques, i.e. with electrophysiological recordings. By contrast, pain as a subjective experience can be verbally or visually described by humans, and it cannot be measured objectively However, animals (as well as humans) show reflex responses to acute noxious stimuh, which can be assessed for the relationship between nociception and pain. 

In all sensory systems, this afferent pathway consists of at least three neurons (nerve cell and nerve fiber) arranged in a continuous chain. The transfer of nervous signals from one neuron to the next occurs in well-defined loci called synapses. 

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