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At Cambridge, Elliott, and then Langley, speculate that impulses liberate chemicals from nerve endings. Others, including Adrian, believe that nerve endings exert their effects by electric currents flowing through the synapses. In Austria, Otto Loewi dreams of a way to detect any chemical released by the endings of the vagus nerve in the frog heart. The method works and the chemical is later identified as acetylcholine—the first neurotransmitter to be discovered. In London, Henry Dale’s pharmacological experiments lead him to suggest that acetylcholine is also the transmitter in the sympathetic ganglia and at the nerve endings on muscle fibres. His later experiments are aided by Wilhelm Feldberg’s sensitive bioassay for acetylcholine. John Eccles, however, is still convinced that electric currents are responsible for part, or all, of the excitatory and inhibitory effects at synapses. Dale and Loewi share the 1936 Nobel Prize.

From early psychophysical studies a variety of exogenous and endogenous substances have been found to induce pain and hyperalgesia, that is, to be algogenic. To study the underlying mechanisms several behavioural and reflex models have been developed in animals. However, it has been difficult to differentiate between peripheral and central mechanisms. Single-fibre recordings provided a tool for isolating the contributions of primary afferents. Controlled application of defined mediator concentrations became feasible allowing for the investigation of the direct effects on nociceptive nerve endings. These sensory terminals, however, comprise receptive membrane sections, action potential generator region(s), and conductive zones of the axon, each of which could be the target of a chemical mediator. To differentiate these would require intracellular recording of the membrane potential or currents; this is not achievable due to the submicroscopic size of nociceptive nerve endings and their embedding in the tissue. Considering the cell soma in dissociated sensory ganglion cultures to be a model of its receptive ending, valuable information can be obtained from patch-clamp recordings of chemically mediated membrane effects.

The previous chapter provided an overview of the anatomy of the CNS, concentrating on structures that can be seen during dissection of the human brain and spinal cord or the study of anatomical models of these structures. Some indication of the function of different components of the CNS has been given in Chapter 15, but this chapter shows how the various anatomical components of the CNS are functionally linked together through sensory and motor pathways. These pathways enable the nervous system to convey information over considerable distances, to integrate the information, and formulate functional responses that coordinate activities of different parts of the body. It will be necessary to introduce some other structures in addition to those described in Chapter 15 during the description of major pathways; most are not visible to the naked eye and even when seen in microscopical sections, they require considerable practice to distinguish them. However, they are important landmarks or relay stations in the central nervous pathways and you need to know of them for a full understanding of pathways. As emphasized in Chapter 14, our views of the structure and function of many aspects of the nervous system are constantly subject to revision in the light of new clinical and experimental observations and methods of investigation. This applies to nerve pathways just as much as any other aspect of the nervous system. This chapter presents a summary of current views on somatic sensory and motor functions and their application to the practice of dentistry. The special sensory pathways of olfaction, vision, and hearing are described in Chapter 18 in the context of the cranial nerves that form the first part of these pathways. The information conveyed from the periphery by the sensory components of spinal and cranial nerves is destined to reach the cerebral cortex or the cerebellum. You will be conscious of sensory information that reaches the cerebral cortex, but mostly unaware of information that does not travel to the cortex. However, this does not mean that sensory information that does not attain cortical levels is of no value. For example, sensory neurons or their collateral processes form the afferent limbs of many reflex arcs.