Sensory receptors encode information about stimulus modality, location, intensity, and novelty. As described earlier, this is achieved through the specific transducer elements within individual receptor endings that convert the energy from chemical, thermal, or mechanical stimuli into bioelectrical signals in the form of generator potentials and action potentials. Generator potentials are localized responses to sensory stimuli that vary in intensity based on the intensity of the stimulus. Generator potentials of sufficient size will initiate action potentials; even larger generator potentials will generate multiple action potentials. Action potentials are not localized responses; rather, they are propagated along the central projection fibers of the neuron. In this way, sensory information passes from peripheral to central elements of the nervous system. Specifics of the transmitted information are encoded as follows: Stimulus modality is encoded by the preferential response of a given sensory neuron to its appropriate stimulus; location is encoded by the receptive field properties of the activated neuron; and intensity is encoded by the frequency of action potentials. Rapidly adapting neurons fire only at stimulus onset (or offset) and thus transmit information about stimulus change or novelty.
Cell bodies of somatic sensory neurons are located in the dorsal root ganglia and are designated as first-order sensory afferents because they provide the first link within the chain of neurons constituting the primary sensory pathways. Their central fibers project within short dorsal spinal rootlets to their synaptic targets on second-order sensory neurons in designated areas of the spinal cord and brain stem. Action potentials, generated by stimulation of receptor end organs, travel back to the cell body and along the central process. This wave of depolarization stimulates the release of a variety of neuroactive substances (including substance P, calcitonin-gene-related product [CGRP], and glutamate) that act as chemical neurotransmitters to excite or inhibit second-order neurons. Sensory information is thus transmitted along individual cells by action potentials and between neurons within the pathway by chemical transmission. This basic plan establishes a polarity for neurons, with input arriving on peripheral dendritic processes and output being provided by central axonal processes with nerve terminals containing stores of neurotransmitters positioned for release. Most sensory and motor pathways operate in this manner.
An interesting exception is represented by certain dorsal root ganglia cells, which apparently release neuro-active substances at dendritic endings as well as central axon terminals, providing direct feedback as well as feedforward responses. As noted in the previous chapter, peripheral release of transmitter substances from nocicep-tive sensory neurons is a trigger for primary hyperalgesia.
Cell bodies from all types of somatic sensory nerves are grouped together within the dorsal root ganglia; however, their central projections maintain distinctly separate positions within the spinal cord. Two major parallel sensory pathways exist: one transmitting information about touch and proprioception, and the other transmitting information about pain and temperature. Subdivisions are observed within the main pathways with specific locations designated for each modality or even submodality (e.g., light-touch and crude-touch fibers are grouped separately). Relative position within the main pathway also reflects the location of the receptive field that is represented. Thus, each pathway has within it a somatotopic arrangement of its fibers, with the lower extremities layered into the tract first, followed by the trunk, arms, and head.
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