23 Autonomic system The autonomic system differs from the somatic system in that the somatic nervous system connects to its target organ via a single neuron, whereas the autonomic nervous system consists of two neurons. The autonomic ganglion forms the synaptic connection between the preganglionic and postganglionic neuron. The efferent autonomic nervous system can be divided into the sympathetic system (stress response), parasympathetic system (rest response) and enteric nervous system (digestive system).45 Preganglionic fibers are myelinated and use acetylcholine as neurotransmitters. Postganglionic nerve fibers are smaller compared to preganglionic fibers, are unmyelinated and use norepinephrine as neurotransmitter. An exception are sweat glands, which use cholinergic nerves (Figure 2).45 Table 2 Overview of different nociception receptors with corresponding small nerve fiber type35 Receptor type Fiber group Modality Cutaneous and subcutaneous mechanoreceptors Hair down Thermal receptors Cold receptors Warm receptors Heat nociceptors Cold nociceptor Nociceptors Mechanical Thermal-mechanical Thermal-mechanical Polymodal Muscle and skeletal mechanoreceptors Stretch-sensitive free endings Aδ Aδ C Aδ C Aδ Aδ C C Aδ Touch Light stroking Temperature Skin cooling (250C) Skin warming (410C) Hot temperatures (>450C) Cold temperatures (<50C) Pain Sharp, pricking pain Burning pain Freezing pain Slow, burning pain Limb proprioception Excess stretch or force 2 25 2
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