The neurological output from the parasympathetic system is the integrated end product of a complex interactive network of neurons spread throughout the mesencephalon, pons, and medulla. Note that all preganglionic axons are myelinated and all postganglionic axons are unmyelinated. It also includes the transmitter (ACH −) of the ventral horn cell and the (nicotinic) receptors in muscle. In contrast to the sympathetic system, the preganglionic parasympathetic neurons are generally longer than the postganglionic neurons as they synapse in ganglia that are further from their origin and closer to the effector than the postganglionic neurons innervate.įigure 8.2 This figure outlines the preganglionic and postganglionic neurotransmitters and receptors of the sympathetic and parasympathetic divisions of the autonomic nervous system.
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The cell bodies of parasympathetic preganglionic neurons are located in discrete nuclei at various levels of the brainstem as described above and in the intermediolateral cell column of levels S2–4 in the spinal cord or vertebral level L1–2. Axons of the preganglionic nerves of the parasympathetic system tend to be long, myelinated, type II fibres and the postganglionic axons tend to be somewhat shorter, unmyelinated, C fibres (see Chapter 7). The vagus nerve and sacral nerve roots compose the major output route of parasympathetic enteric system control ( Furness & Costa 1980).
![autonomic nervous system lens autonomic nervous system lens](https://i.pinimg.com/236x/1d/f7/f5/1df7f5197e4f50541ca1fa73cc4a3929.jpg)
The parasympathetic system communicates via both efferent and afferent projections within several cranial nerves including the oculomotor (CN III) nerve, the trigeminal (CNV) nerve, the facial (CNVII) nerve, the glossopharyngeal nerve, and the vagus (CN X) and accessory (CN XI) nerves ( Fig. Descending autonomic modulatory pathways also arise from the nucleus solitarius, noradrenergic nuclei of the locus ceruleus, raphe nuclei, and the pontomedullary reticular formation (PMRF). They finally terminate on the neurons of the parasympathetic preganglionic nuclei of the brainstem, the neurons in the intermediate grey areas of the sacral spinal cord, and the neurons in the intermediolateral cell column of the thoracolumbar spinal cord. These descending fibres initially travel in the medial forebrain bundle and then divide to travel in both the periaqueductal grey areas and the dorsal lateral areas of the brainstem and spinal cord. Hypothalamic projections that originate mainly from the paraventricular and dorsal medial nuclei influence the parasympathetic and sympathetic divisions as well as the enteric division of the autonomic nervous system. All three systems are modulated by projections from the hypothalamus. The autonomic system can be divided into three functionally and histologically distinct components: the parasympathetic, sympathetic, and enteric systems.
![autonomic nervous system lens autonomic nervous system lens](http://image.slidesharecdn.com/ans-130217134747-phpapp01/95/central-nervous-system-the-autonomic-nervous-system-16-638.jpg)
Although historically only the efferent connections were considered, all of the projections of the autonomic nervous system are reciprocal in nature and involve both afferent and efferent components. The postganglionic component neurons vary in location with some living in the paraspinal or sympathetic ganglia, and others in ganglia distant from the cord, known as stellate ganglia.
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The preganglionic component neurons live in the grey matter of the spinal cord. Anatomically, the autonomic outflow from the spinal cord to the end organ occurs through a chain of two neurons consisting of a pre- and postganglionic component. The autonomic nervous system comprises the major autonomous or non-volitional efferent outflow to all organs and tissues of the body with the exception of skeletal muscle. Organisation of the autonomic nervous system