The preganglionic fibres of the SNS lie within the ventral horn of the spinal cord and emerge from the mid section of the spinal cord, the thoracic and lumbar regions. The axons of these fibres project into the sympathetic chain or collateral ganglia (Bear, 2007). The fibres emerge from the spinal cord as white rami communicants which connect them to the sympathetic chain. They do this through 3 possible pathways.
Postganglionic neurones in the SNS, secrete, in the most part noradrenaline which has widespread affects. When postganglionic neurones reach their target organs, they branch extensively to exert their influence through the release of neurotransmitters which stimulate the activation or inhibition of the tissue (Gabella, 2009; Bear, 2007).
The sympathetic nervous system also plays a role in the phenomenon of referred pain i.e. pain in one part of the body is perceived in another part of the body. It is due to common pathways shared by sensory nerves from different parts of the body. There are several hypotheses for the mechanism of referred pain, it seems to be caused by the pain afferents following sympathetic fibres back to the same segment of the spinal cord and so the brain perceives the pain to come from the somatic region supplied by those spinal cord segments. This is common with angina, where pain from the heart is perceived across the chest and down the left arm. Cardiac pain fibres have their cell bodies within the dorsal root ganglion of the thoracic segments 1-4 (White, 1957).
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The parasympathetic nervous system
The function of the PNS generally opposes that of the SNS and hence the PNS controls so called â€œrest and digestâ€Â functions which maintain homeostasis in the long term (Bear, 2007). Preganglionic fibres of the PNS originate from the brain stem and the sacral region of the spinal cord. Parasympathetic ganglia lie close to or within their target organs so their preganglionic fibres are long and their postganglionic fibres are short. Axons follow cranial nerves and spinal nerves to their ganglia. Neurones enter the oculomotor nerve (lll), the facial nerve (VII), the glossopharyngeal nerve (IX) and the vagus nerve (X). These all synapse in named ganglia. Axons from cranial nerve III terminate in the ciliary ganglion and innervate the muscles controlling pupil dilation. Axons from cranial nerve VII synapse in the submandibular ganglion, IX cranial nerve fibres synapse in the otic ganglion (innervating the parotid gland) and the vagal nerve fibres synapse in ganglia near the tracheal and bronchial muscles (Gabella, 2009). Preganglionic fibres originating in the sacral spine and following spinal nerves synapse in the pelvic ganglia which predominantly innervate urinary and genital organs (Bear, 2007).
Postganglionic parasympathetic fibres release Ach which acts on muscarinic Ach receptors in the localised region. The afferents of the PNS are vital for regulation of the cardiovascular system and gastrointestinal tract (Bear, 2007).
Central Control of the ANS
Higher brain centres are responsible for the integration of visceral information relating to autonomic control. Nuclei in the brain, particularly in the hypothalamus exert control and regulation over the ANS. The hypothalamus is responsible for the maintenance of homeostatic function as well as regulation of feeding and drinking behaviour (Bear, 2007). The hypothalamus contains many nuclei that also exert control over autonomic behaviour such as the Suprachiasmatic nucleus (which regulates circadian rhythm) and projections to cardiovascular centres in the parabrachial nucleus. The ventromedial nucleus has glucose sensitive neurones that detect plasma glucose levels. The integration of incoming visceral information allows regulation of many autonomically derived processes. The nucleus tractus solitaruis receives input from the CVS which exerts an influence over heart rate and blood pressure. NTS has also been demonstrated to project neurones to the parabrachial nucleus in the Pons which is activated when test subjects are asked to perform inspirational tasks (Cechetto and Shoemaker, 2009). The activation of sympathetic fibres in the heart, including those which induce a decrease in blood pressure have been shown, through fMRI studies, to activate the periaqueductal grey matter. The caudal part of the NTS has been implicated to receive information from the GI tract. The Raphe nucleus has been shown to have a role in visceral input such as distension (Cechetto and Shoemaker, 2009).
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The Enteric Division
The enteric nervous system is a third division of the autonomic nervous system and is not always included in the classical description. It is a very distinctive branch of the nervous system since it is embedded within the walls of the intestines and oesophagus. It controls the digestive system in terms of musculature, blood supply and secretion (Bear, 2007). It co-ordinates the gastric responses of the individual to maintain homeostatic function across the GI tract. It is controlled by two plexuses, the myenteric and submucosal, which are embedded within the muscular layers of the intestine (Gabella, 2009). The neural networks monitor the tension and chemical composition of the GI tract which allow it to maintain a healthy homeostatic function. If the pH in the upper intestine increases, emptying of the proximal intestine is slowed to redress the balance (Furness, 2006). Since it also receives input from both the PNS and SNS it is not entirely autonomous (chronic sympathetic activity can override enteric activity) which is where the controversy of classifying it as a third autonomic branch arises (Gabella, 2009).
Pre and Postganglionic fibres
All of the preganglionic fibres in the autonomic use acetylcholine as their neurotransmitter as do the parasympathetic postganglionic fibres. Sympathetic postganglionic, for the most part, uses noradrenaline. The acetylcholine activates both nicotinic and muscarinic receptors. Muscarinic receptors are G protein coupled and have an influence over the opening and closing of other channels in the cascade. In this way only small EPSPs are produced which means that summation is required to stimulate the action potential to threshold allowing some level of regulation over the postganglionic activity that is stimulated (Bear, 2007).
Postganglionic fibres are adapted so that a relatively small number of fibres can have their effects across a much wider area. This is overcome by branching of the fibres within the organ. This extensive branching allows a vast surface area of the organ to be covered by the fibres. The fibres also have varicosities on them which are a swelling which can function as a nerve ending and can release vesicles of neurotransmitter. In this way the ability of the postganglionic fibre to innervate its target is hugely multiplied (Gabella, 2009).