Parasympathetic system

Cholinergic neurotransmission ChEs terminate cholinergic transmission in the central nervous system (CNS), in NMJs and in the autonomic system (the parasympathetic system, somatic motor nerves and pre-ganglionic sympathetic nerves). A few sensory cells and the NMJ in nematodes also include ChEs. 

Because of its motor, i.e., activating effect on vascular smooth muscle and its inhibitory effect on intestinal smooth muscle, the sympathetic nervous system has been cast into the role of the component of the nervous system that executes control of visceral function in times of physical emergency for the organism. The phrase fight or flight has been often used to describe the circumstances in which the adrenergic transmitters of the sympathetic system are dominant over the cholinergic parasympathetic system. This concept is perhaps oversimplified but it has the utility of a first approximation of how the two components of the ANS interact in the periphery. Sensory inputs which lead to increased blood pressure, for example, activate the sympathetic pathways. 

Compare and contrast anatomical features of the sympathetic and parasympathetic systems... 

Describe the overall and specific functions of the parasympathetic system... 

The ANS is composed of two anatomically and functionally distinct divisions the sympathetic system and the parasympathetic system. Two important features of these divisions include ... 

Many tissues are innervated by both systems. Because the sympathetic and parasympathetic systems typically have opposing effects on a given tissue, increasing the activity of one system while simultaneously decreasing the activity of the other results in very rapid and precise control of a tissue s function. Several distinguishing features of these two divisions of the ANS are summarized in Table 9.2. 

Table 9.2 Distinguishing Features of Sympathetic and Parasympathetic Systems Sympathetic system Parasympathetic system...

An important feature of this system, which is quite distinct from the parasympathetic system, is that the postganglionic neurons of the sympathetic... 

Because the terminal ganglia are located within the innervated tissue, there is typically little divergence in the parasympathetic system compared to the sympathetic system. In many organs, the ratio of preganglionic fibers to postganglionic fibers is 1 1. Therefore, the effects of the parasympathetic system tend to be more discrete and localized, with only specific tissues stimulated at any given moment, compared to the sympathetic system in which a more diffuse discharge is possible. 

The two most common neurotransmitters released by neurons of the ANS are acetylcholine (Ach) and norepinephrine (NE). Several distinguishing features of these neurotransmitters are summarized in Table 9.3. Nerve fibers that release acetylcholine are referred to as cholinergic fibers and include all preganglionic fibers of the ANS — sympathetic and parasympathetic systems all postganglionic fibers of the parasympathetic system and sympathetic postganglionic... 

Release autonomic nervous system all postganglionic neurons of parasympathetic system some sympatheticpostganglionicneurons innervating sweat glands (alpha motor neurons innervating skeletal muscle)b adrenal medulla (20% of secretion) secretion)... 

Figure 9.2 Autonomic nerve pathways. All preganglionic neurons release acetylcholine (Ach), which binds to nicotinic receptors (N) on the postganglionic neurons. All postganglionic neurons in the parasympathetic system and some sympathetic postganglionic neurons innervating sweat glands release Ach that binds to muscarinic (M) receptors on the cells of the effector tissue. The remaining postganglionic neurons of the sympathetic system release norepinephrine (NE), which binds to alpha (a) or beta (P) receptors on cells of the effector tissue. The cells of the adrenal medulla, which are modified postganglionic neurons in the sympathetic system, release epinephrine (EPI) and NE into the circulation.

Because cardiac muscle is myogenic, nervous stimulation is not necessary to elicit the heart beat. However, the heart rate is modulated by input from the autonomic nervous system. The sympathetic and parasympathetic systems innervate the SA node. Sympathetic stimulation causes an increase in heart rate or an increased number of beats/min. Norepinephrine, which stimulates ( -adrenergic receptors, increases the rate of pacemaker depolarization by increasing the permeability to Na+ and Ca++ ions. If the heart beat is generated more rapidly, then the result is more beats per minute. 

The autonomic nervous system exerts the primary control on heart rate. Because the sympathetic and parasympathetic systems have antagonistic effects on the heart, heart rate at any given moment results from the balance or sum of their inputs. The SA node, which is the pacemaker of the heart that determines the rate of spontaneous depolarization, and the AV node are innervated by the sympathetic and parasympathetic systems. The specialized ventricular conduction pathway and ventricular muscle are innervated by the sympathetic system only. 

At rest, the parasympathetic system exerts the predominant effect on the SA node and therefore on heart rate. In a denervated heart, such as a trans-... 

The effects of the autonomic nervous system on MAP are summarized in Figure 15.4. The parasympathetic system innervates the SA node and the AV node of the heart. The major cardiovascular effect of parasympathetic stimulation, by way of the vagus nerves, is to decrease HR, which decreases CO and MAP. 

Similar considerations would apply at the synapse between the preganglionic fibre and the nerve cell of the postganglionic fibre of the sympathetic and parasympathetic systems and curare is the blocking agent (fig. 7 A). 

The pupil is supplied with constrictor fibres from the parasympathetic system (via the Illrd or oculomotor nerve and the ciliary ganglion) and with dilator fibres from the sympathetic system (the upper thoracic nerves to the sympathetic and to the inferior and superior cervical ganglia fig. 8). 

Figure 19.17 The biochemistiy and physiology responsible for penile erection. Sexual activity itself begins with a state of arousal that leads to erection. Arousal results in part from stimulation of the sense organs. The hypothalamus coordinates the sensations and activates the autonomic nervous system. Sensory nerves from the skin of the penis and other erogenous zones stimulate the parasympathetic system. This activates nitric oxide synthase and the resultant nitric oxide, via cyclic GMP, causes vasodilation of the arterioles. This increases blood flow through the corpora cavernosa which then expands producing an erection. Pheromones secreted by the female can stimulate the odour detecting system in the nasal cavity of the male (Chapter 12 and see above). Stress, however, activates the sympathetic system releases cyclic AMP which can result in vasoconstriction of the arterioles. Other factors that can interfere with an erection are physical fatigue and alcohol.

Stimulation of the parasympathetic system releases acetylcholine at the neuromuscular junction in the sinoatrial node. The binding of acetylcholine to its receptor inhibits adenylate cyclase activity and hence decreases the cyclic AMP level. This reduces the heart rate and hence reduces cardiac output. This explains why jumping into very cold water can sometimes stop the heart for a short period of time intense stimulation of the vagus nerve (a parasympathetic nerve) markedly increases the level of... 

Sympathomimetics, such as dobutamine and isoprenaline, mimic the sympathetic system. Orphenadrine is an antimuscarinic drug acting as an antagonist to the parasympathetic system. 

Responses to activation of the parasympathetic system. Parasympathetic nerves regulate processes connected with energy assimilation (food intake, digestion, absorption) and storage. These processes operate when the body is at rest, allowing a decreased tidal volume (increased bronchomotor tone) and decreased cardiac activity. Secretion of saliva and intestinal fluids promotes the digestion of foodstuffs transport of intestinal contents is speeded up because of enhanced peristaltic activity and lowered tone of sphincteric muscles. To empty the urinary bladder (micturition), wall tension is increased by detrusor activation with a concurrent relaxation of sphincter tonus. 

Anatomy of the parasympathetic system. The cell bodies of parasympathetic preganglionic neurons are located in the brainstem and the sacral spinal... 

Because their actions are so broad, including blocking of sympathetic and parasympathetic systems, their therapeutic use has been largely supplanted by more specific drags. They may still be used in the control of blood pressure in patients with acute dissecting aortic aneurysm and for the induction of hypotension in surgery. 

In comparison with more modem antihypertensives reserpine causes unpleasant side-effects, such as sedation, depression and various effects reflecting a dominant parasympathetic system (nasal congestion, diarrhea and exacerbation of peptic ulcers). Reserpine should be considered as an antihypertensive of second choice, although in certain countries it is still used because of its low price. 

The parasympathetic system is designed to function more or less on an organ system basis, usually under conditions of minimal stress. For example, the activation of the gastrointestinal tract takes place during digestion of a meal constriction of the pupil and accommodation for near vision are essential for reading. 

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