Parasympathetic division

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. 

---

Forming part of the peripheral nervous system is the autonomic nervous system which controls the glands and non-skeletal muscles that are not under conscious control. This control is provided by two parts of this system the sympathetic and parasympathetic divisions which, in general, bring about antagonistic responses. 

Acetylcholine (ACh) as a transmitter. ACh serves as mediator at terminals of all postganglionic parasympathetic fibers, in addition to fulfilling its transmitter role at ganglionic synapses within both the sympathetic and parasympathetic divisions and the motor end-plates on striated muscle. However, different types of receptors are present at these synaptic junctions ... 

Excitation of the parasympathetic division of the autonomic nervous system causes release of acetylchoUne at neuro-effector junctions in different target organs. The major effects are summarized in A (blue arrows). Some of these effects have therapeutic appUcations, as indicated by the clinical uses of parasympa-thomimetics (p. 102). 

Autonomic ganglia. Ganglionic stimulation occurs in both the sympathetic and parasympathetic divisions of the autonomic nervous system. Parasympathetic activation results in increased production of gastric juice (smoking ban in peptic ulcer) and enhanced bowel motility ( laxative effect of the first morning cigarette defecation diarrhea in the novice). 

Acetylcholine is the primary neurotransmitter in the parasympathetic division of the autonomic nervous system, which mainly innervates the gastrointestinal tract, eyes, heart, respiratory tract, and secretory glands. Although its receptors are crucial for maintaining all normal functions of the body, an extremely small number of illnesses can be explained by the dysfunction of cholinergic regions of the peripheral autonomic system. 

The preganglionic neurons of the sympathetic nervous system have their cell bodies in the thoracic and lumbar regions of the spinal cord, termed the thoracolumbar division. The preganglionic neurons of the parasympathetic division have their cell bodies in the brainstem and in the sacral region of the spinal cord, termed the craniosacral division. The cranial part of the parasympathetic nervous system innervates structures in the head, neck, thorax, and abdomen (e.g., the stomach, part of the intestines, and pancreas). The cranial parasympathetic fibers leave the CNS in the oculomotor, facial, glos-... 

Erection involves a coordinated action of the autonomic nervous system, and certain drugs may interfere with either the sympathetic division (e.g., aj- receptors) or the parasympathetic division (e.g., noncholinergic neurotransmitters). 

Anatomy of the Autonomic Nervous System Sympathetic and Parasympathetic Divisions... 

Except for skeletal muscle, virtually all tissues in the body are innervated in some way by the ANS.9 Table 18-1 summarizes the innervation and effects of the sympathetic and parasympathetic divisions on some of the major organs and tissues in the body. As indicated in Table 18-1, some organs, such as the heart, are innervated by both sympathetic and parasympathetic neurons. Other tissues, however, may only be supplied by the sympathetic division. The peripheral arterioles, for instance, are innervated by the sympathetic division but receive no parasympathetic innervation. 

If an organ is innervated by both the sympathetic and parasympathetic divisions, a physiologic antagonism typically exists between these divisions. That is, if both divisions innervate the tissue, one division usually increases function, whereas the other decreases activity. For instance, the sympathetics increase heart rate and stimulate cardiac output, whereas the parasympathetics cause bradycardia. However, it is incorrect to state that the sympathetics are always excitatory in nature and that the parasympathetics are always inhibitory. In tissues such as the gastrointestinal tract, the parasympathetics tend to increase intestinal motility and secretion, whereas the sympathetics slow down intestinal motility. The effect of each division on any tissue must be considered according to the particular organ or gland. 

CNS—brain and spinal cord—and two separate pathways within the peripheral nervous system (PNS) for two-way communication with the peripheral organs. The PNS subdivisions are the somatic and autonomic nervous systems (Figure 11.2). The latter is further divided into sympathetic and parasympathetic divisions (Figure 11.3). 

The parasympathetic division maintains essential bodily functions, such as digestive processes and elimination of wastes, and is required for life (see Figure 3.3). It usually acts to oppose or balance the actions of the sympathetic division and is generally dominant over the sympathetic system in "rest and digest" situations (see Figure 3.4). The parasympathetic system is not a functional entity as such and never discharges as a complete system. If it did, it would produce massive, undesirable, and unpleasant symptoms. Instead, discrete parasympathetic fibers are activated separately, and the system functions to affect specific organs, such as the stomach or eye. 

C. The parasympathetic division originates from cell bodies in the central nervous system. 

C. The parasympathetic division is involved in accommodation of near vision, movement of food, and urination. 

D. The postganglionic fibers of the parasympathetic division are long, compared to those of the sympathetic nervous system. 

The preganglionic fibers terminating in the adrenal medulla, the autonomic ganglia (both parasympathetic and sympathetic), and the postganglionic fibers of the parasympathetic division use acetylcholine as a neurotransmitter (Figure 4.2). Cholinergic neurons innervate voluntary muscles of the somatic system and are also found in the CNS. 

Q3 What are the anatomical differences between the sympathetic and parasympathetic divisions of the autonomic nervous system ... 

Previous studies have shown that accommodation mediated via ciliary smooth muscle activity also receives sympathetic innervation. Sympathetic nerves reach the ciliary muscle through the uveal blood vessels in close association with arteries and terminal arterioles. The distribution of the adrenergic fibers in the ciliary muscle appears to vary across species. In primates sympathetic nerve terminals, mainly 3 receptors, can generally be found in the anterior portion of the ciliary muscle. The accommodative amplitude significantly decreased in human subjects after instillation of phenylephrine (an a agonist) or hydroxyamphetamine (an a and (3 agonist). Such observations provide evidence that both sympathetic and parasympathetic divisions of the autonomic nervous system can affect accommodation but not equally. Furthermore, the nature of sympathetic innervation can be summarized as follows ... 

Parasympathetic division. Stimulation of cholino-ceptors in autonomic ganglia and at the post-... 

Organ/System Sympadietic division Parasympathetic division ... 

---