Pupil innervation

Symptoms of intoxication in humans caused by accidental ingestion of Kou-Wen plants have been described as follows. The effect on the digestive system starts with loss of appetite and turn of the stomach, and continues to severe abdominal pain and intestinal bleeding. The effect on the respiratory system presents as breathing difficulties which finally lead to death by respiratory failure. The effect on muscle innervation usually results in generalized muscular weakness and paralysis of the limbs. The effect on the circulatory system starts with heartbeat disorders and a drop in blood pressure, but heart failure is not a common cause of death. In addition to dilation of pupils, a drop in body temperature and proliferation of white blood cells have also been obseryed (70). 

Pupil size Accommodation i Constriction of radial muscle causing dilation (mydriasis) No innervation Contraction of circular muscle (miosis) Contraction of ciliary muscle producing accommodation for near vision... 

C. a-Adrenoceptors mediate contraction of the radial muscle of the iris. The shortening of the radial muscle cells opens the pupil. Phentolamine blocks a-adrenoceptors, allowing parasympathetic nerves innervating the sphincter muscle to take over. This leads to a less opposed contraction of the sphincter muscle induced by transmitter acetylcholine and a constriction of the pupil or miosis. 

These drugs are best avoided in patients with cerebrovascular, cardiovascular and hepatic disorders. Some sympathomimetic effects may occur, mainly mild tremor and occasionally cardiac arrhythmias. Apparent anticholinergic effects may also occur but these are the result of sympathetic potentiation in tissues with dual cholinergic/adrenergic innervation, e.g. pupil. Sympatholytic effects can also occur, principally postural hypotension, because of synthesis of relatively inactive false transmitters, e.g. octopamine, in nerve terminals following inhibition of MAO and activation of alternative metabolic pathways. 

The ganglion-blocking drugs cause a predictable cycloplegia with loss of accommodation because the ciliary muscle receives innervation primarily from the parasympathetic nervous system. The effect on the pupil is not so easily predicted, since the iris receives both sympathetic innervation (mediating pupillary dilation) and parasympathetic innervation (mediating pupillary constriction). Ganglionic blockade often causes moderate dilation of the pupil because parasympathetic tone usually dominates this tissue. 

Lowenfeld12 identified the components of the fight reflex that were controlled by parasympathetic and sympathetic innervation of the smooth muscles controlling pupil diameter. They concluded that the parasympathetic nervous system must be intact to observe the light reflex the sympathetic nervous system influences the shape of the reflex. For example, in the absence of sympathetic innervation, the constriction velocity is increased and the dilation velocity is decreased. Conversely, in situations of increased sympathetic tone, the constriction is sluggish and incomplete, and the pupil slowly returns to its baseline size. The effects of abused drugs on these and other components of the light reflex were studied in the experiment described below. 

As was demonstrated in the experiment above and elsewhere, amphetamine26 and its derivatives and cocaine26,27 significantly increased pupil size through an activation of the sympathetic nerve innervation of the iris. Evidence from an animal study indicated that amphetamine-induced mydri-... 

Most blood vessels, the sweat glands, and the spleen are innervated only by one division of the autonomic nervous system. In the salivary glands, the two divisions of the autonomic nervous system supplement one another. In the bladder, bronchi, gastrointestinal tract, heart, pupil, and sex organs, the two divisions of the autonomic nervous system have opposing effects (see Figure 14.5). 

Figure 7.7. Response of the pupil to activation of autonomous innervation. The movements are caused by the radial and the concentric muscle fibers of the pupil dilatator and sphincter muscles, which have sympathetic/adreneigic and parasympathet-ic/cholineigic innervation and receptors, respectively.

The ocular effects of cocaine include anesthesia (see Chapter 6), mydriasis, and vasoconstriction. The mydriatic effect of cocaine depends on the presence of a functioning adrenergic innervation. After topical appUcation to the eye, the pupil begins to dilate within 15 to 20 minutes. The maximum effect, which is typically less than 2 mm of dilation, occurs within 40 to 60 minutes, and the pupil may remain dilated for 6 or more hours. The mydriasis is accompanied by vasoconstriction that causes blanching of the conjunctiva. Cocaine is also readily absorbed through the mucous membranes into the systemic circulation. 

Pupil size is determined by varying degrees of parasympathetic innervation to the sphincter muscle, which contracts accordingly and produces a corresponding degree of pupillary constriction. Sympathetic innervation, which is secondary, maintains a persistent tone in the dilator muscle, aiding relaxation of the sphincter and resulting in dilation. 

Cocaine Test. When topically applied, cocaine produces dilation of the pupil by preventing the reuptake of norepinephrine that has been released into the synaptic junctions of the iris dilator muscle in response to a nerve impulse. If the sympathetic innervation to the eye... 

If, however, the patient exhibits only a unilateral fixed and dilated pupil without evidence of ptosis or extraocular muscle involvement, the clinician should perform the pilocarpine test, first using a 0.125% solution to reveal any cholinergic hypersensitivity as evidence for Adie s pupil. If there is no local iris damage by slit-lamp examination, no sector palsy of the iris sphincter, and no cholinergic hypersensitivity demonstrated by the 0.125% pilocarpine test, then the condition might be associated with interruption of the preganglionic innervation to the iris sphincter (i.e., third-nerve palsy). If the patient has third-nerve palsy, topically instilled pilocarpine in moderate concentrations activates the muscarinic receptor sites on the iris sphincter. Therefore if 0.125% pilocarpine reveals no cholinergic hypersensitivity, the practitioner... 

Pupil size and function can be affected by peripheral autonomic action and by centrally initiated impulses.The iris is an excellent indicator of autonomic activity because of the delicate balance between adrenergic and cholinergic innervation to the iris dilator and iris sphincter muscles, respectively. By acting directly on these muscles, both sympathetic and parasympathetic agents can influence pupil size and activity. 

The size of the pupil is determined by the balance of forces exerted by the dilator muscles fibers (sympathetically innervated and radially arranged) and the constrictor muscle fibers (parasympathetically innervated and circularly arranged) of the iris. Normally both sets of muscle fibers have a constant degree of tonus and act reciprocally to dilate or constrict the pupil. Any substance that paralyzes the constrictor muscle fibers (parasympathol3d ic) allows the unopposed tone of dilator muscle fibers to widen the pupil. 

Acetylcholine is the transmitter between the constrictor muscle fibers and the parasympathetic nerve that innervates them. Therefore, acetylcholine and its congeners stimulate the constrictor muscle fibers of the iris and constrict the pupil. Atropine and related compounds paralyze the constrictor muscle fibers and cause widening or dilatation of the pupil. 

For effector tissues with dual innervation, PANS is dominant. These include the SA and AV nodes of the heart, the pupil, G1 and GU muscles, and sphincters. SANS is dominant only in terms of vascular tone and thermoregulatory sweat glands. 

MIOSIS Morphine and most /i and )Cagonists cause constriction of the pupil by an excitatory action on the parasympathetic nerve innervating the pupd. After toxic doses of jX agonists, the miosis is marked, and pinpoint pupils are pathognomonic however, marked mydriasis occurs if asphyxia intervenes. Some tolerance to the miotic effect develops, but addicts with high circulating levels of opioids continue to have constricted pupils. Therapeutic doses of morphine increase accommodation and lower intraocular pressure in normal and glaucomatous eyes. 

Complex organ control— the eye The eye contains multiple tissues with various functions, several of them under autonomic control (Figure 6-5). The pupil, discussed above, is under reciprocal control by the SANS (via alpha receptors) and the PANS (via muscarinic receptors) acting on two different muscles in the iris. The ciliary muscle, which controls accommodation, is under primary control of muscarinic receptors innervated by the PANS, with insignificant contributions from the SANS. The ciliary epithelium, on the other hand, has important beta receptors that have a permissive effect on aqueous humor secretion. 

The cornea, the protective outer layer of the eye, is heavily innervated with sensory neurons, triggering the blink reflex and tear duct secretion in response to irritation. The cornea is also an essential optical element, supplying two thirds of the total refraction in the eye. Behind the cornea is a clear fluid, the aqueous humor, in which the central aperture of the iris, the pupil, is free to constrict or dilate. The two actions are accomplished by opposing sets of muscles. 

Boris Babkin (center), a pupil of Pavlov, translated the Russian scientist s views into English and was responsible for disseminating many of his ideas in North America. Babkin, himself, developed novel theories regarding the parasympathetic innervation (bottom) of the gastric glands. 

Visceral motor fibers are carried in the occu-lomotor nerve (III) to the muscles of the pupil of the eye. They are carried in the facial nerve (VII) to the glands of the head other than the parotid, which is innervated by the glossopharyngeal nerve (IX). The vagus (X) innervates all thoracic viscera and abdominal viscera to the splenic flexure of the colon. 

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