Pupil parasympathetic system

Inspection of the retina during an ophthalmoscopic examination is greatly facilitated by mydriasis, or the dilation of the pupil. Parasympathetic stimulation of the circular muscle layer in the iris causes contraction and a decrease in the diameter of the pupil. Administration of a muscarinic receptor antagonist such as atropine or scopolamine prevents this smooth muscle contraction. As a result, sympathetic stimulation of the radial muscle layer is unopposed, causing an increase in the diameter of the pupil. These agents are given in the form of eye drops that act locally and limit the possibility of systemic side effects. 

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). 

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. 

Many of the physiological effects of anti-ChEs are attributable to excess neurotransmitter ACh (Taylor 1996). The precise symptoms and the time course depend on the chemicals and the localization of the receptors affected. Early symptoms of cholinergic poisoning represent stimulation of muscarinic neuro-effectors of the parasympathetic system. Effects include slowing of the heart (bradycardia), constriction of the pupil of the eye, diarrhea, urination, lacrimation, and salivation. Actions at nicotinic skeletal neuromuscular junctions (motor end plates) result in muscle fasciculation (disorganized twitching) and, at higher doses. 

Ocular Effects. One study reported that seven children exposed to methyl parathion by inhalation, oral, and possibly dermal routes exhibited pinpoint pupils (miosis) (Dean et al. 1984). This effect is a consequence of the effects on the parasympathetic nervous system. No other studies were located regarding ocular effects in humans after oral exposure to methyl parathion. 

PAM chloride - 2-pralidoxime chloride. See oxime, parasympathetic nervous system -the part of the autonomic nervous system that decreases pupil size, heart rate, and blood pressure and increases functions, such as secretion of saliva, tears, and perspiration. 

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. 

How did scopolamine and atropine, both components of henbane, kill King Hamlet To answer this, let s return to the autonomic nervous system. Recall the functions of the ANS that I mentioned previously. For example, it controls heart and breathing rate, intestinal motility, pupil dilation, salivation, and perspiration The two major components of the ANS, the parasympathetic and sympathetic nervous systems (see Fig. 2—4), essentially function in competition with each other to maintain... 

Recall that scopolamine, an ingredient in henbane, blocks muscarinic acetylcholine receptors. This blockade essentially removes the influence of the parasympathetic nervous system on the body. In the absence of this influence, the balance of forces is upset and the sympathetic nervous system gains the upper hand thus, your heart rate increases, your pupils dilate, salivation stops, your ability to urinate is impaired, and you become constipated overall, things get very uncomfortable. But none of this is directly lethal (unless the constipation makes one commit suicide). If you do die from an overdose of henbane, it is believed to result from either a complex series of events in your brain that lead to the loss of control of your diaphragm, causing death from asphyxiation, or from cardiac arrest. This is why the deadly nightshade is so deadly and how Shakespeare chose to kill King Hamlet with henbane. 

In rats, cyanide poisoning produces overactivity of the parasympathetic nervous system causing miosis, salivation, lacrimation, and twitching. Symptoms of severe poisoning in rats include constricted pupils followed by markedly dilated pupils, congested vessels of the iris and retina, and possibly papilledema. 

In the peripheral nervous system, norepinephrine is an important neurotransmitter in the sympathetic branch of the autonomic system. Sympathetic nerve transmission operates below the level of consciousness in controlling physiological function of many organs and tissues of the body. The sympathetic system plays a particularly important role in regulating cardiovascular function in response to postural, exertional, thermal, and mental stress. With sympathetic activation, the heart rate is increased, peripheral arterioles are constricted, skeletal arterioles are dilated, and the blood pressure is elevated. In addition, sympathetic nerve stimulation dilates pupils inhibits smooth muscles of the intestines, bronchi, and bladder and closes the sphincters. Sympathetic signals work in balance with the parasympathetic portion of the autonomic nervous system to maintain a stable internal environment. 

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