Describe in detail how the diameter of the pupil of the eye is regulated by the parasympathetic an sympathetic nervous systems. In doing so, please describe the neurotransmitters and receptors that are involved

 

The Role of Autonomic Nervous System in pupillary response.

The Sympathetic nervous system causes pupillary dilation through the binding of norepinephrine with the alpha 1a adrenoreceptors of the dilator pupillae muscles.

The Parasympathetic nervous system causes pupillary constriction through the binding of acetylcholine with the m3 receptors of sphincter pupillae muscles.

Step-by-step explanation

Role of the ANS in pupillary response.

SYMPATHETIC

When a person is under stress, the body triggers the "fight or flight" response driven by the sympathetic nervous system; and pupillary dilation is one of its many functions. In response to a stimulus (physical or emotional stress), the hypothalamus sends signal through the 1st-order neuron down to the spinal cord's center of Budge. From here, the 2nd-order neuron (pre-ganglionic sympathetic) - carrying the signal - ventrally exits the spinal cord and synapse with the superior cervical ganglion. The signal is then carried through the 3rd-order postganglionic neuron to synapse with the long ciliary nerves. The axons of these nerves then trigger the release of norepinephrine neurotransmitters, and cause them to bind to the alpha 1a adrenoreceptors (predominant receptor subtype in dilator pupillae muscles). The binding ultimately causes the calcium ion concentration to increase thus, eliciting contraction of the muscle that dilates the pupil (dilator pupillae muscle).

PARASYMPATHETIC

The pupillary constriction pathway begins at the nucleus of Edinger-Westphal in the midbrain. The nerve fibers, together with the oculomotor nerve fibers, enter the orbit and eventually synapse with the ciliary ganglion. From here on, the parasympathetic postganglionic fibers (short ciliary nerves) trigger the release of acytelcholine (Ach) neurotransmitters. Ach binds with the m3 receptors (predominant muscarinic receptor subtype in sphincter pupillae muscles) causing a cascade of events that ultimately leads to an intracellular increase of calcium ion concentration. Ultimately, sphincter pupillae muscles contract, leading to contstriction of the pupils.