The autonomic nervous system can be divided into the parasympathetic system and the sympathetic system. The autonomic system is responsible for regulating the body's unconscious activity. The sympathetic system is responsible for stimulating activities associated with "fight or flight," and includes mydriasis (dilation of the pupil). The parasympathetic response is responsible for "rest and digest," and includes mucus secretion, salivation, lacrimation, urination, digestion, and defecation. 

Though all of these nerves are involved with the eye, only one of them is involved with vision. The optic nerve (cranial nerve II) has a sensory function, allowing us to see. The oculomotor nerve (cranial nerve III) has a motor function, allowing eyelid and eyeball movement. The trochlear nerve (cranial nerve IV) also has a motor function, allowing the eye to turn downward and laterally. Lastly, the abducens nerve (cranial nerve VI) also has a motor function, allowing the eye to turn laterally.

Cranial nerve VIII is also known as the vestibulococchlear nerve. It performs sensory functions, transmitting sound and equilibrium information from the inner ear to the brain. Cranial nerve VI is also known as the abducens nerve. It performs motor functions, controlling the movement of the lateral rectus muscle of the eye. Cranial nerve XI is also known as the accessory nerve. It performs motor functions, controlling the sternocleidomastoid and trapezius muscles. Lastly, cranial nerve VII is also known as the facial nerve. It performs both sensory and motor functions, controlling the muscles of facial expression and conveying taste sensations from the tongue and oral cavity.

The twelve cranial nerves and their origins are given below:

I-Olfactory nerve; origin: frontal lobe

II-Optic nerve; origin: thalamus

III-Ophthalmic nerve; origin: midbrain

IV-Trochlear nerve; origin: midbrain

V-Trigeminal nerve; origin: pons

VI-Abducens nerve; origin: pons

VII-Facial nerve; origin: pons

VIII-Vestibulocochlear nerve; origin: pons

IX-Glossopharyngeal nerve; origin: medulla

X-Vagus nerve; origin: medulla

XI-Spinal Accessory nerve; origin: medulla

XII-Hypoglossal nerve; origin: medulla

The nerves to originate in the pons are V, VI, VII, and VIII (the trigeminal, abducens, facial, and vestibulocochlear nerves).

The parasympathetic nervous system is responsible for the "rest and digest" mechanism. When the body is at rest the following occurs: pupils constrict (ophthalmic branch of the trigeminal nerve), lacrimation and salivation occur (facial nerve and glossopharyngeal nerve), heart rate decreases, respiration rate decreases, and digestion increases (vagus nerve).

In addition to the cranial nerves, sacral nerves 2-4 (pelvic splanchnic nerves) carry parasympathic fibers that control erections, the bladder, and the bowels.

The facial nerve (CN VII) is responsible for the sensation of taste on the anterior two-thirds of the tongue. CN IX, the glossopharyngeal nerve, is responsible for posterior sensation. CN V, the trigeminal nerve, is also involved in certain sensations throughout the tongue.

There are twelve cranial nerves. They are given in order below, with their associated functional classes.

CN I - olfactory nerve - sensory

CN II - optic nerve - sensory

CN III - occulomotor nerve - motor

CN IV - trochlear nerve - motor

CN V - trigeminal nerve - both sensory and motor

CN VI - abducens nerve - motor

CN VII - facial nerve - both sensory and motor

CN VIII - vestibulocochlear nerve - sensory

CN IX - glossopharyngeal nerve - both sensory and motor

CN X - vagus nerve - both sensory and motor

CN XI - accessory nerve - motor

CN XII - hypoglossal nerve - motor

Cranial nerves 3, 7, 9, and 10 are all responsible for parasympathetic nuclei of the brainstem. Oculomotor CN 3 contains the Edinger-Westphal nucleus which controls the eye smooth muscles (which if damaged would result in a dilated pupil that would not constrict in bright light). Facial CN 7 controls the superior salivatory nucleus which controls the submandibular, sublingual, and lacrimal gland secretion. Glossopharyngeal CN 9 innervates the parotid gland secretions. The vagus CN 10 innervates the heart and lungs down into the chest cavity. Helpful hint** - Students often confuse the superior and inferior salivatory nuclei, remember that on the brainstem the facial nerve exits superior to the glossopharyngeal nerve and thus how it is properly named for their nuclei.

Rods are located in every part of the retina except the fovea centralis, which is where the cones that allow us to see most colors and detail are located. Rods are responsible for our peripheral vision, and are very sensitive to motion. Rods do not provide very good spatial differentiation, but their sensitivity to light is essential to providing definition.

The ciliary muscle surrounds the lens in a circular fashion. When contracted, the ciliary muscle will decrease in size, which allows the lens to become more convex in shape. This allows the eye to focus on closer objects.

The iris, or pupillary sphincter, is capable of constricting and dilating the pupil. The main muscle responsible for raising the eyelid is the levator palpebrae superioris. The orbicularis oculi is responsible for closing the eyelid.