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.

Sound vibrations from the air are collected by the outer ear, including the cartilaginous pinna. Air vibrations interface with the tympanic membrane, transferring the vibrations to the bones of the middle ear. These bones interface with the oval window to transfer the vibrations to the fluid of the cochlea found in the inner ear, where nerve endings translate the vibrations into electrical signals.

The malleus, incus, and stapes are the bones of the middle ear and are considered the smallest bones in the human body.

Sounds from the external environment are first met by the pinna which directs them towards and into the external auditory meatus (or opening of the ear). Sound travels towards the tympanic membrane or eardrum, which vibrates against the ossicles. The ossicles then transmit these vibrations to the cochlea which convert the vibrations into nerve impulses which travel to the brain through the vestibulocochlear nerve.

The internal auditory meatus is a canal within the petrous part of the temporal bone. The canal lies between the posterior cranial fossa and the inner ear. This canal provides passage through which the vestibulocochlear nerve, the facial nerve, and the labyrinthine artery pass from inside the skull to the inner ear and face. It also contains the vestibular ganglion. If a tumor were to grow in this area it would have a number of consequences including affecting taste (via chorda tympani of facial nerve), cause dizziness (via the vestibular ganglion), and cause dry eye (via facial nerve).

The semicircular canals detect rotation. They consist of three bony canals at right angles to each other. Each is filled with a fluid called endolymph and movement of the body causes the fluid to move. The fluid's movement against tiny hair cells within the canals allows the body to detect rotational acceleration. The cochlea is involved in hearing and the ossicles collectively transmit sound from the external environment/tympanic membrane to the cochlea.