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.

Mechanoreceptors are associated with the perception of touch/proprioception. With this being said, free nerve endings give nociceptive sensory information to perceive pain and would not be considered mechanoreceptors, while all other options are.

Both Meissner's corpuscles and Merkel receptors are located superficially underneath the top layer of the skin, the epidermis. This allows these receptors to have a smaller receptor field where more sensitive sensory information can be picked up. To help remember this, use the following tip: "M&Ms (Meissner/Merkel) are small (small receptor fields) and lay on top (superficially) of your hand."

The gate theory suggests that mechanoreceptors inhibit nociception. This is done by the mechanoreceptors because they activate an inhibitory neuron that stops signaling of the nociceptors. This theory can apply to the human reaction to when we stub our toe, our natural reaction is to grab the hurt area which stimulates mechanoreceptors and inhibits the nociception.

Muscles spindles and GTO's are both receptors of A alpha fibers. These fibers are associated with proprioception within the body. Furthermore, muscle spindles respond to the stretch of a muscle while Golgi tendon organs respond to tension at the tendinous junctions.