The pigmented part of the eye, called the iris, regulates the amount of light entering the eye. The iris is actually a band of circular muscle capable of regulating the size of the pupil. By dilating or constricting, this ring controls the amount of light that can enter the eye.

The ciliary muscles are responsible for altering the lens shape. When these muscles contract, the lens flattens and the focal length is reduced, allowing focus on objects closer to the eye. While the lens plays an important role in focusing light, it cannot be stimulated directly and relies on action of the ciliary muscles to change its shape.

The optic nerves cross over in a location known as the optic chiasm, located posterior to the eyes in the brain. The optic tract refers to the portion of the optic nerve that carries signals prior to the optic chiasm, and thus is not involved in the cross. The result of the optic chiasm is that information from the right eye is transmitted to the left visual cortex, while information from the left eye is transmitted to the right visual cortex.

The blind spot created by the optic nerve exiting the eye is known as the optic disc.

Cones are responsible for the perception of color. Three different types of cones exist, each type containing a different pigment specifically tuned to a wavelength or color of light.

Rods are predominantly responsible for the transmission of images in low-intensity light, however do nothing to perceive color. The ciliary body is a muscle involved in the dilation of the pupils, and the vitreous humor is the aqueous matrix that occupies the space between the lens and the retina within the eye.

Mechanoreceptors, propioreceptors, photoreceptors, and nociceptors are all classified as somatic sensory receptors, and are responsible for conveying information about the external environment to the brain.

Mechanoreceptors give the perception of touch and pressure. Propioreceptors are responsible for detecting the orientation of the body, for example, knowing where your feet are without looking at them. Photoreceptors are located in the eyes and detect light. Nociceptors are able to detect and transmit the sensations of pain and extreme temperatures.

Proprioceptors relay information about the relative position of the body in space. These receptors are embedded in the skin and musculoskeletal system, and are triggered by movement. The prefix "proprio-" means self, making proprioreceptors "receptors of the self."

Barioreceptors detect pressure changes in the blood and are primarily located in the aorta. Photoreceptors include rods and cones, and are responsible for receiving electromagnetic input in the eyes. Thermoreceptors are primarily located in the skin and detect changes in temperature. Somatic sensors are a general category of receptors that carry afferent signals to the brain.

Somatosensory neurons are most sensitive to mechanical force, temperature change, and tissue damage. Nociception is the processing of pain signals, which could result from any of these stimuli.

While somatosensory neurons are found within specialized touch receptors, the touch receptor organ for the tactile system is the skin. These sensors will detect stimuli for pressure, temperature change, and pain. Though these sensations can be detected in some internal regions of the body as well, the sensation of touch must be triggered by an external stimulus. As such, the receptors must have contact with the external environment (via the skin).

Sensors for proprioception can be found within joint tissues, muscles and tendons. These sensors detect spatial orientation and motion, stretch and compression, and internal pain. Stimuli for proprioception must be triggered by the body's internal environment.

The only entirely true statement is that proprioceptive stimuli are internal and generated by body position and movement, while tactile stimuli are external. Proprioception is the awareness of the body in space, which allows animals to have the sensation of the placement of their bodies without needing to see what they are doing or feel their way. Tactile perception is the body's interaction with an external stimulus, such as pain, temperature, or pressure.

While most tactile stimuli are transmitted through the spinal trigeminal pathway, certain sensations use the main sensory trigeminal pathway.

Taste buds are bulb-shaped and open to the environment through a small pore. Chemical compounds bind to small cellular hairs of taste receptor cells to stimulate an action potential, relaying the taste to the brain. Basal cells are located at the bottom of the taste bud. As taste receptors die, basal cells will differentiate into new receptors to maintain the taste bud.

Options must be considered carefully here, as all answer choices relate to the gustatory system. The only similarity is that salty (sodium ion) and sour (hydrogen ion) tastes both involve the passing of these ions through sodium channels. (Hydrogen can also pass through proton transport channels.) While sour tastes can block the efflux of potassium ions, this is not true for salts, making this an incorrect answer choice.