Sensory Organs - MCAT Biological and Biochemical Foundations of Living Systems

The fovea centralis, also known as the focal point, is the point where an image is centered when you stare at objects. This region of the retina has a high number of cone and rod cells, and explains why we can only clearly focus on one object at a time. Only the object lined up with the fovea centralis will remain clear, while peripheral objects appear blurry.

The cornea is avascular and made primarily of collagen, not lipids. When light first reflects off of a focused object, it strikes the eye on the cornea, where its high refractive index (1.4) allows most of the bending of light to occur at the interface between air and cornea. Unlike the lens, the cornea does not change its shape. The lens is tethered by suspensory ligaments, which are further connected to ciliary muscles. When the ciliary muscles contract, they release tension on the suspensory ligaments, thus making the lens more spherical and bringing the focal point closer to the lens.

The incus, along with the malleus and stapes, are three bones found in the middle ear, which act as a lever system to transmit sound waves to the oval window.

The pinna is simply the skin and cartilage component of the outer ear. The semicircular canals are found in the inner ear, and are responsible for adapting the head and body to positional changes, thus maintaining balance. The perilymph is the fluid found within the inner ear.

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.

Rods and cones are two photoreceptors located on the back of the retina that are responsible for black, white (dark and light), and color sense. Rods are responsible for light and dark and are extremely sensitive, while cones sense color and are less sensitive.

The pupil and retina are both anatomical structures of the eye. The pupil controls the amount of light to pass through the eye, and the retina converts the electromagnetic signal to a neural stimulus.

The malleus, incus, and stapes are the three bones of the ear. Sound enters the external ear by using air as a medium. In the middle ear, vibrations from the air transition to vibrations through bone. The inner ear converts these bone vibrations to fluid vibrations in the cochlea, which converts the fluid vibrations to electrical stimuli for the nervous system.

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.

Rods and cones are the primary photoreceptors in our eyes. The rods are responsible for night vision, allowing us to differentiate different grays and blacks, whereas the cones are responsible for sensing color. There are three main pigments in cones, each perceiving one type of color: red, green, or blue. Deficiency in one type of cone can result in color blindness.

The retina contains both rods and cones and is responsible for converting electromagnetic stimulation to electrical impulses. These impulses are fired through the optic nerve to the occipital lobe for processing. The lens helps to focus light on the retina.

Rods and cones are the primary photoreceptors in our eyes. The rods are responsible for night vision, allowing us to differentiate different grays and blacks, whereas the cones are responsible for sensing color. Rods are concentrated in the periphery of the retina, making them essential for peripheral vision as well.

The retina contains both rods and cones and is responsible for converting electromagnetic stimulation to electrical impulses. These impulses are fired through the optic nerve to the occipital lobe for processing. The lens helps to focus light on the retina.

The optic nerve collects stimuli from the retina and exits through the back of the eye to relay the information to the occipital lobe of the cerebrum. The point at which the optic nerve exits the eye contains no photoreceptors (rods and cones), and is unable to interpret light signals. This leads to a blind spot in the eye at the optic disk.

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.

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.

The two major photoreceptors of the body are cone cells and rod cells. Cone cells are most highly concentrated in the fovea and are best for color vision and visual acuity. In contrast, rod cells are found in higher concentrations away from the fovea (at the periphery of the retina) and used for seeing low levels of light and peripheral vision. Rod cells are therefore the photoreceptors used for night vision.

Ganglion cells and bipolar cells are also cells of the retina. They are used in the transmission of the electrical signals from the photoreceptors to the optic nerve and brain.

The first structure that light will encounter as it enters the eye is the cornea, a transparent protein disc designed to refract light toward the eye interior. The aqueous humor is the liquid medium between the cornea and the lens of the eye. Light will pass through the aqueous humor and the pupil before hitting the lens. The lens then focuses the light onto the back of the eye. The vitreous humor is the liquid medium between the lens and the retina at the back of the eye.

The iris is used to control the shape of the pupil, and does not contain photoreceptors. All photoreceptors are located on the retina, which lines the back wall of the eye. A small region known as the fovea centralis houses a large number of cones (photoreceptors) at the back of the eye, but is part of the greater retinal structure.

The only true statement given is that light will pass through the aqueous humor before passing through the vitreous humor.

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 vestibular apparatus is the name given for the entire inner ear structure that interprets position. It contains substructures such as the semicircular canals, the ampulla, and the crista. The crista are the specialized hair cells that respond to fluid movement; fluid does not move within the crista themselves.

All other listed answers are true.

The cochlear and vestibular nerves join to form the auditory nerve. The crista are specialized hair cells that help in postural equilibrium and send information via the vestibular nerve. The incus is one of the three auditory bones (the others include the malleus and the stapes), the motion of which is part of sound reception. This information is transmitted via the cochlear nerve. Finally, the cochlea is the fluid-filled structure of the inner ear that translates movement into vibrations (also involved in sound reception). All of the given structures take part in transmitting information to the acoustic nerve.

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.

After taste receptor cells contact solutes in the saliva, action potentials cause a calcium influx through ion channels, which leads to the release of the neurotransmitter that induces an action potential in the afferent nerve. This action potential is carried to the brain for integration and interpretation.

Chloride ions do not enter the receptor during this process.