Special Senses & Photoreceptors

Introduction: Sensory System

• The special senses are sight, hearing, smell, taste, touch, and equilibrium.
• All senses rely on specialized sensory receptors and allow the body to detect changes that are external or internal stimuli.
• The senses are linked to the nervous system, interpreting stimuli and producing an appropriate response.

Sensory Receptors

• The five kinds of sensory receptors in the body are chemoreceptors, photoreceptors, mechanoreceptors, thermoreceptors, and pain receptors.
• Chemoreceptors respond to certain chemicals; an example would be tongue taste buds.
• Photoreceptors respond to light wavelengths; an example would be retina rods and cones.
• Mechanoreceptors Respond to touch or movement; an example would be cochlear hair cells.
• Thermoreceptors respond to heat or cold; an example would be skin heat receptors.
• Pain receptors respond to tissue damage; an example would be skin pain receptors.

Sensory Receptors for Senses

• Sight (eye) receptors are rods and cones. Rods and cones are photoreceptors stimulated by light wavelengths.
• Hearing (ear) receptors are hair cells, which are mechanoreceptors stimulated by vibrations.
• Smell (nose) receptors are olfactory cells, which are chemoreceptors stimulated by chemicals in the air.
• Taste (tongue) receptors are taste buds, which are chemoreceptors stimulated by chemicals in food.
• Touch (skin) receptors are Pacinian corpuscles, which are mechanoreceptors stimulated by pressure.
• Equilibrium (ear) receptors are maculae and cristae, which are mechanoreceptors stimulated by deflection.

Sensory Areas of the Brain

• All received stimuli are carried along sensory neurons to specific areas of the brain.
• The sensory cortex includes multiple brain areas where sensory information is received and processed.
• There are six cortices linked to the six different senses: visual, auditory, olfactory, gustatory, somatosensory, and vestibular cortex.

Introduction to the Eye

• The eye is the sensory system's organ of sight, gathering light or radiant energy from the environment and focusing it on the sensory receptors on the retina to form an image.
• The image converts into nerve impulses that are carried along sensory neurons to the brain.
• The visual cortex interprets the impulses to form an image.

Outer Layers of the Eye

• The wall of the human eye consists of 3 layers: the sclera, the choroid, and the retina.
• The outer, fibrous coat consists of the white sclera, and the transparent front area of the sclera is the cornea, allowing light to enter the eye
• The middle layer is the choroid, highly vascular containing numerous blood vessels.
• The retina is the innermost layer, containing the receptors that detect light.

Cavities of the Eye

• The eye contains two fluid-filled cavities at the front and back.
• The anterior cavity is at the front, between the cornea and lens, and contains a watery fluid called aqueous humor.
• The posterior cavity is at the back of the eye, between the lens and retina, and contains a gel-like fluid called vitreous humor.

Anterior Anatomy of the Eye

• The pupil allows light to enter the eye.
• The iris is a ring of smooth muscle that constricts or dilates to alter the diameter of the pupil; it contains pigments that gives the eye its color.
• The lens is a transparent, biconcave disk behind the iris, and it focuses light on the retina.
• The ciliary body consists of muscles that change the shape of the lens for focusing.

Posterior Anatomy of the Eye

• The retina is a sensory layer covering the back of the eye.
• The retina is lined with over 120 million photoreceptor cells, which are rods and cones that detect light and send nerve impulses to the brain along the optic nerve.
• The fovea is a dip in the center of the retina where most color-sensitive cone cells are located.
• The "blind spot" is a part of the retina where the optic nerve passes through and contains no photoreceptor cells, so no light is detected in this area.

Rods and Cones

• Rods and cones are sensory cells that detect the different properties of incoming light.
• Cones detect the color of light and exist in three forms: red, green, and blue. Cones are located in the center of the retina, are highly concentrated at the fovea.
• Rods detect brightness of light, “seeing” in black and white, and are found at the retina’s edges.

Retina Cells

• Light enters the eye and strikes the retina, passing through the neuronal cell layers before reaching/activating the photoreceptors.
• Horizontal and amacrine cells communicate laterally between the neuron layers.
• The photoreceptors then initiate the synaptic communication back toward the ganglion cells.

Photoreceptor Density

• Visual information from peripheral vision is generally detected by rod cells.
• Cone photoreceptor cells facilitate high-acuity vision due to being densely packed at the fovea, which corresponds to the center of the visual field.
• Cone cells outnumber rod cells in the human retina by about 20 times.

Synaptic Convergence

• Photopic vision uses cone photoreceptors of the retina and handles high-acuity sight and color during the daytime.
• Scotopic vision uses rod photoreceptors and enables nighttime visibility under low-light conditions.
• Mesopic vision handles intermediate lighting, indoor lighting or outdoor traffic lighting during the night.
• Rod cells have high synaptic convergence. High-convergence networks add many small signals together for a larger signal. It is difficult to identify exactly which photoreceptor is activated by incoming light, which is why accuracy is poor when seeing stimuli in our peripheral vision.
• Cone cells precisely identify the location of incoming light.

Phototransduction

• Photoreceptors do not fire action potentials but rather respond to light changes with graded receptor potentials (depolarization or hyperpolarization).
• The amount of glutamate released changes along with the membrane potential
• Photoreceptors hyperpolarize in light and depolarize in dark.
• When the photoreceptor moves to light, the cell hyperpolarizes.
• Light reaches the photoreceptors and causes a conformational change in opsin (a special receptor protein)
• The opsin receptor has a pre-bound chemical agonist called retinal (vitamin A).
• The opsin + retinal comprises the photopigment rhodopsin.
• In the dark, the photoreceptor's membrane potential is more depolarized than the "typical" neuron (approximately -40 mV).

Eye Accessory Structures

• Eyebrows and eyelashes protect against foreign particles entering the eye and scratching its surface.
• Eyelids protect the anterior portion of the eye.
• The conjunctiva is a thin, clear membrane that covers the front surface of the eye and inner eyelid, keeping the eye moist and lubricated.
• The lacrimal apparatus has glands producing tears, which moisten the eye, kill microbes, and spread across the eye during blinking.

Vision Step One

• Light rays pass through the transparent cornea.
• The cornea focuses the rays throught the pupil into the lens.
• The pupil dialates (widens) in dim light to allow more light in
• The pupil constricts (narrows) in bight light, to allow less light in

Vision Step Two

• Light passes through the lens, which focuses it onto the retina.
• The ciliary muscles constrict or relax, making the lens thicker or thinner depending on the object's distance.
• For distant objects, the ciliary muscles contract, and the lens is pulled thinner.
• For nearby objects, the ciliary muscles relax, and the lens is made thicker.

Vision Step 3

• Light rays hit a layer of sensory receptor cells in the retina.
• Cone cells detect the color of light, exist in three types, and the colors of light they detect are red, green, and blue.
• Rod cells detect brightness or light intensity and only "see" in black and white.

Vison Step 4

• Rods and cones send nerve impulses along the optic nerve to the visual cortex, which interpretss the signals as visual images.

How the Brain "Sees"

• Sensory information from each eye is carried by the optic nerve.
• Optic nerves from each eye cross at the optic chiasm
• Information from the right visual field of both eyes goes to the left side of the brain
• The brain sends information from the left visual field of both eyes to the right side.
• Neurons in the visual cortex flip the image horizontally so humans "see" images correctly

Visual Fields

• The fovea separates the retina into sections: the nasal and temporal retina
• The nasal retina is the medial/located toward the nose
• The temporal retina is the lateral oortion/located toward the temples/temporal lobe

Pathways to the Brain

• All information from the left visual field enters the right hemisphere, whereas visual information from the right visual field enters the left hemisphere.
• The nasal retina from one eye combined with the temporal retina from the other view an entire hemifield.

Striate Cortex

• The organizational patter is where a section of retinal inputs maps onto nuerons of particular section of V1. This is known as retinotopic organization.

Post-Striatal Processing

• Ventral stream is responsible for object recognition, and fusiform face area.
• Dorstal stream is responsible for motion and spatial recognition.

Test your knowledge of special senses, sensory receptors, and their functions. Explore mesopic vision, photoreceptor activity, and the roles of rod cells in the retina. Learn about light detection, conformational changes, and synaptic convergence in rod cells.