CH 16

Sensory Receptors

• Provide information about external and internal environments.
• Respond to a stimulus.
• Each type of receptor responds best to a specific stimulus (e.g., light energy for eye receptors, sound energy for ear receptors).
• Transducers: Convert stimulus energy into electrical energy.
• Have a resting membrane potential.
• Receptor membranes have modality gated channels that respond to their type of stimulus.
• Action potentials are conveyed to the CNS for interpretation.

Structure of Sensory Receptors

• Receptors convey signals to the CNS via sensory neurons.
• Receptive field: The distribution area of the endings of a sensory neuron.
• Smaller receptive fields allow more precise stimulus localization.

Sensory Information

• Sensation: A stimulus we are consciously aware of.
  • Signals must reach the cerebral cortex to enter consciousness.
  • Only a fraction of stimuli result in sensations.
  • A lot of sensory input goes to other areas of the brain (e.g., blood pressure signals relayed to the brainstem).
• Receptors provide the CNS information about stimulus:
  • Modality
  • Location
  • Intensity
  • Duration

Modality

• Type of stimulus based on the "labeled line".
  • For example, the brain interprets optic nerve signals to the occipital lobe as visual, and cochlear nerve signals to the temporal lobe as auditory.

Location

• Determined by which receptive field is active.
• The postcentral gyrus has a body map represented by a homunculus.

Intensity

• Determined by the frequency of nerve signals to the CNS.
• Stronger stimuli:
  • Cause more neurons to fire.
  • Cause sensitive neurons to fire more frequently.

Duration

• Receptor adaptation helps determine stimulus duration.
• Adaptation: Decreased sensitivity to a continuous stimulus.
• Tonic receptors: Show limited adaptation and respond continuously.
  • Examples: Head position receptors in the inner ear; all pain receptors.
• Phasic receptors: Adapt rapidly and only respond to new stimuli.
  • Example: Pressure receptors.

Sensory Receptor Classification

• Categorized by distribution, stimulus origin, and stimulus modality.

Categorization by receptor distribution:

• General sense receptors:
  • Simple structures distributed throughout the body.
    • Somatic sensory receptors: Tactile receptors of the skin and mucous membranes; proprioceptors of joints, muscles, and tendons.
    • Visceral sensory receptors: Found in walls of internal organs, they monitor stretch, chemical environment, temperature, and pain.
• Special sense receptors:
  • Specialized receptors in complex sense organs of the head.
  • 5 special senses: olfaction, gustation, vision, audition, equilibrium.

Categorization by stimulus origin:

• Exteroceptors: Detect stimuli from the external environment.
  • Skin and mucus membranes; special sense receptors.
• Interoceptors: Detect stimuli from internal organs.
  • Visceral sensory receptors monitoring the internal environment.
• Proprioceptors: Detect body and limb movements.
  • Somatosensory receptors of muscles, tendons, and joints.

Categorization by modality of stimulus (stimulating agent):

• Five types: chemoreceptors, thermoreceptors, photoreceptors, mechanoreceptors, and nociceptors.
  • Chemoreceptors: Detect chemicals dissolved in fluid.
    • Include receptors for the external environment (e.g., smell of food) or internal environment (e.g., oxygen levels in blood).
  • Thermoreceptors: Detect changes in temperature.
    • Include receptors in the skin and hypothalamus.
  • Photoreceptors: Detect changes in light intensity, color, and movement.
    • In the retina of the eye.
  • Mechanoreceptors: Detect distortion of the cell membrane.
    • Include touch, pressure, vibration, and stretch receptors.
    • Function as baroreceptors, proprioceptors, tactile receptors, and specialized receptors in the inner ear.
  • Nociceptors: Detect painful stimuli.
    • Somatic nociceptors detect chemical, heat, or mechanical damage to the body surface or skeletal muscles.
    • Visceral nociceptors detect internal organ damage.

Tactile Receptors

• Abundant mechanoreceptors of the skin and mucous membranes.
• Endings can be encapsulated or unencapsulated.

Unencapsulated tactile receptors:

• Dendritic ends of sensory neurons with no protective cover.
  • Free nerve endings: Terminal ends of sensory neuron dendrites.
    • Simplest tactile receptors.
    • Reside close to the skin surface and in mucous membranes.
    • Mainly for pain and temperature but also light touch and pressure.
    • May be phasic or tonic.
  • Root hair plexuses: Wrap around hair follicles.
    • Located in the deeper layer of the dermis.
    • Detect hair displacement.
    • Phasic receptors.
  • Tactile discs: Flattened endings of sensory neurons extending to tactile cells (Merkel cells).
    • Tactile cells are specialized epithelial cells in the basal layer of the epidermis.
    • Respond to light touch.
    • Tonic receptors.

Encapsulated tactile receptors:

• Neuron endings wrapped by connective tissue or covered by connective tissue and glial cells (neurolemmocytes).
  • End (Krause) bulbs: Ensheathed in connective tissue.
    • Located in the dermis and mucous membranes.
    • Detect pressure and low-frequency vibration.
    • Tonic receptors.
  • Lamellated (Pacinian) corpuscles: Wrapped in neurolemmocytes and concentric layers of connective tissue.
    • Located deep in the dermis, hypodermis, and some organ walls.
    • Detect deep pressure, coarse touch, and high-frequency vibration.
    • Phasic receptors.
  • Bulbous (Ruffini) corpuscles: Wrapped in CT.
    • Within the dermis and subcutaneous layer.
    • Detect deep pressure and skin distortion.
    • Tonic receptors.
  • Tactile (Meissner) corpuscles: Intertwined endings wrapped in modified neurolemmocytes, covered in connective tissue.
    • In dermal papillae (especially in sensitive regions of the body).
    • Discriminative light touch—allow recognition of texture and shape.
    • Phasic receptors.

Proprioceptors

• General sensory receptors located in muscles, tendons, and joints.
• Specialized mechanoreceptors relay sensory information regarding body position and movement.
• All are tonic receptors (adapt slowly).
• Proprioception (the "sixth sense") – sense of body position and movement.
• Three types:
  • Muscle spindle – detect stretch in skeletal muscle.
  • Golgi tendon organ – detect stretch in tendon.
  • Joint kinesthetic receptor – detect stretch in the articular capsule.

Referred Pain

• Inaccurate localization of sensory signals.
• Signals from viscera perceived as originating from skin or muscle.
• Many somatic and visceral sensory neurons send signals via the same ascending tracts within the spinal cord.
• The somatosensory cortex is unable to determine the true source.

Clinical relevance:

• Heart attack pain may be referred to the pectoral region and medial arm.
  • Sympathetic innervation of the heart and somatic innervation of those skin regions both come from T1–T5 segments of the spinal cord.
• Kidney and ureter pain may be referred to the inferior abdomen.
  • T10–L2 spinal nerves.
• Visceral pain is often conveyed along sympathetic nerves, but occasionally on parasympathetic nerves.
• Bladder pain can be conveyed via sacral parasympathetic nerves and referred to the buttocks.

Phantom Pain

• Sensation associated with a removed body part.
• Occurs following amputation of a limb.
• Experience of pain from the removed part.
• Stimulation of the sensory neuron pathway on the remaining portion.
• The cell body of the sensory neuron is still alive.
• Pain is sometimes quite severe.

Olfaction: The Sense of Smell

• Detection of odorants dissolved in the air.
• Odorants (volatile molecules) dissolved in nasal mucus are detected by chemoreceptors.
• Provides information about food, people, and danger.
• We can distinguish thousands of different odors.
• Olfactory epithelium—sensory receptor organ.
  • Located in the superior region of the nasal cavity; has three types of cells:
    • Olfactory receptor cells: Detect odorants.
    • Supporting cells: Sustain receptors.
    • Basal cells: Continually replace olfactory receptor cells.
• Replacement and sensitivity of receptors decline with aging.
• Lamina propria:
  • Areolar connective tissue layer internal to the olfactory epithelium.
  • Houses blood vessels, nerves, and olfactory glands.
• Olfactory (Bowman) glands:
  • Help form the mucus covering the olfactory epithelium.
• Olfactory receptor cells:
  • Primary neurons in the sensory pathway for smell.
  • Bipolar structure: a single dendrite and unmyelinated axon.
  • Olfactory hairs: cilia projecting from the receptor cell dendrite.
    • House chemoreceptors for a specific odorant.
    • Perceived smell depends on which cells are stimulated.
• Olfactory nerves (CN I):
  • Bundles of olfactory cell axons.
  • Project through the skull’s cribriform plate and enter olfactory bulbs.
• Olfactory nerve structures and pathways:
  • Olfactory bulbs (pair):
    • Ends of olfactory tracts located under the brain’s frontal lobes.
    • Olfactory nerve fibers synapse here with mitral cells and tufted cells.
    • Connections form olfactory glomeruli.
  • Olfactory tracts (pair):
    • Axon bundles of mitral and tufted cells on the inferior frontal lobe surface.
    • Project directly to the primary olfactory cortex (in the temporal lobe), hypothalamus, amygdala, and other regions.
    • Does not project through the thalamus like other sensory pathways.
• Detecting smells:
  • Sniff repeatedly or breathe deeply.
  • Mucus contains odorant-binding proteins.
  • Olfactory sensations begin when an odorant binds to a protein and the protein stimulates a receptor cell (rapidly adapting receptor).
  • G-protein in the receptor cell activates adenylate cyclase, converting ATP to cAMP.
  • cAMP leads to the opening of ion channels for Na^+ and Ca^{2+} (depolarization).
  • An action potential is triggered on the axon and conducted to the glomerulus.
  • A secondary neuron conducts signals to various CNS areas:
    • Cerebral cortex (perceive, identify smell).
    • Hypothalamus (visceral reaction to a smell).
    • Amygdala (smell recognition, emotional reaction).

Gustation: The Sense of Taste

• Gustation = sense of taste; detection of tastants.
• Gustatory cells are chemoreceptors within taste buds.
• Papillae of the tongue:
  • Filiform papillae: short and spiked.
    • No taste buds (no role in gustation); help manipulate food.
    • Located on the anterior two-thirds of the tongue surface.
  • Fungiform papillae: mushroom-shaped.
    • Each contains a few taste buds.
    • Located on the tip and sides of the tongue.
  • Foliate papillae: leaflike ridges.
    • Not well developed.
    • House a few taste buds in early childhood.
    • Located on the posterior lateral tongue.
  • Vallate (circumvallate) papillae: largest, least numerous.
    • Contain most of the taste buds.
    • Located in a row of 10 to 12 along the posterior dorsal tongue surface.
• Taste buds: onion-shaped organs housing taste receptors.
  • Gustatory cells: receptor cells detect tastants.
  • Supporting cells: sustain gustatory cells.
  • Basal cells: neural stem cells that replace gustatory cells.
• Gustatory cells: neuroepithelial chemoreceptive cells of taste buds.
  • Gustatory microvillus (taste hair) forms a dendritic ending.
  • The microvillus often extends through the taste pore to the tongue surface.
  • Tastants (tasty molecules) dissolve in saliva and stimulate the microvillus.
• Gustatory pathways:
  • Sensory neurons connect to multiple gustatory cells in the tongue and project to the medulla.
  • In anterior parts of the tongue, sensory neurons are part of the facial nerve (CN VII).
  • In the posterior two-thirds of the tongue, sensory neurons are part of the glossopharyngeal nerve (CN IX).
  • Secondary medullary neurons project to the thalamus.
  • Tertiary thalamic neurons project to the primary gustatory cortex in the insula.
• Five basic taste sensations spread over broad regions of the tongue:
  • Sweet: Produced by organic compounds, for example, sugar or artificial sweeteners.
  • Salt: Produced by metal ions, for example, Na^+ and K^+.
  • Sour: Associated with acids, for example, vinegar.
  • Bitter: Produced by alkaloids, for example, unsweetened chocolate.
  • Umami: Taste related to amino acids producing savory or meaty flavor.
• Transduction in gustatory cells:
  • For sweet, bitter, and umami, the tastants are molecules.
    • The tastant binds to a specific cell membrane receptor.
    • G protein is activated, causing the formation of a 2nd messenger.
    • Results in cell depolarization.
  • For salt and sour, the tastants are ions.
    • The tastant depolarizes the cell directly.
  • The depolarized gustatory cell releases a neurotransmitter, stimulating a primary neuron (in CN VII or CN IX).
• Gustatory pathway:
  • A primary neuron in the cranial nerve brings a signal to the nucleus solitarius within the medulla.
  • Medullary activity triggers salivation and stomach secretions.
    • Nauseating stimuli instead trigger gagging or vomiting.
  • The signal is relayed to the thalamus.
  • Then relayed to the primary gustatory cortex for conscious taste.
  • Taste is integrated with temperature, texture, and especially smell.
  • Food has less taste if olfaction is blocked (e.g., having a cold).

Accessory Structures of the Eye

• Accessory structures are attached to or around the eye.
  • Include: six extrinsic eye muscles, eyebrows, eyelids, eyelashes, conjunctiva, and lacrimal glands.
• Eyebrows:
  • Located along the supraorbital ridge.
  • Aid in nonverbal communication (e.g., surprise) and prevent sweat from dripping into eyes.
• Eyelashes:
  • Extend from the margins of the eyelids.
  • Prevent objects from coming into contact with the eye; can initiate a blink reflex.
• Eyelids (palpebrae):
  • Made of:
    • Fibrous core (tarsal plate).
    • Orbicularis oculi muscle.
    • Thin skin.
  • The upper eyelid is raised by the levator palpebrae superioris muscle.
    • Widens the palpebral fissure (eyeslit).
  • Eyelids join at medial and lateral palpebral commissures (canthi).
  • Near the medial commissure is the lacrimal caruncle: a small pink body.
  • Tarsal glands: sebaceous glands of the tarsal plate.
    • Release an oily secretion at the eyelid edge.
    • Infection = chalazion
  • Sebaceous and sweat glands at the eyelid base.
    • Infection = stye
• Conjunctiva: a transparent lining of eye and lid surfaces.
  • Specialized stratified columnar epithelium.
  • Ocular conjunctiva covers the anterior sclera (white of eye).
  • Palpebral conjunctiva covers the internal surface of the eyelid.
  • Conjunctival fornix: junction of the ocular and palpebral conjunctiva.
  • Contains numerous goblet cells to moisten the eye, many blood vessels to nourish the sclera, and abundant nerve endings.
  • Does not cover the cornea so as not to interfere with light passage.
  • Pink eye = conjunctivitis.

Eye Infections:

• Chalazion: cyst within the eyelid; forms from infected tarsal gland.
• Stye: reddened, pus-filled area beneath the skin of the eyelid; forms from infection of a sebaceous gland or modified sweat gland.
• Conjunctivitis: inflammation of the conjunctiva due to infection or irritant.
• Lacrimal apparatus: produces, collects, and drains fluid.
  • Lacrimal fluid: water, Na^+, antibodies, and lysozyme (antibacterial enzyme).
    • Lubricates, cleanses, and moistens the eye, reduces eyelid friction, defends against microbes, and oxygenates and nourishes the cornea.
  • Lacrimal gland: produces fluid and secretes it through ducts.
    • Located in the superolateral orbit.
    • Blinking (15 to 20 per minute) washes fluid over the eye.
  • Fluid drains into lacrimal puncta (holes by lacrimal caruncle).
    • Each punctum has a lacrimal canaliculus draining to the lacrimal sac.
  • The sac drains to the nasolacrimal duct to the nasal cavity.
    • Fluid then mixes with mucus and is swallowed.
  • Excess lacrimal fluid produces tears.

Eye Structure

• The eye is almost spherical.
  • 2.5 cm diameter.
  • Located in the skull’s orbit, padded by orbital fat.
  • The interior contains two cavities:
    • The posterior cavity (behind the lens) contains permanent vitreous humor.
    • The anterior cavity (in front of the lens) contains circulating aqueous humor.
    • Subdivided into the anterior chamber and posterior chamber, separated by the iris.
  • The wall is formed by three tunics:
    • Fibrous (external).
    • Vascular (middle).
    • Retina (inner).
• Vitreous humor (vitreous body):
  • Transparent gelatinous fluid in the posterior cavity (behind the lens).
  • Permanent fluid first produced in embryonic development.
  • Helps the eye maintain its shape.
  • Supports the retina—keeps it flush against the back of the eye.
• Aqueous humor:
  • A transparent watery fluid in the anterior cavity (in front of the lens).
  • Continuously produced by ciliary processes.
  • Nourishes and oxygenates the lens and inner cornea.
• Aqueous humor production circulation and drainage:
  • Plasma filtered across capillary walls of ciliary processes in the posterior chamber.
  • Aqueous humor circulates through the pupil into the anterior chamber.
  • It drains from the chamber via the scleral venous sinus and then to nearby veins.
  • Drainage failure can lead to glaucoma.

Glaucoma:

• Characterized by increased intraocular pressure
  • Angle-closure glaucoma:
    • Involves the angle in the anterior chamber formed by the union of the choroid and corneal-scleral junction.
    • If narrow, aqueous humor and pressure build.
  • Open-angle glaucoma:
    • Impaired fluid transport out of the anterior chamber.
  • Congenital glaucoma:
    • Rare, due to heredity or intrauterine infection.
• May cause compression of the choroid layer, constrict blood vessels nourishing the retina.
• May cause reduced field of vision, dim vision, halos around light.
• Lens: changes shape to focus light on the retina.
  • Cells within it have lost organelles and are filled with crystallin protein.
  • Lens enclosed by a dense, fibrous elastic capsule.
  • The shape determines the degree of light refraction.
  • The shape is determined by the ciliary muscle and suspensory ligaments.
• Fibrous tunic: tough outer layer.
  • Composed of the sclera and cornea.
  • Sclera: white of the eye.
    • Composed of dense irregular CT.
    • Provides eye shape.
    • Protects internal components.
    • Attachment site for extrinsic eye muscles.
  • Cornea: anterior convex transparent “window”.
    • Inner layer of simple squamous epithelium; middle layer of collagen; outer layer of stratified squamous epithelium (corneal epithelium).
    • No blood vessels.
    • Limbus: corneal scleral junction.
    • Refracts light
• Vascular tunic (uvea): the middle layer with many vessels.
  • Houses blood vessels, lymph vessels, and intrinsic muscles.
  • Three regions: choroid, ciliary body, and iris.
  • Choroid: extensive, posterior region.
    • Many capillaries nourish the retina.
    • Many melanocytes make melanin to absorb extraneous light.
  • Ciliary body: ciliary muscles and processes.
    • Located just anterior to the choroid.
    • Ciliary muscles: bands of smooth muscle connected to the lens.
    • Muscle contraction loosens suspensory ligaments, altering lens shape.
    • Ciliary processes: contain capillaries secreting aqueous humor.
  • Iris: gives eye color; most anterior region of the uvea.
    • Contains smooth muscle, melanocytes, vessels, and neural structures.
    • Divides the anterior segment into the anterior chamber (between the cornea and iris) and the posterior chamber (between the iris and lens).
    • The pupil is an opening in the center of the iris connecting the two chambers.
    • The iris controls pupil diameter.
    • Sphincter pupillae muscles: concentrically circular fibers constrict the pupil with parasympathetic nervous system activity (CN III).
    • Dilator pupillae muscle: radially organized smooth muscle dilates the pupil with sympathetic nervous system activity.
    • The pupillary reflex alters pupil size in response to light (increased brightness leads to constriction).
• Retina: internal or neural tunic.
  • Pigmented layer:
    • Attached to the choroid (internal to it).
    • Provides vitamin A for photoreceptors.
    • Absorbs stray light to prevent light scatter.
  • Neural layer:
    • Houses photoreceptors and associated neurons.
    • Receives light and converts it to nerve signals.
    • Ora serrata – jagged edge.
    • Boundary between photosensitive and nonphotosensitive parts of the retina.
    • The nonphotosensitive part is anterior—covers the ciliary body and the posterior side of the iris.
  • Retina: cells of the neural layer form 3 sublayers:
    • Photoreceptor cell layer: outermost neural layer.
      • Contains rods and cones.
      • Contain pigments that react to light.
    • Bipolar cell layer:
      • Their dendrites receive synaptic input from rods and cones.
    • Ganglion cell layer: innermost neural layer.
      • Their axons gather at the optic disc and form the optic nerve.
      • Capable of action potentials.
    • Other retinal interneurons:
      • Horizontal cells regulate signals sent between photoreceptors and bipolar cells.
      • Amacrine cells regulate signals between bipolar and ganglion cells.
      • Capable of action potentials.
  • Components of the retina:
    • Optic disc:
      • Contains no photoreceptors—blind spot.
      • Where ganglion axons exit toward the brain.
    • Macula lutea:
      • Rounded, yellowish region lateral to the optic disc.
      • Contains fovea centralis (central pit).
      • The highest proportion of cones (hardly any rods).
      • Area of sharpest vision.
    • Peripheral retina:
      • Contains primarily rods.
      • Functions most effectively in low light.

Retina clinical views

*   Detached retina:
    *   Occurs when outer pigmented and inner neural layers separate.
    *   May result from head trauma.
    *   Increased risk in diabetics and nearsighted individuals.
    *   Results in nutrient deprivation in the inner neural layer.
    *   Symptoms of “floaters” and “curtain” in the affected eye.
    *   Symptoms of flashes of light and decreased vision.
*   Macular degeneration:
    *   Physical deterioration of the macula lutea.
    *   Leading cause of blindness in developed countries.
    *   May be associated with:
        *   Diabetes
        *   Ocular infection
        *   Hypertension
        *   Eye trauma
    *   Loss of visual acuity in the center of the visual field.
    *   Diminished color perception and “floaters”.

Physiology of Vision: Refraction and Focusing of Light

• Refraction of light:
  • Sharp vision requires light rays to be bent (refracted) as they pass toward the retina.
  • Refraction results when light passes:
    • Between media of different densities such as air and cornea.
    • Through curved surfaces such as the lens.
  • Refractive index: a number that represents its comparative density.
• Focusing of light:
  • For objects 20 feet away and further:
    • Eyes directed forward.
    • Ciliary muscles relaxed, tensing suspensory ligaments and flattening the lens (it’s resting position).
    • The dilator pupillae of the iris contracts, dilating the pupil.
  • Focusing of light (continued):
    • For objects closer than 20 feet (near response):
      • Eyes directed medially, so the image of the object is directed onto the fovea centralis.
      • Extrinsic eye muscles weaker in one eye may lead to diplopia (double vision).
      • Ciliary muscles contract, decreasing tension on suspensory ligaments, and the lens becomes more spherical.
      • Light refracted to a greater extent; process called accommodation.
      • Sphincter pupillae contract, the pupil constricts.
  • Cataracts:
    • Small opacities within the lens.
    • Usually as a result of aging.
    • Difficulty focusing on close objects.
    • Reduced visual clarity and reduced color intensity.
    • Needs to be removed when it interferes with normal activities. *New surgical techniques include phacoemulsification
      • The opaque center of the lens is fragmented using ultrasonic sound waves, making it easier to remove.
• Functional Visual Impairments:
  • Emmetropia: normal vision.
    • Parallel light rays focused on the retina.
  • Hyperopia: far-sighted.
    • Trouble seeing up close; the eyeball is too short.
    • Only convergent rays from distant points are brought to focus.
    • Corrected with a convex lens.
  • Myopia: near-sighted.
    • Trouble seeing faraway objects; the eyeball is too long.
    • Only rays close to the eye focus on the retina.
    • Corrected with a concave lens.
  • Astigmatism:
    • Unequal focusing.
    • Unequal curvatures in one or more refractive surfaces
  • Presbyopia: age-related change in vision.
    • The lens is less able to become spherical.
    • Reading close-up words becomes difficult.
    • Corrective convex lens.
    • Can be treated with various surgical techniques.

Physiology of Vision: Phototransduction

• Phototransduction: converting light to electrical signals
  • Performed by photoreceptor cells (rods and cones)
• Photoreceptor parts (from outer to inner)
  • The outer segment extends into the pigmented layer of the retina
    • Hundreds of photopigment-containing discs that absorb light
    • Discs are continually replaced
  • The inner segment contains cell organelles
  • The cell body contains the nucleus
  • Synaptic terminals contain vesicles storing glutamate neurotransmitter
• Photoreceptors: rods and cones
  • Rods are longer and narrower than cones; more numerous
    • Each rod is highly sensitive: activated by even dim light
    • The periphery of the retina contains many rods
    • Many rods converge on fewer bipolar cells, which converge on fewer ganglion cells
    • Results in sensitivity to dim light but a blurry image
  • Cones are concentrated at the fovea centralis
    • Activated by high intensity light, allow color vision
    • Cones have a one-to-one relationship with bipolar cells and ganglion cells
    • Results in a sharp image but only possible in bright light
  • Photopigments: light-absorbing molecules
    • Found within membranes of outer segments of rods and cones
    • Made of opsin protein and light-absorbing retinal (made from Vitamin A)
    • Different pigment types have different opsins transducing different wavelengths (colors) of light
    • Each photoreceptor has only one photopigment type
    • Rods contain rhodopsin
    • Three types of cones each containing a type of photopsin with a different sensitivity
      • Blue cones detect short wavelengths, green cones absorb intermediate wavelengths, and red cones best detect long wavelengths
  • Color Blindness:
    • X-linked recessive condition is more common in males
    • Absence or deficit in one type of cone cell
    • Red and green are most commonly affected
      • Results in difficulty distinguishing red and green
  • Phototransduction: light converted to an electrical signal
    • In the dark, rhodopsin contains cis-retinal
    • Light causes reconfiguration to trans-retinal, which dissociates from opsin (bleaching reaction)
    • After bleaching, rhodopsin must be rebuilt for the rod to function
    • Trans-retinal is transported to the pigmented layer and is actively converted to cis-retinal
    • Cis-retinal is transported back into the rod and combined with opsin
    • The process is slow for rhodopsin—rods don’t function in bright light
    • The process is similar for cone photopsins, but quicker
    • More intense light is needed for bleaching
    • Photopsin regenerates rapidly
  • Dark adaptation
    • Return of sensitivity to low light levels after bright light
    • Bleached rods must regenerate rhodopsin
    • May take 20 to 30 minutes to see well
  • Light adaptation
    • Process of adjusting from low light to bright conditions
    • Pupils constrict, but cones are initially overstimulated
    • It takes about 5 to 10 minutes for full adjustment
  • Initiating nerve signals
    • In the dark, rods are depolarized (membrane at –40 mV)
    • Cyclic guanosine monophosphate (cGMP) is produced
    • cGMP binds to cation channels and keeps them open
    • Na^+ and Ca^{2+} enter the cell (“the dark current”)
    • Voltage-gated Ca^{2+} channels in the rod’s synaptic terminal stay open
    • Glutamate transmitter is continuously released by the rod
    • Glutamate hyperpolarizes bipolar cells, preventing them from exciting ganglion cells
  • Initiating nerve signals (continued)
    • When exposed to light:
      • Light hyperpolarizes rods
      • Light splits rhodopsin and a G protein 2nd messenger system is activated
      • Phosphodiesterase is activated and breaks down cGMP
      • cGMP-gated cation channels close
      • Na^+ and Ca^{2+} stop entering the cell, and the cell becomes more negative
      • Voltage-gated Ca^{2+} channels in the rod’s synaptic terminal close
      • The rod stops releasing glutamate
      • Bipolar cells are no longer inhibited, now release glutamate to ganglion cells
      • Ganglion cell excitation leads to impulses being sent along its axon to the brain

Visual Pathways

• In the retina:
  • Photoreceptors → bipolar cells → ganglion cells
  • Ganglion cell axons bundle at the disc to form the optic nerve
• Optic nerves
  • Exit backs of eyes and converge at the optic chiasm
  • Medial axons cross to the opposite side of the brain
  • Lateral axons remain on the same side
• Optic tracts (ganglion cell axons from both eyes)
  • Most axons go to the lateral geniculate nucleus of the thalamus
  • Thalamic neurons’ axons project to the visual cortex of the occipital lobe
• Left and right eyes have overlapping visual fields
  * Allows stereoscopic vision (depth perception)
• Some optic tract axons project to the midbrain * Superior colliculi coordinate reflexive eye movements * Pretectal nuclei coordinate pupillary reflex and lens accommodation reflex
  • Ganglion cells projecting to pretectal nuclei are directly photoresponsive and contain melanopsin pigment

Ear Structure

• The ear detects sound and head movement
  * Signals transmitted via the vestibulocochlear nerve (CN VIII)
• External ear * Auricle: funnel-shaped visible part of ear with elastic cartilage
  • Protects ear entryway and directs sound waves inward
    • External acoustic (auditory) meatus: ear canal
      • Extends to the tympanic membrane
      • Ceruminous glands produce cerumen
        • Ear wax impedes microorganism growth
    • Tympanic membrane: eardrum
      • Funnel-shaped epithelial sheet separating external and middle ear
      • Vibrates when sound waves hit it
• Middle ear
  * Contains air-filled tympanic cavity
  * Bony wall separates it from the inner ear
  * The wall has two membrane-covered openings: the oval window and the round window
  * Auditory tube (Eustachian tube)
  * Passage extending from the middle ear to the nasopharynx (upper throat)
  * Middle ear infections often result from infections spreading from throat through the auditory tube