Anatomy and Physiology

Last Class Reminders

Last day of classes, focusing on special senses, with lab review. Assignments (drawn label quiz, discussion checkpoint) due tonight. Late assignments accepted until Friday; no extensions. Drawn label grading in progress. Grades finalized by Friday. Final exam: Friday at 10:30 AM in the usual room. Extra credit available via anonymous survey on Canvas. AU survey feedback is crucial for class improvement, impacting the department and the instructor's evaluation.

Feedback and Office Hours

Constructive feedback is encouraged, focusing on improvements. Office hours are available, with a schedule announcement pending due to final exam schedule conflicts. Regular hours through Monday, adjusted hours through next Thursday.

Sensory Receptors by Modality of Stimulus

Chemoreceptors

Detect chemicals, vital for smell and taste. Neurotransmitter receptors like nicotinic and muscarinic acetylcholine receptors are examples. The chemical stimulus binds to the receptor to cause a change.

Thermoreceptors

Detect temperature changes via sensory neurons in the skin. Specialized receptors respond to specific temperature ranges, decoded by the brain for hot or cold sensations.

Photoreceptors

Detect light, located in the retina.

Mechanoreceptors

Detect mechanical changes, anchored in membranes (internal, external, or plasma). Membrane touch/pressure stretches them open, initiating flow, typically through ion channels. Examples include touch, pressure, vibration, stretch, baroreceptors, proprioceptors, tactile receptors, and inner ear receptors.

Nociceptors

Detect painful stimuli via free nerve endings. Pain perception is cognitive, while the actual feeling is nociception. Somatic nociceptors in skin/muscle and visceral nociceptors detect internal organ damage (e.g., heart attack felt as referred pain).

Tactile Receptors in the Skin

Two types of endings: unencapsulated and encapsulated.

Unencapsulated Endings

Free nerve endings:

  • Tactile discs: have a little disc on the end.

  • Free nerve endings: A bouton ending in the tissue.

  • Root hair plexus: Innervating hair, detecting vibrations.

Encapsulated Endings

Bulbous, with a capsule at the end (cone or balloon-like):

  • Tactile corpuscle.

  • End bulb.

  • Bulbous corpuscle.

  • Lamellar corpuscle.

Olfaction (Smell)

Chemicals in the air (odorants) are sensed by receptors on dendrites in the nasal cavity roof. A mucus layer dissolves odorants for receptor contact (chemoreceptors). Information transmits to the olfactory bulb, then via the olfactory nerve and tract to the cerebral cortex (olfactory cortex in the temporal lobe), hypothalamus, amygdala, and other regions. This pathway bypasses the thalamus. Human smell is poor (2,000-4,000 odors) compared to animals. Strong links between smell and memories, especially emotional.

Gustation (Taste)

Chemoreceptors sense food tastes. Papillae on the tongue contain taste buds. Taste buds have receptor cells that bind to tastants.

Types of Papillae

  • Filiform papillae: Short, spiked; aid food manipulation; no taste buds.

  • Fungiform papillae: Mushroom-shaped; taste buds present; anterior region, sides of tongue.

  • Foliate papillae: Leaf-like ridges; taste buds present; closer to the back and on the lateral sides of tongue.

  • Valate papillae: Circular; taste buds present (especially for bitter tastes); posterior dorsal tongue surface.

A single papilla can have hundreds of taste buds. Damaged taste buds regenerate via basal cells. Taste buds are not neurons, but specialized receptor cells called gustatory cells.

Five Basic Tastes

Sweet, salty, sour, bitter, umami (delicious, meaty/savory). A single taste bud excels at detecting one tasteant but can recognize others. The brain integrates information to identify foods.

  • Sweet: Detects sugar, artificial sweeteners.

  • Salt: Detects sodium chloride, sodium/potassium ions.

  • Sour: Detects high hydrogen ion concentrations (acidity).

  • Bitter: Detects alkaloids (e.g., unsweetened chocolate, quinine, coffee).

  • Umami: Detects savory, meaty flavors.

Taste Transduction

Tasteant activates taste receptor cell, causing depolarization and neurotransmitter release, signaling a sensory neuron.

  • Ionotropic channels: Ions diffuse through channels (salt, sour).

  • Metabotropic receptors: Trigger protein cascades (sweet, bitter, umami); typically G protein-coupled receptors, leading to ion channel opening and cell depolarization.

Gustatory Pathway

Cranial nerves VII and IX innervate the tongue, projecting to the medulla, then thalamus, then cortex (insula). Taste integrates with smell, temperature, texture, and nociception (e.g., capsaicin in spicy foods). Sight is integrated, too. A cold impairs smell and taste.

Clicker Question

How is a sour taste transduced? Hydrogen ions enter the receptor to depolarize the cell.

Vision

Conjunctiva

Clear layer protecting the eye. Ocular conjunctiva covers the sclera, palpebral conjunctiva covers the inner eyelid surface, and the conjunctival fornix is the junction. No conjunctiva covers the pupil. Pinkeye is inflammation of the conjunctiva.

Lacrimal Apparatus

Tears produced by the lacrimal gland contain water, sodium, antibodies, and lysozyme (antibacterial enzyme). Tears oxygenate and nourish the cornea, reduce friction, and drain into the nasal cavity.

Eye Anatomy

Roughly spherical (2.5 cm diameter), padded by orbital fat. The cornea allows light to pass through, entering the aqueous humor (watery fluid in the anterior cavity). The pupil, a hole in the iris, appears black due to the pigmented epithelium at the back of the retina. The iris dilates/constricts to control light amount. Aqueous humor drains through sclerovenous sinuses. The lens focuses light into the vitreous humor (thick fluid in the posterior cavity). Vitreous humor maintains eyeball shape.

Three Tunics of the Eye

Fibrous tunic, vascular tunic, and retina.

Fibrous Tunic

Outermost layer:

  • Sclera: White of the eye; dense irregular connective tissue; provides shape and protects.

  • Cornea: Clear outer covering; simple squamous, collagen, and stratified squamous epithelium; main light refraction; no blood vessels. The limbus is the corneal-scleral junction.

Vascular Tunic

Middle layer:

  • Choroid: Posterior; capillaries nourish the retina; melanin absorbs extraneous light.

  • Ciliary body: Ciliary muscles control lens shape; ciliary processes make aqueous humor.

  • Iris: Eye color; dilates/constricts to control light amount; divides anterior chamber from lens. Sphincter pupillae muscle (CN III, parasympathetic) constricts pupil. Dilator pupillae muscle (sympathetic) dilates pupil. Pupillary reflex is a response to light.

Retinal Layer

Innermost layer (neural tunic):

  • Pigmented layer: Pigmented epithelium catches stray light; provides vitamin A for photoreceptors.

  • Neural layer: Houses photoreceptors (rods, cones) and associated neurons; the aura serrata is a jagged edge.

Light Pathway Through the Retina

Light passes through ganglion cells, bipolar cells, then to photoreceptors (rods, cones). Rods respond to dim light; cones recognize color (blue, green, red) and function in bright light. Cones are sharp, concentrated at the fovea. Bipolar cells receive information from photoreceptors and pass it to retinal ganglion cells. Horizontal and amacrine cells connect information for low-level processing.

Optic Nerve and Visual Cortex

Exits through the optic disc (blind spot; no photoreceptors). Enters the thalamus (relay center), then to the cortex (occipital lobe). The fovea centralis has the most cones for best color vision. The macula lutea surrounds it. Peripheral retina has mostly rods, best for low-light vision.

Eye Structure: Lens

Focuses light onto the retina. Ciliary muscles relax to make the lens taut (far vision) or contract to make it plump (close-up vision). Impaired vision includes:

  • Emmatropia: Normal vision. Eyes can focus light on the retina.

  • Hyperopia: Farsightedness; eyeball too short; needs corrective lens.

  • Myopia: Nearsightedness; needs concave lens.

  • Astigmatism: Unequal corneal curvature.

Clicker Question

What is the correct path of light through the retina? Retinal ganglion cells, bipolar cells, photoreceptors.

Conditions of the Eye

  • Color blindness: X-linked recessive; missing red or green cone; more common in males.

  • Macular degeneration: Degeneration of the macula; causes central blindness; leading cause in developed countries.

  • Retinal detachment: Retina peels away; causes blindness in the region; treatments include pneumatoretinoplexy, scleral buckle.

  • Glaucoma: Increased intraocular pressure; degeneration of retinal ganglion cells; treated with eye drops.

  • Cataracts: Clouding of the lens; caused by dead cells piling up; preventable by wearing sunglasses.

  • Presbyopia: Age-related vision change; close-up vision becomes hard; needs corrective lenses.

  • LASIK surgery: Doesn't last forever (10-20 years).

Retinal Ganglion Cells and Optic Nerve

Medial cells cross at the optic chiasm; temporal cells stay ipsilateral. Go to the thalamus (lateral geniculate nucleus), then the optic radiation to the optic cortex for visual decoding.

Hearing

Pitches is determined by the sound frequency with humans able to hear 20 to 20,000 hertz. Loudness is detected by the amplitude.

Basilar Membrane

Inside the cochlea, low frequencies excite the apex, high frequencies excite the base, determined by amplitude. Loud sounds can damage hair cells measured in decibels. The basilar membrane vibrates, moving hair cells. Mechanosensitive channels open with movement. Potassium floods into channels, depolarizing the cell, which excites the neuron, sending the signal to the brain. Fluid is the endolymph that has a lot of calcium in it.