(FE) The Special Senses: Chapter 16

Sensation

A stimulus we are consciously aware of. To enter consciousness, signals must reach cerebral cortex. Only a fraction of stimuli result in sensations

Sensory Receptors

Provide information about external and internal environments. Respond to a stimulus. Each types of receptor responds best to a type of stimulus. (E.g., light energy for eye receptors; sound energy for ear receptors) Action potentials are conveyed to CNS for interpretation.

Transducers

Covert stimulus energy into electrical energy. Receptors have a resting membrane potential. Receptor membranes have modality gated channels that respond to their type of stimulus. Receptive field.

Receptive Field

The distribution area of the endings of a sensory neuron

Exteroceptors

Detect stimuli from external environment. Skin and mucus membranes; special sense receptors.

Interoceptors

Detect stimuli from internal organs. Visceral sensory receptors monitoring internal environment.

Proprioceptors

Detect body and limb movements. Somatosensory receptors of muscles, tendons, and joints.

Five Types of Sensory Receptor Classification by Modality of Stimulus

Chemoreceptors, Thermoreceptors, Photoreceptors, Mechanoreceptors, and Nociceptors.

Chemoreceptors

Detect chemicals dissolved in fluid. Include receptors for external environment (e.g., smell of food) or internal environment (e.g., oxygen levels in blood)

Thermoreceptors

Detect changes in temperature. Include receptors in skin, hypthalamus.

Photoreceptors

Detect changes in light intensity, color, movement. In retina of the eye.

Mechanoreceptors

Detect distortion of 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 skin and mucous membranes. Endings can be: Unencapsulated or Encapsulated.

Unencapsulated Tactile Receptor Endings

Dendritic ends of sensory neurons with no protective cover. Includes: Tactile disc, Free nerve ending, and root hair plexus.

Encapsulated Tactile Receptor Endings

Neuron endings wrapped by connective tissue or covered by connective tissue and glial cells (neurolemmocytes). Includes: Tactile corpuscle, End bulb, Bulbous corpuscle, Lamellated corpuscle/

The Special Senses

Taste, Smell, Hearing/Equilibrium, Vision

Olfaction

Sense of smell. Detect ordorants (chemicals in air). Adaptive.

Olfactory Cells

(receptor) found in olfactory epithelium of nasal passageways. Chemoreceptors. Olfactory nerve (CN I). About 2,000 to 4,000 odors distinguished.

Olfactory Nerve

(CN I). Goes directly to cerebral cortex. Rimary olfactory cortex (in temporal lobe), hypothalamus, amygdala, and other regions. Does not project through thalamus like other sensory pathways.

Gustation

The Sense of Taste. Detection of tastants. Gustatory cells are chemoreceptors within taste buds on papillae.

Papillae of Tongue

Filiform Papillae, Fungiform Papillae, Foliate Papillae, Vallate (Circumvallate) Papillae.

Filiform Papillae

Short and spiked. No taste buds (no role in gustation); help manipulate food. Located on anterior two-thirds of tongue surface.

Fungiform Papillae

Mushroom-shaped. Each contains a few taste buds. Located on tip and sides of tongue.

Foliate Papillae

Leaflike ridges. Not well developed. House a few taste buds in early childhood. Located on posterior lateral tongue.

Vallate (Circumvallate) Papillae

Largest, least numerous. Contain most of the taste buds. Located in a row of 10-12 along posterior dorsal tongue surface.

Taste Buds

Onion-shaped organs housing taste receptors

Gustatory Cells

Receptor cells detect tastants (live 7 to 9 days)

Supporting Cells

Sustain gustatory cells

Basal Cells

Neural stem cells that replace gustatory cells.

Five Basic Taste Sensations

1) Sweet = Produced by organic compounds. 2) Salt= Produced by metal ions (e.g., Na+ and K+) 3) Sour = Associated with acids (e.g., vinegar, H+) 4) Bitter = Produced by alkaloids (e.g., unsweetened chocolate) 5) Umami = Taste related to amino acids producing a meaty flavor.

Transduction in Gustatory Cells

For salt and sour the tastants are ions. The tastant depolarizes the cell directly (through iron channel). For sweet, bitter, and umami the tastants are molecules. Tastant binds to specific cell membrane receptor (GPCR). Signaling results in cell depolarization. Depolarized gustatory cell releases neurotransmitter stimulating primary neuron (in CN VII or CN IX) projecting to medulla;

Taste

Integrated with temperature, texture, and especially. Food has less taste if olfaction is blocked (e.g., having a cold)

Conjunctiva

Transparent lining of eye and lid surfaces. Specialized stratified columnar epithelium. Includes: Ocular conjunctive, Palpebral conjuctiva, Conjuctival fornix. Does not cover cornea so as not to interfere with light passage.

Ocular Conjuctiva

Covers anterior sclera (white of eye)

Palpebral Conjunctiva

Covers internal surface of eyelid

Conjunctival Fornix

Junction of ocular and palpebral conjunctiva.

Conjunctivitis

Pink eye

Lacrimal Apparatus

Produces, collects, drains fluid.

Lacrimal Fluid

Water, Na+, antibodies, lysozyme (antibacterial enzyme) Lubricates, cleanses and moistens eye, reduces eyelid friction, defends against microbes, oxygenates and nourishes cornea.

Lacrimal Gland

Produces fluid and secretes it through ducts.

Lacrimal Sac

Drains to nasolacrimal duct to nasal cavity. Excess lacrimal fluid produces tears.

Eye Structure

Almost spherical. 2.5 cm diameter. Located in skull’s orbit, padded by orbital fat. Interior contains two cavities: Anterior cavity and Posterior cavity.

Anterior Cavity

(In front of lens) contains circulating aqueous humor.Transparent watery fluid. Nourishes and oxygenates lens and inner cornea. Continuously produced by ciliary processes. Aqueous humor circulates through pupil into anterior chamber. It drains from the chamber via the scleral venous sinus and then to nearby veins. Drainage failure can lead to glaucoma.

Posterior Cavity

(Behind lens) Contains permanent vitreous humor. Transparent gelatinous fluid in posterior cavity. Permanent fluid first produced in embryonic development. Helps eye maintain shape. Supports retina— keeps it flush against back of eye.

Eye Wall

Formed by three tunics. Fibrous (external), Vascular (middle), and Retina (inner).

Fibrous Tunic

Tough outer layer. Composed of 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). Middle layer with many vessels. Houses blood vessels, lymph vessels, intrinsic muscles. Three regions: choroid, ciliary body, iris.

Choroid

Extensive, posterior region. Many capillaries nourish retina. Many melanocytes make melanin to absorb extraneous light.

Ciliary Body

Ciliary muscles and processes. Located just anterior to choroid. Includes Ciliary muscles and Ciliary processes.

Ciliary Muscles

Bands of smooth muscle connected to lens. Muscle contraction loosens suspensory ligaments, altering lens shape.

Ciliary Processes

Contain capillaries secreting aqueous humor

Iris

Gives eye color; most anterior region of uvea. Contains smooth muscle, melanocytes, vessels, neural structures. Divides ther anterior segment into the anterior chamber. (between cornea and iris) and posterior chamber (between iris and lens). Pupil is opening in center of iris connecting the two chambers. Iris controls pupil diameter: Sphincter pupillae muscles, Dilator pupillae muscle, and Pupillary reflex.

Sphincter Pupillae Muscles

Concentrically circular fibers constrict pupil with parasympathetic nervous system activity (CN III)

Dilator Pupillae Muscle

Radially organized smooth muscle dilates pupil with sympathetic nervous system activity.

Pupillary Reflex

Alters pupil size in response to light (increased brightness leads to constriction)

Retina

Internal and neural tunic. Includes: Pigmented layer, Neural layer, and Ora serrata. Cells of the neural layer form 3 sub layers: Photoreceptor cell layer, Bipolar cell layer (middle), and Ganglion cell layer.

Pigmented Layer

Attached to choroid (internal to it). Provides vitamin A for photoreceptors. Absorbs stray light to prevent light scatter.

Neural Layer

Houses photoreceptors and associated neurons. Recieves light and converts it to nerve signals.

Ora Serrata

Jagged edge. Boundary between photosensitive and nonphotosensitive parts of retina. Nonphotosensitive part is anterior— covers ciliary body and posterior side of iris.

Light Going Through Structures of the Eye

Cornea, Aqueous humor containing anterior chamber, Pupil, Lens, Vitreous humor containing posterior chamber, Retina.

Photoreceptor Cell Layer of Retina

Outermost/posterior neural layer. Contain pigments that react to light. Includes rods and cones.

Rods

Highly sensitive; activated by even dim light. Longer and narrower than cones; more numerous. The periphery of the retina contains many rods. High sensitivity to dim light but a blurry image.

Cones

Activated by high intensity light, allow color vision. Concentrated at fovea centralis. Sharp image but only possible in bright light.

Bipolar Cell Layer

(Middle). Their dendrites receive synaptic input from rods and cones.

Ganglion Cell Layer

Innermost/anterior neural layer. Their axons gather at optic disc and form optic nerve. Capable of action potentials.

Other Retinal Interneurons

Horizontal cells and Amacrine cells

Horizontal Cells

Regulate signals sent between photoreceptors and bipolar cells

Amacrine Cells

Regulates signals between bipolar and ganglion cells. Capable of action potentials.

Photoreceptors in Eye Navigation

Transduce light into signal, send to bipolar cells, to retinal ganglion cells, out optic nerve to thalamus and then cortex.

Optic Disc

Contains no photoreceptors—blind spot. Where ganglion axons exit toward brain.

Macula Lutea

Rounded, yellowish region lateral to optic disc. Contains fovea centralis (central pit), which is the highest proportion of cones (hardly any rods) and area of sharpest vision.

Peripheral Retina

Contains primarily rods. Functions most effectively in low light.

Lens

Changes shape to focus light on retina. Cell within it have lost organelles and are filled with crystallin protein. Lens enclosed by dense, fibrous elastic capsule. Shape is determined by ciliary muscle and suspensory ligaments.

The Eye When Viewing Objects 20 or More Feet Away

Muscle relaxes, suspensory ligaments are tense, lens flattened.

The Eye When Viewing Objects Closer than 20 Feet

Accommodation. Muscle tenses, suspensory ligaments loosen, lens more spherical.

Emmetropia

Normal vision. Parallel light rays focused on retina.

Hyperopia

Far-sighted. Trouble seeing up close; eyeball too short. Only convergent rays from distant points brought to focus. Corrected with convex lens.

Myopia

Near-sighted. Trouble seeing far away objects; eyeball too long. Only rays close to eye focus on retina. Corrected with concave lens.

Astigmatism

Unequal focusing, halos around light. Unequal curvatures in one or more refractive surfaces.

Color Blindness

X-linked recessive condition more common in males. Absence or deficit in one type of cone cell. Red and green most commonly affected. Results in difficulty distinguishing red and green.

Macular Degeneration

Physical deterioration of macula lutea. Leading cause of blindness in developed countries. May be associated with diabetes, ocular infection, hypertension, eye trauma. Loss of visual acuity in center of visual field. Diminished color perception and “floaters”.

Detached Retina

Occurs when outer pigments and inner neural layers separate. May result from head trauma. Increased risk in diabetics and nearsighted individuals. Results in nutrient deprivation in inner neural layer. Symptoms of “floaters and curtain” in affected eye. Symptoms of flashes of ligh, decreased vision. Treatments include pneumatic retinopexy with laser reattachment and the scleral buckle.

Glaucoma

Characterized by increased intraocular pressure. May cause compression of choroid layer, constrict blood vessels nourishing retina. May cause reduced field of vision, dim vision, halos around light. Eye drops to widen apertures and increase drainage.

Cataracts

Small opacities within the lens (accumulated dead lens cells). Usually as a result of aging. Difficulty focusing on close objects. Reduced visual clarity and reduced color intensity. Needs to be removed when interferes with normal activities. New surgical techniques include phacoemulsification: Opaque center of lens is fragmented using ultrasonic sound waves, making it easier to remove.

Presbyopia

Age-related change in vision. Lens less able to become spherical. Reading close-up words becomes difficult. Corrective convex lens. Can be treated with various surgical techniques (LASIK lasts 10-20 years).

Visual Pathways In Retina

Light ravels through retinal ganglion cells, bipolar cells, to photoreceptors. Photoreceptors > bipolar cells > ganglion cells. Ganglion cell axons bundle at optic disc to form optic nerve.

Visual Pathway for Optic Nerves

Exit backs of eyes and converge at optic chiasm. Medial axons cross to opposite side of brain. Lateral axons remain on same side.

Visual Pathways for Optic Tracts

(Ganglion cell axons from both eyes). Most axons go to lateral geniculate nucleus (LGN) of thalamus. Thalamic neuron’s’ axons project to visual cortex of occipital lobe.

Hearing

Sound is the perception of pressure waves established from vibrating objects. Vibration pushes molecules, which transfer the energy from one molecule to the next. Includes Pitch and Loudness.

Pitch

Depends on the frequency of vibrating object.

Frequency

The rate of vibration in Hertz (Hz; cycles per second). High-frequency sounds excite cells in stiff basilar membrane near oval window. Low-frequency sounds excite cells in flexible basilar membrane near apex.

Loudness

Depends on wave amplitude (degree of molecular compression). Louder sounds create larger movements of basilar membrane. Larger movements cause faster rate of nerve signals and a larger number of stimulated cells. Temporal lobe’s auditory cortex interprets this as loudness. Loudness is measured in decibels (dB). Zero dB is the threshold for hearing. Energy of sound increases ten times for every 10 dB increase.

Cochlear Hair Cell Stimulation

Inner hair cells contain ion channels at their tips and tip link proteins that connect them. Hair cells are bathed in K+ endolymph that is far more positive than the fluid inside the cell. When basilar membrane moves up, hair cells are pushed into tectorial membrane and their tips are tilted, pulling tip links. Tip links pull open ion channel allowing K+ to diffuse into the hair cell and depolarize it. Hair cell releases more neurotransmitter from its base, exciting the sensory neuron, which can fire action potentials. When basilar membrane moves down, the process quickly reverses.

Deafness

Any hearing loss. Two types: Conductive deafness and Sensorineural deafness.

Conductive Deafness

Interference of wave transmission in external or middle ear

Sensorineural Deafness

Malfunction in inner ear or cochlear nerve

Equilibrium

Coordination, balance, and orientation in three-dimensional space.