Sense Organs: Tongue & Eye

Receptors

structures that detect stimuli

Receptor classification by modality (5)

thermoreceptors, photoreceptors, chemoreceptors, nociceptors, mechanoreceptors

Thermoreceptors

detect temperature

Photoreceptors

detect light

Chemoreceptors

detect chemicals

Nociceptors

detect pain

Mechanoreceptors

detect pressure or stretch

Receptor classification by distribution

general senses (widely distributed) and special senses (complex, cranial nerves)

Receptor classification by stimulus origin (3)

exteroceptors, interoceptors, proprioceptors

Exteroceptors

detect stimuli from outside the body

Interoceptors

detect stimuli from internal organs

Proprioceptors

detect body position and movement

Unencapsulated nerve endings

dendrites lacking connective wrapping

Free nerve endings

detect warm, cold, and pain (nociceptors)

Tactile (Merkel) discs

detect light touch and pressure

Hair receptors

detect movement of hairs

Encapsulated nerve endings

dendrites wrapped by glia or connective tissue

Tactile (Meissner) corpuscles

oval masses in dermal papillae; sense light touch and texture

End bulbs (Krause)

similar to tactile corpuscles; located in mucous membranes

Bulbous (Ruffini) corpuscles

flat receptors that sense pressure, skin stretch, and joint movement

Lamellar (Pacinian) corpuscles

onionlike; sense deep pressure, stretch, tickle, and vibration

Muscle spindles

fusiform receptors that sense skeletal muscle stretch

Tendon organs

leaflike receptors that sense tendon stretch from muscle activity

Receptive field

area supplied by a single neuron

Receptive field size

small receptive fields (e.g., fingers) allow fine two-point discrimination

Referred pain

pain from viscera perceived as coming from superficial sites due to shared neural pathways

Gating of pain

pain signals from periphery are blocked from reaching brain (affected by stress, tension, or inactivity)

Taste buds

sensitive to all five taste qualities

Gustation

sense of taste; tongue contains most taste buds, others in soft palate, pharynx, epiglottis, and cheeks

Lingual papillae (4)

filiform, foliate, fungiform, vallate

Filiform papillae

numerous, tiny spikes; no taste buds; for texture

Foliate papillae

ridges on tongue sides; taste buds in children only

Fungiform papillae

mushroom-shaped bumps; contain taste buds

Vallate papillae

large bumps in a row at back of tongue; contain many taste buds

Taste pore

opening on epithelial surface of tongue for taste detection

Taste cells

banana-shaped cells with taste hairs; act as receptors; live 7–10 days; synapse with sensory nerves

Cranial nerves for taste (3)

facial nerve (VII), glossopharyngeal nerve (IX), vagus nerve (X)

Facial nerve (VII)

carries taste signals from anterior tongue

Glossopharyngeal nerve (IX)

carries taste signals from posterior tongue

Vagus nerve (X)

carries taste signals from palate, pharynx, and epiglottis

Accessory structures of the eye (6)

eyebrows, eyelids, lacrimal caruncle, tarsal glands, eyelashes, conjunctiva

Eyelids

include palpebral fissure and medial/lateral commissures (canthi)

Conjunctiva

transparent mucous membrane covering inner eyelid and eye surface

Lacrimal apparatus

produces, distributes, and drains tears

Lacrimal gland

secretes tears across conjunctiva and cornea

Lacrimal punctum

small pore in eyelid that drains tears into canal

Lacrimal canal

channels tears into lacrimal sac

Lacrimal sac

collects tears before draining

Nasolacrimal duct

drains tears into nasal cavity

Extrinsic eye muscles (6)

superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, inferior oblique

Superior rectus

moves gaze upward

Inferior rectus

moves gaze downward

Medial rectus

moves gaze medially

Lateral rectus

moves gaze laterally

Superior oblique

rotates eye downward and laterally

Inferior oblique

rotates eye upward and laterally

Fibrous tunic (2)

sclera and cornea

Sclera

white of the eye

Cornea

transparent anterior portion of eye

Vascular tunic (uvea) (3)

choroid, ciliary body, iris

Choroid

pigmented layer behind retina

Ciliary body

ring of smooth muscle controlling lens shape

Iris

pigmented diaphragm controlling pupil size

Pupillary constrictor

circular muscle that narrows pupil

Pupillary dilator

spoke-like myoepithelial cells that widen pupil

Inner layer

retina, lines posterior two-thirds of eye

Optical components (3)

aqueous humor, lens, vitreous body

Aqueous humor

fills anterior and posterior chambers between cornea and lens

Lens

focuses light onto retina; suspended by suspensory ligaments

Vitreous body

transparent gel filling space behind lens

Retina

light-sensitive layer containing photoreceptors

Ora serrata

anterior margin of retina

Optic nerve (CN II)

exits eye at optic disc (blind spot without receptors)

Macula lutea

central region for detailed vision

Fovea centralis

pit within macula with highest visual acuity

Photoreceptor cells (2)

rods and cones

Rods

responsible for night vision and monochromatic vision; contain rhodopsin pigment

Cones

responsible for day vision and color vision; contain photopsins (three pigments for color wavelengths)

Bipolar cells

interneurons receiving input from rods and cones

Ganglion cells

receive signals from bipolar cells; axons form the optic nerve