Senses

The Senses
Objective: Describe the modalities of the general and special senses.
Sensory receptors- cells that detect stimuli to monitor internal and external conditions.
Sensory modalities- are the different categories of sensory information detected by specific receptors
and relayed through specific afferent neurons.
1) ___General _________ senses- detected by somatosensory receptors widely distributed throughout
the body. Subdivided into different somatosensory modalities: fine touch, crude touch, itch, pain,
and temperature, and proprioception.
Special senses- detected by receptors within the specialized sensory organs: eye, ear, nasal cavity, and
tongue. Information is relayed into the brain through cranial nerves. Perception of each modality
involves processing in dedicated regions of the cerebral cortex.
The special sensory modalities:
2) ___Vision __________ (sight)- light is the stimulus detect by photoreceptor cells in the retina of the
eye.
3) ___Audition ________ (hearing)- sound is the stimulus detect by hair cell receptors in the cochlea
of the inner ear.
4) ___Equilibrium ______ (vestibular sense)- acceleration is the stimulus detect by hair cell receptors
in the vestibule of the inner ear.
Olfaction (smell)- chemicals dissolved in the mucous of the nasal cavity are the stimuli detect by
olfactory receptor cells in the olfactory epithelium of the nasal cavity.
Gustation (taste)- chemicals dissolved in the saliva are the stimuli detect by gustatory receptor cells in
the taste buds of the tongue.
Objective: Explain how sensory transduction takes place at a sensory receptor.
5) Receptive field_______________- region of the environment that is monitored by a sensory
receptor.
6) Sensory transduction___-A stimulus opens or closes ion channels in the membrane of the
receptor cell producing receptor potentials.
7) ___Mechanoreceptors___- detect forces such as pressure, vibration, and stretching. The stimulus
pulls open an ion channel to produce a receptor potential.
8) ___Chemoreceptors____- detect chemicals dissolved in extracellular fluid. Includes the olfactory
receptors for smell and gustatory receptors for taste. For olfactory receptors and some of the
gustatory receptors the stimulus is a ligand that binds to a membrane protein receptor on the surface
of the sensory receptor cell. These membrane proteins produce second messenger signals inside the
receptor cells to regulate ion channels.
Photoreceptors- detect light. Light is detected by membrane proteins that produce second messenger
signals to regulate ion channels.
9) ___Thermoreceptors____- detect changes in temperature. Thermoreceptor cells have membrane
proteins that are ion channels stimulated to open in response to a change in temperature.

Objective: Describe the sensory transduction mechanisms and sensory pathways for the olfactory
modality.
10) ___Olfaction__- is commonly known as the sense of smell. Airborne odorant molecules dissolve
into the mucus of the olfactory epithelium in the nasal cavity and bind to G protein-coupled receptor
proteins on the surfaces of the dendrites of olfactory neurons. There are around 350 distinct types of
olfactory receptor genes that code for different G protein-coupled receptors that are activated by
different shaped ligands of odorant molecules. We can distinguish far more odorants than the
number of distinct receptors because an odorant may activate one or a combination of olfactory
receptors. Binding of an odorant by the receptor activates a G-protein that detaches from the receptor
and activates the enzyme adenylate (adenylyl) cyclase within the cytoplasm of the olfactory neuron.
11) cAMP_______- the second messenger molecule produced by the enzyme adenylate cyclase
that stimulates opening of ion channels allowing sodium and calcium ions to enter producing
depolarization of the membrane potential in the olfactory neuron. Depolarization triggers action
potentials in the axons of olfactory neurons that travel into the brain through the olfactory nerve
(CNI) and synapse with mitral cells in the olfactory bulb.
12) ___ Olfactory tract_________- white matter structure posterior to the olfactory bulb that connects
to olfactory cortex and contains the axons of mitral cells.
13) ___Temporal______ lobe- region of the cerebrum that contains primary olfactory cortex
responsible for awareness and identification of an odor. This is the only sensory pathway that has no
synapse in the thalamus. Neurons from the olfactory cortex then contact other neurons in the
amygdala, hippocampus, hypothalamus, and other parts of the limbic system. This evokes emotional
responses to odors as well as visceral reactions such as salivation, sexual arousal, and nausea.
Objective: Describe the sensory transduction mechanisms and sensory pathways for the gustatory
modality.
14) Gustation________- is commonly known as the sense of taste. Molecules dissolved in saliva
activate specialized receptor cells in taste buds. There are 5 distinct tastes that have been identified
by physiologists: sweet, savory (umami), bitter, salty, and sour.
Each taste has its own transduction mechanism that leads to influx of calcium in the gustatory receptor
cells to stimulate release of neurotransmitters onto the dendrites of the afferent neurons.
Gustatory Transduction Mechanisms:
Sweet: (sucrose, lactose, fructose, or glucose) bind to G protein-coupled receptors on the surface of
gustatory receptor cells.
Bitter: Bitter molecules (for example: alkaloids in coffee or chocolate) bind to G protein-coupled
receptors on the surface of gustatory receptor cells.
Savory: Amino Acids (glutamate) bind to G protein-coupled receptors on the surface of gustatory
receptor cells to produce the savory (umami) taste.
15) ___Second_______ messengers- signals produced inside the gustatory receptor cells following
activation of G-protein coupled receptors that stimulate calcium influx and neurotransmitter release.
16) Salty: Sodium (Na+)___ ions enter the gustatory receptor cells through cation channels
depolarizing the gustatory receptor cells to produce the salty taste.
17) Sour: Hydrogen (H+)_ ions enter the gustatory receptor cells through cation channels
depolarizing the gustatory receptor cells to produce the sour taste.

Sensory pathway:
18) Facial___ (CN VII), Glossopharyngeal (CNIX), and Vagus (CNX) nerves- contain axons of
afferent neurons that travel from the taste buds to form synapses in the solitary nucleus of the
medulla oblongata. Some neurons in the solitary nucleus connect with parasympathetic neurons of
the facial and glossopharyngeal nerves to control reflexive production of saliva. Neuron in solitary
nucleus relay information to neurons in the thalamus. Neurons in the thalamus then relay
information to the gustatory cortex in regions of the parietal lobe and insula responsible for the
perception of taste.
Objective: Identify the structures of the eye and describe their functions.
Three layers (tunics) compose the wall of the eye:
19) ___Fibrous_____ layer – tough fibrous superficial layer
· Sclera – white of the eye, dense irregular connective tissue provides structure and connects to the
tendons of eye muscles.
· Cornea – clear anterior portion contains a thin epithelial tissue surrounding a layer of dense regular
connective tissue with collagen fibers arranged with parallel alternating spaces to allow light to enter
the eye. This tissue produces most of the refraction (bending) of light that enters the eye.
20) ___Vascular____ layer- middle layer, rich with blood vessels.
· Choroid – region deep to sclera in the posterior of the eye contains pigment that prevents reflection
of light that would blur an image. Blood vessels here provide nutrients to the retina.
· Ciliary body – structure that connects the choroid to the iris. Contains a ring of smooth muscle
(ciliary muscle) attached to the lens by suspensory ligaments, controls the shape of the lens. Also
secretes fluid called aqueous humor that provides nutrients to the lens and cornea.
· Iris – smooth muscle which controls the diameter of the pupil.
o Circularly arranged pupillary sphincter (constrictor) muscles stimulated by parasympathetic
efferent fibers decrease the diameter of the pupil.
o Radially arranged pupillary dilator muscles stimulated by sympathetic efferent fibers increase
the diameter of the pupil.
21) _____Neural____ layer (Retina) – deepest layer, portion of the eye where an image is formed, it
extends anteriorly to the posterior margin of the ciliary body.
· Pigmented epithelium- most superficial layer of neural layer epithelial cells contain melanin
granules to prevent reflection of light.
· Photoreceptor layer- located just deep to pigmented epithelium contains photoreceptor cells that
detect light.
· Macula (macula lutea)- yellow colored region that contains a high density of photoreceptors.
o Fovea centralis – small pit in the center of macula lutea which contains the greatest
concentration of cone photoreceptors that produce color vision.
· Optic disc- location where axons of retinal ganglion neurons exit the eye, creates a blind spot in the
visual field because there are no photoreceptors in this region of the retina.
22) _____Anterior______ cavity - in front of the lens, contains aqueous humor which provides
nutrients to the lens and cornea.

23) ____Posterior_____ cavity -posterior to the lens, filled with jelly like vitreous humor that helps
hold the retina in place by pushing it against the choroid.
Lens - clear biconvex disc that refracts light to help focus an image on the retina. Contains elastic fibers
that increase its curvature to focus light.
Near point accommodation- contraction of ciliary muscles releases tension on the suspensory
ligaments allowing elastic fibers of the lens to recoil increasing curvature of the lens and refraction of
light.
Presbyopia- decreased near point accommodation that results from age related stiffening of the lens.
Objective: Identify the accessory structures of the eye and describe their functions.
Palpebrae- commonly known as the eyelids these skin folds protect the anterior surface of the eye and
distribute tears across the surface of the eye during blinking.
24) Levator______ Palpebrae Superioris- skeletal muscle that elevates the superior palpebrae to
open the eyes.
25) Orbicularis___ Oculi- skeletal muscle with a circular fascicle arrangement that closes the
palpebrae.
Conjunctiva- mucous membrane that functions to lubricate and protect the eye contains goblet cells in
the epithelium secrete mucus. Leukocytes in the connective tissue layer help protect against infections.
Palpebral Conjunctiva – mucous membrane covering the inside of the eyelids.
26) Bulbar_______ Conjunctiva – mucous membrane covering the anterior surface of the sclera
(white part) of the eye.
27) Conjunctivitis___- “pink eye” inflammation of the conjunctiva. Most commonly results from
viral or bacterial infections.
Lacrimal gland – exocrine gland that produces tears. Found on the superior / lateral surface of the eye.
Lacrimal puncta – tiny holes on the medial edge of the palpebra that open into lacrimal canaliculi that
drain tears into the lacrimal sac.
28) Nasolacrimal duct – canal between the lacrimal bone, maxilla, and inferior nasal concha that
drains tears from the lacrimal sac into the nasal cavity.
Extrinsic eye (Extraocular) muscles – skeletal muscles, movement of the eye
29) ____Superior____ rectus- directs gaze superiorly, excited by CNIII
30) ____Medial_____ rectus- directs gaze toward midline, excited by CNIII
31) ____Inferior____ rectus- directs gaze inferiorly, excited by CNIII
32) ____Lateral_____ rectus- directs gaze away from midline, excited by CNVI
33) ____Superior___ oblique- directs the gaze inferior and lateral, excited by CNIV
34) ____Inferior____ oblique- directs gaze superior and lateral, excited by CNIII
35) _____Strabismus___ - “lazy eye” a vision disorder in which the eyes do not properly align with
each other when looking at an object. Can result from weakness of the extraocular muscles or
damage to the cranial nerves (CNIII, IV, or VI).
Objective: Describe the visual sensory transduction mechanism.
36) ____Rods_______ – photoreceptors sensitive to a broad wavelength range providing black and
white vision.

37) ____Rhodopsin__- the G protein-coupled receptor protein found in the membranous discs of rod-
shaped photoreceptors that contains a cofactor molecule known as retinal (form of vitamin A) that
responds to light. Light causes a shape change in the retinal, altering its structure from a bent (cis)
form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the G-protein
transducin.
38) Phosphodiesterase ___- enzyme stimulated by the G-protein tranducin to break down the
second messenger cyclic guanosine monophosphate (cGMP) in photoreceptors that are exposed to
light. Decreased levels of cGMP lead to closing of cyclic nucleotide gated ion channels.
39) Hyperpolarization ___- the membrane potential of the photoreceptor becomes more negative.
This decreases the release of neurotransmitter from the photoreceptors onto the dendrites of bipolar
cells in the retina.
40) ____Cones________ – photoreceptors sensitive to narrow wavelength range providing color vision.
· The opsins photopigment proteins in discs of cones are similar to the opsin that forms rhodopsin.
However, the range of wavelengths of visible light that can activate the opsins of cones are more
narrow enabling cones to respond to specific colors of light (blue, green, red).
· Color vision deficiency - decreased ability to see color. Most commonly results from a mutation
in one of the cone opsin photopigment genes (green or red) on the X-chromosome.
Objective: Describe the sensory pathways for the visual modality.
Following visual sensory transduction photoreceptors relay information to cells in a deeper layer called
41) ____Bipolar_____ cells (neurons) that relay information from rods and cones to neurons called
42) ____Retinal_____ ______Ganglion____ cells (neurons) in the deepest layer of retina that extend
axons through the optic nerves and optic tracts.
43) ___Optic______ nerve (CNII): afferent fibers travel from the retina into the cranium and connect to
the brain.
44) ___Optic Chiasm________: PNS/CNS boundary where the medial fibers cross over the midline.
45) ___Optic tract__: axons of the retinal ganglion cells continue through the CNS to reach the
46) ___Thalamus__________: region of the diencephalon that contains lateral geniculate nucleus
(LGN) where the axons of retinal ganglion cells synapse with thalamic neurons. Neurons in LGN
send visual information to the cortex.
47) ____Occipital________ lobe: contains primary visual cortex the first region of the cerebrum to
receive visual information. In addition to conscious awareness of visual stimuli, the primary visual
cortex also provides initial processing of location, movement, shape, and color, and combines
images from each eye for binocular vision. From the primary cortex, visual information is then sent
into two general pathways:
48) ___Dorsal_____ stream (posterior pathway)- regions of the parietal lobes receive visual
information and interpret motion. “Where is the object.”
49) Ventral___ stream (anterior pathway) regions of the inferior temporal lobes receive visual
information and process details like color and shape needed to identify objects. “What is the object.”
Objective: List and describe the five steps of the pupillary reflex.
50) Photoreceptors_ in the retina detect light and relay the signal through a series of neurons in the
retina.
51) Opticnerve contains the afferent fibers that relay information from the retina to the brain.
The brain (midbrain) contains the integration center. If light levels are too high, the midbrain
activates a bilateral parasympathetic efferent pathway.

52) Oculomotor__ nerves, ciliary ganglia, and ciliary nerves contain the parasympathetic
efferent pathway activated to stimulate the smooth muscles of the iris in both eyes.
53) Pupillary sphincter muscles of both eyes contract decreasing the diameter of the pupils and
reducing the amount of light entering the eyes.
Objective: Identify the structures of the ear and describe their functions.
External ear
54) ___Pinna (Auricle)____ – outer visible portion of the ear, channels sound waves into the auditory
canal
External acoustic meatus – commonly known as the ear (auditory) canal this is the pathway directing
sound through the temporal bone towards the middle ear.
Tympanic membrane – commonly known as the ear drum this cone shaped structure made of epithelial
and connective tissue forms the external/middle ear boundary and sound waves cause it to vibrate.
Middle ear – small air-filled cavity within the petrous portion of temporal bone, deep to the tympanic
membrane.
· Ear ossicles – tiny bones that bridge the tympanic membrane to the oval window on the cochlea
of the inner ear.
55) ____Malleus_____ (hammer)- most lateral ossicle, attached to tympanic membrane.
56) ____Incus__________ (anvil)- middle ossicle
57) ____Stapes___________ (stirrup)- most medial ossicle, attached to the oval window.
· Oval window – opening between middle and inner ear.
58) ___Pharyngotympanic___ (eustachian) tube - connects the middle ear and superior portion of the
throat (nasopharynx).
· Equalizes pressure in the middle ear
· Opens when swallowing or yawning
·
Inner ear - bony labyrinth filled with perilymph fluid, contains a membranous labyrinth filled with
endolymph fluid.
59) ______Vestibule___ – central cavity of the bony labyrinth.
· Responsible for the vestibular sense – receptors function as inertial sensors to detects linear and
angular acceleration.
· Important for reflexes that maintain posture and move the eyes to stabilize an image as the head
moves.
60) _____Macula__- structures in the vestibule that contain the vestibular hair cell receptors that detect
linear acceleration. The stereocilia are embedded in a gelatinous mass referred to as the otolithic
membrane, which is suspended in endolymph. The otolithic membrane contains otoliths, which are
crystals of calcium carbonate that increase its density. Linear accelerations such as the force of
gravity cause movement of the otoliths bending the stereocilia to alter neurotransmitter release.
Together with information from proprioceptors, this vestibular information about gravity enables
perception of head position.
61) ______Semicircular_______ canals – three canals orientated in one of the three planes (x, y, or z)
contain semicircular ducts of the membranous labyrinth.
62) _____Ampulla__- regions of the semicircular ducts that contain the vestibular hair cell receptors
that detect angular acceleration. Within the ampulla vestibular hair cells have their stereocilia

embedded in a gelatinous mass called the cupula. With angular acceleration, endolymph lags and
pushes on the cupula bending the stereocilia to alter neurotransmitter release.
63) Motion sickness- dizziness, nausea, and sometimes vomiting that occurs when the brain
receives mismatched sensory information from the eyes and the vestibular system.
Vertigo- “dizziness” a condition in which a person has the sensation of movement or of surrounding
objects moving when they are not. May be accompanied by nausea and vomiting.
64) ____Cochlea_______ – spirals around a central axis and is divided into three ducts
· Vestibular duct– begins at the oval window and contains perilymph.
· Tympanic duct- ends at the round window and contain perilymph.
· Cochlear duct – part of the membranous labyrinth, filled with endolymph and contains the
Spiral Organ of Corti where auditory hair cell receptors extend between the flexible basilar
membrane and the rigid tectorial membrane.
65) The mechanical properties of the basilar membrane change along its length, such that different
regions of the basilar membrane vibrate according to the frequency (pitch) of the sound.
66) Auditory Hair __cells- specialized receptor cells found in the basilar membrane of the cochlea.
Vibration of the basilar membrane causes opening and closing of the tip link mechanically-gated ion
channels on the stereocilia (extensions of the apical cell membrane that contact the overlying
tectorial membrane). Cations (potassium) enter the auditory hair cells causing depolarization that
stimulates the release of neurotransmitters onto the dendrites of afferent neurons.