special senses

Sensory receptors

• Provide information about external and internal environments

• Respond to a stimulus could be touch, sounds, colors, movement, smell, taste, pressure, temperature, solute concentration

• convert stimulus energy into electrical energy. i.e. light energy in eye receptors is converted to electrical energy (the neuron cells graded and action potentials)

Sensation

a stimulus of any type we are consciously aware of

Modality

type of stimulus based on “labeled line” then the brain will interpret a given stimulus as a specific type based on which receptors and which sensory pathway receives the signal. 

• For example, brain interprets optic nerve signals to occipital lobe as visual, cochlear nerve signals as to temporal lobe as auditory

Adaptation

decreased sensitivity to continuous stimulus, so the amount of action potentials will decrease as a stimulus continues

Tonic receptors

receptors that show limited adaptation; respond continuously throughout a stimulus's duration

Phasic receptors

receptors that adapt rapidly; only respond to new stimuli

General sense receptors

• Simple structures distributed throughout the body 

• Somatic sensory receptors: tactile receptors of skin and mucous membranes; proprioceptors of joints, muscles, and tendons

• Visceral sensory receptors: found in walls of internal organs, they monitor stretch, chemical environment (ex. Hypothalamus), temperature (neurons in the hypothalamus), pain

Special sense receptors

• Specialized receptors in complex sense organs of the head

• 5 special senses: olfaction, gustation, vision, audition, equilibrium

Exteroceptors

• detect stimuli from external environment

• Skin and mucus membranes; special sense receptors (eyes for ex)

Interoceptors

• detect stimuli from internal organs

• Visceral sensory receptors monitoring internal environment (hypothalamus for example)

Proprioceptors

• detect body and limb movements

• Somatosensory receptors of muscles, tendons, and joints

Chemoreceptors

• detect chemicals dissolved in fluid

• Include receptors for external environment (for example, smell of food) or internal environment (for example, oxygen levels in blood)

Thermoreceptors

• detect changes in temperature

• Include receptors in skin, hypothalamus

Photoreceptors

• detect changes in light intensity, color, movement

• In the 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

• The most abundant type of mechanoreceptors of skin and mucous membranes (found in mouth, genitalia, nose, body cavities)

• Provide sense of touch; sensory input from tactile receptors received by primary somatosensory cortex in the parietal lobe

Unencapsulated tactile receptors

• Dendritic ends of sensory neurons with no protective cover (CT or glial cells)

• There are 3 main types of unencapsulated tactile receptors: free nerve endings, root hair plexus, tactile discs

Free nerve endings

• terminal ends of sensory neuron dendrites

• Simplest tactile receptors

• Reside close to skin surface and surface of mucous membranes

• Mainly for pain and temperature but also light touch and pressure

• May be phasic (reduced response over time) or tonic (continuous response)

Root hair plexuses

• wrap around hair follicle, so are associated with individual hairs

• Located in deeper layer of dermis

• Detect hair displacement. If the hair moves, the tactile receptor responds

• Phasic receptors putting on a hat or shirt does not provide continuous stimulation because they are phasic receptors. 

Tactile discs

• flattened dendrite endings of sensory neurons extending to specialized tactile cells (also called Merkel cells)

• Respond to light touch, ex. Can be found in high density in the fingertips and lips. 

Encapsulated tactile receptors

sensory nerve endings wrapped in a specialized connective tissue

End bulbs

• ensheathed in connective tissue

• Tonic receptors in dermis and mucous membranes. I.e. in the nose, genitalia, mouth. 

• Detect pressure and low-frequency vibration

Lamellated corpuscles

• are wrapped in neurolemmocytes and concentric layers of connective tissue

• Phasic receptors deep in dermis, hypodermis, some organ walls

• Detect deep pressure, course touch, high-frequency vibration. What structure might help detect these high frequency vibrations?

Bulbous corpuscles

• are wrapped in CT

• Tonic receptors within dermis and subcutaneous layer

• Detect deep pressure and skin distortion. I.e. could detect if skin is being stretched or otherwise distorted.

Tactile corpuscles

• are intertwined dendrite endings wrapped in modified neurolemmocytes, covered in connective tissue

• Phasic receptors In dermal papillae (especially in sensitive regions of the body). So for example the hands

• Discriminative light touch—allow recognition of texture, shape

Muscle Spindle

• detect stretch in skeletal muscle

• can be involved in stretch reflexes

Tendon organs

• detect stretch in tendon

• can be involved in tendon reflexes

Joint kinesthetic receptors

• detect stretch in articular capsule

• relevant to body positioning because detects stretch in long bone joints

Referred pain

• inaccurate localization of sensory signals. i.e. pain is being felt in a region that is not the actual source of the pain

• Signals from internal organs perceived as originating from skin, muscle

• Many somatic and visceral sensory neurons send signals via the same ascending tracts within spinal cord. So since the sensory pathway is shared, sensation localization can be confused. 

• Somatosensory cortex unable to determine true source

Olfaction

detection of odorants (molecules dispersed in air) dissolved in the air

Olfactory epithelium

the primary sensory receptor organ responsible for detecting odors

Lamina propria

• Areolar connective tissue layer internal to olfactory epithelium

• Houses blood vessels, nerves, and olfactory glands so this CT layer is important in supplying the olfactory epithelium with nutrients, removing waste, and signaling. 

Olfactory glands

Help form mucus covering olfactory epithelium the mucus is what will help bind and dissolve odors or molecules that are dispersed in air

Olfactory receptor cells

• Primary neurons in sensory pathway for smell

• Bipolar neuron structure: a single dendrite and unmyelinated axon

Olfactory bulbs

• Ends of olfactory tracts located under the brain’s frontal lobes

• Olfactory nerve fibers synapse here with specialized  mitral cells and tufted cells

• Connections form olfactory glomeruli within the bulbs. There are about 2000 different olfactory glomeruli so each one corresponds to a different smell

Olfactory tracts

• Axon bundles of mitral and tufted cells on inferior frontal lobe surface

• Project directly to primary olfactory cortex (in the temporal lobe), hypothalamus, amygdala, and other regions. These connections to the hypothalamus and amygdala are responsible for emotional connections to different scents

  • Does not project through thalamus like other sensory pathways so smell is not filtered by the thalamus

Gustation

sense of taste; detection of tastants. So for example, NaCl, sodium chloride would be the chemical tastant that is responsible for a salty taste

Papillae

elevations of connective tissue on surface of tongue that perform a different function for the tongue. There are 4 types

Filiform papillae

• short and spiked

• No taste buds (no role in gustation); help manipulate food

• Located on anterior two-thirds (close to the mouth) of tongue surface

Fungiform papillae

• mushroom-shaped 

• Each contains a few taste buds but not very many

• Located on tip and sides of tongue

Foliate papillae

• leaflike ridges

• Not well developed in humans in other mammals can be used for grooming behaviors

• These house a few taste buds in early childhood

• Located on posterior lateral tongue

Vallate papillae

• largest, least numerous of the papillae

• Contain most of the taste buds

• Located in a row of 10 to 12 along posterior dorsal tongue surface

Taste buds

onion-shaped organs housing taste receptors, found in all of the papillae types except for filiform.

Gustatory cells

receptor cells detect tastants. Each individual neuron will correspond to a specific tastant chemical once the appropriate matching tastant binds the neuron will signal this specific taste

Eyebrows

• Located along superciliary arch above the eyes

• Aid in nonverbal communication (for example, surprise, anger) and prevent sweat from dripping into eyes

Eyelashes

• Extend from margins of the eyelids

• Prevent objects coming into contact with eye (dust, insects, dirt, rain…); can initiate blink reflex if a root hair plexus neuron (unencapsulated tactile sensor) senses an eye lash moving this can cause a blink reflex

Eyelids

• Protect the eyes, function in blinking, close your eyes while resting or sleeping= darkness and “cutting off” all of the stimuli coming in from your eyes. 

• Made of fibrous core (called a tarsal plate), the orbicularis oculi muscle, and a thin layer of skin

Conjunctiva

• transparent lining of eye and lid surfaces

• helps protect, lubricate, detect foreign objects, and supply nutrients to the eye. 

Tarsal glands

secrete oil to help keep eyes moist

Lacrimal apparatus

produces, collects, drains fluid. 

Posterior cavity

(behind lens) contains permanent vitreous humor

Anterior cavity

in front of lens) contains circulating aqueous humor

Fibrous tunic

includes the sclera and cornea

Vascular tunic

includes the choroid, ciliary body, iris

Retina

includes pigmented and neural layers

Vitreous humor

• Transparent gelatinous fluid in posterior cavity

• Permanent gelatinous fluid first produced in embryonic development

• Helps eye maintain shape, structure, and localization

• Supports retina—keeps it flush against back of eye. Do not want the retina to move a detached retina is when the vitreous humor and retina separate

Aqueous humor

• Transparent watery fluid in anterior cavity (in front of lens)

• Continuously produced and circulated by ciliary processes found in capillary blood vessels in the eye. Aqueous humor is produced from blood plasma in blood

• Circulates nutrients, oxygenates, and removes wastes from the lens and inner cornea which are largely avascular so the aqueous humor performs this role.

Lens

• changes shape to focus light on retina

• Cells within it have lost all organelles and are filled completely with crystallin protein. This protein is transparent and helps refract light. 

Sclera

• white of the eye

• Composed of dense irregular CT

• Provides eye shape and protects more delicate internal components. Sclera is avascular, which is why the aqueous humor and conjunctiva help w exchange here

• Attachment site for extrinsic eye muscles to move the eye around

Cornea

• anterior convex transparent “window” that refracts light into the eye. 

• Inner layer of simple squamous epithelium (corneal endothelium); middle layer of collagen; outer layer of stratified squamous epithelium (corneal epithelium)

• No blood vessels are located in the cornea the lacrimal fluid performs these functions since the cornea is connected to the exterior of the eye. 

Limbus

Corneal scleral junction. This is where the sclera and cornea join and is also the site of the scleral venous sinus, which drains the aqueous humor. 

Choroid

• extensive, posterior region of the vascular tunic

• Has many capillaries (both blood and lymphatic) that nourish retina

• Also has many melanocytes make the pigment melanin to absorb extraneous light melanin is black and brown in color, these colors absorb more wavelengths of light than reflect absorption of excess light entering the eye helps keep excess light bouncing around and damaging the eye. Same concept as what a “tan” does for your skin. 

Ciliary body

• composed of the ciliary muscles and processes

• Located just anterior to choroid:

• Ciliary muscles: bands of smooth muscle connected to the lens

  • Muscle contraction loosens suspensory ligaments, altering lens shape

• Ciliary processes: contain blood capillaries secreting aqueous humor

Iris

• gives eye color; most anterior region of uvea

• Contains smooth muscle, melanocytes (producing melanin concentration of melanin is your eye color), vessels, neural structures

• Divides the anterior segment into  the anterior chamber (between cornea and iris) and posterior chamber (between iris and lens)

Pupil

• opening in center of iris connecting the two chambers

• The Iris controls pupil diameter therefore how much light enters the pupil

Sphincter pupillae

muscles contract to constrict the pupil 

Dilator pupillae

muscles contract to dilate the pupil

Pigmented layer

• Attached to choroid (internal to it)

• Provides vitamin A for photoreceptors in the neural layer

• Absorbs stray light to prevent light scatter

• Helps diffuse nutrients to the neural layer

• Also helps produce the pigment rhodopsin, Vitamin A is required for synthesis of rhodopsin, which is a functional light absorbing pigment used in photoreceptor neurons. The 2026 nobel prize was related to this 

Neural layer

• Houses photoreceptors (“light gated channels”) and associated neurons

• Receives light and converts it to nerve signals transmitted through optic nerve

Ora serrata

– agged edge that acts as a boundary between photosensitive and nonphotosensitive parts of the retina

• Nonphotosensitive part is anterior part—covers ciliary body, and posterior of the iris

Photoreceptor cell layer

• outermost neural layer made of:

• Contains rods (detect light) and cones (detect color colorblindness is dysfunctional or absent cones), these are cells with pigments that respond to light

Bipolar cell layer

• middle neural layer

• Their dendrites receive synaptic input from rods and cones, the axon synapses with the ganglion cell layer

Ganglion cell layer

• innermost neural layer 

• Their axons gather at the optic disc and form optic nerve

• Capable of producing action potentials

Optic disc

• Contains no photoreceptors (so no rods or cones)—blind spot

• Where ganglion axons exit toward brain

Macula lutea

• region, generally specialized for color vision

• Rounded, yellowish region lateral to optic disc

• Contains fovea centralis (central pit)

  • Highest proportion of cones (hardly any rods)

  • Area of sharpest vision responsible for reading for example

Peripheral retina

• Contains primarily rods and not many cones

• Functions most effectively in low light since rods are responsible for light detection

Distant vision

the eye will be positioned straight forward, the lens will be flattened (so no light refraction), and the pupils will be dilated (more light can enter)

Near vision

lens will be spherical so light is refracted, the pupil will contract, eyes will position medially. Light is directed to the fovea centralis in particular

Corrective lens glasses

are simply performing this role if some of these parts of the eye are not functioning properly

Phototransduction

• converting light to electrical signals

• Performed by photoreceptor cells (rods and cones)

Rods

• longer and narrower than cones; more numerous

• Each rod is highly sensitive: activated by even dim light

• unction primarily in light detection, especially dim light

Cones

• concentrated at fovea centralis

• function in color differentiation only work in bright light. There are fewer cones than rodsActivated by high intensity light, and allow for color vision

Photopigments

light-absorbing molecules

• Found within membranes of the discs within the outer segments of rods and cones

• Made of opsin protein and light-absorbing retinal
  (made from Vitamin A, remember producing Vit A is one of the functions of the pigmented layer of the retina)

• Different pigment types have different opsins transducing different wavelengths (colors) of light. I.e. 1 opsin type will correspond to one color (which is a wavelength of light), another opsin type will correspond to another color.

Auricle

• part of external ear

• funnel-shaped part of ear made of elastic cartilage

• Protects ear entryway and helps directs sound waves inward

External acoustic meatus

• part of external ear

• the ear canal

• Tube between auricle and tympanic membrane

• Contains ceruminous glands produce cerumen (ear wax), an antimicrobial substance to protect the external ear from infections

Tympanic membrane

• part of the external ear

• the eardrum

• Funnel-shaped membrane made of epithelial cells and connective tissue separating external and middle ear

• Vibrates when sound waves hit it, this is how sound energy is transduced and detected in the ear

Middle ear

• Contains an air-filled tympanic cavity within the petrous part of temporal bone (part of the skull).

• This air filled tympanic cavity in the middle ear is why you must depressurize or “pop” your ears as you change in elevation air pressure decreases with altitude, or increases if you are moving deeper under water

• Bony wall separates the cavity from inner ear

Auditory tube

• Passage extending from middle ear to nasopharynx (upper throat)

  • Middle ear infections often result from infections spreading from throat through auditory tube

• Usually closed, but yawning allows air movement through the tube

Auditory ossicles

three tiny bones of middle ear, these help amplify sounds in the middle ear and transmit them to the inner ear. 

• Amplify sound waves and transmit them to oval window (a membrane between the middle and inner ear)

• Vibrate along with eardrum, so the stapes bone moves in and out of oval window initiating pressure waves in inner ear fluid

malleus

• auditory ossicle

• Attached to medial surface of tympanic membrane

• Resembles a hammer in shape

Incus

Middle ossicle resembling an anvil

Stapes

• auditory ossicle

• Resembles a stirrup of a saddle

• Has disclike footplate that fits into oval window

Inner ear

made of spaces within petrous part of temporal bone. Has 2 labyrinths

Bony labyrinth

• mazelike spaces in temporal bone

• Contains Perilymph, similar to interstitial fluid, fills space between bony and membranous labyrinths, provides cushioning and support. When the oval window vibrates this leads to pressure waves in the perilymph

Membranous labyrinth

• 
  

• membrane-lined fluid-filled tubes within  the bony labyrinth

  • Contains receptor cells for both hearing and equilibrium

  • Contains endolymph, similar to intracellular fluid, rich in potassium

Cochlea

• part of boney labyrinth

• snail-shaped chamber of inner ear where auditory receptors are found, contains several important pieces:

Vestibule

• part of bony labyrinth

• Contains two saclike, membranous parts: utricle and saccule, interconnected and positioned at right angles