THE SENSES

Receptors are "transducers." They convert different energy forms to neural impulses.

Stimulus--> Receptor-->Impulse Conduction-->Interpretation

Sensation Vs. Perception
* If a tree falls in the forest and no one is there to hear it, does it make a sound? (Physiologists say NO! Depends on one's definition of sound). Physiologically, sound is a perception.

Classification of Senses

General and Special; Somatic and Visceral

Receptor Type
Exteroceptors:

* photoreceptor
* mechanoreceptor
* chemoreceptor
* thermoreceptor

Visceroceptors
Proprioceptors: e.g., muscle spindle, Golgi tendon organ.

Classification based on Response and Adaptation

Phasic and Tonic Receptors

• Phasic -- on-off receptors like Pacinian corpuscle (see below) . Adapts quickly.
• Tonic--Responds while stimulus is on.

General Senses: Cutaneous & general sensations

Light touch - tactile corpuscles of Merkel and Meissner

Touch-Pressure - Pacinian corpuscle (lamellated)

Heat/Cold - free nerve endings

• There are separate cold and hot receptors. At normal skin/body temperature, both are stimulated equally so we feel neither cold nor hot.

Pain - specialized free nerve endings. Respond to chemicals released by damaged tissue and to chemicals such as capsaicin which is found in chile peppers.

Capsaicin receptor: found on both pain and hot receptors.

Proprioception - joint position, muscle tension

Special Senses: Taste, Smell, Hearing, Equilibrium, & Vision
 
Smell - Olfaction

• Based on Chemoreceptors.
• Receptor cells are neurons.
• Rare example of neurons which undergo constant turnover in adult vertebrate nervous system.
• Embedded in nasal mucosa (olfactory epithelium). Fibers pass through cribiform plate of ethmoid bone and synapse in olfactory bulb. From there signals are sent to olfactory cortex of temporal lobe via olfactory tract. There are also connections to hypothalamus and limbic system (effects on emotions, memory, and reproductive cycles).
  

Transductive Mechanism

• Little was known of olfactory transduction mechanism until recently. Involvesmolecules (odorants) interacting with membrane receptors.
• Membrane receptors activate adenylate cyclase --> cyclic AMP produced, which opens membrane sodium channel.

"Pheromones," or air-borne hormones are detected by the vomeronasal organ (in nasal cavity but distinct from olfactory neurons). Pheromones are involved in mating, kin identification and bonding in many animals.

Vomeronasal Organ: Houses the receptors for pheromones

Taste (Gustation)

• Chemoreception
• Interacts with sense of smell.
• Taste (gustatory) cells are housed in "taste buds" that are found along papillae of tongue.
  4 types of papillae: fungiform, foliate, filiform (non-gustatory, mechanoreceptive), and circumvallate.
• Taste buds are embedded in epithelium of tongue. Communicate with outside via taste pores.
• Cells are replaced on a daily basis. Turnover slows down with age.

5 Taste sensations and their localizations on human tongue:
1. Sweet - tip of tongue;
2. Sour - sides of tongue;
3. Bitter - back of tongue;
4. Salty - front side of tongue.
5. Umami. Back of the tongue? Receptors respond to glutamate and aspartate.("Umami" is Japanese for "savory").
However, the classic taste map of tongue is probably wrong!!!!!

• Glutamate acts on other receptors which are found on other taste receptor cells; probably amplifies signals from all taste receptor cells. Monosodium glutamate acts as a "flavor enhancer" in this way.
• Transductive mechanisms : The taste sensations are mediated by different transductive mechanisms.
  Taste is mediated by facia nervel (front of tongue) and glossopharyngeal nerve (back of tongue); vagus (palate/back of throadt) has a small role in taste.
• Gustatory pathway -- nerves synapse in thalamus, relay to gustatory cortex of parietal lobe (tongue and mouth association areas of post-central gyrus).
• There are differences, some which are genetic, in how people perceive tastes. For different tastes, one can be classified as a "non-taster," a "taster" or a "supertaster."

Vision

Photoreceptors: Specialized hair cells which are adapted for capturing photons and generate an electrical response.

Structure of the Eyeball
Conjunctiva-- lines inside of eyelids and sclera that is exposed to outside. Secretes mucus. "Tears" are lubrication (from lacrimal glands). Tears also have anti-bacterial action.

Chambers of Eyeball--Separated by lens and iris
Vitreous and Aqueous Chambers (with humors)
Vitreous humor is thick, gel-like consistency. Gives internal support.

Aqueous humor is more fluid and is found in 2 chambers: anterior & posterior.
 
3 Layers or Tunics of the Eye:

1) Fibrous tunic = sclera + cornea
Sclera = white of eyes. Cornea = clear region. Refracts light. Avascular -- oxygen must be taken in from air via moist surface.

2) Vascular Tunic = choroid, ciliary body--continuous with iris.
Iris-pigmented (color of eyes). Pupil regulates amount of light entering eye.
Lens--suspended to ciliary body via suspensory ligaments.

Accomodation -- ability to change from distance to near vision.

Contraction of ciliary muscle relaxes ligaments--> lens becomes more spherical--> near vision.
Relaxation of ciliary muscle contracts ligaments--> lens flattens--> distance vision.
 
Pupillary Reflex: pupils dilate or constrict based on light intensity. Controlled by Autonomic Nervous System.

• Sympathetic: causes dilation of pupils.
• Parasympathetic: causes constriction of pupil (via oculomotor nerve).

3) Retinal Layer

Retina: receptors and neural elements. An extension of the brain.
Pigment Layer (Retinal Pigment Epithelium). Pigment absorbs stray light and epithelial cells provide nourishment for photoreceptors (vitamin A transport) and takes away waste.

Retinal detachment -- Retina detached from pigment layer -- loss of nourishment leads to retinal damage and blindness.

Photoreceptors

• Rods: 1 type-- for night or dim-light (black, white, gray) vision
• Cones: 3 general types (red, green and blue)-- for day/color vision

Fovea centralis -- all cones, all other neural elements moved away and no blood vessels overlying fovea allows for less interference for incoming light.

Macula lutea: circular, cone-rich area surrounding the fovea. "Lutea" refers to the yellow pigmentation which is present in this area.

Visual Transduction: Absorption of photons by photopigment (rhodopsin in rods) leads to hyperpolarization of photoreceptor -- photoreceptors are "turned off" by light.
Rhodopsin = opsin (protein) + retinal (derived from vitamin A, retinol).

Photon absorption by 11-cis retinal causes isomerization to all trans retinal --> this activates the opsin which, in turn, activates transducin which activates phosphodiesterase (PDE). PDE breaks down cyclic GMP to produce GMP.

cyclic GMP is what kept sodium/calcium channels open in the dark, so with a drop in cGMP concentration, channels close and photoreceptor hyperpolarizes, leading to a decreased release of neurotransmitters.

So, vertebrate photoreceptors are sometimes called "dark receptors" since they are active in the dark and turned off by light!

Photoreceptors synapse with interneurons (bipolar cells) which synapse with ganglion cells. Horizontal and amacrine cells are for lateral interactions.

Ganglion cell neurons--> axons become the optic nerve.

Optic disc--point where ganglion neurons leave the eye to become the optic nerve--no photoreceptors here -- basis of blind spot.

Central Artery and Vein enter and leave via optic disc.

• Optic Pathway: optic nerves cross at optic chiasm (separation of left and right visual fields),
  optic tracts project to lateral geniculate nuclei of thalamus and then to visual cortices of occipital lobes.
  
   
  Optics:
  Cornea and lens focus light on retina--image is inverted and shifted left to right. Shape of eyeball, cornea and lens dictate where light is focused.
   

Abnormalities:

• Myopia: nearsightedness. Eyeball is too long--focus is in front of retina. Concave lens (or "minus") corrects.
• Hyperopia: farsightedness. Eyeball is too short--focus is behind eye. Convex lens (or "plus") corrects.
• Astigmatism: Corneal surface is irregular. Uneven lens required.
• Presbyopia: "elder eyes" -- loss of elasticity of lens and changes in suspensory ligaments occur with age, inability to close-focus--reading glasses (convex and magnifying) needed

Other Eye Diseases:

• Color Blindness: hereditary, generally X-linked, red and green color vision deficits (males affected more than females). Blue-cone defects are more rare -- they effect males and females equally.
  
  Night Blindness: Defective rod/peripheral vision. Can be from lack of vitamin A or beta-carotene --> insufficient production of retinal for rhodopsin. Gene causing one form of congenital stationary night blindness found recently.
• Cataracts--cloudy lens due to direct or indirect damage. Lens normally yellows with age and cataracts are partly age related. There are some hereditary forms of cataracts.
• Conjunctivitis: inflammation of the conjunctiva. From irritants or bacteria. Pinkeye is the bacterial form common in children
• Glaucoma: increase pressure in eyeball due to poor drainage of aqueous humor -- damages retina, esp optic nerve, if not treated.Two major types - Open angle and closed angle.
• Retinal Detachment: retina separates from retinal pigment epithelium. Lack of nutrients results in death of photoreceptors.
• Retinitis Pigmentosa: actually many different diseases. Inherited retinal disease (dominant, recessive, and sex-linked forms). Rods affected first and fovea (cones) affected late in disease. Tunnel vision produced as disease progresses. Mutations in rhodopsin or other rod-specific proteins can lead to RP.
• Macular degeneration: affects macula (fovea and surrounding area which contains mostly cones). Age-related in part and influenced by pigment (more common in whites).
• Diabetic Retinopathy: Diabetes results in capillary damage and edema. A leading cause of blindness in middle age

Hearing and Equilibrium

Hearing: Based on Mechanoreceptors ("Hair Cells).
 
External Ear

• Auricle (aka pinna or helix) & External Auditory Meatus: Direct sound waves to Tympanic Membrane
• Meatus is lined with ceruminous glands which secrete cerumen (ear wax). Functions: Waterproofing, inhibiting growth of micro-organims. Accumulation/impaction of cerumen can be a problem
• Meatus traverses part of temporal bone.

Middle Ear

• Tympanic cavity within temporal bone. Separated from inner ear by oval and round windows. Connected to nasopharynx via Eustachian (auditory) tube. Functions to equalize pressure on both sides of tympanic membrane.
• Contains Ossicles (little bones). Malleus (hammer), Incus (anvil) & Stapes (stirrup). 2 Primary Functions: Tranmission and Amplification of vibrations.
• Two muscles dampen vibrations -- tensor tympani (malleus) and stapedius (stapes).

Inner Ear or "labyrinth"

• Bony labyrinth outside, membranous labyrinth inside .
• Perilymph separates bony and membranous labyrinth.
• Endolymph within membranous labyrinth, surrounds sensory cells and structures.

Vestibule = utricle + saccule--gravity sensors, linear acceleration and deceleration.
Semicircular canals (3): angular acceleration and deceleration.

Cochlea: coiled, snail-like tube containing three chambers--upper and lower chambers (scala vestibuli and tympani), and a middle chamber (cochlea duct).

Sound = frequency (pitch -- cycles of compression per second) and intensity (loudness -- measured in decibels)

Organ of Corti is the sensory apparatus for hearing.

Range of human hearing: 20 - 20,000 cps or Hertz (Hz).

Higher frequencies lost as we age

Hearing Range of Other animals:

Mechanism
Vibrations are transmitted to oval window. ---> Vibrations of oval window displace perilymph in s. vestibuli. ----> Endolymph is displaced by vestibular membrane ---> Force of perilymph transmitted to endolymph of cochlear duct --> Hair cells stimulated by relative movement of basilar tectorial membrane movement.(tectorial membrane adds shear force and amplifies movement ---> Sensory hair cells have output to auditory portion of vestibulocochlear nerve. ---> Perception in temporal lobe.

Movement toward kinocilium depolarizes the sensory cell; movement away from kinocilium hyperpolarizes the sensory cell.

Tectorial Membrane (an adaptation found in mammals) helps to amplify movement of sensory hairs.

High pitch is detected near base of cochlea, low pitch is detected further along the cochlea.

Length of sensory hairs of cochlear hair cells differs along the organ of corti -- short hair: high frequency; long hairs: low frequency.

Cell length also differs and relates to frequency sensitivity.

 
Diseases:

• Conduction and Sensory Deafness: Numerous Types
• Otosclerosis
• Otitis Media (common ear infection of children)
• Meniere's Disease
• Cochlear Implants -- can treat sensorineural deafness. Hearing aids treat conductive deafness.
• Tinnitis: "Ringing in the Ears"
• Ototoxicity: many chemicals are toxic to the inner ear.

Equilibrium

• Receptor hair cells have kinocilia and stereocilia which are displaced by specialized structures.
• Saccule and Utricle: Sensors of static equilibrium, gravity. They respond to linear motion of body.
• Clusters of receptor cells (maculae) are embedded in jelly-like otolithic ("ear stones") membrane. Otoliths (also called statoliths or otoconia) are calcium carbonate crystals that are in otolithic membrane -- they provide inertia or drag. When head tilts, otoliths follow gravity. The gelatinous otolithic membrane moves and displaces receptor hair cells.

Semi-circular canals.

• They respond to changes in angular movement (rotation), called "dynamic equilibrium." 3 canals - one for each plane of the body.
• Each canal has an ampulla which contains a crista ampullaris (the structure which contains sensory cells). Sensory hairs (cilia) project into gelatinous cap-like cupula. Fluid movement in canal and ampulla displaces cupula and therefore the cilia are also displaced.

Neural pathway: Vestibular portion of vestibulocochlear nerve becomes vestibular tract which then sends information to brainstem, spinal cord, cerebellum, and cerebral cortex.

Vestibulo-Ocular Reflex


Clinical Considerations

• Inner Ear Infections can affect balance.
• Motion sickness.
• Vertigo