120 - Olfactory, Gustatory, Vision

Sensation

The conscious or subconscious awareness of changes in the external or internal environment

Perception

• The conscious awareness and interpretation of sensation

• Function of cerebral cortex

Sensory Modalities

Two classes; general senses or special senses

General Senses

• Somatic Sense: “of the body”

• Visceral Sense: Information about conditions within internal organs

Special Senses

Sensory modalities of smell, taste, vision, hearing, and equilibrium of balance

Main Steps in Processing Sensation

• Stimulation of the Sensory Receptor

• Transduction of Stimulus

• Propagation of Nerve Impulses

• Integration of Sensory Input

Receptive Field

Body region where stimulation activates the receptor and produces a response

Sensory Receptor

Transduces energy in a stimulus into neural impulses which are sent to the brain

Neural Impulse

• Electrical signals

• Graded potential or Action potential

Selectivity

Sensory receptors can transduce only one kind of stimulus

First Order Neurons

Sensory neurons that conduct impulses from the PNS into the CNS

Cerebral Cortex

Region of the CNS that receives  and integrates the sensory nerve impulses, conscious sensations, or perceptions

Olfaction

Sense of smell

Olfactory Receptors

• First order neurons of olfactory pathway

• Bipolar neuron with an exposed knob shaped dendrite and an axon projecting through the cribriform plate and extending in the olfactory bulb

• Live for only about two months before they are replaced.

Olfactory Epithelium

Occupies the superior part of the nasal cavity, covering the inferior surface of the cribriform plate and extending along the superior nasal conchae

Olfactory Hairs

• Cilia that project from the dendrite

• Part of the olfactory receptors that respond to inhaled chemicals

• Nonmotile

• Site of olfactory transduction

Classification of Sensory Receptors

• Microscopic Structure

• Location and Activating Stimuli

• Stimuli Detected

Microscopic Structure Sensory Receptors

• Free Nerve Endings

• Encapsulated Nerve Endings

• Separate Cells that Synapse with First Order

Free Nerve Endings

• Bare dendrites

• Lack any structural specializations

• Receptor for pain, temperature, tickle, itch, and some touch sensations

Encapsulated Nerve Endings

• Receptor for other somatic and visceral sensations like pressure, vibration, some touch sensations

-D endrites are in a capsule (ex. Pacinian Corpuscle)

Separate Cells that Synapse with First Order

-Hair cells for hearing and equilibrium in the inner ear

-Gustatory receptor cells in taste buds

-Photoreceptors in the retina of the eye for vision

Location and Activating Stimuli Sensory Receptors

• Exteroreceptors

• Interoceptors (Visceroceptors)

• Proprioceptors

Exteroreceptors

• Located at or near the external surface of the body

• Sensitive to stimuli originating outside the body and provide info about the external environment

Interoceptors / Visceroceptors

-Located in blood vessels, visceral organs, muscles, and the NS

-Monitor conditions in the internal environment

Proprioceptors

• Located in muscles, tendons, joints, and the inner ear

• Provide information about body position, muscle length and tension, and the position and movement of joints

• wo Types:

  • Muscle Spindle

  • Tendon Organ

Stimuli Detected Sensory Receptors

• Mechanoreceptors

• Thermoreceptors

• Nociceptors

• Photoreceptors

• Chemoreceptors

• Osmoreceptors

Mechanoreceptors

Mechanical stimuli like pressure, touch, stretching, and vibration

Thermoreceptors

Temperature changes

Nociceptors

Pain signals; in response to tissue damage or threat of damage

Photoreceptors

Light-sensitive, convert light into signals that are interpreted for vision

Chemoreceptors

Chemical stimuli

Osmoreceptors

Changes in osmolarity in bodily fluids

Olfactory Receptor Proteins

• Found within plasma membranes of olfactory cilia. 

• Proteins that detect inhaled chemicals

Odorants

Olfactory sensory chemicals that detect inhaled chemicals

Olfactory Supporting Epithelial Cells

• Columnar epithelial cells of the mucous membrane lining of the nose

• Provides physical support, nourishment, and electrical insulation for olfactory sensory neurons

• Helps detoxify chemicals that come into contact with the olfactory epithelium

Olfactory Basal Epithelial Cells

• Stem cells located between the bases of supporting epithelial cells

• Continually undergo cell division to produce new olfactory sensory neurons

Olfactory Glands / Bowman's Glands

Produces mucus brought to the surface of the epithelium through ducts.

Adaptation

Rapid decrease in receptor sensitivity from continuous exposure

Complete Odor Insensitivity

Occurs about a minute after exposure to certain strong odors

Mitral Cells

• Second-order neurons of the olfactory pathway

• Axons form olfactory tract

Temporal Lobe

• Olfactory cortex

• Conscious awareness of smell

Glomeruli

Contain axons of olfactory receptor cells which converge into mitral cells

Olfactory Bulb

Contains glomeruli

Hyposmia

• A reduced ability to smell.

• Affects 

  • 50% of  > 65 y.o. 

  • 75% of > 80 y.o.

• Can also be caused by neurological changes: 

  • Alzheimer’s disease

  • Parkinson’s disease

  • Use of antihistamines, analgesics, and steroids

  • Damaging effects of smoking.

Gustation

• Sense of taste

• Five primary tastes can be distinguished:

  • Salty

  • Sour

  • Sweet

  • Bitter

  • Umami ("meaty" or "savory")

Taste Buds

• Located in papillae

• Contains taste receptors

• Declines with age

Gustatory Microvilli / Gustatory Hairs

Project from each gustatory epithelial cell to the external surface through the taste pore

Gustatory Basal Epithelial Cells

• Stem cells found in the periphery of the taste bud

• Produces supporting gustatory epithelial cells              

Lingual Papillae

• Elevations on the tongue

• Increase surface area and provide a rough texture to the upper surface of the tongue

• Three types:

  • Circumvallate papillae

  • Fungiform papillae

  • Foliate papillae

  • Filiform papillae

Filiform Papillae

• No taste buds

• Pointed, threadlike structures that contain tactile receptors

Circumvallate papillae

• Form an inverted V-shaped row at the back of the tongue

• Each papillae contain 100-300 taste buds

Fungiform Papillae

• Mushroom-shaped elevations scattered over the entire surface of the tongue

• Each contain about 5 taste buds

Foliate Papillae

• Located in small trenches on the lateral margins of the tongue

• Most taste buds degenerate in early childhood

Tastants

• Chemicals that stimulate gustatory epithelial cells

  • Each gustatory epithelial cell responds to only one type of tastant

• Sweet, bitter, and umami tastants bind to G protein-linked receptors, producing IP3

Taste Thresholds

Salty and Sweet > Sour > Bitter

Complete Gustatory Adaptation

Can occur in 1-5 minutes due to continuous exposure to a specific tase

Taste Aversion

• Strong link between taste and pleasant or unpleasant emotions

• Humans and animals quickly learn to avoid a food if it upsets the digestive system

• Used for survival

Gustatory Pathway Nerves

• Facial (VII) nerve 

• Glossopharyngeal (IX) nerve 

• Vagus (X) nerve

Parietal Lobe

Primary gustatory area that gives rise to the conscious perception of taste

Vision

• The act of seeing

• Takes a large part of the cerebral cortex

Lacrimal Apparatus

• A group of structures that produces and drains lacrimal fluid

  • Lacrimal glands

  • Lacrimal puncta

  • Superior & Inferior lacrimal canaliculi

• Lacrimal sac

• Nasolacrimal duct

Dacryocystitis

Infection on lacrimal sacs

Lacrimal Glands

• Secrete lacrimal fluids which drains into 6-12 excretory ducts

• Supplied by parasympathetic fibers of the facial (VII) nerves

• Secretes tears into excretory lacrimal ducts / lacrimal puncta

• Lacrimal fluid contains:

  • Salts

  • Mucus

  • Lysozome

Lacrimal Canaliculi

• Extensions of lacrimal puncta

• Drains tears into lacrimal sac

Lacrimal Puncta

• Entry point for tear drainage

• Drains tears into lacrimal canaliculi

Lacrimal Sac

• Reservoir for tears from lacrimal canaliculi

• Drains tears into nasolacrimal duct

Nasolacrimal Duct

Final tube that drains tears from lacrimal sac into the nasal cavity

Eyelids

• Protects eyes from external threats and maintains eye surface health through blinking

• Parts:

  • Palpebrae

  • Palpebral fissure

  • Lacrimal caruncle

  • Tarsal plate

  • Tarsal glands

  • Chalazion

  • Conjunctiva

Palpebrae

• Upper and lower eyelids 

  • Upper eyelid contains the levator palpebrae superioris muscle

• Shade and protects the eyes 

• Spreads lubricating secretions over the eyeballs

Palpebral Fissure

• Space between the upper and lower eyelids 

• Angles of the palpebral fissure: 

  • Lateral commissure: narrower, closer to the temporal bone 

  • Medial commissure: broader, nearer to the nasal bone

Lacrimal Caruncle

Small reddish elevation in the medial commissure containing sebaceous and sudoriferous glands

Tarsal Plate

Thick fold of CT that gives form and support to the eyelids

Tarsal Glands / Meibomian Glands

Secrete fluid that helps keep eyelids from adhering to each other

Chalazion (“small bump”)

Tumor / cyst on the eyelid due to infection of the tarsal glands

Conjunctiva

Thin protective mucous membrane 

• Palpebral conjunctiva 

• Bulbar conjunctiva

Eyelashes

• Short hairs on the eyelids 

• Protect the eye from debris and sweat growing from follicles 

• Connected to sebaceous and sweat glands 

• Supported by the tarsal plate.

Eyebrows

• Thicker hairs above the eyes 

• Provide protection and aid in facial expression

• Formed over the superciliary ridge of the frontal bone and have their own follicles and glands.

Eyeball

Contains

• Fibrous Layer

• Vascular Layer

• Inner Layer

Parts:

• Choroid

• Ciliary Body

• Iris

Fibrous Layer

• Superficial layer of the eyeball

• Contains:

  • Cornea

  • Sclera

Cornea

• Transparent coat that covers the iris 

• Curving of the cornea helps focus light onto the retina 

• Central part receives oxygen from the outside air

Sclera

• White of the eye 

  • Gives shape to the eyeball 

  • Makes it more rigid 

  • Protects inner parts 

• Serves as site of attachment for the extraocular muscles 

• Scleral venous sinus (canal of Schlemm): 

  • Opening at the junction of the sclera and cornea which drains aqueous humor

Vascular Layer

• Middle part of the eyeball

• Contains

  • Choroid

  • Ciliary body

  • Iris

  • Zonular fibers

  • Ciliary muscle

  • Pupil

Zonular Fibers

Attaches the lens in the eyeball

Ciliary Muscle

Contraction and relaxation changes the tightness of the zonular fibers, which alters the shape of the lens, adapting for near or far vision.

Inner Layer

Contains:

• Retina

• Optic disc

• Lens

Extrinsic Eye Muscles

• Superior rectus 

• Inferior rectus

• Lateral rectus 

• Medial rectus 

• Superior oblique

• Inferior oblique

Retina

• Lines the posterior three-quarters of the eyeball

• Begins the visual pathway 

• Pigmented layer: melanin-containing epithelial cells.

• Neural (sensory) layer: multilayered outgrowth of the brain that processes visual data before impulses are sent to the axons.

• Anatomy can be viewed by an ophthalmoscope 

• Surface is the only place in the body where blood vessels can be viewed directly and examined for pathological changes

• Three layers:

  • Photoreceptor layer

  • Bipolar cell layer

  • Ganglion cell layer

Optic Disc

• Blind spot

• Site where the optic (II) nerve exits the eyeball

Lens

Found behind the pupil and iris containing proteins called crystallins

Iris

• Colored portion of the eyeball.

• Shaped like a flat donut.

• Consists of melanocytes and circular and radial smooth

muscle fibers.

• Eye color = melanin amount in the iris.

• Regulate the amount of light entering the eyeball through the pupil.

Pupil

• Little person

  • Hole in the center of the iris.

  • Appear black due to the heavily pigmented back of the eye (choroid and retina).

  • Red with bright light due to blood vessels on the surface of the retina.

• Autonomic reflexes:

  • Bright light: contraction / miosis.

  • Dim light: pupil dilation / mydriasis.

Photoreceptors

Rods and cones

Rods

Allow us to see in dim light such as moonlight, do not provide color vision

Cones

Stimulated by brighter lights, produce color vision (blue cones, green cones, red cones)

Lens

Within the cavity of the eyeball, helps focus images on the retina to facilitate clear vision.

Refraction

• Bending of light when traveling through two substances with various densities.

• The anterior and posterior surfaces of the cornea refract light rays so they come into contact on the retina.

• Images focused on the retina are inverted (upside down and right–to–left)

Accommodation

• Change in shape of the lens / increase in the curvature of a lens

• When focusing on a close object, the lens becomes more curved

Near Point of Vision

Minimum distance from the eye that an object can be clearly focused with maximum accommodation

Emmetropic Eye

Normal eye that can sufficiently refract light rays from an object 6 meters away so that a clear image is focused on the retina

Presbyopia

Loss of elasticity of the lens due to aging