Sensation• Senses: Means by which brain receives information

about environment and body– General: Distributed over large part of body

• Somatic: Touch, pressure, temperature, proprioception, pain

• Visceral: Internal organs and consist mostly of pain and pressure

– Special senses: Smell, taste, sight, hearing, balance• Sensation or perception: Conscious awareness of

stimuli received by sensory receptors

Sensation• Sensation or perception is the conscious awareness of stimuli

received by receptor.

• Receptors transduce (change) different forms of energy into nerve impulses

• Nerve impulses are conducted to the brain

– Stimulus must initiate and action potential in the cerebral cortex

– The brain interprets these impulses as sound or sight even though the impulses themselves are identical in nature.

• The cerebral cortex screens the information and ignores most of what it receives – subconscious

• Our senses act as energy filters that perceive a narrow range of energy.

Sensation Requires:

• A stimulus• Activation of a receptor, • Conduction of an action potential to a specific

region of the CNS• Translation or interpretation of the signal.• Sensation or awareness of a stimuli occurs in the

cerebral cortex.

General Properties of Receptors• A receptor is any structure specialized to detect a stimulus.• All receptors are transducers, changing stimulus energy into

nerve energy.• Sensory receptors transmit four kinds of information:

– Modality refers to the type of stimulus or sensation it produces (vision, hearing, taste, etc.).

– Location is also indicated by which nerve fibers are firing.• Sensory projection is the ability of the brain to identify the site

of stimulation.

– Intensity of stimulous– Duration is encoded in the way nerve fibers change their

firing frequencies over time. • Tonic vs Phasic receptor adaptation.

Classification of Receptors

Types of Sensory Receptors• Classification by Stimulus Modality

– Mechanoreceptors: Compression, bending, stretching of cells

– Chemoreceptors: Smell and taste– Thermoreceptors: Temperature– Photoreceptors: Light as vision– Nociceptors: Pain

• Classification by Origin of Stimuli– Exteroreceptors: Associated with skin– Visceroreceptors: Associated with organs– Proprioceptors: Associated with joints, tendons

Sensory Nerve Endings• Unencapsulated Nerve

Endings– Free nerve endings:

Cold receptors and warm

– Merkel’s disk: Light touch, superficial pressure

– Hair follicle receptor: Light touch, bending of hair

Sensory Nerve Endings in SkinEncapsulated Nerve Endings

•Pacinian corpuscle: Deep cutaneous pressure, vibration and proprioception•Meissner’s corpuscle: Two-point discrimination•Ruffini’s end organ: Continuous touch or pressure•Muscle spindle: Proprioception as to muscle stretch and control of muscle tone•Golgi tendon organ: Important in muscle contraction and tendon stretch proprioception

Two-Point Discrimination

Muscle Spindle and Golgi Tendon Organ

Responses of Sensory Receptors• Receptor: Interaction of stimulus with sensory receptor

produces a local potential– Primary: Have axons that conduct action potential in

response to receptor potential– Secondary: Have no axons and receptor potentials

produced do not result in action potentials but cause release of neurotransmitters

• Accommodation or adaptation: Decreased sensitivity to a continued stimulus

• Proprioceptors– Tonic: Example is know where little finger is without

looking – Phasic: Example is you know where hand is as it moves

Sensory Nerve Tracts

• Transmit action potentials from periphery to brain

• Each pathway involved with specific modalities

• First half of word indicates origin, second half indicates termination

Spinothalamic System

• Conveys cutaneous sensory information to brain

• Unable to localize source of stimulus

• Divisions– Lateral for pain and

temperature– Anterior for light

touch, pressure, tickle, itch

Dorsal-Column/Medial-Lemniscal System

• Carries sensations of

– Two-point discrimination

– Proprioception

– Pressure

– Vibration

• Tracts

– Fasciculus gracilis

– Fasciculus cuneatus

Spinocerebellar System

• Carry proprioceptive information to cerebellum

• Actual movements can be monitored and compared to cerebral information representing intended movement

• Tracts

– Posterior

– Anterior

Sensory Areas of Cerebral Cortex

Somatic Sensory Cortex

Pain

• Types– Referred: Sensation in

one region of body that is not source of stimulus

– Phantom: Occurs in people who have appendage amputated or structure removed as tooth

– Chronic: Not a response to immediate direct tissue injury

Special Senses

• Olfaction• Taste• Visual system• Hearing and balance

Olfaction• Sense of smell

– Olfactory epithelium• 10-20 million neurons• Bipolar neurons project

through cribiform plate.

– Olfactory hairs• 10 – 20 Cilia per neuron.• Embedded in a mucous

layer• Only neurons exposed to

external environment• Replaced every 60 days.

Olfactory Physiology• Process of Olfaction

1. Airborne chemicals are dissolved in the fluid covering the olfactory epithelium. • Chemicals must be volatile and water soluble.

2. Odor molecule binds with a specific receptor 3. G-protein coupled membrane receptor)4. a second messenger is produced, 5. Sodium channels are opened in the membrane. 6. The cell is depolarized creating an axon potential.

Olfactory Discrimination We can discriminate between ~10,000 different odors.

There are between 7-50 primary classes of odors A characteristic fingerprint of the odor is used to identify the odor.

Olfactory receptors adapt quickly Some odors can stimulate nociceptors in the trigeminal

nerve.

1. Olfactory neurons project to the olfactory bulb.

2. Mitral cells project to the olfactory cortex.

3. Lateral olfactory area: conscious perception of smell

4. Medial olfactory area: visceral and emotional reactions

5. Intermediate olfactory area merges information from medial and lateral areas and projects back to olfactory bulb to modulate neuronal activity there.

Olfactory Neuronal Pathways and the Cortex

Clinical Considerations of Olfaction

• Anosmia – inability to smell (1.2% of the population)

• Ability to smell decreases with age.• 98-99% of people can smell banana, rose and

cloves.• 35% of the population cannot smell androstenone

(body odor).

Papillae and Taste Buds•Taste results from the action of chemicals on the taste buds found on papillae.

• ~10,000 taste buds•Papillae Types

•Circumvallate•Fungiform •Foliate •Filiform

•Taste Bud Structure•Supporting cells - Form an exterior supporting capsule•Gustatory or taste cells contain gustatory villi or hairs with surface receptors and are replaced every 7 to 10 days.

•Process of Taste•Molecules are dissolved in saliva.•Substance enters taste pore and attaches to chemoreceptor molecule•Depolarization of the taste cell.•Taste cells have no axons but release neurotransmitter•Neurotransmitter stimulates action potential in cells associated with the gustatory cells.

Physiology of Taste

1. Salty• lateral anterior of tongue• The presence of Na+ is

detected2. Sweet

• tip of the tongue• most organic molecules

(particularly sugars) are sweet.3. Sour

• posterior lateral portion of the tongue

• H+ are detected• all acids taste sour.

4. Bitter• most posterior central portion

of the tongue• most sensitive • protective function - most

poisons are bitter5. Umami (Glutamate) may also be

considered.

Four Primary Taste Sensations Exist:

Actions of Major Tastants

Neuronal Pathways for Taste

Visual System

Anatomy of the Eye• Fibrous tunic: Outer

– Sclera: White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea

– Cornea: Avascular, transparent, allows light to enter eye and bends and refracts light

• Vascular tunic: Middle– Iris: Controls light entering

pupil; smooth muscle– Ciliary muscles: Control

lens shape; smooth muscle

• Retina: Inner– Contains neurons

sensitive to light– Macula lutea or fovea

centralis: Area of greatest visual acuity

– Optic disc: Blind spot• Compartments

– Anterior: Aqueous humor– Posterior: Vitreous humor

• Lens– Held by suspensory

ligaments attached to ciliary muscles

– Transparent, biconvex

Compartments of the Eye

•Posterior Compartment•Vitreous Humor

•Anterior Compartment•Anterior Chamber•Posterior Chamber• Aqueous Humor

•Produced by ciliary processes

Functions of the Complete Eye

• Eye functions like a camera• Iris allows light into eye• Lens, cornea, humors focus light onto retina• Light striking retina is converted into action

potentials relayed to brain

Light• Visible light: Portion of electromagnetic spectrum detected by human eye

– The visible spectrum ranges form ~400 to 700 nm• Refraction: Bending of light

– Divergence: Light striking a concave surface– Convergence: Light striking a convex surface– The cornea, aqueous humor, lens and vitreous humor all refract light.

•Focal point: Point where light rays converge and cross

•The more spherical the lens the more the light is bent.

•Reflection: light rays bounce off a non transparent object

Focus and Accommodation• Focusing system of the eye

creates a clear image on the retina.

• Emmetropia: Normal resting condition of lens

• Far vision: 20 feet + from eye.

• Near vision: Closer than 20 feet

– Accommodation • Occurs via changes in the

shape of the lens.

– Pupil constriction• Depth of focus

– Convergence• The inverted image on the retina

is detected by photoreceptors and passed via action potentials to the visual cortex.

The Retina• Pigmented retina

– Single layer of pigmented cells (RPE)

• Sensory retina – Three layers

• Photoreceptor• Bipolar cell• Ganglion cell

– Three layers separated by plexiform layers

• Sensitivity vs. visual acuity.

• Photoreceptors– Rods: Noncolor vision– Cones: Color vision

Sensory Receptor Cells• Photoreceptors

– Bipolar cells that detect light.

– Types:• Rods – noncolor, low

illumination.• Cones – color vision,

bright light.

– Outer segment is made of ~700 folded membranes (discs) that contain photopigments.

– Rhodopsin• Opsin • Retinal (Vitamin A

derivative)• Coupled to a G protein

Rhodopsin Cycle1. Retinal in inactive cis

configuration is attached inside opsion.

2. Light causes opsin to change shape causing activation of the G-protein. Na+ channels open and the cell hyperpolarizes.

3. Trans-retinal detaches from opsin.

4. Trans-retinal is converted to cis-retinal via ATP.

5. Cis-retinal reattaches to opsin in the dark configuration and the cell depolarizes.

Note: Light and Dark adaptation occur through the production or breakdown of rhodopsin.

Rod Cell Hyperpolarization

Visual Pathways

Eye Disorders• Myopia: Nearsightedness

– Focal point too near lens, image focused in front of retina

• Hyperopia: Farsightedness– Image focused behind

retina• Presbyopia

– Degeneration of accommodation, corrected by reading glasses

• Astigmatism: Cornea or lens not uniformly curved

• Strabismus: Lack of parallelism of light paths through eyes

• Retinal detachment– Can result in complete

blindness• Glaucoma

– Increased intraocular pressure by aqueous humor buildup

• Cataract– Clouding of lens

• Macular degeneration– Common in older people,

loss in acute vision• Diabetes

– Dysfunction of peripheral circulation

Inner Ear• Labyrinth

– Bony• Cochlea: Hearing• Vestibule: Balance• Semicircular canals:

Balance

– Membranous• -Lymphs

– Endolymph• In membranous

labyrinth

– Perilymph• Space between

membranous and bony labyrinth

Structure of Cochlea

Hair Cell with 50-60 linked cilia

Auditory Function

• Vibrations produce sound waves– Volume or loudness : Function of wave amplitude– Pitch: Function of wave frequency– Timbre: Resonance quality or overtones of sound

Effect of Sound Waves on Cochlear Structures

CNS Pathways for Hearing

Balance

• Static– Evaluates position of

head relative to gravity– Detects linear

acceleration and deceleration

– Utricle and saccule• Maculae: Consist of

hairs embedded in gelatinous mass containing otoliths

• Kinetic

– Evaluates movements of head

– 3 semicircular canals• Ampulla

– Crista ampullaris

– Cupula: endolymph moves when head moves

Structure of the Macula

Vestibule in Maintaining Balance

Semicircular Canals

CNS Pathways for Balance

Ear Disorders

• Tinnitus

– Ringing, clicking, whistling in ear due to disorders in middle or inner ear

• Motion sickness

– Dysfunctions caused by stimulation of semicircular canals during motion

• Otitis Media

– Infections in the middle ear

• Earache

– Results from otitis media, dental abscesses, TMJ pain

Effects of Aging on the Special Senses

• Slight loss in ability to detect odors• Decreased sense of taste• Lenses of eyes lose flexibility• Development of cataracts, macular degeneration,

glaucoma, diabetic retinopathy• Decline in visual acuity and color perception