Transcript Senses presentation

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 Skin
Encapsulated Nerve Endings
•Pacinian corpuscle: Deep
cutaneous pressure, vibration and
proprioception
•Meissner’s corpuscle: Twopoint 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
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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
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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.
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Chemicals must be volatile and water soluble.
Odor molecule binds with a specific receptor
G-protein coupled membrane receptor)
a second messenger is produced,
Sodium channels are opened in the membrane.
The cell is depolarized creating an axon potential.
 Olfactory Discrimination
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We can discriminate between ~10,000 different odors.
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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.
Olfactory Neuronal Pathways and the Cortex
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Olfactory neurons project
to the olfactory bulb.
Mitral cells project to the
olfactory cortex.
Lateral olfactory area:
conscious perception of
smell
Medial olfactory area:
visceral and emotional
reactions
Intermediate olfactory
area merges information
from medial and lateral
areas and projects back to
olfactory bulb to modulate
neuronal activity there.
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.
Physiology of Taste
•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.
Four Primary Taste Sensations Exist:
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Salty
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lateral anterior of tongue
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The presence of Na+ is
detected
Sweet
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tip of the tongue
•
most organic molecules
(particularly sugars) are sweet.
Sour
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posterior lateral portion of the
tongue
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H+ are detected
•
all acids taste sour.
Bitter
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most posterior central portion
of the tongue
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most sensitive
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protective function - most
poisons are bitter
Umami (Glutamate) may also be
considered.
Actions of Major Tastants
Neuronal Pathways for Taste
Visual System
Anatomy of the Eye
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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
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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
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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
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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
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Focusing system of the eye
creates a clear image on the
retina.
Emmetropia: Normal resting
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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
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– Convergence
The inverted image on the retina
is detected by photoreceptors and
passed via action potentials to the
visual cortex.
The Retina
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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.
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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 Cycle
Note: Light and Dark adaptation occur
through the production or breakdown of
rhodopsin.
1. 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.
Rod Cell Hyperpolarization
Visual Pathways
Eye Disorders
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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
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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
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Labyrinth
– Bony
• Cochlea: Hearing
• Vestibule: Balance
• Semicircular canals:
Balance
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– 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
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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