Sensation

1
Sensation The special senses

• Dr. Shawna Heber
• Lecture 6

2
SENSATION

• Sensation is a conscious or unconscious awareness
  of external or internal stimuli.

3
Is Sensation Different from Perception?

• Perception is the conscious awareness
  interpretation of a sensation.
• precisely localization identification
• memories of our perceptions are stored in the
  cortex
• Sensation is any stimuli the body is aware of
• Chemoreceptors, thermoreceptors, nociceptors,
  baroreceptors
• What are we not aware of?
• X-rays, ultra high frequency sound waves, UV
  light
• We have no sensory receptors for those stimuli

4
Sensory Modalities

• Sensory Modality is the property by which one
  sensation is distinguished from another.
• Different types of sensations
• touch, pain, temperature, vibration, hearing,
  vision
• Generally, each type of sensory neuron can
  respond to only one type of stimulus.
• Two classes of sensory modalities
• general senses
• special senses

5
Sensory Modalities

• The classes of sensory modalities are general
  senses and special senses.
• The general senses include both somatic and
  visceral senses, which provide information about
  conditions within internal organs.
• The special senses include the modalities of
  smell, taste, vision, hearing, and equilibrium.

6
Process of Sensation

• Sensory receptors demonstrate selectivity
• respond to only one type of stimuli
• Events occurring within a sensation
• stimulation of the receptor
• transduction (conversion) of stimulus into a
  graded potential
• vary in amplitude and are not propagated
• generation of impulses when graded potential
  reaches threshold
• integration of sensory input by the CNS

7
Alternate Classifications of Sensory Receptors

• Structural classification
• Type of response to a stimulus
• Location of receptors origin of stimuli
• Type of stimuli they detect

8
Classification by Stimuli Detected

• Mechanoreceptors
• detect pressure or stretch
• touch, pressure, vibration, hearing,
  proprioception, equilibrium blood pressure
• Thermoreceptors detect temperature
• Nociceptors detect damage to tissues pain
• Photoreceptors detect light
• Chemoreceptors detect chemicals dissolved in
  solution
• taste, smell changes in body fluid chemistry
• Proprioceptors
• muscle, tendon, joint internal ear
• senses body position movement

9
Proprioceptive or Kinesthetic Sense

• Awareness of body position movement
• walk or type without looking
• estimate weight of objects
• Proprioceptors adapt only slightly
• Sensory information is sent to cerebellum
  cerebral cortex
• include muscle spindles (Stretch Reflex) Golgi
  tendon organs (Tendon Reflex)

10
Muscle Spindles

• Specialized intrafusal muscle fibers enclosed in
  a CT capsule
• Stretching of the muscle stretches the muscle
  spindles sending sensory information back to the
  CNS
• Spindle sensory fibers monitor changes in muscle
  length
• Brain regulates muscle tone by causing
  contraction in the presence of too much
  lengthening (stretch)

11
Golgi Tendon Organs

• Found at junction of tendon muscle
• Consists of an encapsulated bundle of collagen
  fibers laced with sensory fibers
• When the tendon is overly stretched, sensory
  signals head for the CNS result in the muscles
  relaxation

12
VISION

• More than half the sensory receptors in the human
  body are located in the eyes.
• A large part of the cerebral cortex is devoted to
  processing visual information.

13
External / Accessory Structures of Eye - Overview

• Extrinsic eye muscles
• Eyelids or palpebrae
• protect lubricate
• Conjunctiva
• Delicate membrane lining eyelids part of the
  outer eyeball surface
• Lacrimal apparatus
• Secrete tears

14
Extraocular Muscles

• Six muscles that insert on the exterior surface
  of the eyeball
• Innervated by CN III, IV or VI.
• 4 rectus muscles -- superior, inferior, lateral
  and medial
• 2 oblique muscles -- inferior and superior
• Responsible for gross eye movements

15
Eyelids

• The eyelids shade the eyes during sleep, protect
  the eyes
• Meet at the medial and lateral canthus (corners)
• Eyelashes and eyebrows help protect the eyeballs
  from foreign objects, perspiration, and the
  direct rays of the sun.
• The meibomian glands lubricate the eye
• The ciliary glands lubricate the hair follicles
• When blocked forms a sty
• The conjunctiva is a thin mucous membrane that
  lines the inner aspect of the eyelids and is
  reflected onto the anterior surface of the
  eyeball.
• Inflammation Conjunctivitis, contagious form
  is pink eye

16
Lacrimal Apparatus

• Includes lacrimal glands located above the
  lateral end of each eye
• Secrete tears at a continuous rate
• Lacrimal canals located medially drain the tears
  into the lacrimal sac, to the nasolacrimal duct,
  which empties into the nasal cavity
• Tears contain water, salt, antibodies, and
  antibacterial enzymes

17
Lacrimal Apparatus

• About 1 ml of tears produced per day. Spread over
  eye by blinking. Contains bactericidal enzyme
  called lysozyme.

18
Internal Structures The Eyeball

• The eyeball is a hollow sphere with 2
  fluid-filled chambers.
• The wall is composed of 3 coats called Tunics.

19
Tunics (Layers) of Eyeball

• The eye is constructed of three layers
• Fibrous Tunic(outer layer)
• Vascular Tunic (middle layer)
• Nervous Tunic(inner layer)

20
Fibrous Tunic -- The Cornea

• Transparent
• Helps focus light (refraction)
• astigmatism
• Avascular
• Transplants
• common successful
• no blood vessels so no antibodies to cause
  rejection
• Nourished by tears aqueous humor

21
Fibrous Tunic -- The Sclera

• White of the eye
• Dense irregular connective tissue layer
• Provides shape support
• Posteriorly pierced by Optic Nerve (CN II)

22
Vascular Tunic -- Choroid Ciliary Body

• Choroid
• pigmented epithilial cells (melanocytes) blood
  vessels
• provides nutrients to retina
• black pigment in melanocytes absorb scattered
  light
• Ciliary body
• ciliary processes
• folds on ciliary body
• secrete aqueous humor
• ciliary muscle
• smooth muscle that alters shape of lens

23
Vascular Tunic -- Iris Pupil

• Colored portion of eye
• Shape of flat donut suspended between cornea
  lens
• Hole in center is pupil
• Function is to regulate amount of light entering
  eye
• Autonomic reflexes
• circular muscle fibers contract in bright light
  to shrink pupil
• radial muscle fibers contract in dim light to
  enlarge pupil

24
Vascular Tunic -- Muscles of the Iris

• Constrictor pupillae (circular) are innervated by
  parasympathetic fibers while Dilator pupillae
  (radial) are innervated by sympathetic fibers.
• Response varies with different levels of light

25
Vascular Tunic -- Description of lens

• Avascular
• Crystallin proteins arranged like layers in onion
• Clear capsule perfectly transparent
• Lens held in place by suspensory ligaments
• Focuses light on retina

26
Vascular Tunic -- Suspensory ligament

• Suspensory ligaments attach lens to ciliary
  process
• Ciliary muscle controls tension on ligaments
  lens

27
Nervous Tunic -- Retina

• Posterior 3/4 of eyeball
• Optic disc
• optic nerve exiting back of eyeball
• Central retina BV
• fan out to supply nourishment to retina
• visible for inspection
• hypertension diabetes
• Detached retina
• trauma (boxing)
• fluid between layers
• distortion or blindness

View with Ophthalmoscope
28
Photoreceptors

• Receptor cells located in the retina that respond
  to light
• Found over entire retina except at the OPTIC DISK
  where the optic nerves leave the eyeball
• AKA The blind spot

29
Photoreceptors

• Rods
• specialized for black-and-white vision in dim
  light
• allow us to discriminate between different shades
  of dark and light
• permit us to see shapes and movement
• Most dense in periphery-peripheral vision
• Cones
• specialized for color vision and sharpness of
  vision (high visual acuity) in bright light
• most densely concentrated in the center of the
  retina, the FOVEA CENTRALIS
• The area of highest visual acuity

30
Rods Cones--Photoreceptors

• Rods----rod shaped
• shades of gray in dim light
• 120 million rod cells
• shapes movements
• distributed along periphery
• Cones----cone shaped
• sharp, color vision
• 6 million
• Fovea centralis
• densely packed region
• at exact visual axis of eye
• sharpest resolution (acuity)

31
Pathway of Nerve Signal in Retina

• Light penetrates retina
• Rods cones transduce light into action
  potentials
• Rods cones excite bipolar cells
• Bipolars excite ganglion cells
• Axons of ganglion cells form optic nerve leaving
  the eyeball (blind spot)
• To thalamus then the primary visual cortex

32
Lens

• The eyeball contains the nonvascular lens, just
  behind the pupil and iris.
• The lens fine tunes the focusing of light rays
  for clear vision.
• With aging the lens loses elasticity and its
  ability to accommodate resulting in a condition
  known as presbyopia.
• Diminished ability to focus the eyes on nearby
  objects

33
Cavities of the Interior of Eyeball

• Anterior cavity (anterior to lens)
• filled with aqueous humor
• produced by ciliary body
• continually drained
• replaced every 90 minutes
• 2 chambers
• anterior chamber between cornea and iris
• posterior chamber between iris and lens
• Posterior cavity (posterior to lens)
• filled with vitreous body (jellylike)
• floaters are debris in vitreous of older
  individuals

34
Eye Anatomy

• The pressure in the eye, called intraocular
  pressure, is produced mainly by the aqueous
  humor.
• The intraocular pressure, along with the vitreous
  body, maintains the shape of the eyeball and
  keeps the retina smoothly applied to the choroid
  so the retina will form clear images.
• Glaucoma
• increased intraocular pressure
• problem with drainage of aqueous humor
• may produce degeneration of the retina and
  blindness

35
Near Point of Vision and Presbyopia

• Near point is the closest distance from the eye
  an object can be still be in clear focus
• 4 inches in a young adult
• 8 inches in a 40 year old
• lens has become less elastic
• 31 inches in a 60 to 80 year old
• Reading glasses may be needed by age 40
• presbyopia
• glasses replace refraction previously provided by
  increased curvature of the relaxed, youthful lens

36
Refraction Abnormalities

• Myopia is nearsightedness
• Hyperopia is farsightedness
• Astigmatism is a refraction abnormality due to an
  irregular curvature of either the cornea or lens.

37
Correction for Refraction Problems

• Emmetropic eye (normal)
• can refract light from 20 ft away
• Myopia (nearsighted)
• eyeball is too long from front to back
• glasses concave
• Hypermetropic (farsighted)
• eyeball is too short
• glasses convex (coke-bottle)
• Astigmatism
• corneal surface wavy
• parts of image out of focus

38
Application Color Blindness Night Blindness

• Most forms of colorblindness result from an
  inherited absence of or deficiency in one of the
  three cone photopigments
• More common in males.
• A deficiency in rhodopsin may cause night
  blindness
• Color blindness
• inability to distinguish between certain colors
• absence of certain cone photopigments
• red-green color blind person can not tell red
  from green
• Night blindness (nyctalopia)
• difficulty seeing in low light
• inability to make normal amount of rhodopsin
• possibly due to deficiency of vitamin A

39
Visual Pathway

• Horizontal cells transmit inhibitory signals to
  bipolar cells
• bipolar or amacrine cells transmit excitatory
  signals to ganglion cells
• ganglion cells which depolarize and initiate
  nerve impulses
• Impulses are conveyed through the retina to the
  optic nerve, the optic chiasma, the optic tract,
  the thalamus, and the occipital lobes of the
  cortex

40
Brain Pathways of Vision
41
Processing of Image Data in the Brain

• Visual information in optic nerve travels to
• hypothalamus to establish sleep patterns based
  upon circadian rhythms of light and darkness
• midbrain for controlling pupil size
  coordination of head and eye movements
• occipital lobe for vision

42
Visual fields

• Fibers from nasal 1/2 of each retina cross in
  optic chiasm
• Left occipital lobe receives visual images from
  right side of an object through impulses from
  nasal 1/2 of the right eye and temporal 1/2 of
  the left eye
• Left occipital lobe sees right 1/2 of the world
  and Right occipital lobe sees left 1/2 of the
  world.

43
(No Transcript)
44
Eye Reflexes

• Convergence
• Eyes move inward to view close objects
• Photopupillary reflex
• Pupil constricts with bright light
• Accomodation pupillary reflex
• Pupil constricts to view close objects

45
CHEMICAL SENSES

• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex limbic system
• evokes strong emotional reactions

46
Anatomy of olfactory receptors

• The receptors for olfaction, which are
  chemoreceptors, are in the nasal epithelium in
  the superior portion of the nasal cavity
• Supporting cells are epithelial cells of the
  mucous membrane lining the nose
• For smell to occur, chemicals must dissolve in
  the nose mucus and stimulate the olfactory
  receptors

47
Olfactory Epithelium

• 1 square inch of membrane holding 10-100 million
  receptors
• Covers superior nasal cavity and cribriform plate
• Cilia project out of epithelium and are bathed by
  mucus secreted by underlying glands

48
Cells of the Olfactory Membrane

• Olfactory receptors
• Stimulated by chemicals dissolved in mucus
• Impulses travel via the olfactory nerve (CN I) to
  cortex

49
Physiology of Olfaction - Overview

• Genetic evidence suggests there are hundreds of
  primary scents.
• In olfactory reception, a generator potential
  develops and triggers one or more nerve impulses.
• Adaptation to odors occurs quickly, and the
  threshold of smell is low only a few molecules
  of certain substances need be present in air to
  be smelled.
• Olfactory receptors convey nerve impulses to
  olfactory nerves, olfactory bulbs, olfactory
  tracts, and the cerebral cortex and limbic
  system.
• Hyposmia, a reduced ability to smell, affects
  half of those over age 65 and 75 of those over
  80. It can be caused by neurological changes,
  drugs, or the effects of smoking .
• Anosmialack of smell

50
Olfaction Sense of Smell

• Odorants bind to receptors
• Na channels open
• Depolarization occurs
• Nerve impulse is triggered

51
Adaptation Odor Thresholds

• Adaptation decreasing sensitivity
• Olfactory adaptation is rapid
• 50 in 1 second
• complete in 1 minute
• Low threshold
• only a few molecules need to be present
• methyl mercaptan added to natural gas as warning

52
Olfactory Pathway

• Axons from olfactory receptors form the olfactory
  nerves (Cranial nerve I) that synapse in the
  olfactory bulb
• pass through 40 foramina in cribriform plate
• Neurons within the olfactory bulb form the
  olfactory tract that synapses on primary
  olfactory area of temporal lobe
• conscious awareness of smell begins
• Other pathways lead to the frontal lobe where
  identification of the odor occurs

53
GUSTATORY SENSE OF TASTE

• Taste is a chemical sense.
• To be detected, molecules must be dissolved.
• Taste stimuli classes include sour, sweet,
  bitter, and salty.

54
Gustatory Sensation Taste

• Taste requires dissolving of substances
• Four classes of stimuli--sour, bitter, sweet, and
  salty
• Other tastes are a combination of the four
  taste sensations plus olfaction.
• 10,000 taste buds found on tongue, soft palate
  larynx
• Chemoreceptors (gustatory cells) are located on
  taste buds
• Found on sides of circumvallate fungiform
  papillae

55
Anatomy of Taste Buds

• An oval body consisting of 50 receptor cells
  surrounded by supporting cells
• A single gustatory hair projects upward through
  the taste pore
• Basal cells develop into new receptor cells every
  10 days.

56
Physiology of Taste

• Receptor potentials developed in gustatory hairs
  cause the release of neurotransmitter that gives
  rise to nerve impulses.
• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4 primary
  tastes
• most sensitive to bitter (poisons)
• least sensitive to salty and sweet
• Mechanism
• dissolved substance contacts gustatory hairs
• receptor potential results in neurotransmitter
  release
• nerve impulse formed in neuron

57
Gustatory Pathway

• Gustatory fibers found in cranial nerves
• VII (facial) serves anterior 2/3 of tongue
• IX (glossopharyngeal) serves posterior 1/3 of
  tongue
• X (vagus) serves palate epiglottis
• Signals travel to thalamus or limbic system
  hypothalamus
• Taste fibers extend from the thalamus to the
  primary gustatory area on parietal lobe of the
  cerebral cortex
• provides conscious perception of taste

58
4 Basic Taste Sensations

• Sweet
• Sugar, saccharine, amino acids
• Anterior portion of tongue
• Sour
• H or acidity
• Sides of tongue
• Bitter
• Alkaloids
• Back of tongue
• Salty
• Metal ions
• Tip of tongue