WEEK NINE - EYES AND VISION

Question 1

Define sensation & discuss the process of sensation

Answer 1

Conscious or subconscious awareness of changes in external or internal environment

1. process begins in a sensory receptor [stimulus must occur in receptive field of receptor to be detected]
2. receptor tranduces energy eg mechanical, thermal into a graded receptor potential [electrochemical energy
3. if graded potential reaches threshold = AP is triggered = propagates to CNS via 1st order neuron
4. sensory input takes place in CNS via 2nd/3rd order neuron. [conscious sensation perceived in cerebral cortex only]

Question 2

List the 4 kinds of information that sensory receptors transmit & describe how the CNS encodes each one

Answer 2

1. modality [encoded by labelled line code] eg taste, hearing, vision, pain
   eg if signal comes from retina = must be visual signal
   all AP are the same
2. location [receptor receives input from receptive field and brain identifies site of stimulation (sensory projection]
   Area of cortex dedicated to sensations from specific body parts = proportional to sensitivity i.e number of sensory receptors per unit area
3. intensity [encoded in three ways]
   Frequency of AP can +/-
   Number of nerve fibres stimulated + with + in intensity
   Which nerve fibres activated, depends on weak or strong stimuli [differ in sensitivity]
   = activation of high threshold receptors indicates increased levels of a particular stimulus
4. duration [encoded by changes in firing frequency of nerve fibre over time]
   All receptors exhibit sensory adaptation = if stimulus prolonged then firing of neuron = slower over time eg jumping into hot bath

phasic receptor = quickly adapts, may stop signalling if stimulus continues

tonic receptor = slowly adapts, produces constant rate of firing if stimulus applied eg nociceptors

Question 3

Outline 3 ways in which receptors can be classified

Answer 3

1. modality
   chemo,thermo, mechano, photo-receptors and nociceptors
2. origin of stimuli
   interoceptors [internal stimuli from organs]
   proprioceptors [sense body position/movements]
   exteroceptors [external stimuli]
3. distribution in body
   general senses [widely distributed - somatic]
   special senses [limited to head, innervated by CNS]

Question 4

List & identify the different types of somatosensory receptors

Answer 4

encapsulated receptor [dendrites wrapped in CT or glial cells]
1. tactile corpuscles [light touch and texture]
2. karuse end bulb - phasic, tactile in mucous membrane
3. lamellated corpuscles - phasic, concentrated layers of CT [deep pressure stretch, tickle, vibration
4. ruffini [bulbous] corpuscles - tonic [heavy touch, pressure, joint movements

unencapsulated receptors
1. free nerve endings [nociceptors, thermoreceptors]
2. tactile [markel discs] - cells at base of epidermis eg fingertips
3. hair receptors [monitor movement of hair

Question 5

Differentiate between somatic pain, visceral pain & referred pain

Answer 5

somatic - from skin, muscles, joints

visceral - from internal organs [can result from stretch, chemical irritants, ischemia of viscera/insufficient blood flow]

referred - misinterpreted pain from convergence of neural pathways from sensory input

Question 6

Describe the receptors for taste & explain how they are stimulated

Answer 6

gustation = sensation of taste
chemoreceptor = taste buds - banana shaped epithelial cells located within papillae [tongue protrusions]

• Synaptic vesicles at base of cell = release neurotransmitter when stimulated by tastant
• neurotransmitter molecules trigger nerve impulses in 1st order sensory neurons that synapse with base of gustatory receptor cells

Question 7

List the 5 primary taste sensation & state their location on tongue

Answer 7

1. sweet - concentrated on tip
2. salty - lateral margins
3. sour - lateral margins
4. bitter - posterior
   5.unami - tip/ overall regions

Question 8

Describe the projection pathways for taste

Answer 8

taste buds innervated by
facial nerve - anterior 2/3 of tongue
glossopharyngeal nerve - posterior 1/3
vagus - palate, pharynx, epiglottis

1st order neurons from cranial nerves –> medulla
2nd order project signals to thalamus
3rd order project to gustatory cortex in insula cortex

Question 9

Describe the receptors for smell & explain how they are stimulated

Answer 9

olfactory receptors = 1st order bipolar neurons
found in olfactory mucosa in superior part on nose under cribriform plate

olfactory epithelium lines nasal cavity - contains millions of olfactory receptor cells
- cells have cilia hairs that project from dendrites and spread over the epithelium
- odors stimulate/bind to and depolarise the receptors on cilia - binding opens ion channels for Na+ for Ca+ =
creating an Receptor Potential that travels down the axon, through cribriform plate into the olfactory bulb

Question 10

Describe the projection pathways for smell

Answer 10

sensory neurons project via the olfactory nerves to synapse with mitral cells in olfactory glomeruli in olfactory bulb

mitral cells [2nd order neurons] project via the olfactory tract → the olfactory cortex in the medial temporal lobe directly OR through the thalamus to orbitofrontal cortex.

cortex signals to insula, frontal cortex, hippocampus, amygdala and hypothalamus

Question 11

Explain the terms light & vision & discuss wavelength ranges

Answer 11

vision - perception of visible light
light - visible electromagnetic radiation with wavelength from 400-700nm [only this range causes reaction in photoreceptors]

Question 12

Describe the anatomy of the conjunctiva & lacrimal apparatus

Answer 12

conjunctiva - transparent mucous membrane
lines inner surface of eyelid + anterior surface of eye except cornea
lubricates eye and richly vascular = heals quickly

lacrimal apparatus - lacrimal gland produces tear and released in lacrimal ducts
tears flow medially and wash particles = lubricate eye
prevents infection - contains bactericidal enzyme
eventually drains into nose via nasolacrimal duct

Question 13

Name & identify the 6 extrinsic eye muscles, their innervation & actions

Answer 13

superior rectus
CNIII
elevate eye

inferior rectus
CNIII
depress eye

lateral rectus
CNVI
lateral abduction

medial rectus
CNIII medial adduction

superior oblique
CNIV
intorsion [anticlockwise rotation]

inferior oblique
CNIII
extorsion [clockwise rotation

Question 14

List, identify & describe the 3 tunics of the eyeball

Answer 14

outer fibrous later
1. sclera = white of eye
2. transparent cornea [most anterior part of eye-admits light]

middle vascular layer
1. choroid - tissue layer posterior to retina
2. ciliary body - thick extension of choroid - supportive ring around lens and iris - secretes aqueous humor
3. iris -adjustable diaphragm - controls diameter or pupil

internal layer
consists of retina and beginning of optic nerve

iris
pigmented tissue - controls aperture of eye

Question 15

Name, identify & describe the optical components of the eye

Answer 15

cornea - transparent convex on anterior surface of eyeball - admits light into eye and slightly refracts

aqueous humor - fluid secreted by ciliary body, forms anterior and posterior chamber

lens - flattened compresses cells - changes shape to focus light on retina

vitreous humor - jelly that fills vitreous chamber between lens and retina

Question 16

Name & describe neural components of the eye including blind spot and fovea centralis

Answer 16

retina - inner layer of eyeball pressed against rear of eye by vitreous humor
contains photoreceptor cells
centre of retina = macula lutea [3mm diameter] contains fovea centralis - sharpest vision due to density of cones

optic nerve - CNII
originates on retina at optic disc
where the optic nerve attaches to retina = blind spot [no photoreceptor cells here]

Question 17

Name & describe the 3 processes necessary to focus an image on the retina during the near response

Answer 17

near response = allows eyes to refocus gaze from distant to nearby object

1. convergence of eyes
   medial rectus of eyeball helps eye orient visual axis [look at near object]
2. constriction of pupil
   blocks periphereal light/spherical abberation [blurry edges]
   contraction of smooth muscles = alters diameter of eyeball
3. accomodation of lens
   change in lens curvature = focus on nearby object
   contraction of ciliary muscles = relaxes suspensory ligament = relaxes lens and assumes convex shape = GREATER refraction of light and BETTER focusing

Question 18

Name, describe & identify the photoreceptor cells of the eye

Answer 18

photoreceptor cells - situated on most posterior layer of retina

rods - best adapted for night vision and grey images
best absorb light at 500nm wavelength

cones - colour vision [based on mixture of nerve signals]
three types

1. red - longest wavelength = peak at 558 nm [orange-yellow]
2. green - peak at 531 nm
3. blue - peak at 420 nm

Question 19

Describe the visual pathway from photoreceptors to visual cortex

Answer 19

1. begins at photoreceptor cells
2. cells [particularly rods] release neurotransmitter
   glutamate = determines whether synapsing
   bipolar cells will be excited
3. bipolar cells [1st order neurons]
   Synapse on ganglion cells
4. axons of ganglion cells [2nd order neurons]
   form optic nerve [CNII]
5. optic nerve fibre axons hemidecussate in optic chiasm
   - ½ of the fibres from each optic nerve of the right and left eye cross over to the opposite side of brain
6. optic nerve 2nd order neurons synapse –> third order neurons in lateral geniculate nucleus of thalamus
   - these nerves = optic radiation –> primary visual cortex in occipital lobe [conscious visual sensation occurs]
   - some fibres also –> midbrain for visual reflexes [photopupillary and accommodation]