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Neuroscience Block > The Visual System > Flashcards

The Visual System Flashcards

1
Q

List the bones that contribute to the formation of the orbit

A

Roof: Frontal Bone and Lesser Wing of Sphenoid bones

Floor: Maxilla, Zygomatic and Palantine bones

Lateral Wall: Zygomatic and Greater Wing of Sphenoid bones

Medial Wall: Maxilla, Lacrimal, Ethmoid and Body of Sphenoid

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2
Q

What is the structure and function of the sclera of the eye?

A

The sclera is the opaque and highly fibrous collagen layer of the eye. It is also known as the white of the eye - forming 5/6th of the eyeball.

The sclera maintains the shape of the eye globe, providing resistance to both internal and external forces on the eye. It also provides attachments for extraocular muscles.

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3
Q

Why is the sclera opaque?

A

The sclera is opaque as a result of the random and variable organisation of collagen fibres in the sclera.

The level of opacity varies with:

  1. Composition of the stroma
  2. Hydration
  3. Size and distribution of the collagen
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4
Q

Why is the cornea transparent?

A

The cornea is transparent as a result of the structure and organisation of the collagen fibrils that compose it.

Collagen fibrils of uniform diameter are evenly spaced in parallel running bundles (lamellae).

The 200-300 lamelle layered within the stroma lie at angles to each other (but remember that all fibrils within a single lamellae are parallel)

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5
Q

What is the structure and function of the cornea?

A

The cornea represents the anterior 1/6th of the eye and is the principle refracting component of the eye.

To optimise the refraction of light, the cornea is avascular and transparent.

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6
Q

How many layers of tissue comprise the cornea?

A

The five layers, from superficial to deep, are:

  1. Epithelium
  2. Bowman’s Membrane
  3. Stroma
  4. Descemet’s Membrane
  5. Endothelium

Note:

  • The epithelium connects the tear film to the outside world
  • The endothelium controls water balance of the eye. Deficiencies result in severe pathology and these endothelial cells are incapable of regeneration.
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7
Q

What is the anterior chamber angle?

A

The anterior chamber angle represents the junction between the iris and cornea

The chamber here is the site at which aqueous humour is drained from the eye.

The key structures of the **anterior chamber angle **include the cornea, trabecular meshwork, canal of Schlemm and ciliary body.

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8
Q

What are the three layers of the eye?

A

Outer layer:

  • Sclera + Cornea
  • Function: Strength

Middle Layer: Uvea

  • Ciliary Bodies + blood vessels

Inner Layer: Retina

  • Neural cells and receptors of vision
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9
Q

What is a ciliary body?

A

Ciliary bodies are circumferential tissues composed of ciliary muscle and ciliary processes (or ciliary epithelium) within the uvea layer of the eye.

Ciliary bodies, functionally:

  1. Form the aqueous humour
  2. Tether the lens (via ciliary processes)
  3. Accomadation (contraction altering the convexity of the lens)
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10
Q

What is the importance of aqueous humour?

A

Aqueous humour is a transparent, gelatinous fluid similar to plasma, but containing low protein concentrations. It is produced by the ciliary epithelium.

It is important for:

  1. Maintaining the health of the lens and cornea
  2. Creating intraocular pressure

It flows through the narrow cleft between the front of the lens and the back of the iris, to escape through the pupil into the anterior chamber. From here, it drains out of the eye via the trabecular meshwork of the anterior chamber angle to the venous system.

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11
Q

What is involved in accomodation of the eye?

A

Accomodation is the process by which the eye changes optical power in order to maintain a clear image or focus on an object as its distance varies.

The mechanism by which this occurs involves ciliary muscles contracting and altering the shape of the lens via zonules (ligaments connecting the lens to ciliary muscle)

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12
Q

What is ciliary muscle and what is important for?

A

Ciliary muscle is a ring of striated smooth muscle that acts a sphincter muscle around the iris in the uvea layer of the eye.

It is contained within ciliary bodies

It is innervated by the parasympathetic nervous system (involuntary muscle)

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13
Q

In accomodation, are the ciliary muscles contracted or relaxed for long vision? What about short vision?

A

For long sightedness, the ciliary muscle sphincter is relaxed - increasing tension in the zonules. This causes the lens to stretch and become thin.

For short sightedness, the cilirary muscle sphincter is contracted - removing tension in the zonules. This causes the lens to thicken.

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14
Q

What is presbyopia?

A

Presbyopia refers to the normal loss of accomodation with age.

It results from reductions in the flexibility of the lens capsule and zonules.

Treatment involves the wearing of ‘plus’ lenses - ‘reading glasses’ etc.

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15
Q

What is the iris? What is it’s role?

A

The iris is a thin, circular structure of the eye, responsible for controlling the diameter of the pupil - thus the amount of light reaching the retina.

It is made up of two muscles:

  1. Sphincter pupillae: constricts pupil & innervated by parasympathetic NS.
  2. Dilator pupillae: dilates pupil & innervated by sympathetic NS
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16
Q

What is the choroid?

Where is the choroid?

What is its primary role?

A

Choroid is a term incompassing three layers of blood vessels that sit immediately below the retina

The most important blood vessel layer is the **choriocapillaris **which is directly below the retina.

These blood vessels are important in supplying nutrients to the retina - particularly the neurons of the outer retina where alternative supplies are not present.

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17
Q

What is special about the fovea?

A

The fovea is a specialised area of the eye for high visual acuity.

Particularly, it is:

  • Avascular (recieves nutrients from choroid)
  • High density of cone cells
  • No rod cells
  • Reduced number of neuron layers preceding it.
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18
Q

How is the optic nerve region of the eye specialised for it’s function?

Involve the relevance of the lamina cribosa

A

The optic nerve is formed by axons of ganglion cells as they exit the retina to transmit visual information to higher cortical areas.

The lamina cribosa is the structure at which the ganglion cell axons form the optic nerve and exit the eye. It is a band of dense connective tissue arranged in a sieve pattern. It occupies a region in the eye where there is a deficiency in the continuency of the retina.

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19
Q

From what vessels of the head and neck is the blood supply to the eye derived from?

A

Internal carotid artery - > Opthalamic artery.

The opthalamic artery gives rise to the central retinal artery; as well as long posterior ciliary arteries, short posterior ciliary arteries and anterior ciliary arteries.

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20
Q

Which arteries provide arterial supply to the retina?

A

The central retinal artery supplies the inner retina - it transmits in the optic nerve through the lamina cribosa to be on the internal surface of the retina.

The posterior ciliary arteries supply the outer retina (including the photoreceptors). Long PCA’s supplies can extend around the choroid and supply the peripheral retina, while short PCA’s supply the central retina.

Anterior ciliary arteries do not supply the retina - it supplies the anterior structures of the eye and extraocular muscles.

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21
Q

List the important structures of the eyelid

A
  1. Skin
  • Glands
  • Eyelashes
  • Conjuctivae
  1. Muscles
  • Orbicularis oculi
  • Levator palpebrae superiosis
  1. Lacrimal Apparatus
  • Lacrimal gland and ducts
  • Nasolacrimal sac and duct
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22
Q

What are the two muscles of the eyelid?

How are these two muscles controlled and what innervations are responsible for this?

A

**Levator palpebrae superiosis **elevates the upper lid. It is innervated by CN III. Anatomically, it is rectangular shaped muscle on the upper eyelid.

**Obicularis Oculi **depressed the upper lid. It is innervated by CN VII. Anatomically, it is a spherical muscle surrounding the orbit of the eye.

Both are straited muscles

These muscles never work simultaneously. One is always relaxed while the other is contracting.

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23
Q

What is the macula of the eye?

A

The macula is an oval-shaped, pigmented yellow spot near the centre of the retina.

In the centre of the macula lies the fovea

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24
Q

What is visual acuity?

What is the test and formula utilised to determine an individual’s visual acuity?

A

Visual acuity refers to the ability to resolve fine detail.

Testing is conducted by the recognition of letters on a Snellen or LogMAR chart.

VA= D’ (test distance) / D (where each letter subtends the 5 min arc)

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25
Q

What is the normal visual acuity of a person?

When is somebody legally blind?

Put these two visual acuities in perspective by comparing their ability to see an object.

A

6/6 is the normal visual acuity

A person is legally blind at a visual acuity of 6/60

This implies that a normal person can see an object from 60m away; whereas a legally blind person could only see it from 6m away.

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26
Q

What optical factors affect visual acuity?

A

Visual acuity is affected by the following optical factors:

  • Pupil size
  • Clarity of optical media
    • Cataracts, corneal opacities, scarring etc.
  • Refractive errors
    • Myopia (eye too long or short)
    • Presbyopia
    • Hypermetropia
    • Astigmatism
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27
Q

Compare the best visual acuities capable during the day and night; and what areas of the retina are responsible for such acuity?

A

At phototopic light levels (day), the best VA= 6/6.

This is achieved via the fovea which is specialised for high resolution acuity. It has a dense population of cones and low density of neural transduction cells. There is demonstratable VA loss 5 degrees from the fovea.

At scotopic light levels (night), the best VA= 6/60.

This is achieved via rod densities located 5-15 degrees off centre (thus, better night vision is obtained by looking off centre of the visual fields).

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28
Q

What are the six types of neural cells found within the retina?

A
  1. Rods
  2. Cones
  3. Bipolar Cells
  4. Gangion Cells
  5. Horizontal Cells
  6. Amacrine Cells
29
Q

Compare and contrast the two different types of photoreceptors in the retina

A

Rods

  • Night vision
  • “Scotopic”
  • Very sensitive
  • One type
  • No colour vision
  • 100 million in the retina
  • Absent from fovea
  • Thin and slender in shape

Cones

  • Day vision
  • “Phototopic”
  • Less sensitive
  • Three types (Blue, Green, Red)
  • Colour vision
  • Densest in fovea
  • Large and bulky in shape
30
Q

What cells occupy the inner nuclear layer of the retina?

What do they do and what types are there?

A

Bipolar cells occupy the inner nuclear layer and are important in the “through” pathway - recieves inputs from the photoreceptors and provides outputs to ganglion cells.

There are 10 types of bipolar cells: 1x rod bipolar cells and 9x cone bipolar cells.

These bipolar cells can be further classified as OFF BC’s (producing a hyperpolarising signal) or ON BC’s (producing a depolarising signal)

Are important for spatial vision and colour vision

31
Q

What are horizontal cells important for?

A

Horizontal cells are important in integrating the inputs from multiple photoreceptor cells as part of lateral inhibition - particularly in adjusting to both bright and dim light conditions.

They are located in the outer plexiform layer.

HC’s recieve input from photorecptors and provide output to photoreceptors in what is essentially a negative feedback mechanism.

HC’s, when stimulated, release GABA to hyperpolarise photoreceptors.

32
Q

What are amacrine cells?

A

Amacrine cells are located in the inner plexiform layer of the retina are involved in lateral inhibition of the visual transduction pathway.

They are axonless cells and are considered inhibitory. Little is known of specific functionality.

33
Q

What cells are found within the ganglion cell layer?

What does the principle cell type in this area do?

A

This layer contains the cell bodies of ganglion cells and some displaced **amacrine cells **(normally in inner plexiform layer)

Ganglion cells are the principle output neuron of the retina with their axons forming the optic nerve.

34
Q

Discuss the importance of a ganglion cell’s receptive field

A

Ganglion cells respond to light by increasing or decreasing their rate of AP propagation.

The **receptive field **of ganglion cell (or of a bipolar cell) is the area of retina that when stimulated by light changes the cell’s membrane potential and alter AP propagation.

This property allows the comparison of light hitting different receptive fields - thus comparison/contrast of the objects being viewed

35
Q

Discuss phototransduction within the photoreceptors of the retina

A
  1. Light hitting the all-cis retinal protein in photoreceptors causes the isomerisation into all-trans retinal
  2. Retinal is no longer able to bind it’s opsins and activates/binds transducin
  3. Transducin is a GPCR that, when activated, activates phosphodiesterases that break down cGMP
  4. Breakdown of GMP results in the closure of cGMP-dependent Na+ channels in the photoreceptor’s membrane.
  5. This hyperpolarises the cell and Glutamate NT is not released - no conduction

In the absence of light, glutamte is continuously released.

36
Q

What is the difference between ON and OFF bipolar cells?

A

OFF Bipolar cells are HYPERPOLARISED by light.

ON Bipolar cells are DEPOLARISED by light.

A single photoreceptor transmits information to both an ON and OFF bipolar cell simultaneously - this is the beginning of the parallel processing mechanism

Note: a signal includes the absence of action potential propogation if AP propagation occurs basally.

37
Q

What is the centre-surround pathway concept?

A

There are two pathways through which information of the retina is delivered to a ganglion cell for relay to the CNS.

Central Response

Central response is the ‘through’ pathway of aphotoreceptor to bipolar cell to ganglion cell in series.

Surround Response

The surround response is determined by inputs from horizontal cells. Horizontal cells recieve inputs from many photoreceptors and provide **output to other **photoreceptors. Thus, information is not necessarily conveyed from a central photoreceptor of specific ganglion cell.

The size of the surround is determined by the extent of electrical coupling between horizontal cells.

38
Q

Demonstrate the visual pathway taken by the optic nerves

A

Each optic nerve transmits nerve fibres in bundles that discretely transmit information from the right and left hemifields of vision.

Only the nasal fibres cross at the optic chiasm. Temporal fibres enter the chiasm, but do not cross.

The axons of ganglion cells that comprise the optic nerves terminate in the thalamus at the lateral geniculate nucleus

From the LGN, massive white matter tracts known as optic radiations synapse onto the primary visual cortex.

39
Q

What is a hemifield of vision?

Which hemispheres of the brain are the left and right visual hemifields viewed?

A

A hemifield represents either the left or right half of the visual field seen by a person. Each eye obtains and transmits information on both the right and left hemifields discretely.

The right visual hemifield is “viewed” by the left hemisphere V1 cortex

The left visual hemifield is “viewed” by the right hemisphere V1 cortex.

40
Q

Describe the anatomical region surrounding the optic chiasm

A

The optic foramen demarcate the entrance into the **optic canal **which exits into the middle cranial fossa.

Immediately posterior and superior to the optic canal exit, is the optic chiasma where the two optic nerves cross over.

The pituitary gland, nestled in the Turkish Saddle of the Sphenoid Bone, is immediately inferior to the optic chiasm. Pituitary masses can compress the optic nerves in this region and cause **bi-temporal loss of vision **most commonly.

The internal carotid arteries also run adjacent to this region - pathology of these arteries can interfere with optic nerve transmission.

41
Q

What is the Lateral Geniculate Nucleus?

A

The lateral geniculate nucleus is located in the posterior Thalamus and is the relay station of information from most **retinal ganglion cellsdestined for theprimary visual cortex**

The LGN contains six layers of cells which contain either magnocellular cells or parvocellular layers.

Magnocellular = Layers 1,2 ; Parvocellular = Layers 3-6

M-Ganglion cells synapse to magnocellular layers, P-Ganglion cells to parvocellular layers.

At the LGN there is NO mixing of visual information: segregation of inputs by eye and ganglion cell type are maintained.

Right eye: Layers 2,3,5,

Left eye: Layers 1,4,6

Cells of the LGN give rise to massive white matter tracts known as the **optic radiations **that convey visual information to the visual cortex.

42
Q

How is the primary visual cortex (V1) organised?

A

V1 is located on the occipital lobe on each side of the **calcarine fissure **(area 17).

Each half of the visual field is represented on the contralateral visual cortex.

Retinotopic organisation occurs in the visual pathways: neighbouring cells within the retina project to neighbouring cells in the LGN and V1

43
Q

What visual defects would result from varying lesion sites of the visual pathway?

A

Before the optic chiasm:

Vision deficits in one eye only

At the optic chiasm:

Visual defects in both eyes but opposite sides of the visual field (bi-temporal vision loss)

Post optic chiasm

Defects in both eyes but same sides of the visual field

note: Macula sparing is indicative of pathology (esp. arterial pathology) of the occipital lobe/V1

44
Q

Discuss the pain pathway of migraines

A

Migraine pain is due to nerves signalling from the dura (blood vessels in the meninges)

Dural pain is transmitted by the **trigeminal nerve **to the brainstem before the posterior nucleus of the thalamus (PTN)

The **posterior nucleus of the thalamus **is also the site of synapse of the ipGC’s - there are light sensitive neurons in the PTN

45
Q

What are ipGC’s?

A

ipGC’s are **intrinsically photosensitive ganglion cells. **Also known as melanopsin ganglion cells.

They are a small population of GC’s that contain light sensitive melanopsin pigment (similar to invertebrates) that leads to depolarisation of GC’s in light.

The ipGC’s transmit non-vision related information regarding:

  • Circadian rhythm
  • Sleep regulation
  • Pupil responses
  • General information about light levels
  • Light allodynia (photophobia associated with migraines, ocular injury or infection)
46
Q

How are ipGC’s related to pupil response to light?

A

ipGC’s/melanopsin GC’s provide retinal input to the brainstem that controls the pupil responses to light - the contraction of pupils.

ipGC’s/melanopsin GC’s project to the Optical **Pretectal Nucleus (OPN) **in the midbrain.

This information is then relayed to the Edinger-Westfal Nucleus (also in midbrain) that gives rise to CN III (oculomotor) that co-ordinates the **sphincter pupilae **muscle.

47
Q

What is the consensual response observed in the pupillary light reflex

A

The consensual response is where light _stimulation of one eye produces the reflex pupil constriction in both eyes. _

Afferent ipGC signalling through one optic nerve connects to the pretectal nucleus, whose axons run to both the right and the left Edinger-Westfal Nuclei which produces the two CN III Oculomotor Nerves

48
Q

How are ipGC’s invloved in circadian rhythms?

A

ipGC’s transmit light information to the **suprachiasmatic nucleus (SCN) **of the **hypothalamus **- which is responsible for setting the circadian rhythms of the body.

49
Q

How are ipGC’s involved in the photophobia involved in migraine pain?

A

Migraine aura are often visual and light exposure worsens the pain of a migraine.

ipGC’s project to the posterior thalamic nucleus; the same nucleus that recieves migraine pain information from the Dura (blood vessels in the meninges)

Light causes increased excitement of the posterior thalamic nucleus - thus, increased pain sensitivity results

Blind people without photoreceptors still experience increased pain when subjected to light during a migraine - because ipGC’s are still present.

50
Q

Describe the location of M & P Cell inputs from the LGN into V1
Describe the location of outputs from V1

A

M & P Cell inputs to the visual cortex remain segregated by synapsing into different regions of the 4th layer of the visual cortex. There are six layers in the V1.

**M cells ** terminate in the layer 4Ca

**P cells **terminate in the layer 4Cb

51
Q

Describe the organisation of output neurons from the V1

A

**Layer 3 and 4B: **Other cortical areas

Layer 5: Superior colliculus and pons

**Layer 6: **LGN

52
Q

Discuss the pathway of M-Cell information from the retina to the V1

A

**M ganglion cells **are large diameter and wavelength insensitive neurons that project from the retina to M Cells in the LGN

**M Cells **in the LGN project to layer 4Ca

Cell in V1 layer 4Ca project to V1 layer 4B

53
Q

​What is directional selectivity?

In what layer of the V1 is directional selectivity processed?

A

Certain neurons show preference for a particular direction of motion of objects.

Cells in layer 4B of the V1 respond to M-cell derived information in this way.

An object passing through a receptive field from left to right may excite a population of these neurons - but right to left a different population.

54
Q

How much of the cortex is thought to be involved in vision?

A

40%

The cortical visual system is composed of multiple visual areas with different functions

55
Q

What are the two parallel visual streams of the cortex?

A

The two streams of cortical visual processing:

Dorsal Pathway

  • Processes where objects in the visual field are
  • M-cell information is processed by this pathway

Ventral Pathway

  • Processes **what **objects are in the visual field
  • P-cell information is processed by this pathway
56
Q

What is the importance of the middle temporal lobe / Area MT in visual processing?

A

Area MT is a specialised area of the middle temporal lobe that is specialised for processing object motion.

It recieves input from layer 4B of the V1; as well as other cortical areas including V2 and V3

Neurons in Area MT have large receptor fields, respond to stimulus movement, are directional selective and respond to differnt types of motion.

57
Q

Discuss the relevance of individual cone absorption profiles in the perception of colour

A

There are three types of cone cells responsible for detecting light colour within the retina: **blue, green and red cones. **

Each have their own wavelength of light at which they are maximally active:

Blue = 430nm ; Green = 530nm and Red = 560nm

There is overlap in the wavelengths at which these cones absorb light - leading to the combination of photoreceptor signals.

It is the combination of relative activation of different cones that constitutes the colour percieved

**Note: **blue, green and red light mixing makes white

58
Q

How does the ‘colour opponent centre-surround’ mechanism confer coulour information?

A

The colour a person percieves is largely determined by the activity of ganglion cells

P-Ganglion cells are responsible for the propagation of colour vision from the retina and exhibit ‘colour opponent centre-surround’ activity which compares the colour of light.

These P ganglion cells are ON centre & OFF surround ganglion cells - are excited by central photoreceptor pathways and inhibited by surround photoreceptor pathways by horizontal cells.

Comparisons are made between pairs of colours: red & green, blue & yellow and black & white.

59
Q

What areas of the cortex are involved in the Ventral Stream of visual processing?

A

**Area V4 **recieves input from the blob and interblob regions of V1 via V2. Neurons in V4 are orientation selective and colour selective for the perception of shape and colour.

The major output of V4 neurons is to Area IT (inferior temporal lobe). Area IT is important for the visual memory and perception - particularly of faces.

This pathway process P Cell information

60
Q

What are the different groups of colour blindness differentiated by?

A

There are three groups of colour vision deficiencies that be either acquired or inherited:

  1. Monochromacy - only have one type of cone
  2. Dichromacy - only two functional cones
  3. Anomalous trichromacy - have all three cones, but one expresses
    abnormal pigment and doesn’t work the same as normal
    cones.

Specific colour deficiencies are categorised by colour:

**Protan: **

Protanope-no red cone
Protanomal-abnormal red cone

Deutan

Deutanope-no green cone
Deutanomal-abnormal green cone

Tritan
Tritanope-no blue cone
Tritanomal-abnormal blue cone

61
Q

Describe the primary actions of the extraocular rectus muscles

A

Superior rectus = elevation

Inferior rectus = depression

Medial rectus = adduction

Lateral rectus = abduction

Note: these muscle also have secondary and teritary actions (but are beyond our need clinically)

62
Q

Describe the primary actions of the oblique extra-ocular muscles of the eye

A

Superior Oblique

  • inserts well behind the equator at an oblique angle close to LR
  • its tendon runs through a fibrocartilage pulley or trochlea
  • primary action = intorsion

Inferior Oblique

  • inserts behind the equator close to LR
  • primary action = extorsion
63
Q

Why are there multiple muscles that elevate and depress the eye?

A

The presence of secondary and tertiary actions of muscles results form the angles made between the muscle and the eyeball.

Need to isolate individual muscles to assess there functional state/capacity

64
Q

How do you test each ocular muscle in isolation?

A
65
Q

How are the movements of one eye coordinated with the other?

A

Coordination of a person’s two eyes occurs in the reticular fromations of the brainstem.

The **vertical gaze centre **is housed by the midbrain reticular formation and coordinates the SO and SR muscles of each eye via CN IV and CN III. Do not need to know specifics of vertical gaze.

The horizontal gaze centre is housed by the pontine paramedian reticular formation and coordinates MR and LR of each eye via CN III and CNVI

You must know the mechanism of the horizontal gaze saccade

66
Q

Illustrate the mechanism of the horizontal gaze saccade

A

Horizontal Gaze Saccade

  1. Higher cortical structures provide input into the pontine horizontal gaze centre on the ipsilateral side of gaze direction
  2. Pontine reticular formation has two populations of neurons that are activated:

Burst Neurons:

  • High frequency firing immediately prior to eye movement
  • Provide excitatory innervation of ipsilateral abducens nucleus
  • Provide inhibitory innervation of contralateral abducens nucleus

Omnipause Neurons

  • Fire continuously throughout eye movement
  • Are GABAergic neurons that provide inhibitory innervation to contralateral abducens nucleus

  1. Ipsilateral Abducens nerve nucleus projects excitatory abducens nerve to ipsilateral lateral rectus to gaze to the ipsilateral side.
  2. Abducens nerve nucleus projects excitatory medial longitudinal fasciculus (MLF) to contralateral occulomotor nucleus in midrain.
  3. Contralateral occulomotor nucleus contracts contralateral medial rectus muscle to gaze to the ipsilateral side
67
Q

How are we able to maintain focus on stationary objects when turning our head?

A

The co-ordination of eye movement with head movements is achieved by the vestibular system providing information about the position of our head in space - thus, co-ordinating the position of our head and eyes.

The movement of the head results in fluid movement within the semi-circular canals of the vestibular system. Hair cells within the ampulla of these structures detect the fluid movement and transmit neural impulses via the vestibulocochlear nerve about the heads orientation in space while the the otolith organs (saccule and utricle) determines linear acceleration.

In horizontal movements of the head, the only semi-circular canal to transmit information is the horizontal semi-circular canal

When turning the head to the right, it is the right semi-circular canal that increases firing while the left reduces firing (vice versa).

68
Q

Illustrate the mechanism of the vestibular-ocular reflex to maintain focus on an object while the head is moving

A

The vestibulo-ocular reflex is a compensatory response of eye movement following the movement of the head in order to maintain gaze - allows tracking.

The reflex builds upon the horizontal gaze centre control of eye movement; adding:

  1. direction of head movement is towards the contralateral side of eye movement.
  2. the horizontal semi-circular canal on the side of head movement senses movement and signals to the vestibular nucleus of that side via CN VIII.
  3. This vestibular nucleus excites the opposite side’s abducens nucleus** and **inhibits the same side’s abducens nucleus
  4. For rapid operation, there is also a direct projection from the vestibular nucleus to the occulomotor nucleus of the same side
  5. Otherwise, same mechanism as the horizontal gaze saccade
69
Q
A

Neuroscience Block (16 decks)

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