(G1) Anatomy & Physiology of the Motor Mechanism Flashcards
The visual axis passes through the nodal point and the fovea centralis, thus crossing the optic axis making a small angle, commonly spoken as?
Angle Gamma
Clinically this angle is assessed at the pupillary plane and is referred to as
Angle Kappa
angle between optical axis & the visual axis at the nodal point
Angle Alpha
line passing through the center of the cornea & the lens & meets the retina
Optic axis
object of fixation passes thru the nodal point & meets the fovea
Visual axis
line joining the fixation point & the center of rotation of the eye
Fixation axis
In emmetropic eye, the angle kappa is said to be?
Positive. Optic axis usually intersects the retina inside the fovea centralis
In hypermetropic eye, the angle kappa is?
More positive, gives the appearance of pseudoexotropia or pseudodivergent squint. (eyeball is shorter)
In myopia the angle kappa is?
Absent or negative, leading to pseudoesotropia or convergent squint (eyeball is longer).
Neither of these lines can be seen, and direction of line of vision is judged by the position of angle, hence?
The greater the size of a positive angle gamma and kappa the more the eye will appear to look outwards.
The eye appear to look inwards when?
The angle gamma is negative.
The emmetropic eye has a positive angle gamma of?
Positive angle gamma of 5°
The emmetropic eye has a positive angle gamma of 5° producing an apparent divergence of?
Apparent divergence of 10°, regarded as the normal position of the eyes.
Four rectus muscles
Superior, Inferior, Lateral, and Medial Rectus
Two obliques
Superior and Inferior Oblique
Primary action of rotating the eye in the four cardinal directions; up, down, out, in.
Rectus Muscles
arise in fibrous ring around the optic foramen to the nasal side of the axis of eye
Rectus Muscle
RM are inserted in the sclera by flat tendinous insertions about?
10mm broad
Where is the medial rectus inserted and how many mm?
MR is inserted into the sclera and about 5.5mm to the nasal side of corneoscleral margin
Inferior Rectus insertion
IR 6.6m below
Lateral Rectus insertion
LR 7 mm to the temporal side
Superior Rectus insertion
SR 7.75 mm above
Inferior Oblique insertion
IR 18mm
Superior Oblique insertion
SR 13.8mm - 18.8mm
Primary function is rotation of the globe, and are differently arranged.
Oblique Muscles
Superior Oblique arises from common origin at?
Apex of the orbit, runs forward to the trochlea (cartilaginous ring at upper & inner angle of orbit) having threaded through this, becomes tendinous.
(SO) The tendon changes its direction completely & runs over the globe under what rectus muscle?
Superior Rectus, to attach itself above and lateral to the posterior pole
(SO) The action of the muscle is determined by?
Oblique direction of its tendon after it has left the trochlea.
Oblique muscle that maintains a similar direction throughout its course & is the only muscle not arising from the apex of the orbit.
Inferior Oblique
Inferior Oblique arises from?
IO arises anteriorly from the lower & inner orbital walls near the lacrimal fossa &, running below the Inferior Rectus (IR lies between the glove & IO), finds an insertion in the sclera below & lateral to the posterior pole of the globe.
What are the two types of small muscle cells in EOM
Small fibres & Large fibres
A type of small muscle cell in EOM that is located peripherally, have slow twitch response, capable of graded contractions in absence of action potential & have multiple motor end plates (en grappe)
Small Fibres
Multiple motor end plates in the small fibres is also known as
en grappe
A type of small muscle cell in EOM that is located centrally, have a fast twitch response and have a single motor end plate.
Large fibres
The angle gamma is to the nasal side in?
Hypermetropia & emmetropia
Primary, Secondary & Tertiary Action of MR & its Innervation
Primary: Adduction
Secondary & Tertiary: None
Innervation: CNIII
Primary, Secondary, & Tertiary Action & Innervation of LR
Primary: Abduction
Secondary & Tertiary: None
Innervation: CNVI
Primary, Secondary, & Tertiary Action & Innervation of SR
Primary: Elevation (best when eye is abducted position)
Secondary: Incyclotorsion
Tertiary: Adduction
Innervation: CNIII
Primary, Secondary, Tertiary Action & Innervation of IR
Primary: Depression (best when eye is abducted position)
Secondary: Excyclotorsion
Tertiary: Adduction
Innervation: CNIII
Primary, Secondary, Tertiary action & Innervation of SO
Primary: Incycloduction (intortion)
Secondary: Depression
Tertiary: Abduction
Innervation: CNIV
Primary, Secondary, Tertiary action & Innervation of IO
Primary: Excycloduction (extortion)
Secondary: Elevation
Tertiary: Abduction
Innervation: CNIII
EOM rotate around a “centre of rotation” which lies in?
Horizontal plane some 12 or 13m behind the cornea, in every movement of the globe each muscle is involved by either contraction or inhibition
What are the three types of rotation or “degrees of freedom” possible around the center of rotation?
- Rotation around the vertical axis (where the globe is turned from side to side) - [Z axis] (MR & LR)
- Rotation around the horizontal axis (where the globe is turned upwards & downwards) - [X axis] (SR, IR, IO, SO)
- Rotation around the anterioposterior axis (an involuntary movement of torsion; intorsion (upper pole of cornea rotates nasally), extorsion(rotates temporally). [Y axis] (IO, SO, SR, IR)
Every movement of the eye is a “synkinesis”. Not only there is uniocular synkinesis, there is also?
In normal circumstances there is always binocular synkinesis
Abduction of one eye is accompanied by adduction of the other- which is also known as?
Conjugate movement
The only exception to conjugate movements is?
Bilateral adduction of the eyes in convergence and abduction of both eyes in divergence (dysconjugate movements)
Abduction of both eyes in divergence is also known as?
Dysconjugate movement
Elevation or depression of one eye is always accompanied by?
Elevation or depression of the other.
Elevation of both eyes is accompanied by?
Slight abduction (divergence), depression by slight adduction (convergence)
In these movements the muscles which contract together are called
Synergists
Muscles which suffer inhibition or muscles that would work in a directly opposite direction are relaxed are called
Antagonists
Which muscles are involved or synergists & antagonists in Dextroversion?
Synergist: RLR & LMR
Antagonist: RMR & LLR
Muscles involved (synergist/antagonist) in Laevoversion?
Synergist: RMR & LLR
Antagonist: RLR & LMR
Synergist & Antagonist in Dextroelevation
Synergist: RSR & LIO
Antagonist: RIR & LSO
Synergist & Antagonist in Dextrodepression
Synergist: RIR & LSO
Antagonist: RSR & LIO
Synergist & Antagonist in Laevodepression
Synergist: RSO & LIR
Antagonist: RIO & LSR
Synergist & Antagonist in Laevoelevation
Synergist: RIO & LSR
Antagonist: LIR & RSO
both eyes open, attempting to fixate a target, and moving in the same direction
Versions
both eyes open, fixing a target but moving in opposite directions synchronously, (e.g. convergence or divergence)
Vergences
only one eye is open, the other covered or closed (monocular movement)
Ductions
quick fixation movements
Saccadic movements
slow following movements
Pursuit movements
During the initiation of eye movement, increased innervation to an extraocular muscle is accompanied by simultaneous inhibition (a reciprocal decrease in innervation) of the direct antagonist of the contracting muscle of the same eye
Sherrington’s law of reciprocal innervation
During any conjugate eye movement, equal & simultaneous innervation flows to the yoke muscles
Hering’s law of equal innervation
Supplies all the extrinsic muscles except Lateral Rectus & Superior Oblique, as well as Sphincter Pupillae & Ciliary Muscle.
Oculomotor / Third Cranial Nerve (CNIII)
Which cranial nerve supplies the Superior Oblique?
Trochlear / Fourth Cranial Nerve (CNIV)
Which cranial nerve supplies the Lateral Rectus?
Abducens / Sixth Cranial Nerve (CNVI)
located in the mid-brain, forms a large, continuous mass of nerve cells situated near the midline in the floor of aqueduct of Sylvius beneath the superior colliculus
Oculomotor nucleus
supplies fibres to the ciliary muscle (accommodation) & sphincter pupillae (constriction of pupil)
Edinger-Westphal (& Perlia)
a nucleus (central control center) in the brain that manages that lift our upper eyelids
Levator Palpebrae
this nucleus is further back in midbrain & is unique because its signals cross over to the opposite side before reaching the muscle that moves the eyes
Trochlear Nerve
this nucleus is even further back in the brainstem & is close to the facial nucleus (related to facial movements)
Abducens Nerve
A part of Supranuclear center which contains an area responsible for rapid eye movements towards the opposite side for the purpose of fixation
Frontal Cortex
refers to regions in the Central Nervous System that are located in the cerebral cortex
Supranuclear centers
A part of Supranuclear center which has an are that controls slow pursuit eye movements to the same side
Occipital Cortex
initiated in the pyramidal cells of the motor area of the frontal cortex in the second & third frontal convolutions of both sides
Voluntary ocular movements
depend on vision (fixation, fusional movements, convergence, etc) the psycho-optical reflexes- are centered in the visual cortex of the occipital lobe
Involuntary reflex
coordinates the position of the eyes when the head is moved in space
Statokinetic reflexes
coordinates movement of the eyes in respect to movement of the head upon the body
Static reflexes
if chin is depressed, eyes normally elevate if fixation is maintained, if head is rotated on a vertical axis the eyes maintain fixation as a result of Statokinetic reflexes. These movements are referred to as
“Doll’s head”
rapid eye movements to direct the fovea to a target whose image is falling peripherally on the retina
Saccades
During prolonged rotations the optokinetic system sustains compensatory eye speed at the same speed as the head
Optokinetic movements
following movements to maintain the image of a slowly moving small object on the fovea
Smooth pursuit
required to maintain foveal position of image an object which may be moving away or towards the observer or may be located near or far away
Vergence
prevents slipping of the retinal images when the head moves, it moves the eyes at the same speed but in opposite direction as head
Vestibulo-ocular reflex
maintaining the image of regard on the fovea
Fixation
an object to one side of the fixation target forms its retinal images upon the temporal side of one retina & upon the nasal side of the other
Correspondence
same visual direction, objects on the horopter, gives rise to binocular vision, fall within Panum’s fusional area
Corresponding points
different visual direction, objects in front/behind the horopter, gives rise to binocular diplopia fall outside of Panum’s area
Non-corresponding points (disparate points)
part of the eye with the sharpest vision
Fovea
can be demonstrated by rapid-to-and-from movements of the eyes of a person watching passing objects while looking out of the window of a moving train
Optokinetic nystagmus (visually induced nystagmus)
describes the way the two eyes work together to integrate images seen by each into one image
Binocular Vision
happens automatically to maintain singe binocular vision
Involuntary fusional movements
ability to tolerate the prism without developing double vision
Reflex fusional capacity
occurs when a farther object is bifixated with a near object in view, temporal disparity
Heteronymous Physiological Diplopia (Crossed diplopia)
when objects are beyond the point of bifixation, nasal disparity
Homonymous Diplopia (Uncrossed Diplopia)
in context of vision, refers to the inward rotation of both eyes to focus on a near object
Convergence
Convergence is measured by which unit
Metre angle
exerted by each eye when eyes are directed to object at distance of 1m of meridian line between two eyes
Meter angle convergence
closest point at which an object can be seen singly when maximum convergence is exerted
Near point of convergence
refers to relative position of the eyes when completely at rest, usually at infinity
Far point of convergence
mechanism by which the eye changes its refractive power by altering the shape of the lens
Accommodation
What are the normal values for near point & far point of convergence range?
Near: 5cm to 10cm
Far: 6 meters or infinity
the difference in convergence between far point & near point
Amplitude of convergence
used to measure objective & subjective convergence & accommodation in 1mm increments
RAF Rule
Hofstetter formula
Min: 15-0.25(age)
Ave: 18.5-0.30(age)
Max: 25-0.40(age)
He found out that the AA is from 0.25 to 1D greater when two eyes are used separately
Duane
when object of regard is placed at a distance of 100cm, this can be seen by an emmetrope clearly by?
Placing +1.00D lens in front of the eye or by increasing refractive power of eye by 1.00D
pupils become smaller to increase depth of focus & reduce spherical aberration
Pupil constriction (Miosis)
eyes adjust their focus to see clearly near objects, involves change in shape of the lens controlled by ciliary muscles & zonule fibers
Contractions of ciliary muscle
eyes move inward to maintain single binocular vision as object becomes closer
Convergence
When viewing a far object, circularly arranged Muller’s ciliary muscle is relaxed, allowing the lens zonules & suspensory ligaments to pull on the lens, flattening it in the periphery
Helmhotz theory
Accommodation occurred through increase of zonular tension at the lens equator with contraction of ciliary muscle, & therefore a bulging of the lens in accommodation was created by compression rather than by passive relaxation
Tscherning’s theory
Pupillary Light pathway
Retina-Optic nerve-Optic chiasm-Optic tract-Lateral geniculate body-Pretectal nucleus- Edinger Westphal Nucleus-Ciliary body-Sphincter & papillae
If the eyes are abducted 23°, which rectus muscles only elevates & depress not intort, extort or adduct?
Superior & Inferior Rectus
Which oblique muscle is when adducted 51° will only depress, doesn’t intort or abduct?
Superior Oblique
angle between the optic axis & the visual axis at the nodal point
Angle alpha
angle between the visual axis & pupillary plane
Angle kappa
angle between the optical axis & fixation axis at the center of rotation of the eye
Angle gamma
