Exam 3: Eye movement and Limbic system Flashcards
6 fundamental tasks of ocular motor system
- fixation
- saccades
- smooth pursuit
- vestibule-ocular reflex (VOR)
- optokinetic reflex (OKR)
- vergence system
what is the main goal of eye movement?
alignment of the two foveae on objects of interest in the visual field
- The fovea is the part of the retina with the sharpest or best vision (cones)
Fixation
maintenance of focus on a particular spot in the visual world
- Your eyes need to stay motionless for a brief moment in order for you to see
Saccades
rapid conjugate shifts in gaze attention
- These are the fastest movements human muscles can make ⇒ 660-900 degrees/sec
smooth pursuit
fixation on slowly moving objects when head is stationary
vestibule-ocular reflex (VOR)
fixation on stationary objects during brief head movements
- It’s critical for this reflex to be in tact ⇒ the world will not look stable
optokinetic reflex (OKR)
lets you fixate your eyes on stationary images during sustained head rotations or track nearby moving images with the head fixed
- Lets you track moving images close to you
vergence system
fixation on near points in the visual world
To view stationary objects that are close to you with your head stationary
what is the goal of the 6 fundamental ocular motor tasks?
to ensure a stable image of the same part of the visual world on the same parts of the fovea of each retina
conjugate movements
eyes move in the same direction at the same time
which fundamental movements are conjugate? (4)
- Saccades
- Smooth pursuit
- Vestibulo-ocular reflex
- Optokinetic movements
disconjugate movements
the eyes move in opposite direction from each other at the same time
examples of disconjugate movements
Vergence movements ⇒ near vision
what are the 6 extra ocular muscles and what cranial nerves control them?
4 rectus muscles
- Superior ⇒ 3
- Medial ⇒ 3
- Inferior ⇒ 3
- Lateral ⇒ 6
2 oblique muscles
- Superior ⇒ 4
- Inferior ⇒ 3
agonist/antagonist pairs
the lateral and medial rectus muscles attached on either side of one globe ⇒ when one contracts the other relaxes
yoked muscle pairs
each pair of medial and lateral rectus muscles attached to the same side as the direction of movement in each orbit ⇒ both contract in unison
what are saccades used for? (5)
- Gaze shift in response to a novel stimulus in the visual field
- Gaze shift during reading
- Searching novel scenes
- Return gaze to remembered locations
- Quick reset phase of VOR and - OKR ⇒ head movements, eyes move in opposite direction
pulse
burst of activity determining the velocity of eye movement and the position
- To overcome the inherent viscosity of orbital tissues
tonic firing
determines fixation in neurons ⇒ called the step which is the flat line
- To overcome inherent elasticity of the orbital tissues
- Without this then your eyes won’t fixate and your eyes drift back
- Constant firing but at a lower rate
internuclear neurons in the abducens excite what?
contralateral oculomotor neurons
where is the abducens and where is the oculomotor nerve?
abducens is in the pons and oculomotor is at the superior colliculus level
where does the abducens internuclear neuron cross the midline?
through the medial longitudinal fasciculus
what does the oculomotor nuclei coordinate?
the contraction of yoked extraocular muscles and the relaxation of antagonist extraocular muscles
paramedian pontine reticular formation (PPRF)
has excitatory burst neurons which send excitation to the abducens motor neurons which turn on motor neurons so the eyes move
What brain region controls the motor neurons in horizontal gaze shift?
excitatory burst neurons for horizontal gaze shifts are controlled by the cerebral cortex frontal eye fields
what initiates saccades and smooth pursuit?
the cortex
hypometric
when we make a saccade but we cannot quite get to the level we want so there is a second tiny saccade that occurs right after
- Could be basal ganglia or SC
- Can be manifestations of other issues
hypermetric
too far in the first saccade and then brings the eye back
- Almost always an injury to the cerebellum
how does the visual pathway influence gaze movement? (pathway)
- the area of the cortex right above where the eyes are will send a signal to an excitatory burst neuron in the paramedian pontine reticular formation
- the burst neuron synapses on the abducens nucleus neuron which will synapse on the contralateral oculomotor nucleus or go right to the lateral rectus of the ipsilateral eye
- the oculomotor nucleus then synapses on the medial rectus
what type of eye movements happen during reading?
- saccades
- fixation => only can perceive here
- also uses vergence
what movements does the superior oblique let you do?
out and down as a depressed
what muscles are used for eye elevation?
the superior rectus and inferior oblique
what movements do the superior rectus let you do?
eye looks up and in
what does the inferior rectus let you do?
eye looks down and in
what does the inferior oblique let you do?
eye looks up and out
what is the VOR latency?
between 7-15 milliseconds because it is mediated by only 3 neurons
what organs allow for your vestibular ocular reflex (VOR)?
the vestibular end organs
- semicircular cancels
- saccule
- utricle
ampule
contain hair cells with fluids in the semicircular canals which moves the fluid to bend the hair cells
- The sensory activates information to your brain when you head moves
what do the semicircular canals align with?
align with the main directions of pull of the extraocular muscles
horizontal canals are in the place of what muscles?
horizontal muscles => medial and lateral rectus
what do hair cells in the ampules head do?
send head acceleration information to the brain
- Each side sends either excitatory or inhibitory information via projections to the vestibular nuclei, which in turn project to the oculomotor nuclei
what is the 3 neuron arc for VOR?
- excited hair cells in horizontal semicircular canal send ipsilateral excitatory input into the vestibular and nuclear neurons
- vestibular nuclear neurons send excitatory signals to ipsilateral oculomotor nucleus and the contralateral abducens nucleus and inhibitory signals to the contralateral oculomotor nucleus and ipsilateral abducens nucleus
- these send excitatory signals to the ipsilateral medial rectus and contralateral lateral rectus muscles
what happens when your head moves too fast for VOR?
You use the optokinetic reflex (OKR)
- transitions between slow phases (eye maintains gaze) and quick phases (when eye is resetting)
does optokinetic reflex decrease or increase in the dark?
in darkness the vestibular nystagmus response decays as the hair cells in the semicircular canals (SCC) habituate to a constant rotation (zero acceleration)
- in light with constant rotation the optokinetic reflex will not decay due to the continued visual input to the ocular motor system
strabismus
leads to loss of visual acuity in the turned eye (amblyopia = lazy eye) if untreated
- There is a critical period during development of binocular vision where binocular vision must be restored prior to this time (8-10 years) for normal visual acuity
Duanes syndrome
strabismus can be caused by a mutation in the alpha2-chimaerin gene that has been implicated in axon pathfinding
- No abducens nerve on one side and no lateral rectus muscle
- Have binocular input but they cannot see as well due to restricted movements
infantile nystagmus syndrome
involuntary eye oscillations that prevent stable images from forming on the retina
- A number of genes are associated with this ⇒ no known cure
limbic system
interconnected set of brain structures important for emotion, motivation, memory, and automatic control
- Collectively the limbic system integrates these processes to control behavior
telencephalon limbic structures
- Cingulate cortex/gyrus
- Insular cortex
- Nucleus accumbens
- Septal nuclei ⇒ thin next to LV
- Hippocampus
- Amygdala
- Olfactory system
diencephalon limbic structures
- Mammillary bodies in the hypothalamus
- Anterior nucleus of the thalamus
- Mammillothalamic tract
- Fornix tract (out of the hippocampus)
prefrontal cortex
roles in executive function such as complex cognition, abstraction, planning, decision making, social awareness ⇒ governing area
what lobe contains core memory and emotional expression systems?
medial temporal
- Amygdala and hippocampus with associated connections
hippocampus
has to do with memory and sends axons out through the fornix
- the fornix is outside of the hippocampus
fornix
fibers wrap around the diencephalon and terminate the mammillary bodies to make a “C” shape ⇒ comes out of hippocampus
mammillary bodies project to ___ which project to the ______
Mammillary bodies project to the anterior thalamus which projects to the cingulate gyrus
- Can see it in dorsal and ventral locations
mammillary bodies
dangle off the bottom of the diencephalon in a bean shape which the fornix communicates to so they can project to the anterior nucleus of the thalamus via the mammillothalamic tract (white)
functional roles of the limbic system (5)
- Motivation and emotion ⇒ fear, anger, etc.
- Self control
- Learning and memory
- Dysfunction contributes to psychiatric disease ⇒ addiction, mood disorders, schizophrenia
- Much more of these functions in upcoming lectures on learning, memory, and addiction
what process motivation and reward
septal nuclei and nucleus accumbens
- Stimulation of these structures is rewarding or reinforcing
septal rage
septal lesions enhance aggression
selective prefrontal cortex damage can do what?
decrease aggression
Phineas gage
an explosion blasted a rod through his left cheek and out of the top of his head
- extensive damage to prefrontal cortex and anterior cingulate gyrus (mostly unilaterally)
- Prior to the accident Gage was described as responsible, hard working, and generally agreeable
- After he became short tempered, impulsive, and socially awkward
what brain regions involve learning and memory
- Amygdala
- Hippocampus
Patient H.M
treated for temporal lobe epilepsy with a bilateral lesion of part of his medial temporal lobes
- removal/disconnection of his hippocampi and amygdala
- After he was unable to form new declarative memories but his procedural memory was intact
- His presurgical memories and general intellectual abilities were intact
Declarative memories
memories of facts, events, people ⇒ encyclopedia information
Procedural memories
motor memories
Korsakoff syndrome
damage to the mammillary bodies and thalamus from malnutrition, usually from severe alcoholism
- Disruption in the ability to form new declarative memories
confabulation
inventing memories ⇒ makes new declarative fake memories
- People will black out and then try to piece together their events
papez circuit
is critical for declarative memory formation ⇒ broadly the limbic system
Amygdala
stimulation of the amygdala can evoke fear responses in animals and humans
- Elevated amygdala activity is associated with fear and anxiety and disgust
- Damage or lesion to the amygdala reduces fear/threat responses, reduces ability to recognize fear in other people
Kluver-bucy syndrome
temporal lobe lesions
- Docility, low fear, irresponsible to threats
- Other symptoms are hypersexuality, compulsive eating
Urbach Wiethe disease
bilateral loss of amygdala ⇒ low/no fear
what suggests fear is not fully localized to the amgydala?
fear can be induced in people with these disorders under the right conditions
rewarding stimuli
things that feel good and stimulate dopamine release in the nucleus accumbens
- Eating, sex, exercise, social activities, etc.
- The signal is needed for learning and motivational drive
drugs
exploit the endogenous reward system, producing exaggerated dopamine release in the nucleus accumbens
- All drugs do this ⇒ nicotine, cocaine, heroin, marijuana, ethanol
