Neuro Anatomy Flashcards
Where does CN XI enter the cranium?
Foramen magnum
What muscles does hypoglossal innervate?
Intrinsic and extrinsic muscles of tongue except palatoglossus
What type of nerve is CN XII?
Somatic motor
Where does CN XI exit the cranium?
Jugular foramen
Where does CN XII exit?
Hypoglossal foramen
What type of nerve is CN XI?
Somatic motor
Which reflex tests medulla function?
Gag reflex
What part of brainstem does gag reflex test?
Medulla
What CN provides taste to posterior 1/3 of tongue?
Glossopharyngeal (IX)
What nerve is responsible for sensory input of gag reflex?
CN IX glossopharyngeal - sensory to pharynx
What nerve type is the most important component of vagus?
Visceral motor but also has bronchial motor, somatic sensory, visceral special sensory and visceral sensory components
What CN innervates the vocal cords?
Vagus (recurrent laryngeal branch)
What CN provides sensory input from pharynx?
IX glossopharyngeal
What CN provides visceral motor to parotid gland?
Glossopharyngeal
What muscle does glossopharyngeal innervate?
Stylopharyngeus (muscle of soft palate)
How many vestibular nuclei are there?
4
What muscles does spinal accessory innervate?
Trapezius and sternocleidomastoid
Where does CN IX exit the skull?
Jugular foramen
What nerve is responsible for motor output of gag reflex?
CN X vagus - motor to pharynx
How cochlear nuclei are there?
2 - dorsal and ventral
Which CN is responsible for taste at anterior 2/3 of tongue?
Facial
What are the 2 nerves that form the facial nerve?
Facial nerve proper and nervus intermedius
What is the visceral efferent component of CN VII?
Parasympathetic to lacrimcal, sublingual and sublingual glands (not parotid gland!)
What CN innervates stapedius muscle?
Facial
Where does facial nerve exit the skull?
Internal acoustic meatus
Through which foramen does facial nerve enter the face?
stylomastoid foramen
What are the 5 terminal branches of facial nerve?
Temporal Zygomatic Buccal/maxillary Mandibular Cervical
Which CN passes through the parotid gland?
Facial
Which CN innervates the parotid gland?
Glossopharyngeal
Which reflex tests pontine function?
Corneal blink reflex
What is the motor output to eyelid of the corneal blink reflex?
CN VII - facial
Which CN supplies the branchial motor component to the muscles of mastication?
Trigeminal
What is the motor component of facial nerve?
innervates muscles of facial expression and stapedius muscle
“Seventh for Stapedius”
Where does CN VII exit the skull?
Internal acoustic meatus
What portion of brainstem does corneal blink reflex test?
Pons
What is the somatic sensory component of trigeminal nerve?
sensory to face up to apex of the head
What muscles does trigeminal innervate?
muscles of mastication
tensor tympani in middle ear (Trigeminal for Tensor)
Which CN supplies sensory to the face?
Trigeminal
What are the 3 major sensory divisions of CN V?
V1 - opthalmic
V2 - maxillary
V3 - mandibular
Which trigeminal nuclei are responsible for sensory to face and head?
Chief sensory nucleus
Spinal trigeminal nucleus (STN)
Which trigeminal nucleus is responsible for proprioception?
mesencephalic nucleus
How many nuclei does trigeminal nucleus consist?
4 running from midbrain to upper spinal cord
Where does CNV V1 exit the skull?
superior orbital fissure
Which division of trigeminal exits foramen ovale?
V3 - mandibular
Where does the sensory root of trigeminal travel to?
trigeminal ganglion
Where does CNV V2 exit the skull?
foramen rotundum
Which trigeminal division exits the superior orbital fissure?
V1 - opthalmic
What is the sensory input of the corneal blink reflex?
Opthalmic division (V1) of trigeminal
Where does CNV V3 exit the skull?
foramen ovale
What is the sensory input of the pupillary light reflex?
optic
Which division of trigeminal exits foramen rotundum?
V2 - maxillary
What is the motor output of pupillary light reflex?
Oculomotor
Which muscle is innervated by CNIII during pupillary light reflex?
sphincter pupillae
Which CNs exit cribiform plate?
CNI
Which CNs exit optic canal?
CNII
Which CNs enter the orbit via the superior orbital fissure?
CNIII,IV,V1,VI
oculomotor, trochlea, opthalmic division of trigeminal, abducens
Which CNs exit auditory canal?
CNVII, VIII
facial, vestibulocochlea
Which CNs exit jugular foramen?
CNIX, X, XI
glossopharyngeal, vagus, spinal accessory
Which CNs exit hypoglossal foramen?
CNXII - hypoglossal
What extraocular muscles does oculomotor supply?
4 out of 6
sup and inf rectus
medial rectus
inf oblique
Which CN elevates the eyelid?
oculomotor
Which CN depresses the eyelid?
facial
Which CN constricts the pupils?
oculomotor
What is the visceral motor component of oculomotor?
constrict pupils
focusing via ciliary muscles
What extraocular muscle does abducens innervate?
lateral rectus (LR6)
Which extraocular muscle does trochlea innervate?
superior oblique (SO4)
Where do the nerves to the extraocular muscles exit the skull?
superior orbital fissure
Where does olfactory nerve exit the skull?
cribiform plate of ethmoid bone
Where does optic nerve exit?
optic canal
Which column in brainstem is nucleus solitarius located?
visceral sensory
Which CNs provide inputs to nucleus solitarius?
CNVII, IX, X
facial, glossopharyngeal, vagus
What are the 3 columns of sensory nuclei
from medial to lateral:
visceral sensory
somatic sensory
special sensory
Which CNs send outputs to nucleus ambiguous?
CN IX, X
glossopharyngeal, vagus
Which column in brainstem if nucleus ambiguous located?
branchial motor
Where does nucleus ambiguous lie?
behind inferior olive in medulla
What are the 3 motor columns in the brainstem?
From medial to lateral:
somatic motor
branchial motor
visceral motor
Where are the motor nerve nuclei located?
close to the midline
Where are the sensory nerve nuclei located?
lateral
How many layers of neurons are in the cortex?
6
What is contained within grey matter?
Cell bodies
What is the function of myelin and what cell is responsible for myelinating axons?
provides electrical insulation for neurons - accelerate conduction
oligodendrocytes are responsible for myelinating axons in the CNS and Schwann cells are responsible for myelinating axons in the PNS
What is contained within white matter?
Myelinated axons
Where are cells of the choroid plexus located?
ventricles
What forms CSF
choroid plexus
Where are ependymal cells located and what do they do?
line central canal of spinal cord and ventricles
aid CSF flow
What are the passive support functions of astrocytes?
- NT uptake and degradation (i.e. glutamate transporters)
- potassium homeostasis
- neuronal energy supply
- maintenance of BBB
- injury response and recovery
What are the active functions of astrocytes and how do they act?
-modulate neuronal function:
glia show modulations in intracellular calcium
calcium waves inhibit neurons
-modulate blood flow by regulating vascular tone
astrocytes surround arterioles and venules
calcium wave within astrocyte causes vasoconstriction or vasodilation
How do oligodendrocytes compare to Schwann cells?
Oligodendrocytes extend processes that wrap around parts of several axons whereas a Schwann cell wraps around one axon
How are neurons morphologically distinct from other cells?
cytoskeleton contains actin, intermediate filaments and microtubules
cell body is the metabolic centre of the cell
cell body gives rise to axons - major output site- and dendrites -major input site
high proportion of total cell volume consists of axons and dendrites
What are the immune cells of the CNS?
microglia derived from bone marrow and of macrophage origin - part of the innate immune system
What are the boundaries of the brainstem?
extends from mammillary bodies rostrally to pyramidal decussation caudally
What are peripheral nerve fibres composed of?
- one or more fascicles of nerve fibres
- each fascicle surrounded by perineurium CT
- epineurium surrounds multiple fascicles
- endoneurium (loose vascular CT) surrounds each nerve fibre and Schwann cell within each fascicle
What are autonomic ganglia?
cell bodies of post-ganglionic neurons
Where in the cranium if the brainstem located?
posterior cranial fossa
What are the gross anatomical features of the medulla oblongata?
dorsal columns and gracile and cuneate tubercles - dorsal
inferior olives and pyramids - ventral
What are the gross anatomical features of the pons?
linked with cerebellum by cerebellar peduncles
limited by 4th ventricle
What are sensory ganglia?
Cell bodies of sensory neurons
What comprises the tectum?
colliculi
At what level do the spinal nerves end?
C2
What are the gross anatomical features of the midbrain?
superior colliculi (eye movements) and inferior colliculi (hearing) - dorsal cerebral peduncles - ventral
What does the tegmentum contain?
Cranial nerves and caudal reticular formation (reflexes and autonomic functions)
What is the embryological origin of sensory cranial nerve nuclei?
Alar plate
What is the embryological origin of motor cranial nerve nuclei?
basal plate
What are the 2 major ascending long fibre tracts and where do they cross?
dorsal column-medial lemniscus system (fine touch and proprioception) crosses in medial lemniscus in medulla
anterolateral system (pain) crosses in spinal cord
What are the orbital margins and what are they formed by?
Supraorbital margin formed by frontal bone
Infraorbital margin formed by zygomatic bone and maxilla
What bones form the floor of the orbit?
maxilla, zygomatic and palatine orbital plates
What is the main descending long fibre tract and where does it cross?
corticospinal tract (motor function) crosses at pyramidal decussation in medulla
What bones form the lateral wall of the orbit?
Zygomatic orbital plate and greater wing of sphenoid
What do the dorsal aspects of the brainstem contain?
cranial nerve nuclei and reticular formation
What forms 5/6t of the eyeball?
sclera
What bones form the medial wall of the orbit?
maxilla orbital plate, lacrimal bone, ethmoid bone, body of sphenoid
What are the functions of sclera?
maintains shape of globe
provides attachments for extraocular muscles
protection: collagen confers toughness
What bones from the roof of the orbit?
frontal bone and lesser wing of sphenoid
What are the important features of the cornea?
avascular and transparent
5 histological layers
endothelial layer controls water balance within eye
What are the key structures in the anterior chamber angle?
cornea
trabecular meshwork
canal of Schlemm
ciliary body
What forms 1/6th of the eyeball?
cornea
Why is the cornea transparent?
Highly organised structure provides transparency:
Collagen fibrils are uniform in diameter, evenly spaced, and run parallel to each other in bundles (lamellae)
all fibrils within a lamella run parallel while adjacent lamellae lie at angles to each other
How does sclera confer protection?
Collagen is whirled
Where is the anterior chamber angle?
Junction between iris and cornea where aqueous humour drains out of eye into the venous supply
What are the functions of the ciliary body?
- ciliary epithelium forms aqueous humour
- ciliary processes tether lens
- ciliary muscles accommodate the eye
What is the function of aqueous humour?
maintains heath of lens and cornea
creates intraocular pressure
What components accommodate/focus the eye?
- ciliary muscle
- zonules (ligaments) that attach to lens and ciliary processes
How does the eye focus?
Ciliary muscles relax to view objects in the distance
Ciliary muscles contract to view objects close up
What is presbyopia and what causes it?
Loss of accommodation with age caused by reduced flexibility in the lens capsule and zonules
What are the components of the middle layer of the eye?
ciliary body, choroid and iris
What is the iris?
aperture of the eye
What muscles control pupil size and what is their innervation?
sphincter pupillae - constricts pupil (parasympathetic)
dilator pupillae - dilates pupil (sympathetic)
Where is the choroid located and what is its function?
Underneath retina
supplies nutrients to retina
How many layers make up the choroid and which layer is most important?
3 layers of blood vessels
choriocapillaris is most important because it sits immediately deep to retina
What are the components of the retina?
Optic nerve/disc Fovea Macula Posterior pole Orra serrata
What are the features of the fovea?
- avascular (nutrients derived from choroid) so thin layered
- high density of cones (high visual acuity)
- no rods
What are the components of the optic nerve?
- lamina cribosa
- central retinal artery and vein
- dura
- sclera
What is the lamina cribrosa made of and what is its function?
- band of dense CT
- seive-like structure which transmits nerve fibres
What is a consequence of glaucoma on lamina cribrosa?
High pressure can displace lamina cribrosa and damage axons
What is the main blood supply to the orbit?
branch of ophthalmic artery: central retinal artery long posterior ciliary artery short posterior ciliary artery anterior ciliary artery
Which ciliary artery does not pierce the globe?
anterior ciliary artery
What is the blood supply to the retina?
Central retinal artery supplies inner retina
posterior ciliary arteries supply photoreceptors
What CN innervates levator palpebrae superiosis?
CNIII
Which muscle elevates the upper lid?
levator palpebrae superiosis
Which muscle depresses the upper lid?
orbicularis oculi
Which CN innervates orbicularis oculi?
CNVII
What do the long posterior ciliary arteries supply?
photoreceptors far from optic nerve head
supplies more anterior structures like iris and ciliary body
What are the 3 basic layers of the eyeball and what are their functions?
Outer coat: sclera and cornea - strength
Middle coat: uvea - nutrition
Inner coat: retina - vision
Which structure of the eye is important for central vision?
macula - centre of macula is fovea
What it visual acuity?
ability to resolve fine detail
What optical factors affect visual acuity?
- pupil size (appature)
- clarity of optical media (e.g. corneal opacities and cataracts impair)
- refractive errors
What are the 6 types of neurons in the retina?
rods and cones (photoreceptors) horizontal cells bipolar cells amacrine cells ganglion cells
What are the two synaptic layers of the retina?
outer plexiform and inner plexiform
Which neuron in the retina is the last to receive light information?
photoreceptors
How many types of cones are there and what are they?
3 types which allow colour vision
red, green and blue cones
Which photoreceptor is densest in the fovea?
cones
Which photoreceptor is absent in the fovea?
rods
Which photoreceptor is more numerous in the retina?
rods outnumber cones by 20:1
What is necessary to resolve fine detail?
good optical system and small, closely
What do the short posterior ciliary arteries supply?
- photoreceptors relatively close to optic nerve
- optic nerve head
What is the sequence of the “through” pathway in the retina?
Photoreceptors connect to bipolar cells which connect to ganglion cells. Ganglion cells send message to brain
Which retinal neuron can modulate signals in the outer retina?
horizontal cells
Which retinal neuron can modulate signals in the inner retina?
amacrine cells
What are the different bipolar cells and how many of each are there?
10 different bipolar cells:
1 rod bipolar cell
9 cone bipolar cell
What are the functions of bipolar cells?
spatial vision and colour vision
Which bipolar cell hyperpolarises when light falls on the retina?
OFF bipolar cells
Which bipolar cell depolarises when light falls on the retina?
ON bipolar cells
How do ON bipolar cells respond when light hits the retina?
They always depolarise
How do OFF bipolar cells respond when light hits the retina?
Hyperpolarise
Which neurons input onto horizontal cells and which neurons do horizontal cells send output to?
Photoreceptors input to horizontal cells
Horizontal cells output to photoreceptors
How do horizontal cells respond to light?
Hyperpolarise
Which neurotransmitter do horizontal cells release in response to light?
Release GABA to photoreceptors
What sort of signals do amacrine cells send?
Inhibitory cells that release inhibitory neurotransmitters glycine and GABA
Which neuron in the retina fires action potentials?
ON ganglion cell depolarise and fire action potentials
What is receptive field?
the area of retina that, when stimulated with light, changes cell membrane potential
What protein do rods contain?
rhodopsin
What protein do cones contain?
one of three different types of cone-opsins
Which vitamin do opsins bind?
Vitamin A
How do photoreceptors respond to dark?
cGMP gates sodium channels and allows continuous influx of sodium ions causing depolarisation
How do photoreceptors respond to light?
Phosphodiesterase breaks down cGMP to GMP
Sodium channels close
Influx of sodium ceases
Cell becomes hyperpolarised and releases glutamate
What is the sequence of the “through” pathway in the retina?
Photoreceptors connect to bipolar cells which connect to ganglion cells. Ganglion cells send message to brain
What creates the centre-surround phenomenon?
Some ganglion cells depolarise when light is shone on the centre of the receptive field while some ganglion cells hyperpolarise when light is shone on the peripheral part of receptive field
What is the surround response?
Light shone on peripheral part of receptive field
Surround photoreceptors hyperpolarise releasing glutamate
Horizontal cells receive input, become hyperpolarised releasing GABA
GABA causes central photoreceptors to depolarise
Eventually ganglion cells become hyperpolarised
What is the centre response?
Light shone to centre of receptive field
Photoreceptors hyperpolarise releasing glutamate
Causing the depolarisation of ON bipolar cells
Which causes depolarisation of ganglion cells
What do M ganglion cells encode?
Motion
What do P ganglion cells encode?
Colour vision and visual acuity
What forms the optic chiasm and where does it lie?
fibres from left and right optic nerves combine to form optic chiasm
optic chiasm lies at the base of the brain, anterior to pituitary and is surrounded by the carotid arteries
Where do nasal fibres of optic nerve cross?
Optic chiasm
Which fibres pick up light from the temporal visual field?
All the cells at nasal half of retina
Where do temporal fibres cross?
Temporal fibres from each eye travel through chiasm and don’t cross
Which hemisphere interprets information from the left visual hemifield?
Right hemisphere
Where do most ganglion cell axons involved in vision terminate?
Lateral geniculate nucleus in the thalamus
How many layers are within the LGN and which cell types make up the layers?
6 layers within LGN:
Layers 1 and 2 are magnocellular layers
Layers 3-6 are parvocellular layers
Where do M and P ganglion cell axons terminate in the LGN?
M cells target magnocellular layers of LGN (layers 1-2)
P cells target parvocellular layers of LGN (layers 3-6)
T/F
Inputs from each eye mix at the LGN
False
Inputs from each eye are segregated
Mixing of info from each eye does not occur at LGN
Where do LGN neurons project to?
Primary visual cortex in the occipital lobe around the calcarine fissure
What are the optic radiations?
Ganglion cell axons that exit the LGN form white matter tracts which travel to occipital lobe
Where is central vision encoded?
Encodes at the most posterior (occipital) part of the primary visual cortex
Where is peripheral vision encoded?
More anterior part of the primary visual cortex, along the banks of the calcarine fissure
If a lesion only affect vision in only 1 eye, what is the location of the lesion?
In the eye itself or in the ipsilateral optic nerve
before the optic chiasm
If a lesion impairs vision in both eyes in some form or another, what is the location of the lesion?
The lesion may be at the chiasm or posterior to the chiasm
What is the location of a lesion causing a bitemporal hemianopia - affecting opposite sides of the visual field?
At the optic chiasm
What is the location of a lesion causing vision impairment in half the eye on the same side of both eyes - affecting same side of the visual field?
Optic tracts
What is the location of a lesion which impairs vision in 1 quadrant of each eye on the same side of the visual field?
Optic radiations
What is the general location of a lesion that impairs vision on the same side of the visual field?
Post chiasm
What is the location of a lesion which causes visual impairment in half of each eye on the same side of the visual field but has macula sparing?
Visual cortex:
Macula sparing means normal central vision and is indicative of a vascular problem in the visual cortex
What are melanopsin ganglion cells?
Intrinsically photosensitive ganglion cells in the retina
What visual pigment do ipGCs contains?
melanopsin
How do ipGCs respond to light?
Activation of melanopsin causes depolarisation of ipGCs
Which muscle causes the pupil to constrict and what is its innervation?
Sphincter pupillae innervated by parasympathetic fibres which travel with CN III
Which muscle causes pupil dilation and what is its innervation?
Dilator pupillae innervated by sympathetic fibres
What is the pathway controlling pupil response?
ipGC process travels down optic nerve through the chiasm
Travels to optical pretectal nucleus in the midbrain
Then sends a process to each Edinger Westfal nucleus
A process from each Edinger Westfal nuclues heads back towards the eye which synapses at ciliary ganglion which travels to iris
Where is the location of a lesion causing an afferent pupil defect?
Lesion can be from ipGC to pretectal nucleus in the midbrain
What is the location of a lesion causing an efferent pupil defect?
Lesion can be located anywhere from the Edinger Westfal nucleus to the ciliary ganglion
Where do ipGCs project to control circadian rhythm?
Suprachiasmatic nucleus in the hypothalamus
What is the pain pathway for migraine and how are ipGCs involved?
Pain signal from dura is carried by trigeminal nerve to the brainstem and to the posterior nucleus of the thalamus
ipGCs also project to posterior thalamic nucleus
the posterior thalamic nucleus is light sensitive and becomes excited
Which retinal neuron can modulate signals in the outer retina?
horizontal cells
Where does superior oblique insert?
Inserts far behind the equator at an oblique angle to lateral rectus
Where does the tendon of superior oblique run through?
The tendon runs through a fibrocartilage pulley (trochlea)
What is the primary action of superior oblique?
Torsion - depression
Where does inferior oblique insert?
Behind the equator, close to lateral rectus
What is the primary action of inferior oblique?
torsion - elevation
What is the effect of contracting superior rectus?
Superior rectus elevates the globe an also imposes a twisting action as well as a pulling action outward
How to test superior rectus in isolation?
Abduct the eye, then look up
How to test inferior rectus in isolation?
Abduct eye then look down
How to test superior oblique in isolation?
Adduct eye then look up
How to test inferior oblique in isolation?
Adduct eye then look down
How to test medial rectus in isolation?
Adduct eye
How to test lateral rectus in isolation?
Abduct eye
Which extraocular muscle does trochlear nerve innervate?
Contralateral superior oblique
Which brainstem centre is responsible for horizontal gaze control?
Pontine paramedian reticular formation
Which brainstem centre is responsible for vertical gaze control?
Midbrain paramedian reticular formation
What are the neuron types in the pontine reticular formation?
Burst neurons and omnipause neurons
What are the functions of burst neurons?
- Fire at high frequency just before a horizontal eye movement
- excite ipsilateral abducens
- suppress contralateral abducens
What are the functions of omnipause neurons?
- fire continuously during saccade except when burst neurons activated
- GABAergic neurons that project to contralateral abducens nucleus
What is the neural pathway for looking to the left?
Cortex signals to pontine paramedian reticular formation
In left eye,
Excitatory burst neurons input to left abducens nucleus to cause contraction of left lateral rectus and the right abducens nucleus sends inhibitory signals via medial longitudinal fasciculus to left oculomotor nucleus to cause relaxation of left medial rectus
In right eye,
Inhibitory burst neurons signal to right abducens nucleus to cause relaxation of right lateral rectus and the left abducens nucleus sends excitatory signals via MLF to right oculomotor nucleus to cause contraction of right medial rectus
In what direction do frontal eye fields control saccades?
Contralateral direction
Which UMNs drive brainstem gaze centres?
Frontal eye fields
Posterior parietal cortex
Superior colliculus
Basal ganglia
What do semicircular canals detect?
Head position
What do otolith organs detect?
Linear acceleration
What is the pathway for the vestibular-ocular reflex when turning the head to the right?
Information must be coordinated with CNIII, VI, and VIII:
What is parallel processing?
When cones are hyperpolarised by light, this hyperpolarising signal is communicated to both ON and OFF bipolar cells
Which retinal neuron can modulate signals in the inner retina?
amacrine cells
What are the different bipolar cells and how many of each are there?
10 different bipolar cells:
1 rod bipolar cell
9 cone bipolar cell
What are the functions of bipolar cells?
spatial vision and colour vision
Which bipolar cell hyperpolarises when light falls on the retina?
OFF bipolar cells
Which bipolar cell depolarises when light falls on the retina?
ON bipolar cells
What are saccadic eye movement?
Fovea shifts rapidly to a new visual target
How do ON bipolar cells respond when light hits the retina?
They always depolarise
How do OFF bipolar cells respond when light hits the retina?
Hyperpolarise
Which neurons input onto horizontal cells and which neurons do horizontal cells send output to?
Photoreceptors input to horizontal cells
Horizontal cells output to photoreceptors
How do horizontal cells respond to light?
Hyperpolarise
Which neurotransmitter do horizontal cells release in response to light?
Release GABA to photoreceptors
What sort of signals do amacrine cells send?
Inhibitory cells that release inhibitory neurotransmitters glycine and GABA
Which neuron in the retina fires action potentials?
ON ganglion cell depolarise and fire action potentials
What is receptive field?
the area of retina that, when stimulated with light, changes cell membrane potential
What protein do rods contain?
rhodopsin
What protein do cones contain?
one of three different types of cone-opsins
Which vitamin do opsins bind?
Vitamin A
How do photoreceptors respond to dark?
cGMP gates sodium channels and allows continuous influx of sodium ions causing depolarisation
How do photoreceptors respond to light?
Phosphodiesterase breaks down cGMP to GMP
Sodium channels close
Influx of sodium ceases
Cell becomes hyperpolarised and releases glutamate
What creates the centre-surround phenomenon?
Some ganglion cells depolarise when light is shone on the centre of the receptive field while some ganglion cells hyperpolarise when light is shone on the peripheral part of receptive field
What is the surround response?
Light shone on peripheral part of receptive field
Surround photoreceptors hyperpolarise releasing glutamate
Horizontal cells receive input, become hyperpolarised releasing GABA
GABA causes central photoreceptors to depolarise
Eventually ganglion cells become hyperpolarised
What is the centre response?
Light shone to centre of receptive field
Photoreceptors hyperpolarise releasing glutamate
Causing the depolarisation of ON bipolar cells
Which causes depolarisation of ganglion cells
What do M ganglion cells encode?
Motion
What do P ganglion cells encode?
Colour vision and visual acuity
What forms the optic chiasm and where does it lie?
fibres from left and right optic nerves combine to form optic chiasm
optic chiasm lies at the base of the brain, anterior to pituitary and is surrounded by the carotid arteries
Where do nasal fibres of optic nerve cross?
Optic chiasm
Which fibres pick up light from the temporal visual field?
All the cells at nasal half of retina
Where do temporal fibres cross?
Temporal fibres from each eye travel through chiasm and don’t cross
Which hemisphere interprets information from the left visual hemifield?
Right hemisphere
Where do most ganglion cell axons involved in vision terminate?
Lateral geniculate nucleus in the thalamus
How many layers are within the LGN and which cell types make up the layers?
6 layers within LGN:
Layers 1 and 2 are magnocellular layers
Layers 3-6 are parvocellular layers
Where do M and P ganglion cell axons terminate in the LGN?
M cells target magnocellular layers of LGN (layers 1-2)
P cells target parvocellular layers of LGN (layers 3-6)
T/F
Inputs from each eye mix at the LGN
False
Inputs from each eye are segregated
Mixing of info from each eye does not occur at LGN
Where do LGN neurons project to?
Primary visual cortex in the occipital lobe around the calcarine fissure
What are the optic radiations?
Ganglion cell axons that exit the LGN form white matter tracts which travel to occipital lobe
Where is central vision encoded?
Encodes at the most posterior (occipital) part of the primary visual cortex
Where is peripheral vision encoded?
More anterior part of the primary visual cortex, along the banks of the calcarine fissure
If a lesion only affect vision in only 1 eye, what is the location of the lesion?
In the eye itself or in the ipsilateral optic nerve
before the optic chiasm
If a lesion impairs vision in both eyes in some form or another, what is the location of the lesion?
The lesion may be at the chiasm or posterior to the chiasm
What is the location of a lesion causing a bitemporal hemianopia - affecting opposite sides of the visual field?
At the optic chiasm
What is the location of a lesion causing vision impairment in half the eye on the same side of both eyes - affecting same side of the visual field?
Optic tracts
What is the location of a lesion which impairs vision in 1 quadrant of each eye on the same side of the visual field?
Optic radiations
What is the general location of a lesion that impairs vision on the same side of the visual field?
Post chiasm
What is the location of a lesion which causes visual impairment in half of each eye on the same side of the visual field but has macula sparing?
Visual cortex:
Macula sparing means normal central vision and is indicative of a vascular problem in the visual cortex
What are melanopsin ganglion cells?
Intrinsically photosensitive ganglion cells in the retina
What visual pigment do ipGCs contains?
melanopsin
How do ipGCs respond to light?
Activation of melanopsin causes depolarisation of ipGCs
Which muscle causes the pupil to constrict and what is its innervation?
Sphincter pupillae innervated by parasympathetic fibres which travel with CN III
Which muscle causes pupil dilation and what is its innervation?
Dilator pupillae innervated by sympathetic fibres
What is the pathway controlling pupil response?
ipGC process travels down optic nerve through the chiasm
Travels to optical pretectal nucleus in the midbrain
Then sends a process to each Edinger Westfal nucleus
A process from each Edinger Westfal nuclues heads back towards the eye which synapses at ciliary ganglion which travels to iris
Where is the location of a lesion causing an afferent pupil defect?
Lesion can be from ipGC to pretectal nucleus in the midbrain
What is the location of a lesion causing an efferent pupil defect?
Lesion can be located anywhere from the Edinger Westfal nucleus to the ciliary ganglion
Where do ipGCs project to control circadian rhythm?
Suprachiasmatic nucleus in the hypothalamus
What is the pain pathway for migraine and how are ipGCs involved?
Pain signal from dura is carried by trigeminal nerve to the brainstem and to the posterior nucleus of the thalamus
ipGCs also project to posterior thalamic nucleus
the posterior thalamic nucleus is light sensitive and becomes excited
Where does superior oblique insert?
Inserts far behind the equator at an oblique angle to lateral rectus
Where does the tendon of superior oblique run through?
The tendon runs through a fibrocartilage pulley (trochlea)
What is the primary action of superior oblique?
Torsion - depression
Where does inferior oblique insert?
Behind the equator, close to lateral rectus
What is the primary action of inferior oblique?
torsion - elevation
What is the effect of contracting superior rectus?
Superior rectus elevates the globe an also imposes a twisting action as well as a pulling action outward
How to test superior rectus in isolation?
Abduct the eye, then look up
How to test inferior rectus in isolation?
Abduct eye then look down
How to test superior oblique in isolation?
Adduct eye then look up
How to test inferior oblique in isolation?
Adduct eye then look down
How to test medial rectus in isolation?
Adduct eye
How to test lateral rectus in isolation?
Abduct eye
Which extraocular muscle does trochlear nerve innervate?
Contralateral superior oblique
Which brainstem centre is responsible for horizontal gaze control?
Pontine paramedian reticular formation
Which brainstem centre is responsible for vertical gaze control?
Midbrain paramedian reticular formation
What are the neuron types in the pontine reticular formation?
Burst neurons and omnipause neurons
What are the functions of burst neurons?
- Fire at high frequency just before a horizontal eye movement
- excite ipsilateral abducens
- suppress contralateral abducens
What are the functions of omnipause neurons?
- fire continuously during saccade except when burst neurons activated
- GABAergic neurons that project to contralateral abducens nucleus
What is the neural pathway for looking to the left?
Cortex signals to pontine paramedian reticular formation
In left eye,
Excitatory burst neurons input to left abducens nucleus to cause contraction of left lateral rectus and the right abducens nucleus sends inhibitory signals via medial longitudinal fasciculus to left oculomotor nucleus to cause relaxation of left medial rectus
In right eye,
Inhibitory burst neurons signal to right abducens nucleus to cause relaxation of right lateral rectus and the left abducens nucleus sends excitatory signals via MLF to right oculomotor nucleus to cause contraction of right medial rectus
In what direction do frontal eye fields control saccades?
Contralateral direction
Which UMNs drive brainstem gaze centres?
Frontal eye fields
Posterior parietal cortex
Superior colliculus
Basal ganglia
What do semicircular canals detect?
Head position
What do otolith organs detect?
Linear acceleration
What is the pathway for the vestibular-ocular reflex when turning the head to the right?
Turning head right causes rotation of hair cells in right horizontal semicircular canal which excites right vestibular nucleus, and it causes opposite rotation of hair cells in left horizontal semicircular canal which sends inhibitory signals to left vestibular nucleus.
Right vestibular nucleus sends excitatory input to left abducens nucleus and right oculomotor nucleus and sends inhibitory input to right abducens nucleus:
Excitation of left abducens nucleus causes excitation of left lateral rectus leading to contraction, and excitation of right oculomotor nucleus via MLF leads to contraction of right medial rectus (indirect route)
Excitatory input to right oculomotor nucleus causes contraction of right medial rectus (direct route).
Inhibitory input to right abducens nucleus via right vestibular nucleus causes relaxation of right lateral rectus.
Right abducens nucleus also sends inhibitory input to left oculomotor nucleus to cause relaxation of left medial rectus.
Net result is eyes counteract the right-going head movement by moving in the opposite direction: lateral rectus of right eye and medial rectus of left eye relax and lateral rectus of left eye and medial rectus of right eye contract so that eye movements are left-going.
What is parallel processing?
When cones are hyperpolarised by light, this hyperpolarising signal is communicated to both ON and OFF bipolar cells
What are saccadic eye movement?
Fovea shifts rapidly to a new visual target
What is the smooth pursuit eye movement?
Image of moving target remains on the fovea
Which layer do M ganglion cells terminate in primary visual cortex?`
Layer 4C alpha
Which layer do P ganglion cells terminate in the primary visual cortex?
Layer 4C beta
Where does mixing on visual information occur?
Primary visual cortex, not the LGN
Where do cells in layer 4C alpha project to?
Project to layer 4B
What do the neurons in layer 4B respond to?
these neurons are directionally selective and only respond to bars of light that move in a particular direction. The stimulus must be moving.
What is the dorsal stream of visual processing?
Info carried from M cell pathway from V1, V2 and V3 is carried to Area MT which processes object motion and location
What are characteristics of Area MT?
- neurons have large receptive fields
- neurons respond to movement
- majority of the neurons are directionally selective
- direction-of-motion columns
How is colour detected?
The colour information sent to the retina comes from all the cones combined. Detection of colour is based on relative response levels of each cone. Every cone reacts to a certain extent to each wavelength of light. Colour perception is dependent on comparison.
How do P ganglion cells detect colour?
Some P ganglion cells are excited by red/green on their receptive field centre and others are excited by blue/yellow.
P ganglion cells that are excited by blue light directed at the centre of their receptive field and inhibited by yellow light directed at the periphery. When blue light hits the receptive field centre, blue cones become excited and hyperpolarise which excites ON bipolar cells nd causes them to depolarise. ON ganglion cells then become depolarised causing them to fire action potentials.
Info is carried to LGN and down to V1 by P ganglion cells.
How are ON ganglion cells maximally inhibited?
P ganglion cells that are excited by red light will be maximally inhibited when green light is directed at the receptive field periphery and vice versa.
P ganglion cells that are excited by blue light will be maximally inhibited when yellow light is directed at the receptive field periphery and vice versa.
What is the mechanism by which ganglion cells are maximally inhibited?
If green light is shone at the peripheral part of a P ganglion cell receptive field that is excited by red light, green cones in the periphery will be stimulated and hyperpolarise. Glutamate is sent to horizontal cells which respond by hyperpolarising and sending inhibitory signals to red cones in the receptive field centre. As a result, ganglion cells hyperpolarise and are maximally inhibited.
What happens when light shines over the entire receptive field?
If light covers the entire receptive field, an in-between response is generated and the brain will delete the signals because it can only interpret maximal stimulation and maximal inhibition, nothing in between.
Where is Area IT located?
Medial aspect of inferior temporal lobe
What is a major input to Area IT?
Area V4
What does Area IT process?
Object recognition - perception of faces
What are the three groups of colour vision deficiency?
Monochromacy - having only 1 type of cone
Dichromacy - having only 2 functional cones
Anomalous trichromacy - having all 3 cones but one expresses abnormal pigment
What is the ventral stream?
P pathway:
Info from P cells go down inferior part of temporal lobe to Area V4 which inputs to Area IT to process shape and colour (not motion)
How is sound basically transmitted as a neural response?
Pinna localises sound
Sound travels through the external acoustic meatus to tympanic membrane which vibrates and moves auditory ossicles
Movement of the ossicles is transmitted to the oval window and the cochlea transforms the physical motion of the oval window into a neural response
What is the function of the ossicles?
Matches impedance of air to impedance of fluid within inner ear
How does the basilar membrane respond to sound waves?
Base is stiff and narrow and responds to high frequencies
Apex is wide and responds to low frequency
Basilar membrane is tonotopically coded - adjacent hair cells pick up increasing frequencies
Where is the Organ of Corti and what does it do?
Sandwiched between basilar membrane and reticular lamina
Contains inner and outer hair cells which are the auditory receptors which transduce mechanical sound to neural impulses
How do hair cells transduce neural impulses?
Shearing forces occurring across the tectorial membrane is transferred to stereocilia on the hair cells. Movement of hair bundle towards the kinocilium (tallest sterocilium) causes opening of potassium channels, depolarisation and opening of voltage-gated calcium channels leading to the release of glutamate. Hyperpolarisation of the hair bundle causes closure of potassium channels. Potassium ion concentration must be high in the endolymph of the scala media in order for potassium to flow into the cell
How do inner hair cells compare to outer hair cells?
Inner hair cells are the sensory receptors whose nerve bundles project to brainstem nuclei
Outer hair cells receive efferent inputs from superior olivary complex in order to amplify movements of the basilar membrane
How do outer hair cells respond to stimuli?
They change length in response to voltage change which accentuates the movement of the basilar membrane and amplifies the signal sent to inner hair cells.
When hair cells are depolarised (influx of potassium), the cell contracts and when the hair cells are hyperpolarised, the cell elongates
How do certain drugs affect outer hair cells?
Certain antibiotics and high doses of aspirin are ototoxic. Antibiotics and cause a loss of hair cells and aspirin can inhibit length changes to outer hair cells. As a result, movement of basilar membrane is reduced by 100-fold and loss of hearing may occur
What is the auditory pathways?
Sound mechanical vibration displaces hair cells within the cochlea causing voltage changes which is transmitted as release of glutamate to vestibulocochlea nerve. Cochlear nerve synapses at cochlear nucleus in medulla which sends inputs to superior olivary complex and travels through lateral lemniscus, inferior olive in midbrain, to medial geniculate nucleus in the thalamus which sends any input to auditory cortex
What are the components of the superior olivary complex and what are the functions of each?
Lateral superior olive localises sound by measuring time delay
Medial superior olive localises sound by sensing intensity differences
What is the Duplex theory?
There is a time difference that low frequency sound action potentials reach each ear which is detected by medial superior olive
There are intensity differences (loudness) between high frequency sound reaching each ear which is detected by lateral superior olives and trapezoid bodies
How does auditory cortex process sound?
Neurons within auditory cortex are sensitive to sound frequency
Neurons are organised into columns which that cells within a column are tuned to same frequency
Receives excitatory input from one ear and inhibitory input from opposite ear
Complex sounds are asymmetrically represented in the hemispheres
