Special senses (+ eyes lecture) Flashcards
Which cranial nerve is able to regenerate its neurons?
Olfactory nerve (CN I)
Where does the Olfactory nerve innervate?
Olfactory mucosa in the superior concha of the nasal cavity
What can cause unilateral anosmia?
Meningioma
Anterior cranial fossa trauma (causing damage to the cribriform plate)
Viral infection
Parkinson’s or Alzheimer’s
What is anosmia?
Loss of sense of smell
How do you test for anosmia?
Test each nostril individually with familiar smells (e.g. orange and coffee)
What is phantosmia?
Smelling things that aren’t there
Where does the olfactory tract carry sensory neurons to?
Orbital and piriform cortexes
Where are taste receptors found?
Tongue
Palate
Pharynx
What nerve supplies motor function to the tongue?
Hypoglossal (CN XII)
What nerves supply taste sensation to the tongue?
Anterior 2/3 = CN VII (Chorda Tympani)
Posterior 1/3 = CN IX
What nerves supply sensory input to the tongue?
Anterior 2/3 = CN Vc (lingual branch)
Posterior 1/3 = CN IX
What marks the division between nerve supply between the anterior and posterior portions of the tongue?
Sulcus Terminalis (lined by vallate papillae)
What region of the eye has the greatest visual acuity?
Fovea Centralis (which sits in the middle of the macular)
What is the optic papilla?
Blind spot in visual field caused by optic nerve entering the eye at this point
What are the 3 layers of the eyes?
Retina (sensory region)
Choroid
Sclera (white outer layer)
What would occlusion of the central retinal artery result in?
Blindness in that eye
What is examination of the eye called?
Fundoscopy
What is the optic chiasm and where is it found?
Optic chiasm is point where 2 optic nerves come together - located in the pituitary fossa
[clinical relevance: pituitary tumour can cause compression of the optic nerves]
What is the role of ciliary bodies in accomodation?
Contract to relax suspension ligaments which enables the lens to recoil, making it fatter
What happens to visual information as it enters the eyes?
Information is inverted and flipped as it passes through the lens of the eye (so information from the L visual field of each eye ends up on the R side of the retina of each eye)
How are the retinal fields of the eyes divided?
Nasal (medial) and temporal (lateral) regions
[Nb. nasal and temporal regions of visual fields are opposite way round]
What is the difference between retinal and visual fields?
Visual fields are what an individual sees
Retinal fields are anatomical parts of the retina
How should visual field defects be described?
Described and drawn from patient’s point of view
Does a lesion of the optic pathway cause ipsilateral or contralateral symptoms?
Ipsilateral if optic nerve damaged
Contralateral once passed the optic chiasm
Which part of the optic radiation has a different blood supply?
Occipital pole (of striate area) - Macular vision
What is damage to the occipital pole called and what are the symptoms?
Central scotoma
Loss of central vision (macular region)
What are the steps of the pupillary light reflex?
- Retinal cells send afferent information (via CN II) to pre-tectal nucleus in midbrain (L and R nuclei linked via the posterior commissure)
- PTN linked to Edinger-Westphal Nuclei via interneurons (L receives info from L and R PTN and vice versa)
- Pre-ganglionic parasympathetic fibres enter CN III and synapse in the Ciliary Ganglion (located in the posterior orbit)
- Post-ganglionic parasympathetic fibres in short ciliary nerves enter the iris which controls the sphincter pupillae (to cause contraction of pupils)
What does the pupillary light reflex test?
Function of the retina, midbrain and cranial nerves II and III (optic and occulomotor)
What is the consensual light reflex?
Shining light in one eye should cause pupil constriction in both eyes
How does the lens of the eye fatten during pupil constriction?
Contraction of ciliary body relaxes suspensory ligaments (connecting ciliary body to lens) enabling the lens to recoil thus making it fatter
What is Argyll-Robertson (prostitute’s) pupil?
Absent pupillary light reflex but in tact accommodation reflex seen in tertiary neuro-syphilis and diabetic neuropathy.
Caused by damage to PTN with EWN intact (sparing accommodation reflex)
How does accommodation (of the eyes) occur?
Information from the retina to the 1st visual cortex and the frontal eye field acts on CN III which innervates medial rectus muscles so that vergence can occur
Information from frontal eye field also acts on EWN which causes CN III to innervate the sphincter pupillae (for pupil constriction) and the ciliary body (for lens fattening) via the ciliary ganglion
What can an absent direct and consensual reflex indicate?
Damage to Edinger-Westphal Nucleus or CN III compression on side of absent reflex
How can you differentiate between CN III compression and a CN III vascular lesion?
Compression will cause loss of all CN III functions whilst a vascular lesion will spare pupillary functions
What muscles open the eyelids and what is the consequence of loss of innervation to these muscles?
Levator Palpebrae Superiosis (CN III) - majority of input therefore loss of CN III innervation will cause full ptosis Superior Tarsal (Sympathetic) - smaller input therefore loss of sympathetic innervation here results in small ptosis
What muscle closes the eyelids?
Orbicularis Oculi (CN VII)
What are the extraocular muscles of the eye and their innervation?
Superior rectus (CN III) Inferior rectus (CN III) Medial rectus (CN III) Lateral rectus (CN VI) Superior oblique (CN IV) Inferior oblique (CN III)
How can you differentiate between damage to the EWN and loss of CN III?
Damage to the EWN would not cause ptosis (whilst loss of CN III innervation would due to loss of levator palpebrae superiosis)
How is tear fluid moved across the eye?
Contraction of the orbicularis oculi muscle (CN VII innervated) occurs laterally to medially to sweep tear fluid across the eye into the nasolacrimal duct (removing dust/ grit en route)
What symptoms would occur due to loss of Orbicularis Oculi function?
Inability to blink efficiently would cause sore eyes due to inability to remove dust and grit
What is the trochlea?
Fibrous sheath tethering the superior oblique muscle to the medial wall of the orbit
What neurovascular structures pass through the superior orbital fissure?
CN III, IV, Va and VI
[Nb. compression of CN Va can cause pain in periorbital area and forehead)
Internal Carotid Artery
Where do the extraocular rectus muscles originate from?
Tendinous ring at posterior orbit
Where does the inferior oblique muscle arise from?
Antero-medial floor of orbit
What are the actions of the oblique extraocular muscles (when acting in isolation)?
Superior oblique moves eye downwards
Inferior oblique moves eye upwards
What are the actions of the lateral and medial rectus muscles (when acting in isolation)?
Medial rectus adducts eye
Lateral rectus abducts eye
What are the actions of the superior and inferior rectus muscles (when acting in isolation)?
Superior rectus elevates the eye
Inferior rectus depresses the eye
What is Hering’s Law?
Extraocular muscles normally have equal and simultaneous innervation
What nerve damage can neck pain indicate?
Trochlea nerve damage causing loss of ability to rotate eyes (through loss of innervation to the superior oblique muscle)
Neck pain caused by tilting head to compensate for damage
What are the indicators of CN VI damage?
No lateral movement of affected eye (loss of lateral rectus innervation) Convergent squint (ipsilateral eye rests in adducted position) Horizontal diplopia (worse when looking toward affected side)
What are the indicators of CN III damage?
Complete ptosis on affected side (loss of levator palpebrae superiosis)
“Down and out” position of affected eye
Divergent squint (ipsilateral eye rests in abducted position)
Horizontal and vertical diplopia
Loss of pupillary reflex (direct and consensual) on affected side - will remain dilated
Consensual pupil reflex intact in contralateral eye
What are the indicators of CN IV damage?
Upward deviation and outward rotation of affected eye
Vertical diplopia (worse when looking down)
Torsional diplopia (images twisted apart from each other)
Head tilt to compensate/ prevent diplopia
What areas of the CNS control eye movement?
Vestibular nuclei and parapontine reticular formation
Frontal eye field (frontal cortex)
Saccade centres
Visual association areas
What bones of the skull make up the orbit?
Frontal
Ethmoid
Lacrimal
Zygomatic
Where does the optic nerve (CN II) pass through to enter the orbit?
Optic Canal
How can you test the power of eyelids closing?
Ask patient to screw up eyes to hide eyelashes
What is a saccade?
Quick, simultaneous movement of both eyes scanning from target to target
Brought about by control centres in cerebral cortex
What is the vestibulo-ocular reflex?
Lateral semicircular canals make both eyes look to opposite side when head is rotated axially in a given direction (i.e. right lateral semicircular canal is activated by head movement to the right which makes you look left) - controlled through vestibulor-ocular system and prevents images from jumping around when moving (by keeping eyes on previous target)
[Clinical relevance: can be used to test brainstem function in comatose patients]
What is jerk nystagmus?
Repetitive eye movement with a fast and slow phase
Slow phase = slow drift towards damaged side
Fast phase = eyes move quickly back to midline via action of saccade centres in brainstem and cortex
Named according to direction of FAST phase
What is the cold choloric test?
Cold water put in EAM and see if eyes drift to cold water side and then back to midline
Used to test brainstem function in comatose patients
[Cold water into EAM mimics a vestibular system/ nucleus lesion on that side]
Normal response = nystagmus away from side with cold water
Depressed CNS = slow drift back to midline
Depressed CNS and brainstem = no response
Where is the vestibular and cochlear apparatus?
Within a bony labyrinth in the petrous temporal bone
What fluid is found in the vestibular and cochlear apparatus?
Endolymph (inside spiral membrane)
Perilymph (outside)
[Transmit vibrations and is shared between the 2 systems]
What is the Organ of Corti?
Sensory part of the cochlea that transmits neural signals to the brain
Sensory hair cells that detect vibrations and bend to transmit signals along cochlea nerve (CN VIII)
How do vibrations reach the inner ear?
Pass from the middle ear through the oval window into the perilymph of the cochlea
How does the membrane of the cochlear change in response to changes in frequency?
High frequency sounds cause the membrane to become narrow and stiff
Low frequency sounds cause the membrane to become wide and flexible
What is the inferior colliculus?
Nucleus in the midbrain that is auditory relay centre
Why will a patient not go deaf due to a lesion on one side of their auditory cortex?
Information distributed bilaterally across cortexes
What can cause sensorineural hearing loss?
Drugs
Viral rubella (in utero)
Mumps
What can loss of stereo-placement of sound indicate?
Cortical or thalamic pathology
What is tinnitus?
High-pitched ringing noise in the absence of sound
What can cause tinnitus?
Meniere’s Disease
URTI
Exposure to loud noise
Contractions of the tensor tympani/ stapedius
How is noise transmitted from the external ear to the brain?
- Sound waves travel into ear causing movement of the tympanic membrane
- Movement of the tympanic membrane causes the ossicles of the middle ear to vibrate
- Pivoting movement of the stapes passes vibrations through the oval window into the fluid within the cochlea (perilymph)
- Vibrations in perilymph cause displacement of the basilar membrane which causes vibration of hair cells against tectorial membrane
- Organ of Corti hair cells that detect vibrations
- Vibration converted into nerve impulses which are fired along the Cochlea nerve (a branch of CN VIII)
- Information reaches the Inferior Colliculus (the auditory relay centre in the midbrain)
- Primary auditory cortex receives bilateral auditory sensory input
How do you conduct a Rinne test?
Vibrating tuning fork placed on the mastoid bone until patient can no longer hear the vibrations and then is placed in the air by the external auditory meatus
Air conduction should be better than bone conduction so should be able to hear the vibrations when placed next to the external ear
What is a Rinne-negative finding in the Rinne test?
If no sound can be heard when tuning fork is placed next to the EAM then this is conduction deafness
How do you conduct a Weber test?
Vibrating tuning fork placed on the forehead and ask patient where the sound/ vibrations are loudest (if it is louder on one side or not)
No lateralisation = normal
Sound loudest in affected ear = conduction deafness
Sound loudest in normal ear = sensorineural deafness
What makes up the vestibular system?
Dynamic part = semicircular canals and crista
Static part = maculae (utricle and saccule)
What are the functions of the dynamic and static parts of the vestibular system?
Dynamic = acts on eye movement via the MLF Static = acts via the vestibulospinal pathway to pick up static changes of head position and linear acceleration of your head
What are the individual functions of the maculae found in the static part of the vestibular system?
Utricle macula = detects horizontal acceleration and is active when head is in flexion or extension
Saccule macula = detects vertical acceleration and is active when the head is held to the side - activates vestibulospinal pathway in a fall
What three systems do we need for balance?
Vestibular
Vision
Proprioceptive
[Loss of 2+ will result in deficit]
What is the Romberg test?
Ask patient to stand up with eyes closed
Positive sign = patient sways or falls
Negative = patient remains steady
What might a positive Romberg’s sign indicate?
Damage to the dorsal columns (lack of conscious proprioception)
Damage to inner ear (e.g. fractured petrous temporal bone)
What is strabismus and what causes it?
Squint - caused by visual axes of eyes not being parallel (e.g. due to unequal lengths or abnormal function of extraocular muscles)
Can be convergent (if axes come together) or divergent (move apart)
What is the difference between a concomitant and incomitant strabismus?
Concomitant means the angle of deviation between axes is consistent throughout range of eye movement
Incomitant means the angle varies with direction of gaze
Why don’t children with squints suffer from constant diplopia?
Often compensate during development by weakening vision in affected eye (amblyopia) so that brain ignores input from this eye
Can be corrected by forcing ‘lazy eye’ to work and gain acuity by putting eye patch on stronger eye
What is a horizontal gaze palsy?
Damage to nuclei of CN VI preventing lateral abduction of eye on affected side and medial adduction of eye on contralateral side (e.g. unable to look horizontally in direction of lesion)
What is internuclear opthalmoplegia?
Damage to the medial longitudinal fasciculus (MLF) resulting in loss of adduction due to palsy of medial rectus muscle on affected side (so unaffected eye will look left but affected eye will remain forward for example)
What is the difference between pendular and jerk nystagmus?
Pendular = phases are equal velocity in both directions Jerk = slow movement in one direction followed by quick, jerking movement back in the other direction
What nerve innervates the muscles of mastication?
CN V
What is the function of each muscle of mastication?
Temporalis = closes mouth
Masseter = closes mouth
Lateral Pterygoid = assists with opening mouth
Medial Pterygoid = chewing
What is a Marcus-Gunn pupil?
Relative afferent pupillary defect caused by damage to either the retina or the optic nerve (CN II) - dilation of affected pupil
How is a Marcus-Gunn pupil tested for?
Swinging light test - affected pupil will not constrict as much as unaffected eye (so appears dilated in comparison)
