8. Anatomy of special sense organs Flashcards
Location of the olfactory tract and bulb?
Inferior surface of the frontal lobe
Reasons for acoustic meatus narrowing?
Acoustic neuroma
Also… due to mastoiditis and Paget’s disease
Tumours in the internal acoustic meatus & at the cerebellopontine angle will impinge upon…
Tumours in the internal acoustic meatus & at the cerebellopontine angle will impinge upon CNVII & VIII giving ipsilateral facial signs (possibly extending to include CN V)
Course of CN I on the base of skull?
Arise from olfactory bulb and pass through the cribriform plate of the ethmoid bone.
Then within the superior aspect of the nasal cavity.
Course of CN II on the base of skull?
CN II surrounded by a sleeve of meninges & CSF passes through the optic canal with the ophthalmic branch of the internal carotid artery to reach the orbit
What are the 2 nuclei of the CN VIII?
Vestibular and cochlear nucleus
Hence the name “vestibulocochlear nerve”
Potential structures affecting in a cranial base fracture?
Ear Eye Nose Petrous temporal bone Orbit Cribriform plate
Which bone prevents posterior dislocation of the mandible?
The tympanic plate of the temporal bone
What are the external, middle and internal structures of the ear?
External:
- Auricle
- External acoustic meatus of cartilage and bone (leads sound eaves to the tympanic membrane)
Middle:
- Ossicles (which mechanically transmit sound, connected to pharynx by the auditory tube)
- Tympanic membrane
Internal:
- Semi-circular canals (detect motion)
- Cochlea (detect hearing, converts mechanical signals to electrical. Innervated by CN VIII)
Stylomastoid foramen through which CN ____ emerges
Stylomastoid foramen through which CN VII emerges
Where does the vestibulocochlear apparatus lie within?
Petrous temporal bone
The bony semicircular canals, particularly the anterior, cause slight elevations on the bone surface in the middle cranial fossa
Role of the auricle?
elastic cartilage for support & ‘catching’ the sound & funnelling it into the external acoustic meatus
Composition of external acoustic meatus?
Lateral 1/3 = Cartilaginous
Medial 2/3 = bony
Lined by skin, with hairs, containing ceruminous glands (modified sweat glands) that secrete ear wax
How and why is the external acoustic meatus?
Slightly sinuous course so should be straightened for examination of the tympanic membrane by pulling the ear superiorly, posteriorly & slightly laterally
The internal surface of the tympanic membrane and middle ear are supplied by…
the CN IX
What holds the tympanic membrane in place?
Held within the temporal bone by a fibrocartilaginous ring
Name the 3 ossicles found in the tympanic cavity?
- Stapes
- Incus
- Malleus (attached to the tympanic membrane)
Difference between round and oval window of the cochlea in the inner ear?
The round window acts as a pressure release. Covered by a secondary tympanic membrane
The oval window is covered by the base of the stapes ossicle bones, hence it converts mechanical impulses to electrical signals
Which muscles prevent excess movement of the ossicles?
Tensor tympani and stapedius
Innervation and attachment of tensor tympani?
Innervation: CN V3 via nerve to medial pterygoid muscle
Attachment: From canal of petrous temporal bone above the auditory tube –> handle of malleus ossicle
Role to prevent excess movement of the malleus
Innervation and attachment of stapedius?
Innervation: CN VII
Attachment: Arises within the pyramidal eminence –> neck of the stapes
Role to prevent excess movement of the malleus.
Loss of function causes hyperacusis
Other names for auditory tube?
Eustachian
Pharyngotympanic
At which point does the auditory canal become cartilaginous?
Initially bone, becomes cartilaginous between the petrious temporal bone and the greater wing of the sphenoid
Why does swelling of the tubal tobsil cause otitis media?
It can block the auditory tube
middle ear extending posteriorly via the epitympanic recess & aditus into ….
the mastoid antrum & air cells
How to middle ear infections arise? What is the associated risk?
Infections in the middle ear are common & often caused by obstruction to the auditory tube
Chronic infections may damage the ossicles causing conductive deafness & these may even spread to the mastoid air cells or beyond
Why is there a risk of infection spread between the middle ear and middle cranial fossa?
The roof of bone is very thin and weak
Bony labyrinth..
Where?
Consists of?
Contains
Is the rigid, bony outer wall of the inner ear in the temporal bone.
It consists of three parts:
the vestibule, semicircular canals, and cochlea.
Membranous labyrinth
Perilymph
Endolymph
(In the inner ear)
Role of the cochlear and vestibular aqueducts?
Cochlear & vestibular aqueducts allow the membranous labyrinth to connect with the subarachnoid space for reabsorption of peri & endolymph
Innervation fo the semicircular canals, utricles and saccule of the labyrinth?
From the vestibular part of CN VIII
Leaves these structures to form a ganglia in the internal acoustic meatus
Clinical signs that the balance has been disturbed?
Ataxia
Vertigo
Nausea
Nystagmus
What is the cochlea’s innervation?
The cochlear part of CN VIII is derived from the organ of Corti in the cochlea & lies in the internal acoustic meatus with the vestibular part of CN VIII & CN VII
What separates the anterior and posterior chambers of he eye?
The iris but communicate via the pupil
Contain the aqueous humour
What forms the outer fibrous layer of the eye?
Sclere (replaced by cornea anteriorly)
(Below it is the vascular choird layer
Most inner is the retina)
Choroid continues as the ciliary body which forms a complete…..
Choroid continues as the ciliary body which forms a complete supportive ring around the lens & iris
When ciliaris contracted, what happens to the lens?
When ciliaris muscle (in the ciliary body) contracts (parasympathetic, CN III, Edinger-Westphal nucleus) the ring shrinks, the suspensory ligaments relax, & the lens becomes rounded (accommodation for close vision)
The iris contains both the sphincter (__________, CN III, Edinger-Westphal nucleus) & _____ (sympathetic) pupillae
The iris contains both the sphincter (parasympathetic, CN III, Edinger-Westphal nucleus) & dilator (sympathetic) pupillae
Production and reabsorption of aqueous humour?
Aqueous humour is secreted by the ciliary body & reabsorbed by the scleral venous sinus (canal of Schlemm)
Cause of glaucoma?
Glaucoma results from excess pressure; drugs that constrict the pupil may ‘pull’ the venous sinus open & help
What is the role of ciliary muscles in lens accommodation?
Ciliary muscle relaxation –> zonular fibers under tension –> Lens is stretchde thin to refract light from distance
PS stimulation to ciliary muscles leads to contract–> Zonule fibers relax –> Internal tension and lack of stretching cause lens to become more spherical. Now able to refracted light to near vision
Dilator pupillae under ________ control
Sphincter pupillae under _________ control
Dilator pupillae under sympathetic control
Sphincter pupillae under parasympathetic control
What structures are present at the optic disk? Why is called the blindspot?
• ‘Entry’ of optic nerve & central retinal vein with artery
which is an end/terminal artery with obstruction
causing blindness
• Devoid of photoreceptors & often referred to as the
‘blind spot’
Clinical macula = anatomical ______ centralis
Clinical macula = anatomical fovea centralis
I.e. region of greatest visual sensitivity
Rods sensitive to ____ light and ____ colour
Rods sensitive to dim light and no colour
Cones sensitive to ____ light and ____
Rods sensitive to bright light and colour
inferior and posterio relation relation of optic chiasm?
The pituitary gland.
Pituitary tumour = tunnel vision
Optic nerves —-> optic chiasm —> optic ___ –> _______ bodies of the thalamus –> occipital lobe of ____
Optic nerves —-> optic chiasm —> optic tracts –> geniculate bodies of the thalamus –> occipital lobe of cortex
The right visual field passes to the left optic TRACT & left occipital lobe & vice versa!
The right visual field passes to the left optic TRACT & left occipital lobe & vice versa!
Where is the lesion/problem:
- Left homoymous hemianopia?
- Blind in right eye
- Bitemporal hemianopia
- Right optic tract
- Right optic nerve
- At optic chiasm
Clinical consequences of Horner’s syndrome?
Injury to symp trink –> loss of symp supply to:
- Pupil dilator
- Smooth muscle of levator palpebrae superioris
- Sweat glands
- Blood vessels
Soooo…. presents with:
- Miosis (constriction)
- Ptosis (droopy eye)
- Anhydrosis
- Flushed face
Injury to the oculomotor nerve (CN III) will remove the _________ pupil constriction leading to a dilated pupil with ptosis (complete loss of levator palpebrae _______) & the eye is ______ downwards & outwards (______ oblique & lateral rectus)
Injury to the oculomotor nerve (CN III) will remove the parasympathetic pupil constriction leading to a dilated pupil with ptosis (complete loss of levator palpebrae superioris) & the eye is turned downwards & outwards (superior oblique & lateral rectus)
