Visual and Auditory Pathways Flashcards
What are the 3 layers of the eye?
- outer layer = cornea
- middle layer = vascular layer
- inner layer = retina
What is the outer layer of the eye made up of?
- cornea (front, transparent)
- sclera (back, opaque/white)
What is the outer layer of the eye important for?
protection, refraction and transmission of light
What does the middle layer of the eye include?
- choroid
- ciliary body
- iris
What does the ciliary body control?
the tension of the suspensory ligaments on the lens
What happens when the ciliary body contracts?
the tension on the lens is reduced and becomes more convex (used for looking at near objects; accommodation)
What is the pupil?
an opening within the iris that allows light to pass through to the lens
What does the ciliary body secrete?
aqueous humour
Where is aqueous humour reabsorbed?
scleral venous sinus or canal of Schlemm
What happens if the absorption and secretion of aqueous humour is inhibited?
pressure can build up in the anterior chamber and press against the lens which in turn presses against the posterior compartment and beyond leading to glaucoma (blindness)
What does the iris do?
control the size of the pupils
What are the muscles of the iris?
- circularly arranged sphincter pupillae muscles
- radially arranged dilator pupillae muscles
What does the choroid do?
supply nutrients to the retina via a layer of rods and cones
What are the 2 sources of nutrition (blood supply) to the retina?
- central artery of the retina that supplies the majority of the retinal arterial supply except the layer of cones and rods
- diffusion from the carotid artery
What can renal detachment due to trauma cause?
blindness
What are the 3 layers of the retina?
- rods and cones
- bipolar cells
- retinal ganglion cells
What do the retinal ganglion cells do?
form the axons of the optic nerve and send signals to the lateral geniculate nucleus in the thalamus and then to the primary visual cortex in the occipital lobe
What are the cranial nerves involved in visual and auditory pathways?
- CNII - optic nevre
- CNIII - oculomotor nerve
- CNIV - trochlear nerve
- CNVI - abducens nerve
- CNVIII - vestibulocochlear nerve
What is the optic nerve formed by?
axons of retinal ganglion cells in the retina
What is the sheath that covers the optic nerve made up of?
arachnoid trabeculae, pillars, and septa compartmentalising the subarachnoid space
What does the optic tract do?
relay in the lateral geniculate nucleus before terminating in the primary visual cortex
What happens at the optic chiasma?
- axons from the nasal hemiretina cross the midline to the opposite side
- axons from the temporal hemiretina do not cross the midline
Where is the left visual field from both eyes represented?
on the right visual cortex (and vice versa)
Where do some axons in the retina end up?
the Edinger-Westphal nucleus in the midbrain
What do axons that end up in the Edinger-Westphal nucleus do?
mediate the afferent part of the pupillary light reflex (parasympathetic fibres in the oculomotor nerve mediate the efferent part)
What is consensual pupillary light reflex?
the change in pupil size in the eye opposite to the eye to which the light is directed e.g. if the light is shone in the right eye, the left pupil also constricts consensually
What is the myelin of the optic nerve formed by?
oligodendrocytes
What does damage to the optic nerve result in?
blindness
Describe the pathway of the oculomotor nerve
given off from the midbrain and passes along the lateral wall of the cavernous sinus and enters the orbit
What do the extraocular eye muscles control?
movements of the eyeball and the superior eyelid
What are the intrinsic muscles of the eye?
ciliary body and iris
What does the oculomotor nerve supply motor function to?
most of the extraocular and intrinsic muscles of the eye
What does the oculomotor nerve contain?
preganglionic parasympathetic fibres from neuronal cell bodies located in the Edinger-Westphal nucleus, which are for the supply of intrinsic muscles of the eye
Where do preganglionic parasympathetic fibres from neuronal cell bodies located in the Edinger-Westphal nucleus synapse and supply?
- synapse in ciliary ganglion
- supply sphincter pupillae for constriction and ciliary muscle for accomodation
What does a lesion of the oculomotor nerve result in?
- paralysis of many of the extraocular muscles
- diplopia (double vision)
- loss of pupillary light response
- loss of accommodation
- increased sensitivity to bright light
Describe the pathway of the trochlear nerve
it emerges from the dorsal aspect of the brainstem then passes along the lateral wall of the cavernous sinus and enters the orbit
What does the trochlear nerve supply motor function to?
the superior oblique muscle (extraocular muscle) which makes it possible to look down
What does lesion to the trochlear nerve result in?
- paralysis of the superior oblique muscle
- diplopia
- inability to look inferolaterally
Describe the pathway of the abducens nerve
it leaves the brain via the potomedullary junction and runs across the sharp edge of the petrous temporal bone and passes through the cavernous sinus to enter the orbit
What does the abducens nerve supply motor function to?
the lateral rectus muscles
What does lesion to the abducens nerve result in?
- diplopia
- weakness/paralysis of the lateral gaze due to loss of function of the lateral rectus muscle
- medial deviation of the affected eye
What is the cochlea?
the organ of the inner ear responsible for hearing
Where is the inner ear embedded?
within dense structure of the temporal bone (bone conduction of sound) made up of 3 coils
What are the 3 fluid filled tubes of the interior cochlea?
- upper - scala vestibuli
- middle - scala media
- lower - scala tympani
What does the helicotrema do?
connect the scala tympani and the scala vestibuli and allow fluid to move between them
What does the basilar membrane form?
the partition between the scala media and the scala tympani
What is the basilar membrane?
a mechanical analyser of sound frequency
What is the critical characteristic of the basilar membrane?
its physical properties are not uniform throughout; it is 5x broader at the apex than at the base of the cochlea
What frequency does each end of the basilar membrane respond to?
- apex - 20Hz
- base - 20,000Hz
What is the organ of Corti?
the receptor organ of the inner ear
What happens when the basilar membrane vibrates?
the organ of Corti is carried with it which causes deflection of inner hair cells which then leads to receptor potential
What is meant by the basilar membrane having an ornery arrangement of frequencies?
every hair cell at a particular location is most sensitive to stimulation at a specific frequency
What can cochlear implants do?
stimulate the hair cells at different locations depending on the frequency of the sound
Describe the pathway of the vestibulocochlear nerve
it leaves the pontomedullary junction and enters the internal acoustic meatus together with the facial nerve (CVII)
What does the vestibulocochlear nerve supply sensory function to?
sensory cells of the inner ear responsible for hearing and balance
What does the cochlear portion of the vestibulocochlear nerve come from?
bipolar neurons in the spiral ganglion that innervate the hair cells of the cochlea
What does the vestibular portion of the vestibulocochlear nerve come from?
bipolar neurons in the vestibular ganglion, that innervate sensory cells of the semi-circular canals as well as the saccule
What does lesion of the vestibulocochlear nerve lead to?
- loss of hearing or vestibular sense
- vertigo due to disruptions of the ampellae of the semi-circuluar ducts and the utricle and saccule
- other problems with the labyrinth may cause tinnitus (ringing or buzzing in ears)
What is the most common loss of hearing?
the loss of hair cells of the cochlea (organ of Corti) that occurs for high tones with advanced age (presbycusis)
What are the 5 components of the auditory pathway?
- cochlear nucleus
- superior olivary nucleus
- inferior colliculus
- medial geniculate nucleus
- auditory cortex
How is sound localised in the auditory pathway?
both crossed and uncrossed axons ensure that sound from both ears is being compared
What does the cochlear nerve send branches to?
dorsal and ventral cochlear nuclei
What does the superior olivary nucleus do?
receive input from each cochlear nucleus and localise sounds
What does the medial and lateral portions of the superior olivary nucleus do respectively?
- medial - compares time of arrival of a signal at each ear
- lateral - compares sound intensities
What does the inferior colliculus do?
receive input from the cochlear nucleus or superior olivary nucleus and project to the medial geniculate nucleus of the thalamus
What is the medial geniculate nucleus?
a thalamic relay nucleus that projects to the ipsilateral primary auditory cortex
Where is the primary auditory cortex located?
on the superior aspect of the temporal lobe to receive input from both ears
What are the transverse temporal gyri?
bumps on the lower bank of the lateral fissure
Where is the auditory association cortex?
in the superior temporal gyrus
What is the Wernicke area essential for?
understanding speech
