Neuroanatomy 5 - Auditory and visual pathways and localisation of cortical function Flashcards
Go over the auditory ossciles
Note also refer to handwritten notes on ENT anatomy of the ear
Label the indicated structures of the ear
Note the chochlear nerve is associated with hearing and the vesibular nerve is associated with equilibrium
What is the route of the AP’s of hearing from sound waves to the cochlear nuclei ?
- External ear (auricle to tympanic membrane) collects & conveys sound waves
- Middle ear (tympanic membrane to oval window + eustachian tube) amplifies & conducts sound waves to the inner ear
- Inner ear (oval window to internal acoustic meatus) here sound waves stimulate the basiliar membrane (causing a change from mechanical sound vibrations into electrical impulses (AP’s))
- This causes the organ of corti to depolarise & fibre which inturn stimulates the spiral ganglion & in turn the cochlear nerve which carries AP’s to the cochlear nuclei in the pons
What is the route of hearing AP’s from the first cochlear nuclei in the pons up to the primary auditory cortex ?
- Cochlear nerve from either R or L ear synapses onto the ventral cochlear nuclei on the same side i.e. R cochlear nerve synapses onto the R ventral cochlear nuclei
- The axons sent to the dorsal cochlear nuclei
- Some fibres then decussate (==> input above the level of the cochlear nuclei is bilateral)
- Fibres then travel up both R and L sides to the sup. olivary nucleus
- Then to the fibres now called the lateral leminiscus travel to the inf. colliculus ==> then to medial genticulate body
- Then eventually reaching the sup. temporal gyri (primary auditory cortex)
What are the sup. olivary nucleus and nucleus of nucleus of lateral leminiscus important for ?
Sound localisation & as relays for the stapedial & tensor tympani reflexes e.g. close eyes & hears a sound, is then able to differentiate which direction the sound came from
Where is the primary auditory cortex located ?
On the sup. temporal gyrus in the temporal lobe in the lateral fissure
Where do fibres carrying low frequency sound & high freq sound end up in the auditory cortex?
- Fibres carrying info regarding low freq sound end in the anterolateral part of the auditory cortex
- Fibres carrying info regarding high freq sound end in the posteromedial part of the auditory cortex
Define aphasia
Difficulty/ inability to use language/ speech
Describe how broca/ motor/ expressive aphasia occurs & also how wernickes/ sensory/ receptive aphasia occurs
- Damage to Broca’s area: Patient’s have difficulty in producing language, often using few words and only saying the most important words in a sentence. They do not usually have difficulty comprehending language. Termed: Broca’s, motor or expressive aphasia.
- Damage to Wernicke’s area: Patient’s have difficulty comprehending language. These patients can manifest defects ranging from words out of order to meaningless words.Termed: Wernicke’s, sensory or receptive aphasia
What does the maintenance of equilibrium use info from to do so ?
- Vision
- Proprioception
- The vestibular apparatus (labyrinth)
Go over the structure of the inner ear
Which part of the vestibulocochlear nerve is involved in balance/ equilibrium ?
Vestibular nerve
Describe the vestibular pathway
The vestibular nerve has many different nuclei it projects to such as the thalamus, nuclei of CN’s III, IV & VI, to the cerebellum & spinal cord
Projection of vestibular info onto the cerebral cortex is not well-defined (no primary vestibular cortex) it projects onto different areas such as area of parietal cortex just posterior to the postcentral gyrus, also area just anterior to the primiary auditory cortex & posterior insular cortex
Go over the anatomy of the eye
Describe the visual pathway
- Vision is generated by photoreceptors in the retina
- This info leaves the eye through the optic nerves, the fibres carrying info from the temporal visual field decussate at the optic chaism
- After the chiasm the axons are called the optic tracts which go to the lateral genticulate nucleus where axons synapse (this is at the midbrain where where the optic nerve connects)
- Note that some fibres are sent to the superior colliculi, this nucleus then has Output to: Nuclei of CN III, IV and VI, motor nucleus of VII, and spinal cord
- From here axons from LGN fan out as the optic radiations & travel to the primary visual cortex at the back of the brain (occipital lobe)
Go over visual field defects
What is the location of the primary visual cortex ?
Located in & around either side of the calcarine sulculus in the occipital lobe
Describe the representation of the visual field on the visual cortex
- The lower visual field is projected to the gyrus superior to the calcarine sulculus (these fibres are from the upper retinal quadrant)
- The upper visual field is projected to the gyrus inferior to the calcarine sulculus (these fibres are from the lower retinal quadrant)
Note that since the lens of the eye inverts all images ==> the lower half of the retina sees the upper half of the world
Describe the route of the optic radiations (including meyers loop)
- Fibres of the geniculocalcarine tract initially form part of the internal capsule. Those carrying visual information from the upper half of the visual field first loop anteriorly around the temporal part of the lateral ventricle in Meyer’s loop ending below the calcarine sulcus.
- Fibres carrying info from the lower hald of the visual field pass through the parietal lobe after the internal capsule
Where does the macula project onto the visual cortex?
The posterior pole of the visual cortex
Describe the control of eye movements
The visual cortex controls movements in response to visual stimuli e.g. tracking moving objects
The frontal eyefields (located in the frontal lobe as seen in the pic) control ‘‘movements of command’’ that is movements that are independent of moving stimuli
Tracking movements are controlled by the visual cortex (in occipital lobe) these movements tend to be smooth, whereas movements of command tend to be ‘‘jumpy’’ (saccadic) e.g. track your finger R to L its smooth, and then look from R to L without tracking something (its jumpy)
Describe the pupillary light reflex
Shine light in to R eye, this is sensed, some fibres of the optic tract divert off to the pretectal area & then pretectal fibres project bilaterally to edinger-westphal nucleus & bilaterally stimulate the oculomotor nerves ==> constriction in pupil of R eye (direct light reflex) constriction of L eye (one not shown into) called the consensual light reflex
Describe the accommodation light reflex
Information from the light on each retina is taken to the occipital lobe via the optic nerve and optic radiation (after a synapse in the lateral geniculate body of the posterior thalamus), where it is interpreted as vision.
Interprets accommodation, and sends signals via the Edinger-Westphal nucleus and the 3rd cranial nerve to the ciliary muscle, the medial rectus muscle and (via parasympathetic fibres) the sphincter pupillae muscle
Main difference is in accommodation the info is sent to the visual cortex and then back but in pupillary light reflex as explained it does not
What are the 3 main types of white matter tracts in the brain and what do they do ?
- Association fibres: connect cortical sites lying in the same hemisphere.
- Commissural fibres: connect one hemisphere to the other, usually connecting areas with similar function.
- Projection fibres:connect hemispheres to deeper structures including thalamus, corpus striatum, brain stem and spinal cord.
In the pic; association fibres (blue), commissural fibres (green), projection fibres (red)
