Flashcards in Lesson 8: Cranial Nerves I-IV, VI Deck (69):
Where is smell recieved in the primary olfactory neuron? How is it transduced?
-Smell is received by receptors on the primary olfactory neuron (embedded in the olfactory epithelium lining of the nose) and transduced from a chemical to an electrical signal
Olfactory system: Once an electrical signal is created, where is it received?
The electrical signal is then received by mitral cells, the secondary neurons situated in the olfactory bulb
In the Olfactory bulb, where does information travel? Terminate?
Where does it cross?
Information travels along the olfactory tract, terminating in the temporal lobes bilaterally. It either crosses in the medial olfactory stria (within the anterior commissure) or remains uncrossed traveling in the lateral olfactory stria to end in the primary olfactory cortex on the medial aspect of both temporal lobe
What nerves are involved in the visual system?
Oculomotor, trochlear, and abducens nerves
What systems are involved in the visual system?
Vestibular system, medial longitudinal fasciculus (MLF coordinates eye movements by connecting nuclei for motor signals to muscle controlling eye movements), reticular formation, and spino-tectal and tectospinal info
Is the optic nerve a peripheral nerve?
What is it myelinated by?
Where is the optic nerve sensory receptor?
Where do the neurons from the nasal retina travel? Where do they synapse?
Its sensory receptor is the retina. The neurons from the nasal retina cross in the optic chiasm, joining those uncrossed fibers from the temporal retina to form the optic tract and synapse next in the lateral geniculate body
In the lateral geniculate body of the optic system, where do they terminate?
The lateral geniculate body is the collection of cell bodies for the secondary neurons that terminate in the occipital lobe
Where are objects from the left side of the body going? (temporal visual field, nasal visual field, nasal retina, temporal retina)
Objects on the left side of our body are in the left eye’s temporal visual field and the right eye’s nasal visual field which strikes the left eye’s nasal retina and the right eye’s temporal retina respectively
Where does info from the inner retina cross?
What is the resulting image of light projected into the lens?
The resulting image is both inverted AND reversed by the time it reaches the retina.
When information passes from the left nasal visual field, what are the structures involved up to the right lateral geniculate body?
Information from the left nasal visual field stimulates the left temporal retina, then travels in the left optic nerve, to the left optic tract to synapse in the left lateral geniculate body, while that from the left temporal visual field stimulates the left nasal retina, also traveling in the left optic nerve but crosses at the optic chiasm to the right optic tract to synapse in the right lateral geniculate body.
From the lateral geniculate body, where does light information travel up to the occipital lobe?
LGB: a portion of the fibers swing forward briefly in Meyer’s Loop which is the first part of the optic radiation, then terminate in the primary occipital cortex above and below the calcarine sulcus in the occipital lobe, then terminate in the primary occipital cortex above and below the calcarine sulcus in the occipital lobe.
How is light transduced in the retina?
Light is transduced to electrical energy by the retina
Which structures are for voluntary eye muscles/involuntary pupil constriction?
Oculomotor nucleus is for voluntary eye muscles
Edinger Westphal Nucleus is for involuntary (parasympathetic) pupil constriction
How can bitemporal hemianopia or tunnel vision be caused?
If there is a tumour in the pituitary gland or within the cavernous sinus, this may cause compression on the optic chiasm, where this information crossing occurs.
Macular Sparing: Where is the macula?
If there is a lesion of the posterior cerebral artery, what happens to the macula?
The centre of the retina is represented at the “pole” of the occipital lobe
Therefore, there may be “macular sparing” by the middle cerebral artery despite visual loss due to a lesion of the posterior cerebral artery.
What is Area 17, 18 and 19?
Area 17 (above and below the calcarine sulcus on the medial surface of the occipital lobe) is the primary visual area.
Area 18 is the secondary visual cortex
Area 19 the association cortex
What is the difference between 'blindness' and 'cortical blindness'?
Damages to optic tract causes “blindness”. Damage to the tracts AFTER synapsing in the lateral geniculate body cause “cortical blindness” where only the cortical cells are damaged, but the remainder of the tracts intact.
How is information passed from area 17-19?
What is the result of lesions in area 18 and 19?
- From the primary visual cortex (area 17) information is passed to the secondary visual cortex (area 18) and then visual association are (Area 19).
- Lesions in areas 18 and 19 interfere with the meaning of an object producing visual agnosia, and prosopagnosia.
What would prevent the pupillary light reflex from working?
The oculomotor nerve outer surface is easily damaged and fails to conduct the pupil response to light, i.e., does not get the instruction for constrictor pupillae muscles to constrict the pupil.
What is the pupillary light reflex?
Why is it so important?
This reflex is the pupillary response to light – i.e., pupil constriction
Important because the parasympathetic part (the outer part) of the oculomotor nerve is very vulnerable
Does the pupillary light reflex synapse in the lateral geniculate body?
No. It goes direclty to the midbrain reflex centre where it synapses in the pretectal nucleus on BOTH sides
After synapsing on the pretectal nucleus, where does the pupillary light reflex travel and synapse?
Travels on the outer part of BOTH oculomotor nerves. It then synapses a third time in the ciliary ganglion and terminates in constrictor pupillae (involuntary) muscle on both side.
In the pupillary light reflex, what happens if there is damage in the optic nerve?
There will be NO response in either eye.
What is the first step in the pupillary light reflex?
Does an individual need to be conscious for this test?
1. Light is shone in one eye (left)
Light signal follows the usual light pathway, BUT for the reflex (as shown), the signal goes directly (without synapsing) to the precentral nucleus in the midbrain
Individual can be unconscious for the test
What is the second step in the pupillary light reflex?
2. The information (light stimulus) synapses in the pretectal nucleus bilaterally and then in the Edinger Westphal nucleus (the parasympathetic nucleus portion of the Oculomotor nucleus) bilaterally. It then travels in the parasympathetic fibers along the oculomotor nerve.
In the pupillary light reflex, does the pupil constriction happen in both eyes?
Response occurs in BOTH eyes.
First the pathway from the nasal retina crosses and secondly, there is extensive crossing within the nuclei in the brainstem.
What two nerves are being tested in the pupillary light reflex?
You are testing one optic and two oculomotor nerves because the signal goes “in on CN II, out on CN III”
If the left eye is being tested in the pupillary light reflex, what is constriction of the pupil considered?
What is right pupil constriction considered?
Constriction of the left pupil is described as “presence of a direct response” and right pupil constriction is described as “presence of an indirect response”.
In the Accommodation Reflex, what is the three-part response?
Does it require conscious or unconscious input?
1. Accommodation of the lens
2. Constriction of the pupil
3. Convergence of the eye (turning them inward to focus on the object)
Requires conscious input
Why does the accommodation reflex require conscious input?
Signal must travel to the occipital cortex (conscious level) and then to the Frontal Eyefieldbefore going to the brainstem. Therefore, it synapses in the lateral geniculate body
In the accommodation reflex, where else do the parasympathetic fibres synapse and terminate?
Where do the voluntary fibres synapse?
Synapse in the Edinger-Westphal nucleus and synapse again in the ciliary ganglion and terminate in the constrictor pupillae muscle AND the ciliary muscle
Voluntary fibres synapse in the Oculomotor nucleus in the midbrain and travel to the orbit to stimulate left and right medial rectus muscles
What happens in accommodation of the lens/reflex?
Accommodation of the lens: (lens changing shape) requires active contraction of the ciliary muscle to allow the lens to thicken; Accommodation reflex: (near reflex) requires the interplay of lens accommodation, pupillary constriction and convergence of the eyes to focus on near objects
Oculomotor Nerve: what does it do?
• Is responsible for innervation of medial rectus (eye inward), superior rectus (eye upward), inferior rectus (eye downward), and inferior oblique (eye up and out) muscles.
Its voluntary motor function innervates most of the eye muscles, is involved in the pupillary light reflex.
What is unique about the trochlear nerve?
What does it do?
Is the only cranial nerve to leave the posterior surface of the brainstem
It travels around to the anterior surface of the brainstem where the other cranial nerves exit to supply the superior oblique muscles that directs the eye down and out.
What does the abducens nerve do?
Supplies the lateral rectus muscle that directs the eye outward
Which nerves are involved in eye movement?
Oculomotor, trochlear, and abducens nerves.
Which centres are involved in eye movement? (2)
Coordinated through two centers: the medial longitudinal fasciculus (joining the brainstem nuclei for these nerves) and the Pontine Paramedian Reticular Formation a coordination center within the pons and medulla.
What are the symptoms of a lesion of the oculomotor nerve?
Dilated pupil (unopposed action of the sympathetic dilator pupillae)
Eye directed outward (loss of inward turning medial rectus
Unopposed strong lateral rectus directs eye outward)
Diplopia (double vision)
What are the symptoms of a lesion of the trochlear nerve?
Difficulty looking down toward feet. This is complicated – just remember: “difficulty getting two eyes to both look downward together properly so diplopia looking downward.”
Complains of difficulty going down stairs
What are the symptoms of a lesion of the abducens nerve?
The eye directed inward (loss of lateral rectus to direct eye out unopposed intact medial rectus turns eye inward)
What is the function of the medial longitudinal fasciculus?
Bundle of fibres connecting the motor nuclei for CNs III, IV and VI bilaterally, with connections to gaze centers and the vestibular system
Which muscle raises the eyelid? What types of muscle does it have?
Muscle– skeletal (voluntary – innervated by the oculomotor nerve) and smooth (involuntary, innervated by the sympathetic nerves).
Damage to the smooth muscle = Horner's
What type of innervation is the involuntary innervation of the eyelid?
What is the result of injury to the oculomotor nerve?
Damages the voluntary muscles it innervates, including the eyelid, making the eyelid droop a little but you will also see loss of some eye movements and a dilated pupil
What does the frontal eye field, Area 8 do?
What happens with stimulation of the frontal eye field?
Coordinates eye movement (gaze) and must be intact for the accommodation reflex
Stimulation of the frontal eye field causes decimation of the eyes away from the side been stimulated.
With a lesion in Area 8, what will it interfere with? What is involved in testing this area?
Because it involves the conscious level (cerebral cortex), the patient needs to be conscious for this test. It Area 8 is damaged (e.g. stroke), the eyes remain deviated toward the side of the lesion (unopposed, therefore the intact opposite side takes over)
What structures are involved in the cortical olfactory area?
On the basomedial surface of the cerebral hemisphere, includes the uncus, the periamygdaloid nucleus, anterior hippocampal gyrus, and parts of the temporal lobe
What is this lesion: interrupts the olfactory fibres or the primary olfactory cells, in witch the ability to smell is partially or fully impaired; also loss of taste
What are the two important characteristics of the central visual mechanism?
1. a point-to-point representation of the visual field from the retinae through the lateral geniculate body to the primary visual cortex
2. the projection from each eye to both cerebral hemispheres (binocular processing)
What is the central visual pathway?
Includes the pathway from the retina to the primary visual cortex, which is located on the midsagittal surface of the occipital lobe
Where do light rays from the top/bottom of object strike on the retina?
Light rays from the top of the object strike the lower retina, and rays from the bottom of the object strike the upper retina
What are the two rules that account for the partial crossing of fibres at the chiasm?
1. Fibers from the nasal halves of the retina cross the midline to project to the opposite visual cortex
2. Fibers from the temporal half of each retina remain uncrossed and project to the ipsilateral visual cortex
What are the two divisions of the primary visual cortex?
- Lower (inferior) lip of cortex receives projections from the lower portion of the retina (upper quadrant of visual field).
- Upper (superior) lip of cortex receives projections from the upper retina (lower quadrant of the visual field)
What is Horner's Syndrome?
Consensual response without a direct pupil response – lesion in efferent projections from Edinger-Westphal nucleus to the same eye. Interruption of the sympathetic nerve fibers causes paralysis of the dilator fibers of the iris and results in permanently constricted pupillary diameter = miosis
When the lens is rounded, what type of vision does it permit?
How is far vision permitted?
- With no pulls from ligaments, the lens, because of its inherent elasticity, assumes a rounded form, thus acquiring greater refractive power. (Close up vision)
- Relaxed state of ciliary muscles exerts tension on the suspensory ligaments that flatten the lens by pulling it. This reduces refractive power, permitting far vision
What does heteronymous mean?
Two different parts of the visual field being impaired, for example, the left half of the visual field for one eye and the right half of the visual field for the other = bitemporal hemianopia
What type of lesion is this: - Complete severing of the optic nerve at any point between eyeball and optic chiasm results in total blindness – none of optic nerve fibers from retina are spared
What type of vision loss is this: Loss of vision in the temporal visual field. Associated with pathology of the optic chiasm – interrupts fibres from both nasal retinas. Produces blindness in the temporal visual fields for both eyes = tunnel vision
Bitemporal (heteronymous) hemianopia?
What type of vision loss is this: Loss of vision in the nasal field of only one eye. Associated pathology encroaches on lateral edge of optic chiasm and selectively interrupts the fibers from the ipsilateral temporal portion of the retina. Result is hemianopia in corresponding eye
What type of vision loss is this: Loss of vision in homonymous – either left or right fields for both eyes. Interruption of fibers at any point in the course of the optic tract, LBG, or geniculocalcarine fibers.
What type of vision loss is this: Loss of vision in the superior left quadrants of the visual fields for both eyes?
Homonymous Left Superior Quandrantanopsia
What type of vision loss is this: vision loss in the left lower visual field quadrants for both eyes?
Homonymous Left Inferior Quandrantanopsia
What is the result of a lesion in the entire optic nerve?
Complete blindness in one eye
What occurs as a result of a lesion that affects the nucleus and/or oculomotor nerve?
Weakness/paralysis of three recti, inferior oblique, and palpebrae superioris muscles.
What is this oculomotor nerve dysfunction: paralysis of extrinsic ocular muscles, the affected eye deviates to the lateral side. Eye deviates laterally and ventrally because of unopposed action of the intact superior oblique (trochlear nerve) and lateral rectus muscles (abducens nerve). Failure to direct both eyes toward an object (strabismus) in the direction opposite to the paralyzed side results in double vision
What is this oculomotor nerve dysfunction: interruption of the parasympathetic projections to the constrictors of the iris, the pupil is permanently dilated because of unopposed activity of the dilator pupilae with sympathetic innervation