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Neuroanatomy > Special Senses > Flashcards

Special Senses Flashcards

1
Q

Equilibrium

A

Ability to maintain orientation of the body & its parts in relation to external space

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2
Q

Special senses involved in Equilibrium

A

Proprioceptive, Visual & Vestibular system

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3
Q

Proprioceptive systems

A

Joint, muscle, & other somatosensory information

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4
Q

Visual systems
(receptors)

A

retina of the eye
▪ retina is an extension of the diencephalon
➢ thus, optic nerve is really a CNS pathway, not a true nerve
▪ contains several cell types
➢ some sense light (rods/cones)
➢ some begin processing of the visual signal (e.g., lateral & amacrine cells)
▪ e.g., lateral inhibition
➢ others are relay & integration (e.g., bipolar & ganglion cells)

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5
Q

Visual systems
(rods and cones)

A

▪ transduce light energy into electrical energy
▪ rods = night, grayscale vision
▪ cones = daylight, color vision

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6
Q

Where does visual information leave the eye?

A

Via the optic nerve

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7
Q

Overview of Geniculostriate pathway for visual information

A

▪ primary visual pathway; high resolution system
▪ vast majority of axons follow this pathway

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8
Q

Overview of Tectal pathway for visual information

A

▪ to superior colliculus
▪ visual tracking & some reflexes

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9
Q

Overview of Hypothalamic pathway for visual information

A

helps entrain circadian rhythms (body clock)

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10
Q

Geniculostriate pathway Summary

A

Optic nerve → optic chiasm → optic tract → LGN → internal capsule & optic
radiation → primary visual (striate) cortex

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11
Q

Geniculostriate pathway
1st order neurons

A

▪ optic nerve → optic chiasm → optic tract
▪ optic chiasm is the site of hemidecussation
➢ half of visual information from each eye crosses (and half does not)
➢ information from medial retina decussates
▪ i.e., allows almost all information from right visual field to reach left
cerebral cortex

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12
Q

Geniculostriate pathway
Synapse in lateral genicular nucleus of thalamus (LGN)

A

➢ LGN has two components:
▪ parvocellular
* superior layers
 information is still largely segregated
 2 for each eye
* primarily encode color & form (→→ ventral pathway)
▪ magnocellular
* 2 inferior layers
 1 for each eye
* primarily encode movement & contrast (→→dorsal pathway)

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13
Q

Geniculostriate pathway
2nd order neurons from LGN

A

Travel through the internal capsule, then optic radiation
▪ optic radiation splits into superior & inferior pathways
➢ superior pathway carries information from inferior visual field
➢ inferior pathway carries information from superior visual field
▪ Meyer’s loop
* part of inferior pathway of optic radiation
* passes laterally into temporal lobe before turning posteriorly
 i.e., temporal lobe damage may cause a deficit to superior (and contralateral) visual field

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14
Q

Geniculostriate pathway
Synapse in primary visual cortex (area 17) in occipital lobe

A

➢ superior pathway projects to region of cortex superior to calcarine sulcus
▪ i.e., inferior visual field
➢ inferior pathway projects to region of cortex inferior to calcarine sulcus
▪ i.e., superior visual fields
➢ also, foveal inputs reach more caudal areas & peripheral visual fields
progressively more rostral

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15
Q

Geniculostriate pathway
3rd order neurons (& beyond)

A

▪ from the primary visual (area 17) & association visual (areas 18/19) areas
maintain functional distinctions seen in LGN
-Ventral and dorsal pathway

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16
Q

Ventral “what” pathway

A

▪ color & detailed form
* LGN parvocellular → ventral extrastriate cortex →→ temporal lobe
* lesion = color blindness (achromatopsia) & inability to recognize faces
(prosopagnosia

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17
Q

Dorsal “where” pathway

A

▪ motion & location
▪ LGN magnocellular → dorsal extrastriate cortex →→ parietal lobe
* lesion = motion blindness
➢ note: there is a great deal of intercommunication between the two pathways; this is not a clean division

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18
Q

Lesions of the visual pathways
Hemianopia

A

▪ loss of half of visual field
* e.g., right temporal hemianopia = loss of lateral visual field in right eye

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19
Q

Lesions of the visual pathways
Homonymous hemianopia

A

▪ loss of same half of visual field in both eyes
* e.g., right homonymous hemianopia = loss of right visual field in both
eyes

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20
Q

Lesions of the visual pathways
Homonymous quadrantanopia

A

▪ loss of same ¼ of visual field in both eyes
* e.g., right homonymous inferior quadrantanopia = loss of lower right
visual field in both eyes

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21
Q

Lesions of the visual pathways
Macular sparing

A

▪ with almost all lesions of pathway beyond optic tract some macular vision is maintained
 i.e., always some left…
▪ thought to be the result of the massive size of macular representation
▪ also due to overlap of blood supply from middle & posterior cerebral arteries

22
Q

Tectal pathway
1st order neurons

A

▪ optic nerve → optic chiasm → midbrain/brainstem regions
▪ bypass the LGN

23
Q

Tectal pathway
2st order neurons

A

▪ synapse in the pretectal nucleus
➢ which projects bilaterally to the Edinger-Westphal nucleus
▪ in midbrain
➢ controls (via parasympathetic fiber & ciliary ganglion) constriction of the iris
▪ subserves the pupillary light reflex
* bilateral iris contraction to light
▪ or, synapse in superior colliculus

24
Q

Tectal pathway
Synapse in superior colliculus

A

➢ multiple outputs, but two major efferent pathways:
▪ ascending projections to visual pathway (via LGN & pulvinar)
* affects visual tracking & orienting responses; e.g., saccades
 “blindsight”
 basically no ‘pure’ tectal lesions – so these functions are presumed from animal studies
▪ descending projections to cervical muscles
* via the tectospinal tract
* affects muscular reflexes to visual stimuli
 e.g., looming

25
Q

Hypothalamic pathway

A

Subset of light-sensitive ganglion cells (not rods/cones) that reach the suprachiasmatic nucleus in the hypothalamus
▪ help entrain circadian rhythms to light-dark cycle
➢ helps explain jet lag & sleep disturbances in patients with total blindness
▪ without visual cues, internal clocks are ~25.3 hours

26
Q

Vestibular System

A

Helps sense and coordinate head position/movement with posture, balance, &
equilibrium

27
Q

Vestibular System Overview

A

Semicircular canals/ducts
▪ measure angular acceleration (rotation) of the head
▪ attached to vestibule

Vestibule
▪ contains utricle & saccule
➢ otolithic organs
▪ measure linear acceleration

28
Q

Semicircular ducts

A

▪ sense angular acceleration (rotation) - dynamic
▪ 3 ducts – orthogonal to each other
➢ anterior, posterior, & horizontal receptors

29
Q

Hair cells

A

➢ extend from ridge = crista
➢ extend into gelatinous cap = cupula
▪ movement (i.e., rotation in the proper plane) bends hair cells
➢ movement in one direction increases AP frequency
➢ movement in other direction decreases AP frequency
▪ can tell direction of rotation

30
Q

Utricle & Saccule

A

▪ sense linear acceleration (e.g., gravity)
➢ both static & dynamic receptors
▪ macula is found in each
➢ hair cells extending into otolithic membrane
▪ otoliths = calcium carbonate granules
▪ inertia of otoliths pulls on membrane & causes hair cells to bend
➢ in two planes:
▪ utricle = horizontal acceleration
▪ saccule = vertical acceleration

31
Q

Vestibular nuclei

A

Receptors from all regions (semicircular canals, utricle & saccule) synapse in the
vestibular ganglion (partway along the internal auditory canal)

32
Q

Project to 4 vestibular nuclei:

A

▪ inferior, medial, lateral, superior vestibular nuclei
▪ located in the brainstem
➢ combine information from vestibular sensors, cerebellum (flocculonodular),
spinal cord & visual inputs

33
Q

Efferents of vestibular nuclei

A

➢ motor nuclei of extra-ocular muscles
▪ vestibuloocular reflex (VOR)
➢ cerebellum (eye regions)
➢ thalamus → cerebral cortex (conscious awareness of movement & proprioception)

34
Q

Efferents travel on

A

➢ lateral vestibular tract
▪ projects to antigravity muscles in all spinal levels
▪ is the primary pathway for postural changes to body tilts and movement
➢ medial vestibular tract
▪ project to cervical region
▪ stabilizes/coordinates head/eye movements
* via medial longitudinal fasciculus (MLF)

35
Q

Vestibuloocular reflex (VOR)

A

Gaze stays fixed even when head is moving (or being moved)
Extremely fast
▪ faster than visual tracking
➢ shake your book vs shake your head
▪ short reflex; 3 neurons
➢ inputs from CN VIII
➢ outputs control eye movements via CNs III, IV, & VI

36
Q

Nystagmus

A

Rapid eye movements to opposite direction that VOR causes
▪ Physiological (normal)
➢ e.g., when head rotations are too large for VOR compensation
➢ keeps image stable on retina
▪ Pathological
➢ e.g., damage to vestibular system
“Caloric nystagmus”
▪ warm or cold water put into ear induces nystagmus

37
Q

Control of eye muscles

A

▪ Overall, similar to control of other skeletal muscles
➢ upper motor neurons, lower motor neurons, central pattern generators,
influence by basal ganglia & cerebellum

38
Q

Six extraocular muscles

A

Controlled by 3 cranial nerves
1) oculomotor n. (III) – medial, superior, & inferior rectus, and inferior oblique
2) trochlear n. (IV) – superior oblique
3) abducens n. (VI) – lateral rectus

39
Q

Upper motor neurons

A

Frontal eye fields; along with supplementary & parietal eye fields
Initiate saccades
▪ damage to frontal eye field
➢ =inability to voluntarily look to contralateral side
▪ parietal eye fields (part of ‘where’ pathway) & cerebellum (flocculus) are necessary for smooth pursuit

40
Q

Basal ganglia

A

▪ caudate nucleus → SN reticularis → thalamus & superior colliculus
➢ both Parkinson’s & Huntington’s disease may have visual deficits
▪ e.g., involuntary saccades, slowed smooth pursuit

41
Q

Cerebellum

A

▪ also adjust gain on VOR
➢ e.g., getting new glasses
➢ convergence on near or far objects

42
Q

Lower motor neurons

A

in brainstem nuclei for each cranial nerve i.e., oculomotor, trochlear, and abducens nuclei

43
Q

Medial longitundinal fasciculus (MLF)

A

▪ bilateral tracts from upper cervical region to midbrain
➢ ascending fibers
▪ to CNs III, IV, & VI (VOR)
➢ descending fibers (= medial vestibulospinal tract)
▪ coordinate head/eye movements
▪ lesions to MLF (e.g., multiple sclerosis)
➢ deficits to lateral conjugate gaze
▪ both eyes moving at the same time
➢ vestibular nystagmus
➢ difficulty coordinating head & eyes
More on control of eyes in Unit 8 (Brainstem & CNs

44
Q

Auditory pathway

A

Sense sound information

45
Q

Auditory pathway
Basic Anatomy

A

Auricle → external auditory canal → tympanic membrane → ossicles → cochlea

46
Q

Middle ear

A

Sound causes tympanic membrane to vibrate which causes movement of the ossicles
a malleus → incus → stapes
▪ tensor tympani & stapedius mm attach to ossicles
▪ contraction limits ossicle movement
➢ protective reflex
▪ too slow for noises with fast onset (e.g., gunshots)
▪ good for building sounds (e.g., thunder)
▪ primarily dampens vibrations from your own voice

47
Q

Inner ear

A

Stapes movement causes oval window to vibrate fluid inside cochlea within cochlea, the organ of Corti (via basilar & tectorial membranes and hair cells) transduce sound information into electrical information

48
Q

Auditory projection pathway

A

Hair cells
→ spiral ganglia (=bipolar neurons of CN VIII)
→ cochlear nuclei (in medulla)
→ inferior colliculus (bilaterally; some is used for auditory startle reflexes)
→ via lateral lemniscus to thalamus (medial genicular body)
→ temporal lobe (primary & association auditory cortex)

49
Q

Olfaction

A

▪ sense of smell
▪ CN I
➢ only sensory information that bypasses thalamus
▪ reaches orbitotemporal cortex

50
Q

Gustation

A

▪ sense of taste
▪ CNs VII, IX, & X
▪ synapse in nucleus of the solitary tract
▪ reaches thalamus via central tegmental pathway
▪ gustatory cortex in middle insular region

Neuroanatomy (20 decks)

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