Special Senses

Question 1

Equilibrium

Answer 1

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

Question 2

Special senses involved in Equilibrium

Answer 2

Proprioceptive, Visual & Vestibular system

Question 3

Proprioceptive systems

Answer 3

Joint, muscle, & other somatosensory information

Question 4

Visual systems
(receptors)

Answer 4

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)

Question 5

Visual systems
(rods and cones)

Answer 5

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

Question 6

Where does visual information leave the eye?

Answer 6

Via the optic nerve

Question 7

Overview of Geniculostriate pathway for visual information

Answer 7

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

Question 8

Overview of Tectal pathway for visual information

Answer 8

▪ to superior colliculus
▪ visual tracking & some reflexes

Question 9

Overview of Hypothalamic pathway for visual information

Answer 9

helps entrain circadian rhythms (body clock)

Question 10

Geniculostriate pathway Summary

Answer 10

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

Question 11

Geniculostriate pathway
1st order neurons

Answer 11

▪ 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

Question 12

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

Answer 12

➢ 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)

Question 13

Geniculostriate pathway
2nd order neurons from LGN

Answer 13

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

Question 14

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

Answer 14

➢ 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

Question 15

Geniculostriate pathway
3rd order neurons (& beyond)

Answer 15

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

Question 16

Ventral “what” pathway

Answer 16

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

Question 17

Dorsal “where” pathway

Answer 17

▪ 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

Question 18

Lesions of the visual pathways
Hemianopia

Answer 18

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

Question 19

Lesions of the visual pathways
Homonymous hemianopia

Answer 19

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

Question 20

Lesions of the visual pathways
Homonymous quadrantanopia

Answer 20

▪ 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

Question 21

Lesions of the visual pathways
Macular sparing

Answer 21

▪ 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

Question 22

Tectal pathway
1st order neurons

Answer 22

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

Question 23

Tectal pathway
2st order neurons

Answer 23

▪ 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

Question 24

Tectal pathway
Synapse in superior colliculus

Answer 24

➢ 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

Question 25

Hypothalamic pathway

Answer 25

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

Question 26

Vestibular System

Answer 26

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

Question 27

Vestibular System Overview

Answer 27

Semicircular canals/ducts ▪ measure angular acceleration (rotation) of the head ▪ attached to vestibule Vestibule ▪ contains utricle & saccule ➢ otolithic organs ▪ measure linear acceleration

Question 28

Semicircular ducts

Answer 28

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

Question 29

Hair cells

Answer 29

➢ 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

Question 30

Utricle & Saccule

Answer 30

▪ 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

Question 31

Vestibular nuclei

Answer 31

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

Question 32

Project to 4 vestibular nuclei:

Answer 32

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

Question 33

Efferents of vestibular nuclei

Answer 33

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

Question 34

Efferents travel on

Answer 34

➢ 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)

Question 35

Vestibuloocular reflex (VOR)

Answer 35

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

Question 36

Nystagmus

Answer 36

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

Question 37

Control of eye muscles

Answer 37

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

Question 38

Six extraocular muscles

Answer 38

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

Question 39

Upper motor neurons

Answer 39

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

Question 40

Basal ganglia

Answer 40

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

Question 41

Cerebellum

Answer 41

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

Question 42

Lower motor neurons

Answer 42

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

Question 43

Medial longitundinal fasciculus (MLF)

Answer 43

▪ 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

Question 44

Auditory pathway

Answer 44

Sense sound information

Question 45

Auditory pathway Basic Anatomy

Answer 45

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

Question 46

Middle ear

Answer 46

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

Question 47

Inner ear

Answer 47

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

Question 48

Auditory projection pathway

Answer 48

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)

Question 49

Olfaction

Answer 49

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

Question 50

Gustation

Answer 50

▪ 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