Functional neuroanatomy of the neurological examination ...

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Functional

neuroanatomy of

the neurological

examination:

Cranial nerves

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Chris ThomsonBVSc(Hons), Dip ACVIM (Neurol), Dip ECVN, PhD

[email protected]

VTH, IVABS, Massey University

Palmerston North, New Zealand

mailto:[email protected]

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The Neuro Exam

• Aim to assess:

1) Mentation/arousal and

behaviour

2) Posture and gait

Sensory function – proprioception,

tactile

Motor function – gait, spinal reflexes

Coordination

Balance

4) Cranial nerves

5) Visceral function

6) spinal pain-hyperpathia

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Fig 10.1 Thomson and Hahn

Dog brain, ventral aspect, cranial

nerves

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General principles

– Sensory, motor, or mixed

– Parasympathetic – CNN III, VII, IX, X

– Sensory links• reflex function

• sensory reception – somatosensory cortex, cerebellum

• Sensory nucleus – trigeminal sensory complex

– Nuclear arrangement in brainstem• Ξ to fragmented spinal cord columns

– Only one CN that is pure CNS

– Attachment mainly ventral/ventrolateral • except ???

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Cranial nerve Brain attachment Function

sensory, parasympathetic, motor

I Olfactory Telencephalon Olfaction

II Optic Diencephalon Vision

III Oculomotor Mesencephalon Pupil constriction, extraocular muscles (which

ones?)

IV Trochlear Mesencephalon Extraocular muscles (which ones?)

V Trigeminal Pons/myelencephalon Facial sensation, masticatory muscles (which

ones?)

VI Abducens Myelencephalon Extraocular muscles

VII Facial Pons/myelencephalon Muscles of facial expression

Salivary, lacrimal glands, taste, masticatory

muscle (which one?)

VIII Vestibulocochlear Pons/myelencephalon Hearing, balance

IX Glossopharyngeal Myelencephalon Swallowing, salivary glands, taste

X Vagus Myelencephalon Taste, swallowing, laryngeal, salivary glands,

parasympathetic to body viscera

XI Accessory Myelencephalon Laryngeal function, neck muscles

XII Hypoglossal Myelencephalon Tongue muscles

Cranial nerves, attachment and main functions

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Fig 10.2 Thomson and Hahn,

Functional CNN nuclear columns in the brainstem

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Vision –

CN II (p92)

• Optic nerve– Visible CNS

• Optic chiasm– Variable degree of cross over

• Herbivores 80-90%

• Cats 65%

• Inversely related to stereoscopic vision


Optic pathway and binocular vision

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Vision

• Pathway (cat)

– 80% fibres

-> Lateral geniculate nucleus

-> Optic radiation

-> Visual cortex

– 20% fibres to the midbrain

• Cerebral cortex – midbrain connections

– Required for

• Perception of movement

• Spatial orientation

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Visual

Reflexes

• Rostral colliculus

– Tectonuclear (bulbar) – extraocular

muscles

– Tectospinal – cervical muscles

– Function???


Optic pathway and its connections

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Pupillary light

reflex


Pupillary light reflex pathway

Consensual reflex

affected by degree of

cross over

e.g. human versus

horse

Swinging light test

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Menace Response

• CN II, CN VII

• Menace deficit in cerebellar disease

– Mechanism?• Pathway? – visual cortex,

cerebellum, facial nucleus

• Cerebellar influence on cortex permitting the response?

• Ipsilateral cerebellar and menace deficit


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Somatosensory input

from head

Fig 10.6 Thomson and Hahn, trigeminal sensory complex


C7 spinal cord, TS

Afferents: CNN V, VII, IX, X

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Vestibular System

• Proprioceptors

– Hair cells

– Location • Membranous labyrinth

inner ear/petrous temporal bone

– Function to maintain posture

• Head, neck, trunk, limbs, eyes

• During rest and motion

• Anti-gravity function


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How do hair cells function in

head equilibrium?

• Deflection of microvilli – Stimulates sensory nerve endings of

vestibular portion CN VIII

• Static head equilibrium– Detection by hair cells in sac structures

• Saccule – sagittal/vertical plane

• Utriculus – dorsal/horizontal planes

– Detect effect of gravity; constant tonic discharge

• Dynamic head equilibrium– Detection by hair cells in semi-circular

canals

– Detect effect of acceleration in 3 planes

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Effect of gravity on macula of sacculus or utriculus

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Fig 8.3 Thomson and Hahn, effect of acceleration on SCD

– Head rotation

• Causes endolymph flow in 1+ ducts

• Deflects cupula -> bending microvilli

• Stimulating or inhibiting sensory nerve ending

• Microvilli deflected

– Towards kinocilium stimulates nerve endings

– Away from kinocilium, inhibits neural discharge

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Fig 8.4 Thomson and Hahn– SC ducts are bilaterally paired

• e.g. Left anterior-right posterior

– Example: Turning head to left

-> flow of endolymph in opposite directions in paired ducts

• Stimulating from left side and inhibitory from right side

• Uneven neural input to each side of brain; head rotation is perceived

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What are the effects of vestibular nuclei stimulation?

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Vestibular nuclei connections


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Physiological Nystagmus

• Vestibulo-ocular reflex (VOR)

– Mechanism

• Slow phase – vestibular origin

• Fast phase – extraocular muscle stretching,

brainstem reflex activity

– Function

• Visual fixation during head rotation

• Example

– L head rotation -> R CN IV and L CN III

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Vestibular Function and Eyeball

Position

• Pathway– Vestibular apparatus -> VN -> MLF -> CN III, IV, VI

• Head and eyeball position coordinated

• Predator vs prey species

• Vestibular dysfunction– Strabismus

• Afferent vs efferent dysfunction

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Effect of vestibular lesions; uneven stimulation of VN at rest,

with decreased stimulation on side of the lesion

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Paradoxical

Vestibular

Disease

• Signs– Head tilt to opposite side

– Nystagmus (fast phase) to side of lesion

– Ipsilateral ataxia

and proprioceptive deficits

• Lesion location– Area of caudal cerebellar peduncle

Fig A21 Thomson and Hahn

TS through cerebellar

peduncles

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Paradoxical Vestibular Disease


Loss of inhibitory output to vestibular nuclei; XS stimulation

on side of the lesion

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Hearing – CN VIII • Conscious hearing

– auditory cortex, temporal lobe

• Reflex function– Muscles of the middle ear

• CN V to tensor tympanii and CN VII to stapedius mm.

– (t for trigeminal, s for seven)

• Muscle contraction affects compliance of tympanum (tympanometry)

– Caudal colliculus• Head/eye turning in response to auditory stimuli

• Tectonuclear (bulbar) – extraocular muscles

• Tectospinal – cervical muscles

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Cochlear

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Auditory pathway in the brain


Brainstem auditory evoked reflex

I spiral ganglia, CN VIII

II cochlear nuclei

III-V???

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A curious fact about CN VIII

• It’s only an afferent nerve

– right?

• Olivocochlear reflex

– Protective

– Discriminative –

neutralises background

noise http://upsidedowndogs.com

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Other CNN nuclei (VII, IX, X, XI)

Solitary tract and nucleus – sensory input: taste, carotid sinus, thoracic and

abdominal viscera (p101)

•Parasympathetic nucleus of VII and IX (Salivatory n.) – efferent to salivary glands

•Parasympathetic nucleus of X – Visceral efferent to thoracic and abdominal viscera

•Nucleus ambiguus – somatic efferent to larynx and pharynx


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Innervation of the pharynx and

larynx

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Autonomic innervation of the head

• Parasympathetic = craniosacral

– Therefore CNN III, VII, IX, X

– Functions?

• Sympathetic = thoracolumbar outflow

– Therefore not via CNN, but via sympathetic

fibres from the cranial thorax

– Function?

– Dysfunction?

• See next section

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• Olfaction – CN I – Conscious perception

• Olfactory cortex of piriform lobe

• Connections with

– limbic system, hypothalamus, cerebrum

– olfaction stimulates memory, emotion and

behaviour

– Brainstem – olfaction stimulates autonomic

function

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Dog brains, XS

and horizontal

sections, MRI

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Comparative anatomy CN I, dog and human brain

Dogs 100,000-1million time more sensitive than humans

Blood hounds 10-100million times more

(Wikipedia)

Functional neuroanatomy of the neurological examination ...

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