2.6 Principles of Forebrain Disease Flashcards
What is the embryologic name of the forebrain?
prosencephalon: made up of the telencephalon (cerebrum) and the diencephalon (thalamic region)
What are the functional regions of the cerebrum?
named after the bones that overlie them:
- olfactory/piriform
- temporal
- occipital
- frontal
- parietal
What is the function of the cerebrum?
- conscious perception and interpretation of sensory information
- cognition and behavior
- learning and memory
- voluntary motor activity
Explain the general anatomy of the cerebrum.
grey vs white matter
there is grey and white matter
- grey is the outermost layer: cerebral cortex and basal nuclei (neuronal cell bodies)
- white matter is the internal layer: axonal tracts
the grey matter basal nuclei have a role in gait and emotional responses (limbic system): BUT in non-primate species (all vet species) gait generation is mostly extra-pyramidal (forebrain disaease causes minimal/no gait abnormalities)
the white matter has three axon types:
1. association axons: associate different cortical regions
2. projection axons: project to different parts of the CNS (e.g., brainstem)
3. commissural axons: cross the midline to connect the two hemispheres
What is the organization of the forebrain tracts?
the forebrain is a contralateral system:
- motor planning and sensory information each processed in the contralateral hemisphere
- decussation occurs in the brainstem
What are the clinical signs of forebrain disease?
- hemi-neglect: contralateral to lesion
- obtunded and WIDE circling due to hemi-neglect (c.f., tight circling seen in vestibular disease): but normal gait usual
- head turn (ears in line but nose turned in): ipsilateral to lesion because hemi-neglecting other side (does not exist)
- postural reaction deficits: contralateral to lesion
- reduced sensation contralateral to lesion
- seizures: generalized or focal +/- head pressing (HE)
- central blindness, absent menace, but intact PLR: many PLR pathways caudal to the chiasm, menace requires conscious input from the forebrain
What is the function of the thalamic region?
- acts as a relay system for integration of sensory and motor pathways: gateway of the forebrain
- maintains consciousness
- autonomic and homeostatic functions
- regulation of sleep/wake
- seasonal reproductive cycles
What is ARAS of the thalamic region?
poorly defined primitive collection of neurons whose general function is arousal: lesions here can cause profound changes to the level of mentation (stupor, coma)
- thalamic region is the most rostral part of ARAS
- part of the reticular formation
- regulation of sleep/wake (decrease in ARAS = sleep)
- seasonal reproduction
What are the clinical signs of thalamic disease?
- stupor/coma
- endocrine abnormalities (e.g., PU/PD)
- altered thermoregulation
- changes in sleep/wake
- altered appetite
- vestibular signs: some vestibular pathways go through the thalamic region - if seen, would be in conjunction with other talamic clinical signs
similar CS to cerebral, but NO seizures
Explain how the PLR and menace response pathways differ, and how the two can help to localize a lesion within the brain.
the optic nerve is not a true nerve: it is an extension of the diencephalon and so pathologies that affect the forebrain can affect the optic nerve: e.g., MUA causing optic neuritis
- vision: contralateral cerebral cortical relay for perception and interpretation of visual information (decussation at the optic chiasm)
- menace response: NOT a reflex; retina to chiasm (decussates) to occipital cortex to motor cortex to pons to cerebellum to CNVII
- PLR: from retina to optic chiasm (decussates) to rostral colliculi (midbrain) to EW nucleus to CNIII (parasympathetic: ciliary ganglion -> pupil constriction)
in general: PLR is INTACT in forebrain and cerebellar lesions, DIMINISHED or ABSENT in brain stem lesions (midbrain- EW nucleus)
menace may be ABSENT in forebrain, cerebellar, and brain stem lesions
take optic chiasm into account for ipsilateral/contralateral deficits
