Prosencephalon Localization

Question 1

what is the prosencephalon?

Answer 1

contains the cerebrum (telencephalon) and the thalamus (diencephalon); is also called the forebrain

Question 2

what are the 5 functions of the prosencephalon?

Answer 2

1. behavior
2. consciousness/wakefulness
3. fine motor activity (less important for animals)
4. conscious perception of vision, hearing, touch, nociception, temperature, and proprioception
5. SOME gait generation, but its role in this is less important in veterinary species

Question 3

are there any true reflexes involved with the prosencephalon?

Answer 3

nope, just conscious, learned responses

Question 4

what is the path of anything projecting into or out of the prosencephalon?

Answer 4

crosses the midline!! either at the optic chiasm or at the midbrain

Question 5

where do seizures ALWAYS localize to, regardless of etiology?

Answer 5

prosencephalon!

Question 6

describe mentation with a prosencephalon lesion

Answer 6

can be normal OR abnormal!

but if it is abnormal, you know the source of the abnormal mentation is intracranial, so either prosencephalon or ARAS in brainstem

Question 7

how is conciousness maintained?

Answer 7

sensory input detected by ARAS and transmitted to diffuse cerebral cortex stimulation (medulla to pons to thalamus to internal capsule to corona radiata to cerebral cortex)

Question 8

what are the 2 types of abnormal mentation?

Answer 8

1. abnormal actual mentation: decreased consciousness (too sleepy)
2. abnormal/inappropriate behavior: head press, propulsive/compulsive, get stuck in corners or furniture, loss of learned behaviors

Question 9

what are the 4 ways to describe abnormal mentation and which implicate ARAS involvement?

Answer 9

1. dull/obtunded/depressed/lethargic
2. inappropriate
3. semicoma: ARAS
4. coma: ARAS

Question 10

describe gait with a prosencephalon lesion (4)

Answer 10

1. UMN from cerebrum have little input on gait generation in quadrupeds so gait is normal!! no weakness or ataxia
2. may be propulsive (inappropriate mentation) or
3. may see circling toward the lesion
4. may also see head and body TURN toward side of lesion because animal does not know the side contralateral to the lesion (side controlled by lesion side) exists

Question 11

describe GP deficits with a prosencephalon lesion

Answer 11

contralateral postural reaction deficits WITHOUT weakness; may be a lack of interpretation of where the limb is located (lights are on but no one is home)

Question 12

describe cranial nerve responses with a prosencephalon lesion

Answer 12

responses imply cerebral input!!

menace response: may be absent because optic nerve crosses at chiasm to occipital cortex responsible for awareness of vision (make sure 7 is intact!)

might not have response to nasal mucosal stimulation: trigeminal nerve crosses at midbrain to the contralteral cortex for conscious awareness

Question 13

what is hemi-inattention/hemi-neglect?

Answer 13

1. occurs with prosencephalic lesions but is an uncommon presentation
2. abnormal response to stimuli on contralateral side of body from lesion location because just unaware that the contralateral side exists

Question 14

sum up a left-sided prosencephalic lesion (4)

Answer 14

1. +/- seizures
2. +/- abnormal mentation
3. gait normal (+/- propulsive or circling to the left)
4. all deficits right sided: +/- postural reactions, +/- abnormal menace or nasal mucosa response

not all signs happen all at once in all patients!

Question 15

what might cause a bilateral prosencephalic lesions? (3)

Answer 15

1. metabolic: hypertension, hepatic encephalopathy
2. nutritional: thiamine deficiency
3. toxic

Question 16

describe a bilateral prosencephalic lesion

Answer 16

1. abnormal mentation without lateralizing signs (most common presentation, dull)
2. +/- compulsive walking
3. +/- not walking (if mentation too abnormal)
4. +/- postural reaction deficits in all 4 limbs
5. +/- bilateral loss of menace and response to nasal mucosal stimulation

Question 17

how do you determine a lesion is procencephalic or more caudal?

Answer 17

the largely normal gait and lack of other deficits (like vestibular, cerebellar) clues you in to prosencephalon