lecture 26: the diencephalon & internal capsule

what is the diencephalon?

a region of the brain lying b/w the mesencephalon & telencephalon which bridges everything to the cortex

what does the diencephalon consist of?

* epithalamus (pineal gland)
* thalamus
* sub-thalamus
* hypothalamus

what are the major nuclei of the thalamus?

* LGN
* MGN
* dorsomedial nucleus
* anterior nucleus
* pulvinar nucleus
* ventrolateral nucleus
* reticular nucleus

what are the 4 main stream of the thalamic nuclei?

* prefrontal (fronts-limbic)
* somatosensory/motor
* visual
* auditory

*also theory of reticulothalamic (consciousness)*

what is the main role of the thalamus?

it is our **body’s information relay station**. all info from our body’s senses (except olfaction) must be processed through the thalamus before being sent to the cerebral cortex for interpretation. the thalamus also plays a role in s**leep, consciousness, learning, and memory.**

T/F: all projections from the thalamus are excitatory

true; send info → cortex

which 2 re-entrant cortical pathways is the thalamus a hub for, and where do the axons for these pathways run through?

* thalamocortical and corticothalamic
* axons run through the internal capsule

what parts of the diencephalon are involved in reward and learning?

reward = BG

learning = cerebellum

* all mediated through thalamus

T/F: BG and cerebellum are spontaneously active as excitatory

false

BG = spontaneously active as INHIBITORY

cerebellum = spontaneously active as EXCITATORY

how do the BG and cortex interact w the thalamus?

cortex modulates firing rate of BG to facilitate/disfacilitate cortical acitvity

* when BG are MORE active, this SUPPRESSES the thalamus, thus SUPPRESSING cortical activity

*BG influences thalamus which influences cortex*

how do the cerebellum and cortex interact w the thalamus? 

MORE cerebellar activity INCREASES stimulus to thalamus which INCREASES stimulus for movement and movement planning (thru thalamus)

* cerebellar activity is also modulated by proprioceptive info from cortex

Thalamic neurons send axons to the cortex via …..

the internal capsule

T/F: as a general rule, the axons of the each internal capsule carry info to the cortex abt the contralateral side of the body

true

T/F: as the thalamus projects → cortex, info FROM cortex → rest of brain will rerun through thalamus

false; info will cortex → rest of brain will BYPASS thalamus

*thalamus only relays info going UP cortex, not down.*

what are 2 important descending cortical tracts?

cortex → hindbrain motor nuclei (corticobulbar)

cortex → spinal motor nuclei (corticospinal)

what does the prefrontal (pronto-limbic) division consist of?

multimodal assoc nuclei

* dorsomedial
* lateral posterior
* pulvinar

assoc. cortex(pulvinar) also responsible for visual info

*logical analysis, judgement, language, imagination*

what does the somatosensory/motor division consist of?

premotor

* BG & premotor cortex
* ventral anterior, lateral, and posterolateral nuclei
* involved in planning; heavily-BG influenced

primary motor

* motor cortex & cerebellum
* pre-central gyrus
* current movement, accuracy, time, and execution

somatosensory

* spinal sensory (VPL-body) & trigeminal sensory (VPM-face)
* touch/pain/vibration on body mediated by VPL & VPM
* ventral posterolateral & medial posterolateral

what does the visual division consist of?

* visual cortex
* LGN
* relays visual info → thalamus → cortex

what does the auditory division consist of?

auditory cortex (MGN)

* relays auditorial info → temporal lobes

how does the reticulothalamic nucleus operate?

operates in consciousness by **inhibiting particular areas of thalamus**

* inhibition promotes __**rhythmic bursting**__ for deep sleep

what are the main parts of the internal capsule?

* anterior limb
* genu
* posterior limb
* rentrolenticular limb
* visual & auditory radiation

what is the anterior limb of the internal capsule’s role with surrounding structures?

* Motor fibres from the cortex pass through the anterior limb
* It separates the caudate nucleus and putamen from the thalamus
* Lesions in this area can cause contralateral hemiparesis or hemiplegia
* It also contains fibers for sensory and auditory functions
* The anterior limb is part of the corticobulbar and corticospinal tracts
* It plays a role in movement, emotion, and cognition.

what is the genu of the internal capsule’s role with surrounding structures? 

* responsible for relaying sensory information from the thalamus → primary somatosensory cortex'
* info to/from face
* cranial somatosensory connection w VPN
* cortex → cranial motor nuclei via corticobulbar

what is the posterior limb of the internal capsule’s role with surrounding structures? 

* motor & somatosensory signals from brain to spine and cerebellum for arm, trunk, and leg movement
* body somatosensory connections (V posterolateral)
* touch & proprioception
* info from cortex → spine & cerebellum

Damage to this area can result in motor deficits and sensory loss, and is commonly seen in conditions such as stroke.

what is the visual & auditory radiation of the internal capsule’s role with surrounding structures? 

* LGN = reciprocal connections w visual cortex
* MGN = reciprocal connections w auditory cortex

how do the BG & thalamus get their blood supply?

circle of willis

how is the circle of willis at risk of losing blood supply anatomically ?

areas where sharp bends in circle of willis are where seizures occur; the stroke/aneurism is caused by blockages in these sharp bends (thus losing part of blood supply to thalamus & internal capsule)

what symptoms can be seen in lesions to blood supply to the BG, thalamus, and internal capsule?

* visual defects
* homonymous hemianopia
* auditory defects
* bi/unilateral auditory deficits
* balance defects
* motor defects
* hemiplegia/paralysis
* sensory defects
* loss of sensation/numbness/tingling
* memory defects