Thalamus

Question 1

The Diencephalon

Answer 1

Majority of sensory, motor and limbic pathways relay in the diencephalon

Includes 4 major parts:

1. Epithalamus
2. Subthalamus
3. Hypothalamus
4. Thalamus

Question 2

The epithalamus

Answer 2

Includes the Habenula and the pineal gland

Habenula
Pathway for the limbic system to influence brainstem reticular formation

Pineal gland
Endocrine organ
Releases Melatonin
Codes photoperiodism
Inhibited by light
Useful marker in CT scans 
Tumors associated with early puberty

Question 3

Subthalamus

Answer 3

Subthalamic nucleus
Major role in indirect pathway of basal ganglia-cortex modulation

Zona incerta
Rostral continuation of the midbrain reticular formation

Question 4

Thalamus

Answer 4

ALL SENSORY PATHWAYS relay in thalamus
– Exception of olfaction

A portal to the cortex, not just a passive relay
Decisions about what should reach the cortex
10X as many fibers from cortex as from subcortical afferents

Also cerebellar, basal ganglia and limbic pathways involve thalamus

31 nuclei comprise thalamus
Most separated by internal medullary lamina
Thin, curved sheet of myelinated fibers

Separated into three divisions
Anterior
Medial (or “dorsomedial”)
Lateral (Dorsal, Ventral)

Question 5

2 directions information flow (thalamus)

Answer 5

Sending ahead to cortex
Eg. Spinothalamic
THALAMOCORTICAL RELAYS

Regulates if information leaves thalamus
Attention, consciousness
THALAMIC RETICULAR NUCLEI

Question 6

Anterior nucleus

Answer 6

Group of nuclei in the “anterior” division
Afferent input from mammillothalamic tract
Originates in mammillary body
Efferent info to cingulate gyrus
Limbic system info

Question 7

Dorsomedial nucleus = DM

Answer 7

Association nucleus
Gate info between cortical areas
Afferents = Prefrontal cortex, olfactory and limbic structures
Efferents = Prefrontral cortex

Question 8

Laterodorsal nucleus = LD

Answer 8

Association
Similar in function and connection to anterior nucleus
Afferent = hippocampus
Efferent = cingulate gyrus

Question 9

Lateral posterior (LP)/pulvinar

Answer 9

Association nuclei
Pulvinar-LP complex
Largest nucleus
Connected reciprocally with parieto-occipital-temporal association cortex
*Vision

Question 10

Ventral anterior (VA), ventral lateral (VL)

Answer 10

Relay nuclei of the striato-cortical circuit (basal ganglia)

Grouped together, but
VA mostly basal ganglia
VL mostly cerebellum

Question 11

Ventral Posterior Lateral and Medial

Answer 11

(VPL and VPM)

The main processor and relay nuclei for somatosensory input
Somatotopic

VPL
Afferent = medial lemniscus, spinothalamic (trunk and extremities)
Efferent = Somatosensory cortex

VPM
Afferent = trigeminothalamic (head region)
Efferent = Somatosensory cortex

Question 12

clinical point with VPL and VPM

Answer 12

Modality segregation
Proprioceptive (anterior), Tactile (middle) and Nociceptive (posterior)
Vascular accidents in the thalamogeniculate arteries
Major damage to VPL/VPM, thalamic pain
Central pain from otherwise painless tactile stimulus
Extensive damage also cause ataxia and tactile insensitivity. Together with thalamic pain = “thalamic syndrome”
Occurs contralateral to lesion

Question 13

Medial geniculate nucleus (MGN)

Answer 13

Major auditory relay

Tonotopic

Question 14

Lateral geniculate nucleus

Answer 14

Major visual relay
Retinotopic maps
Optic nerve—LGN—1° Visual cortex
Optic nerve fibers post-chiasm = “optic tract”

Question 15

Intralaminar nuclei

Answer 15

Nonspecific nuclei (also paleospinothalamic pathway)
Afferents = brainstem, cortex
Efferents = cortex, basal ganglia
Landmark nuclei:
-- Centromedian 
Efferent to putamen and motor cortex

– Parafascicular
Efferent to caudate and prefrontal cortex

Question 16

Thalamic reticular nuclei = TRN

Answer 16

External medullary lamina is thin sheet of fibers covering lateral surface of thalamus
Between this and internal capsule = TRN
NO CEREBRAL PROJECTION
GABA-ergic projections back to the thalamus
Thalamocortical projections pass through TRN
TRN inhibit specific thalamic neurons, modulating ascending flow
*Mechanism for thalamic gating

Question 17

Tonic state of neurons

Answer 17

state = normal depolarization of thalamic neuron = Action potential chain

Question 18

burst state of neurons

Answer 18

Burst state = thalamic neuron is hyperpolarized, sensory info blocked

due to control by TRN neurons

Question 19

tonic vs burst state

Answer 19

TRN neurons control tonic/burst state of thalamic neurons
Attention = no TRN inhibition, high thalamocortical activity
Sleep = high TRN inhibition, low thalamocortical activity

Question 20

EEG

Answer 20

is a summary of action potentials in an area.

Each large amplitude deflection in “slow wave sleep” is a “slow wave.” Main EEG aspect of slow wave, restorative sleep. Body is devoting its resources to metabolic regeneration, esp. synapses (requires a lot of energy). When thalamocortical neurons are quiet we are good at regenerating.

In wake/ rem sleep, low amplitude EEG (TRNs not as active). REM sleep interesting– behaviorally still, but the brain looks like it’s awake. It’s when there are vivid dreams.

Question 21

types of waves on EEG

Answer 21

Alpha waves = quiet wakefulness
Beta waves = awake, high activity
Theta waves = light sleep
Delta waves = deep sleep

Question 22

Thalamocortical relays and TRN functions regulate behavior state

Answer 22

NE, HA, 5-HT, Ach— all from ascending reticular activating system neurons. Project to layer 6 in cortex which joins it in telling the nRt to shut up. When we go to sleep, this circuit gets shut off so that the thalamic reticular neurons act and tell thalamocortical neurons to stop getting through to the cortex anymore.