Lecture 14: The Thalamus

Thalamic Neurons Toggle

Between open (tonic) and closed (bursting) modes

Internal Capsule

Carries almost all traffic to/from cortex

Blood supply to these structures

clinically critical (stroke risk)

Mid-Sagittal View of the Thalamus

- Sits atop midbrain

- Part of diencephalon

- Gateway to the cortex

Divisions of the Thalamus

- Internal medullary lamina splits the thalamus into 3 zones

--> Anterior nucleus

--> Medial group

--> Lateral group

Anterior Nucleus

Part of circuit of Papez = hippocampus --> mammillothalamic tract --> anterior nucleus --> cingulate cortex

Medial Group

Includes dorsomedial nucleus, relays amygdala input to prefrontal cortex, adding emotional context to decision making

Lateral Group

Most of the thalamus

Coronal View Divisions: Reticular Thalamic Nucleus

- Regulatory input, not a relay nucleus

- Helps gate what makes it to conscious awareness

Thalamic Nuclei

- VPM: face somatosensation (trigeminal)

- VPL: body somatosensation

- MGN: auditory

- LGN: visual

- VA/VL: motor (input from cerebellum and basal ganglia --> motor cortex)

Thalamic Connections

- Relay nuclei

- Association nuclei

- Intralaminar nuclei

Relay nuclei

send specific information to cortex (VPL, MGN, LGN, VA/VL)

Association nuclei

Interconnect cortical association areas (DM, LP-Pulvinar)

Intralaminar nuclei

- Poorly understood; involved in limbic and basal ganglia circuits

- Thought to relay limbic and basal ganglia signals

- Arousal, attention, pain

Association Nuclei

- Dorsomedial Nucleus (DM)

- Lateral-Posterior Pulvinar (LP)

Dorsomedial Nucleus (DM)

Prefrontal cortex (emotion, planning)

Lateral-Posterior Pulvinar (LP)

Links parietal, occipital, and temporal cortices (visual attention/integration)

Regulatory Inputs

- Thalamic reticular nucelus

- Cerebral cortex

- Brainstem reticular formation (cholinergic, dopaminergic, serotonergic systems)

--> Modulate thalamic neurons -- putting them in open or closed states

Tonic State (Open Gate)

- Neuron near -55mV

- Fires steadily

- Passes sensory input accurately

Burst Mode (Closed Gate)

- Hyperpolarized neuron uses T type Ca2+ channels to burst

- Not good for accurate transmission

- Great for detecting change

TTX Experiment

TTX blocks Na+ channels --> isolated slow Ca2+ waves show burst activity

Cortical Projections

- Relay nuclei send precise input to primary sensory or motor cortices

- Association nuclei interconnect association areas (POT and PFC)

What is the Internal Capsule

- Massive fiver highway for cortical input/output

- Carries: Corticospinal, corticobulbar, corticopontine, thalamocortical fibers

Corona Radiata

- Fan out of these fibers into cortex

--> Stroke Danger: damage --> widespread motor/sensory loss

Blood Supply

- Thalamus mostly from posterior cerebral artery deep branches

- Internap capsule mainly from lenticulostriate arteries (LSA) and anterior choroidal artery

Blood Supply Clinical Connection

Stoke in these arteries --> classic thalamic syndrome

Thalamic Syndrome

- Results from PCA or LSA infarct affecting VLP/VPM and internal capsule

- Thalamic pain

- Loss of discriminative touch/proprioception --> sensory ataxia

- Mild motor deficits (corticospinal tract in internal capsule)

Thalamic Pain

Triggered by normal stimuli, diffusely affects contralateral body