Thalamus

Introduction

The thalamus sits on top of hypothalamus, close to the Substantia niagra.

posterior to thalamus – Mammilary Bodies

not shown in the diagram are Intralaminar nuclei (largest is centromedian nucleus)

Dorsal Medial nuclei: these are damaged in Korsikoff psychosis: caused by EtOH, results in no long term memory (>1min) and confabulation.
Projects to prefrontal cortex (involved in executive function, social skills, affection)

Overview of Nuclei

Specific Relay Nuclei

Anterior nuclear group

Principal Afferent Inputs: Mammilary body of hypothalamus

Major projection sites: Cingulate gyrus

Function: Limbic, emotion, visceral

Ventral anterior group

Principal Afferent Inputs: Globus pallidus and Substantia niagra

Major projection sites: Premotor cortex -area 6

Function: Initiate motor; resting tone

Ventral Lateral

Principal Afferent Inputs: Same as VA + Dentate nucleus of cerebellum via brachium conjunctivum and Motor and Premotor Cortex

Major projection sites: Motor and Premotor Cortex: Area 6

Function: Coordination of motor activity

Ventral Posterolateral

Principal Afferent Inputs: DCML and Spinothalamic pathways

Major projection sites: Somatic sensory parietal cortex

Function: Somatic Sensation of body

Ventral Posteromedial

Principal Afferent Inputs: Principle and Spinal sensory nuclei of V

Major projection sites: Somatic sensory parietal cortex

Function: Somatic sensation of face and Jaw proprioception

Medial Geniculate

Principal Afferent Inputs: Inferior Colliculus via brachium of Inf Coll

Major projection sites: Auditory Cortex Ipsolateral

Function: Hearing

Lateral Geniculate

Principal Afferent Inputs: Retinal ganglion cells through optic nerve and optic tract

Major projection sites: Visual Cortex (area 17)

Function: Vision

Association Nuclei

Pulvinar

Principal Afferent Inputs: Superior colliculus, temporal, parietal occipital association area and primary visual cortex

Major projection sites: Temporal parietal and occipital association area

Function: Integration of sensory information

Lateral Dorsal

Principal Afferent Inputs: Frontal limbic association cortex including the Cingulate Gyrus

Major projection sites: Frontal limbic association cortex including Cingulate Gyrus

Function: Emotional expression

Lateral Posterior

Principal Afferent Inputs: Parietal lobe

Major projection sites: Parietal lobe

Function: Integration of Sensory information

Medial Dorsal

Principal Afferent Inputs: Amygdaloid nuclear complex, olfactory cortex and hypothalamus

Major projection sites: Prefrontal association cortex or anterior frontal lobe

Function: Emotional expression normal memory function

Diffuse Projection nuclei

Reticular nucleus

Principal Afferent Inputs: Ipsolateral Cerebral cortex, thalamus, brainstem

Major projection sites: Ipsolateral thalamic nuclei

Function: Inhibitory thalamic modulation

Intralaminar nuclei

Principal Afferent Inputs: Reticular formation, spinothalamic tract, trigemino thalamic tract, globus pallidus

Major projection sites: Widely to cortex and basal ganglia

Function: Arousal

Midline nuclei

Principal Afferent Inputs: Reticular formation, hypothalamus

Major projection sites: Basal forebrain

Function: Emotional Arousal

Centromedial nucleus

Principal Afferent Inputs: Globus pallidus

Major projection sites: Caudate and Putamen

Function: Motor control

Sensory Nuclei

VPL

Pain and temp info from body processed in dorsal horn Þ decussates Þ asscends in contralateral spinothalamic tract to VPL

Touch and vibration are carried by large fibers in fasiculis gracilus (legs) and cuneatus (arms) to respective nuclei in caudal medulla Þ decussates and ascends in the Medial lemniscus and joins the spinal thalamic tract in the midbrain on its way to VPL.

VPL projects to sensory cortex – has humunculus with leg closest to midline sagital sulcus and face closest to Sylvian fissure (which is receiving the projections from VPM).

VPM

Pain and temp in Trigeminal system descends in Spinal tract of V (analogous to and continuous with Lissauer’s tract), processed at various levels in the spinal nucleus of V (analagous to and continuous with nucleus proprious) which descends into upper cervical region. So people with high cervical spinal cord injuries can lose pain/temp on the face. secondary neurons decusate and ascend in Trigeminothalamic tract located close to spinothalamic tract
Larger fine touch and vibration fibers are processed in the principle nucleus of V.

Proprioceptive information about the jaw is processed in the Mesencephalic nucleus of V, which ascends into midbrain. These later two nuclei project bilaterally close to medial lemniscus.
All of these fibers project to VPM. Topology of these fibers from caudal to rostral is dorsal to ventral face
VPM projects to sensory cortex near Sylvian fissure.

Note: Lateral lemniscus carries auditory info to the medial geniculate and runs lateral to the medial lemniscus.

Motor Nuclei

VL (cerebrocerebellum pathway)

Fibers from the dentate nucleus of cerebellum project via the superior cerebellar peduncles where they decussate in the lower midbrain (brachium conjunctiva). Some fibers synapse in red nucleus, some bypass and go to VL.
VL projects to motor and premotor cortex which plans motor function by telling motor where limb is, what is load, etc.
Cortex feeds back to motor system via 2 tracts:

(1) corticopontine (very large) – synapse on small cells of pontine nuclei that decussate and project via Middle cerebellar peduncle back to cerebellum
(2) lateral corticospinal tract.

These fibers travel through the anterior limb of the internal capsule. There is also feedback to VL.

Along with VL, recieves projections from substantia niagra, projecting to motor and premotor cortex. This loop initiates movement. Therefore in Parkinson’s (or toxin MPTP), there is slowness of movement due to loss of stimulation from VA and VL.
caudate nucleus and putamen project to globus pallidus which projects to VL, VA and Centromedian nucleus via 2 paths:

(1) Fasiculus lenticularis passes straight through the internal capsule
(2) Ansa lenticularis goes around the internal capsule

These paths are inhibitory to VA and VL (opposite of Parkinson’s, surgical lesions in them balances effects of disease)
Huntington’s Chorrea: degeneration of the caudate nucleus resulting in a loss of inhibition leading to hyper kinesis

centromedian nucleus projects back to Caudate and Putamen. Short feedback loop

Limbic System Loop – Papez’s circuit

Anterior Thalimic Nucleus Þ projects to the cingulate gyrus (lies above corpus callosum) Þ projects to the parahipocampal gyrus Þ projects to adjacent hippocampus Þ projects to the fornix which loops around the anterior commissure Þ mainly projects to the mamilary bodies Þ projects to the anterior thalamic nucleus completing the loop via the Mamillothalamic tract.

Wernicke’s encephalopathy is caused by thiamine defficiency in alchoholics due to damage to the mamilary bodies. Any disruption of the above loop results in marked short term memory loss. (Other structures also affect Þ impared eye movement). Can be treated by thiamine.

Reticular formation (important in sleep/wake)

projections to non-reticular nuclei (VL, VA, VPL, VPM, medial and lateral geniculate) are concentrated in layer IV of cortex

projections of reticular formation to cortex is to many layers and are not concentrated in a single zone.

Diffuse projection Nuclei

Intralaminar nuclei:

input from reticular formation, visual, auditory, spinal thalamic, trigeminal tract. All the sensory pathways. A single neuron can receive projections from many different pathways.
project widely to cortex and basal ganglion

Midline nuclei:

input from reticular formation and thalamus. Project to basal forebrain.

Centromedian nucleus is included in this group but mainly is involved in motor function (See above)