http://d3jonline.tripod.com/29-Clinical_Neuroscience/Thalamus.htm .2010

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.
  • VA
    • 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)