Diencephalon - Thalamus

• thalamus stems from the diencephalon (from the prosencephalon/forebrain)
• thalamus - general

  • the largest part of the mammalian diencephalon
  • receives all sensory information first, with the exception of olfaction
  • major relay to the neocortex
• dividing up the thalamus

  • by projections/circuitry
  • relay, association, non-specific nuclei
  • dorsal vs ventral
  • and then reticular nucleus and epithalamus
• thalamic circuitry

  • 4 features that characterize circuitry
  • projection to cerebral cortex
• classifying thalamic nuclei by projections

  • can classify based on axon terminations
  • projecting to single cortical area
  • dorsal lateral geniculate nucleus
  • projecting densely to one area, but diffusely to another
  • lateralis dorsalis (dorsolateral nucleus)
  • projecting diffusely to several cortical areas, but with a concentration in a single field
  • nucleus ventromedialis (ventromedial nucleus)
  • convergence of several motor pathways
  • projecting diffusely over widespread areas
  • suprageniculate nucleus
  • link between limbic and sensory information/structures
• classifying thalamic nuclei

  • can also classify based on cortical laminar termination of thalamic efferents
  • can project to multiple layers
  • classify nuclear groups according to whether or not it has a projection to the striatum and/or amygdala in addition to the cortex
  • divide into specific and non-specific nuclei
  • specific: relay nuclei that terminate in discrete cortical areas
  • nonspecific: project diffusely to cortex and other groups
• specific nuclei (relay)

  • relay nuclei receive very well-defined inputs and project this signal to functionally distinct areas of the cerebral cortex
  • mostly the lateral nuclear group and geniculate group
  • relay nuclei are specific nuclei
  • most specific nuclei are relay nuclei
• thalamus to cortex

  • VPM = ventral posteromedial nucleus
  • VPL = ventral posterolateral nucleus
  • VA = ventral anterior nucleus
  • VL = ventrolateral nucleus
• types of relays

  • first order
  • send messages to cortex about what is going on subcortically
  • higher order
  • receive their driving input from the cortex itself
  • provide a transthalamic relay from one part of the cortex to the other
  • indirect corticocortical connections
  • approximately 1/2 of the thalamic nuclei in primates are higher order nucleus
• association nuclei

  • receive most of their input from the cerebral cortex and project back to the cerebral cortex in the association areas where they appear to regulate activity
  • dorsomedial (mediodorsal) nucleus
  • pulvinar
  • anterior group
• nonspecific nuclei

  • many of the intralaminar nuclei and midline nuclei have diffuse projections to the cortex and have been termed "nonspecific"
  • project broadly to the cortx
  • also project to more 'nonspecific' regions of the cortex
  • these nuclei are probably mostly involved in arousal and alertness
• subdivisions of the thalamus

  • dorsal thalamus
  • largest part of the thalamus and also largest part of the diencephalon by itself
  • major efferents: telencephalic structures
  • mostly neocortex, weak striatal connections
  • relay cells with axons to telencephalon
  • basically, lots of connections back and forth with cortex
  • all connections ipsilateral
  • no known communication between thalamic nuclei directly, except the reticular nucleus
• medullary lamina

  • external medullary lamina: a layer of axons (white matter) that covers the dorsal thalamus on the lateral sides
  • includes fibers coming to and from the cortex
  • the cells of the reticular nucleus reside here
  • internal medullary lamina
  • white matter fibers that subdivide the thalamus into anterior, medial, lateral, midline, and intralaminar nuclear groups
• internal medullary lamina

  • thin layer of nerve fibers that are made up of afferent and efferent projections of some thalamic nuclei
• subdivisions of the thalamus

  • anterior
  • medial
  • lateral
  • midline
  • intralaminar
• anterior thalamus

  • made of:
  • anteroventral nucleus
  • anterodorsal nucleus
  • anteromedial nucleus
  • treated as functional subunits of the same part
  • receive afferents from mammillary bodies via the mammillothalamic tract and from the subiculum via the fornix
  • limbic, memmory
  • project to the cingulate gyrus (pain, unpleasant)
  • functions associated with memory and alertness
  • association nuclei
• The dorsomedial nucleus (mediodorsal nucleus) is an association nucleus with a medial and lateral subdivision
• The lateral part of the dorsomedial nucleus receives projections from the superior colliculus, olfactory cortex, and the ventral pallidum
• Efferent projections from the dorsomedial nucleus go to the frontal eye fields and the anterior cingulate cortex of the frontal lobes
• The dorsomedial nucleus is involved in controlling eye movements and attending to visual stimuli, as well as playing a role in emotional "tone"
• The medial part of the dorsomedial nucleus receives inputs from several brain areas including the solitary nucleus, substantia nigra reticulata, amygdala, and ventral pallidum
• The medial part of the dorsomedial nucleus projects to the limbic area of the cortex, including the insular cortex, orbital frontal cortex, and subcallosal region
• The dorsomedial nucleus is involved in autonomic regulation and emotions
• Damage to the dorsomedial nucleus can impair memory
• lateral nuclear group

  • posterior
  • pulvinar
  • lateral posterior nucleus
  • ventral
  • ventral posterior nucleus
  • ventral anterior nucleus
  • ventral lateral nucleus
• pulvinar nucleus

  • largest of these association nuclei
  • occupies the posterior part of the dorsal tier of the thalamus
  • receives afferent projections from the superior colliculus as well as from the association cortex
  • projects to secondary visual areas and to association areas in the parietotemporal region
  • this contributes to visual perception and eye movements, probably relating to attention to these stimuli
• ventral posterior nucleus

  • principle somatosensory nucleus
  • receives trigeminothalamic input
  • relays to the inferior portion of the postcentral gyrus
  • sense of touch
  • likely to play majore role in central pain syndromes
• ventral lateral nucleus

  • receives input from the cerebellum
  • small input from the basal ganglia to the rostral part of the VL
  • projects to the primary motor area, area 4, of the precentral gyrus and also has a smaller projection to premotor areas
  • thus involved in motor feedback form the cerebellum and basal ganglia to the cerebral cortex
  • along with ventral anterior nucleus
  • coordination, planning, learning of movement
• ventral anterior nucleus

  • receives most of input from the basal ganglia
  • especially the medial globus pallidus and substantia nigra, parts reticulata
  • projects to premotor cortex including the supplementary motor area of the frontal lobes
  • involved in planning and initiating movements
• lateral geniculate nucleus

  • receives retinotopic input via the optic tract from the contralateral visual world
  • projects in a topographic manner to the primary visual cortex via the optic radiations
  • optic radiations from the upper visual world loop through the temporal lobe white matter on the way to the visual cortex (meyer's loop), while optic radiations from the lower visual world pass just deep to the parietal lobe
• medial geniculate nucleus

  • receives tonotopically organized auditory afferents from the inferior colliculus via the brachium of the inferior colliculus
  • projects to the primary auditory cortex on the superior temporal gyrus (transverse gyrus of heschel)
• midline group of thalamus

  • includes:
  • paratenial nucleus
  • paraventricular nucleus
  • reuniens nucleus
  • rhomboidal nucleus
  • "nonspecific" in that they project widely to the cortex and elsewhere
  • may be involved in general functions such as alerting
  • perhaps also more specific functions
  • paraventricular and paratenial nuclei: viscero-limbic functions
  • reuniens and rhomboid nuclei: multimodal sensory processing
  • midline nuclei:
  • limbic system, memory, arousal
  • brainstem reticular formation
  • hypothalamus
  • spinothalamic tract
  • midbrain
  • hippocampal formation
  • amygdala
  • nucleus accumbens / cingulate gyrus
• intralaminar group of thalamus

  • generally non-specific nuclei
  • intralaminar group is made of:
  • anterior (rostral) group
  • paracentral nucleus
  • central lateral nucleus
  • central medial nucleus
  • posterior (caudal) intralaminar group
  • centromedian nucleus
  • parafascicular nucleus
• centromedian nucleus

  • has reciprocal connections with the globus pallidus and with the premotor cortex
  • voluntary movements
  • appears to function as part of the basal ganglia feedback system
• subdivisions of the thalamus

  • dorsal thalamus
  • ventral thalamus
  • much smaller than dorsal thalamus
  • ventral lateral geniculate nucleus (specialized role in visually related movements)
  • subthalamus
  • zona incerta (limbic-motor integration)
  • subthalamic nucleus - receives input from motor areas of the cerebral cortex, projects to the substantia nigra, and is reciprocally connected with the globus pallidus
  • lesion here can lead to hemiballismus - disorder of involuntary, rapid and forceful flailing
  • main part lies on the pathways that link the dorsal thalamus to the telencephalon
• subthalamus

  • lies beneath the thalamus and dorsolateral hypothalamus
  • ventrolateral aspect is against internal capsule
  • contains the subthalamic nucleus and zona incerta (rostral extension of reticular formation)
  • many fibers pass through the subthalamus on the way to the thalamus
  • medial lemniscus, spinothalamic tracts, trigeminothalamic tracts
  • really part of the basal ganglia
• • Strategically placed over the axons being sent between thalamus and cortex
• Reticular nucleus of the thalamus:

  • Surrounds all but the medial and dorsal surface of the thalamus
  • Pierced by fibers of thalamic radiations
  • Homogenous neurotransmitter phenotype - GABA
  • Receives excitatory branches from corticothalamic/thalamocortical axons and sends inhibitory axons to other parts of the thalamus
  • Each circuit sends to and from through the same part of the reticular nucleus
  • Role in adjusting messages based on the attentive state of the whole animals (wake/sleep) and salience (threat/no threat)
  • strategically placed over the axons being sent between thalamus and cortex
• epithalamus

  • pineal gland: contains modified photoreceptor cells (pinealocytes)
  • release melatonin in a circadian rhythm
  • do not respond directly to light in humans
  • respond to circadian differences in sympathetic nervous system activity
  • sleep wake cycle
  • habenular nuclei: receive input from several limbic structures