Thalamus: location, relations, parts, nuclei and main connections

The Diencephalon: General Overview

Diencephalon is part of the prosencephalon (forebrain).

It corresponds largely to the structures that develop lateral to the third ventricles.

The lateral walls of the diencephalon form the epithalamus most superiorly,

the thalamus (dorsal thalamus) centrally and

the subthalamus (Ventral Thalamus) and 
the hypothalamus  most inferiorly.

The epithalamus:

anterior and posterior paraventricular nuclei,

the medial and lateral habenular nuclei,

the stria medullaris thalami and

the pineal gland

The thalamus undergoes proliferation to form numerous nuclear masses, which have extensive reciprocal connections with the cerebral cortex.

The subthalamic region consists of

the subthalamic nucleus,

zona incerta and

the fields of Forel

The subthalamic nucleus is closely related to the basal nuclei.

The hypothalamic rudiment gives rise to most of the subdivisions of the adult hypothalamus.

The Thalamus

(This note describes the location, relation, parts, nuclei and important connections of thalamus.)

·   

    Thalamus is an ovoid nuclear mass about 4 cm long  and is found in relation to lateral wall of third ventricle.

 

     External Features and Relations: 

      It presents:

 

    Two ends or Poles  

      Anterior and posterior

    Four surfaces 

      Medial, lateral, superior and inferior

  

    Anterior end or Pole:

 
 It forms posterior boundary of interventricular foramen.

    

    Posterior end or pole:  

      

    It is called Pulvinar.  

      

    

     

   Pulvinar extends beyond the third ventricle and overhangs the superior colliculus (Superior colliculus is a small elevation found on each side on the posterior aspect of midbrain). Thus, pulvinar lies just above and lateral to superior colliculus.
   

  Superior brachium (a ridge extending from superior colliculus) separates the pulvinar above from the medial and lateral geniculate bodies below. 

    Superior surface:

     This surface is covered by a layer of white matter called stratum zonale.

     

    The medial part of this surface is separated from the overlying body of the fornix by the choroid fissure with the tela choroidea within it. 

   More laterally it forms part of the floor of the lateral
   ventricle. 

  The lateral border of the superior surface of the thalamus is
  marked by the stria terminalis and overlying thalamostriate vein,
  which separate the thalamus from the body of the caudate nucleus.

Laterally, a slender sheet of white matter, the external medullary lamina, separates the main body of the thalamus from the reticular nucleus.

  Lateral to this, the thick posterior limb of the internal capsule lies between the thalamus and the lentiform complex.

   
     Medial Surface:This surface forms the upper part of lateral wall

  of 3rd ventricle and is lined by ependyma.

      

  This surface is usually connected to the opposite thalamus by an interthalamic adhesion/connection. 

  Inferiorly it is related to the hypothalamus separated by an
  indstinct hypothalamic sulcus – this sulucus extends from the
  upper end of cerebral aqueduct posteriorly to the
  interventricular foramen anteriorly.

   

  At the junction of medial and superior surfaces, the ependyma of third ventricle is reflected (from the lateral wall) to the roof. The line of reflection is marked by a line called taenia thalami, underlying which, there is a narrow bundle of fibers called stria medullaris thalami. (Note: Stria medullares in the floor of 4th ventricle are different from stria medullaris thalami)

Image from Gray's Anatomy.

     Inferior Surface:
This surfacfe is related to the hypothalamus anteriorly and subthalamus posteriorly. The subthalamus separates thalamus from the midbrain tegmentum.

Lateral Surface:
This surface is covered by a layer of myelinated fibers called external medullary lamina.
The lamina separates lateral surface with the reticular nuclei.

 
Subdivisions of Thalamus
Internally, the thalamus is divided by a vertical Y-shaped sheet of white matter, the internal medullary lamina into

•      Anterior nuclear groups,
•      Medial nuclear groups and
•      Lateral nuclear groups

 

     In addition,

•   Intralaminar nuclei lie embedded within, and surrounded by, the internal medullary lamina.
•   Midline nuclei either abut the ependyma of the lateral walls of the third ventricle medially, or lie adjacent to, and to some extent within, the interthalamic adhesion. 
•  Reticular nuclei lie lateral to the main nuclear mass, separated from it by the external medullary lamina. (Note: reticular nuclei are now considered as the part of the subthalamus or ventral thalamus)
•   Medial and lateral geniculated bodies, previously considered as Metathalamus, are now included as part of the Thalamus.

Table: Functional division of major thalamic nuclei

Type	Nucleus
Sensory	Ventral posterolateral Ventral posteromedial Lateral geniculate Medial geniculate
Motor	Ventral anterior Ventral lateral
Limbic	Anterior Dorsomedial
Multimodal	Lateral dorsal (dorsolateral) Lateral posterior (posterolateral) Pulvinar

  

       

      ANTERIOR GROUP OF THALAMIC NUCLEI and THEIR CONNECTIONS
 

      These nuclei lies between the limbs of Y.

      Three subdivisions are recognized.

•      Anteroventral nucleus,
•      Anteromedial and
•      Anterodorsal nuclei

    They are collectively called as Anterior Nucleus

Important Connections:
Afferents:
- receives fibers from the mamillary body through mamillothalamic tract.

Effecrents:
- send fibers mainly from medial surface of the hemisphere, - the areas are mainly anterior part of limbic system (anterior and in front of corpus callosum), cingualte gyrus, parahippocampal gyrus
- the connections are reciprocal

Functions:
- the regulation of alertness and attention, emotional tone and the acquisition of recent memory

       

     MEDIAL GROUP OF THALAMIC NUCLEI and THEIR CONNECTIONS

•The single and largest component of this group of nuclei is Medial dorsal (mediodorsal or dorsomedial)       nucleus.
•lies between intralaminar nuclei laterally and midline nuclei medially

subdivided into:  
•anteromedial magnocellular part and
•posterolateral parvocellular part

Main connection:
•Reciprocal connections to prefrontal cortex, also to anterior cingulated and medial frontal gyri
•Receives olfactory inputs from pyriform and adjacent cortex, ventral pallidum  (part of globus pallidus) and amygdale
•Interconnected with all other thalamic nuclei

Functions:
•involved in olfactory information, in controlling one’s emotional and subjective state
•Damage may lead to a decrease in anxiety, tension, aggression or obsessive thinking (reflecting the defects similar to damage to prefrontal cortex)

      

   LATERAL GROUP OF THALAMIC NUCLEI (The lateral nuclear complex)

 
•The largest major division of thalamus
•are lateral to the internal medullary lamina,
•divided into dorsal and ventral tiers of nuclei.

The Dorsal tier of Nuclei (from before backwards)
•    Lateral dorsal (or dorsolateral)
•    Lateral posterior
•    Pulvinar

Important Connections

 
•Lateral dorsal and lateral posterior nuclei mainly receive afferents from superior colliculus.
•Efferents from these two nuclei reach parietal lobe and part of prahippocampus and cingulated gyri
•Lateral posterior nucleus has reciprocal connection with superior parietal lobule
•The Pulvinar also receives afferents from superior colliculus.
•The inferior part of pulvinar also receives direct fibers from the retina.
•The efferents are projected to association areas in parieto-temporal cortex and visual and visual association areas in the occipital and posterior part of temporal cortex
•Also connects with part of cingulated gyrus, hippocampal gyrus and prefrontal cortex.

Functions:

  The complexity of functions of the association areas to which it projects beside visual areas, particularly in the temporal lobe (e.g. perception, cognition and memory), suggest the role of the pulvinar in modulating these functions.

The Ventral Tier of Nuclei (from before backwards)

•       Ventral anterior
•       Ventral lateral and
•       Ventral posterior
•       Ventral posterior medial or ventral posteromedial
•       Ventral posterior lateral or ventral posterolateral

Ventral anterior Nucleus
 Also called ventral anterior (VA) complex
  consists of a principal part (VApc) and a magnocellular part (VAmc)

Main Connections:
Afferents
•Internal part of globus pallidus – to VApc
•pars reticulate of substantia niagra – to VAmc
•premotor cortex (area 6) – to VApc
•frontal eye field (area 8) – to VAmc

Efferents
•Motor areas of Frontal cortex
•Anterior part of parietal lobe
•Intralaminar thalamic nuclei

Function:
•Unclear, may influence motor cortex

Ventral lateral (VL) nucleus
•Consists of anterior division or pars oralis (VLo) and posterior division or pars caudalis (VLc)

Connections:

 
Afferents 
  •From internal part of globus pallidus (ipsilateral) – to VLo
  •From deep cerebellar nuclei (contralateral) – to VLc
  •From precentral motor areas (area 4 and 6)
  •Spinothalamic tract and vestibular nuclei

Efferents
  •To supplementary motor cortex and premotor cortex (from VLo) and to primary motor area (from VLc)

Function:

   •Probably influences the motor activity

  •Stereotactic surgery of ventral lateral nucleus is sometimes     used in the treatment of essential tremor.

Ventral posterior (VP) nucleus

•  is the principal thalamic relay for the somatosensory  pathways
•   consists of two major divisions
•   ventral posterior lateral or ventral posterolateral (VPl)
•   ventral posterior medial or ventral posteromedial (VPm)

Connections:

 
Afferents

 
•from medial leminscal and spinothalamic pathways  to VPl
•from trigeminothalamic and solitariothalamic (gustatory) pathways to VPm

Topographic representation – sacral laterally and cervical medially on VPl and face on VPm adjacent to cervical area of representation on VPl
Taste fibres most anteriorly and ventromedially within the VPl

Efferents

 
•Passes through posterior limb of internal capsule as Superior thalamic radiation
•To the primary somatosensory area (area 3,2,1)
•Also to secondary somatosensory area located at the lower end of postcentral gyrus in the parietal     operculum (or superior lip o posterior ramus of lateral sulcus)

 

     

     Medial  and Lateral Geniculate bodies
•Small rounded elevations situated below the posterior part of the thalamus

Medial Geniculate body
•relay station for auditory pathway
•consists of medial, ventral and dorsal nuclei

Connections
Afferents
•receives fibres from the lateral laminiscus (part of the auditory pathway) directly or after relay in the inferior colliculus via the inferior brachium
•also receives fibers from the auditory area of cerebral cortex

 
Efferents
•To primary auditory cortex (from ventral nucleus) via the acoustic radiation which passes through the sublentiform part of internal capsule 

Tonotopic representation of different frequencies of sound in the ventral nucleus of medial geniculate body – low to high frequencies from lateral to medial part of the nucleus.

•To auditory areas (from other nucleus) surrounding the primary auditory area

Lateral geniculate body or nucleus
•Relay station for visual pathway

Connections;
Afferents
•From both retinae
•Also receives fibers from the primary visual cortex, secondary visual cortex, superior colliculus and reticular formation of pons and medulla

Efferents
•Optic radiation that passes through the retrolentiform part of internal capsule to primary visual areas
•May project to surrounding cortex also

Intralaminar nuclei
•These are the neurons within the inermedullary lamina
•Consists of anterior and posterior subgroups – centromedian nucleus is part of posterior subgroups

Connections:
Afferents
•From the body through spinothalamic tracts
•Also receives fibers from the reticular formation, cerebellar nuclei and the substantia niagra
•Centromedian nucleus receives fibers from globus pallidus

Efferents
•Those from the anterior group are diffuse and reach the many parts of the cerebral cortex
•Those from the posterior group project to the motor, premotor and supplement motor areas as well as striatum

Functions
•Precise functional role is unclear
•May mediate cortical activation from the brain stem reticular formation (thus may influence the levels of consciousness and alertness in an individual), and play a part in sensorimotor integration

 
Midline nuclei
•Consists of several small group of neurons

Connections:
Afferents
•from the hypothalamus, the periaqueductal grey matter of the midbrain, the spinothalamic tract and the medullary and pontine reticular formation
•from ascending noradrenergic and serotoninergic axons from the locus coeruleus and raphe nuclei respectively (from brainstem)
•also receive a cholinergic input from the midbrain.

Efferents
•To the hippocampal formation, the amygdala and the nucleus accumbens.
•Also to cingulate, and possibly orbitofrontal cortex

Functions
•Closely related to limbic system and may play a role in memory and arousal

Reticular nuclei (part of the ventral thalamus or subthalamus)
•Made up of a thin layer of neurons
•Covers the lateral aspect of thalamus separated by extramedullay lamina
•Laterally it is related to internal capsule


Connections
Afferents
•Collaterals from thalamocoritcal, corticothalamic, thalamostriatal and pallidothalamic fibers
•Also receives cholinergic fibers from reticular formation of midbrain
•Thus reticular nucleus receives somatic, visual and auditory impulses

Efferents
•To the body of the thalamus (mostly reciprocal)


REFERENCES: 
     Gray’s Anatomy, 39th Edition 
     Snell’s Clinical Neuroanatomy 7th Edition 
     Lange Clinical Neuroanatomy 25th Edition