Thalamus -Physiological Anatomy

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                     Thalamus-Physiological Anatomy

Thalamus is a mass of nuclei concerned with the conscious sensation, important relay station, and reflex center. The size of the thalamus is like a small egg. The thalamus is the most significant part of the diencephalon.

Other diencephalon structures are the thalamus, epithalamus, subthalamus, hypothalamus, metathalamus,  optic nerve, optic chiasma, optic tract.

In lower animals, the thalamus is the highest sensory center.

(Singular- Thalamus,pleural –thalami.)

The position of the thalamus: Each thalamus is medial to the posterior limb of the internal capsule. They are two in number and mirror image to each other. The two thalami constitute the lateral walls of the third ventricle. The upper two-thirds of each thalamus lies on either side of the third ventricle. They are joined across the midline by isthmus-mass intermedia.  The mass intermedia is inconstant in size and sometimes may be missing. The lower one-third portions of the thalami are separated by superior corpora quadrigemina.

Each thalamus has two poles 1.Rostral and 2. Caudal.

The rostral pole starts just behind the plane of the foramen of Monro and the genu of the internal capsule.

The caudal pole, also known as Pulvinar( cushioned seat), overhangs the geniculate bodies and superior colliculi.

Each thalamus has four surfaces: 1.Dorsal 2.Ventral 3.Lateral,and

 4. Medial.

Dorsal: Dorsolaterally –body, and tail of the caudate nucleus and the vena stria terminalis.

Dorsomedially: the floor of the body of the anterior horn of the lateral ventricle. The tele chorodiea covers cover the dorsomedial aspect of the thalamus and bridges the lumen of the third ventricle as its roof. The tele chorodiea consists of 1. an inner lining of the ependyma and 2.an outer layer of pia enclosing blood vessels of the choroid plexus.

Ventral: The thalamic fasciculus separates the thalamus from the zona –incerta of the subthalamus.

Lateral: An extension of the zona incerta known as nucleus reticularis thalami separates the thalamus from the posterior limb of the internal capsule.

Medial: The lumen of the third ventricle. The intra thalamic nuclei connect the two thalami across the third ventricle lumen.

 

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                                         Figure 1 Relation of  the thalamus to Internal capsule

         

 

                  

                                            Figure 2 The Thalamus and the lateral ventricle

 

 

 

 

                  

                                             Figure 3 The capsule of the Thalamus

 

 

            

                                                            Figure 4 Nuclei of the thalamus

 

 

 

The internal medullary lamina is Y-shaped and divides the thalamus into medial, lateral, and anterior nuclear groups. This lamina is composed of white matter in which many nuclei are present. The main is the centromedian nucleus. So the nuclear groups are

1.Anterior nuclear group

2.Median nuclear group

3. Centromedian nucleus

4.Lateral nuclear group which is divided into

          a.Dorsal nuclei or Pulvinar (cushion)

          b.Ventral nuclear group is again divided into

                   ventro anterior, ventrolateral, ventro posterior lateral, and ventro posterior median.

5.Meta thalamus –lateral and medial geniculate body or nuclei.

The thalamic reticular nucleus is not a part of the thalamus but functionally attached to the thalamus.

 Most thalamic nuclei release the excitatory glutamic acid.

The inhibitory neurons are also present in the thalamic reticular nucleus and release gamma-aminobutyric acid, an inhibitory neurotransmitter. They are thalamic interneurons and modulate responses of other thalamic nuclei to incoming input from the cerebral cortex.

An incomplete C-shaped capsule surrounds the thalamus. The myelinated capsule is present medially, ventrally, and laterally. On the dorsum of the thalamus, a thin myelinated sheet, the stratum zonale covers the thalamus.

The external medullary lamina is a myelinated sheet outside the thalamus. The thalamic reticular nucleus is present between the thalamus and the external medullary lamina.