The thalamus is an egg-shaped structure atop the brain stem that acts as a sensory relay station. It is composed of several discrete nuclei that are classified anatomically and physiologically. Anatomically, the thalamus is divided into anterior, medial, and lateral groups of nuclei. Physiologically, the nuclei are grouped into specific relay nuclei, association nuclei, nonspecific nuclei, and motor nuclei. The thalamus receives ascending sensory inputs and projects them to sensory cortical areas, functioning as a relay for somatic sensations and special senses. It also plays roles in arousal, memory, emotion, motor functions, and sleep-wake cycles.

1. THALAMUS

2. FUNCTIONAL ORGANIZATION
Thalamus is an egg-shaped nuclear structure sitting
obliquely atop the brain stem. It is known as the
sensory relay station.
It receives ascending sensory inputs and projects
them to the sensory cortical areas. There are two
thalami on both sides.

3. Classification of Thalamic nuclei
Anatomical classification.
Anterior.
Lateral.
Ventral-VA, VL, VP, MGB,LGB
Dorsal-LP, LD, PULVINAR
Medial group
medial nucleus
midline.n
(CM)Centromedian &
(IM)Intra laminar.n.

4. Thalamic Nuclei
The thalamus is composed of a number of discrete nuclei. The thalamocortical and corticothalamic fibers
form the external medullary lamina that covers the lateral surface of the thalamus and passes between the
reticular nucleus and the rest of the thalamus.
1. Anatomically, a Y-shaped internal medullary lamina splits the thalamus into three parts. These are
anterior, medial, and lateral group of nuclei (Fig. 141.1).
2. The anterior group of nuclei is enclosed by the bifurcation of the internal medullary lamina.
3. The medial group of nuclei consists of the medial nucleus, the midline nucleus, centromedian, and the
intralaminar nucleus (Fig. 141.2).
4. The lateral group of nuclei is further divided into dorsal and ventral nuclear groups of nuclei.
5. The dorsal nuclear group contains lateral posterior nucleus, lateral dorsal nucleus, and pulvinar.
6. The ventral nuclear group is comprised of ventral anterior, ventral lateral, and ventral posterior nuclei,
and medial and lateral geniculate bodies (Figs. 141.3A and B)
7. The ventral posterior group, also known as ventrobasal complex, consists of venteroposterolateral and
ventroposteromedian groups of nuclei (Flowchart 141.1).

5. PHYSIOLOGICAL nuclear groups and their connections:
However, physiologically, there are four groups of nuclei:
1.Specific relay nuclei: Ascending afferent inputs synapse on these nuclei. The axons arising from them
transmit the afferent impulse to the cortex.
Ventrobasal complex (VBC),
This nuclear group consists of receives inputs from dorsal column pathway, anterolateral system, and
corticothalamic fibers, and projects to the somatosensory cortex.
Lateral geniculate body (LGB), receives input from the optic tract and projects to the visual cortex.
Medial geniculate body (MGB), receives input from the auditory pathway and projects to the auditory cortex .
Anterior nucleus -receives inputs from the hypothalamus through the mammillothalamic tract and relays
information to the cingulate gyrus

6. The trigeminal lemniscus carries the general sensations from face along with the
gustatory afferents and project to the ventral posterior (VP) nucleus that transmits
the afferent inputs to the postcentral gyrus
2.Association nuclei: This nuclear group consists of pulvinar, dorsal nuclei, and
lateral posterior nucleus.
• These nuclei receive inputs from the sensory cortex and limbic system and
project diffusely to the association cortex or to sub-cortical structures.
• The pulvinar projects to the inferior parietal lobe.
• The dorsolateral nucleus is reciprocally connected to the precuneate gyrus.

7. 3.Nonspecific nuclei: These include intralaminar (IM) midline , centromedian
(CM) nuclei, and reticular nuclei (RET).
• They receive inputs mainly from reticular formation and
paleospinothalamic tract, and also inputs from striatum,
hypothalamus, and other thalamic nuclei.
• They project diffusely to the wide areas of the cerebral cortex, to
thalamus and limbic system.
4. Motor nuclei: These include lateral, ventral anterior, and ventral lateral
nuclei

9. The ascending somatosensory pathways terminate in certain thalamic nuclei. The
sensory signals mostly arise from contralateral receptors of the body.
1. Axons from cells in the dorsal column nuclei cross the brain stem immediately at the
level of the medulla and travel in the contralateral medial lemniscus to terminate
primarily in the ventral-posterolateral nucleus (VPL) of the thalamus.
2. The fibers of the anterolateral system (lateral and anterior spinothalamic tracts)
travel in the contralateral anterolateral funiculus of the spinal cord to terminate in the
VPL, posterior (PO), and intralaminar (IM) nuclei.
3. Reticulothalamic fibers of the spinoreticulothalamic pathway remain largely
uncrossed and terminate in IM, midline, or centromedian (CM) nuclei, and in the
thalamic reticular nuclei (RET).
THALAMIC ORGANIZATION AND FUNCTIONS

10. The VPL and PO project to cortical regions.
The VPL receives input from many of ascending sensory pathways.
Thus, the VPL consists of a core of cutaneous inputs and a shell of deep inputs.
Posterior Nuclei (PO) PO receives nociceptive inputs.
Large lesion in PO produces analgesia and stimulation of PO produces pain sensations.
Intralaminar Nuclei and Reticular Nuclei - Responses in IM are strongly affected by the level
of arousal, attention, and affect.
Cells in reticular nuclei (RET) respond to a variety of somatosensory modalities.

11. All afferent impulses on their way to the sensory cortex terminate in the
thalamus.
1. Relay station for all somatic sensations
2. Relay of special sensations: Thalamus is the relay center for all special
sensations except olfaction.
• The lateral and medial geniculate bodies receive the visual and
auditory afferents, respectively.
• The VPL nucleus receives the taste afferents.
FUNCTIONS OF THALAMUS

12. 3. Arousal mechanisms: Thalamocortical projects from the nonspecific
nuclei to the cortex contribute to the reticular activating system that
activates arousal mechanisms and keeps the individual awake.
4. Subcortical perception of sensations: Subcortical perception of pain,
temperature, and pressure (crude touch) occurs to some extent in the
thalamus.
5. Motor Functions: Thalamus is an integral part of motor loop of the brain.
• Through motor loop, the thalamus influences postural movements.
the thalamus influences planning and programming of movements

13. ,.
6. Memory and emotion: Anterior thalamus is a constituent of Papez circuit. • It
receives input from the mammillary body of the limbic system via the
mammillothalamic tract. • Through this connection, the thalamus is concerned with
recent memory and emotion.
7. Synchronization of EEG: Stimulation of intralaminar thalamic nuclei at low frequency
causes synchronization of EEG waves recorded from the ipsilateral cortex. • This is
called recruiting response.
• However, high frequency stimulation causes arousal and desynchronization.
6. Role in sleep: A circuit linking the thalamus and cortex (thalamocortical loop) is
important in generating the pattern of brain activity in sleep–wake cycle..

14. 9. Sensory motor coordination: The thalamus receives all sensory inputs from the
body and closely interacts with basal ganglia, cerebellum, and motor cortex.
*for coordination between sensory and motor functions, especially in the sensory
feedback for correction and improvement in motor output.
10. Language and speech: Dorsal lateral nucleus of thalamus is reciprocally connected
with the parietal lobe of the brain, and therefore is concerned with language and
speech and complex integrated functions.

16. Functions of thalamus
Sensory relay centre.
Crude centre for perception
of sensation.
Arousal & alertness
reaction.
Language.
Emotional aspect of
behaviour.
Sexual sensation.
EEGSynchronization.
Reflex activity centre
Integration of
Sensory impulses.
Motor functions.
Visceral & somatic
function.

17. APPLIED ASPECTS
Thalamic Syndrome Lesion of the VPL occurs due to thrombosis of the
posterolateral branch of the posterior cerebral artery.
1. This results in severe impairment of the discriminative touch and pressure
sensations of contralateral side, whereas diffuse touch, temperature, and pain
sensations are often less impaired.
2. 2. Also, there will be decreased muscle tone,
profound weakness of the muscles, and
ataxia.
3. The emotion may be affected.
.