Thalamus Anatomy, Function Notes

Thalamus Introduction

• Thalamus is a large ovoid mass of gray matter, situated bilaterally in the diencephalon. Both thalami form 80% of the diencephalon.
• The thalami on both sides are connected in their rostral portions by means of an intermediate-mass. The caudal portions are more widely separated by corpora quadrigemina.

Thalamic Nuclei

Thalamus on each side is divided into five main nuclear groups by means of internal medullary septum.

1. Midline nuclei
2. Intralaminar nuclei
3. Medial mass of nuclei
4. Lateral mass of nuclei
5. Posterior group of nuclei.

Read And Learn More: Medical Physiology Notes

1. Midline Nuclei: It is a group of smalpfluclei, situated on the medial surface of the thalamus neanhe midline.
2. Intralaminar Nuclei: The intralaminar nuclei are smaller nuclei present in the medullary septum of the thalamus.
3. Medial mass of nuclei: Medial mass of nuclei is situated medial to septum and it comprises two nuclei.
   1. Anterior nucleus
   2. Dorsomedial nucleus.
4. Lateral mass of nuclei: This group of nuclei is situated lateral to septum. Lateral mass of nuclei is again divided into two subgroups:
   • Dorsal group of lateral mass with two nuclei:
     1. Dorsolateral nucleus
     2. Posterolateral nucleus
   • Ventral group of lateral mass with three nuclei:
     1. Anterior ventral nucleus
     2. Lateral ventral nucleus.
     3. Posteroventral nucleus. It consists of two parts:
        1. Ventralxposterolateral nucleus
        2. Ventral’posteromedial nucleus.
5. Posterior Group Of Nuclei: It is the continuation of lateral mass of nuclei. It has two subgroups
   • Pulvinar
   • Metathalamus which consists of two structures:
     1. Medial geniculate body
     2. Lateral geniculate body.

Connections Of Thalamic Nuclei

Connections Of Thalamic Nuclei: The connections of different groups of nuclei are given in Table.

Thalamic Radiations

• Thalamic radiation is the collection of nerve fibers connecting thalamus and cerebral cortex. It contains both thalamocortical and corticothalamic fibers. All these fibers between the thalamus and cerebral cortex pass through internal capsule.
• Thalamic radiations are divided into four groups, which are called thalamic peduncles or thalamic stalks.

1. Anterior (frontal) peduncle Or Radiation: It connects the frontal lobe of the cerebral cortex with medial and lateral thalamic nuclei. It contains mostly motor nerve fibers.
2. Superior (Centroparietal) Peduncle Or Radiation: The fibers of this peduncle connect the postcentral gyrus (somesthetic area) of parietal lobe and the adjacent area in frontal cortex with a lateral mass of thalamic nuclei. It contains mainly sensory fibers.
3. Posterior (Occipital) Peduncle Or Radiation: It connects the occipital lobe of cerebral cortex with the pulvinar and lateral geniculate body. It contains the nerve fibers concerned with vision.
4. Inferior (Temporal) Peduncle Or Radiation: Fibers of this peduncle connect the temporal lobe and insula with the pulvinar and medial geniculate body. This peduncle contains the nerve fibers concerned with hearing.

Functions Of Thalamus

Thalamus is primarily concerned with somatic functions and it plays little role in the visceral functions. The various functions of thalamus are:

1. Relay Center: Thalamus forms the relay center for the sensations. The impulses of almost all the sensations reach the thalamic nuclei, particularly in the ventral posterolateral nucleus. After being processed in the thalamus, the impulses are carried to the cerebral cortex through thalamocortical fibers.
2. Center For Processing Of Sensory Information: Thalamus forms the major center for processing sensory information. All the peripheral sensory impulses reaching thalamus are integrated and modified before being sent to specific areas of cerebral cortex. This function of the thalamus is usually called the processing of sensory information.
   • Functional Gateway for Cerebral Cortex: Almost all the sensations are processed in thalamus before reaching the cerebral cortex. Very little information of somatosensory function is sent directly to the cerebral cortex without being processed by the thalamic nuclei. Because of this function, thalamus is usually called a “Functional gateway” for the cerebral cortex.
3. Center For Determining Quality Of Sensations: Thalamus is also the center determining the quality of sensations that is, to determine the affective nature of sensations. Usually, the sensations have two qualities
   1. The discriminative nature: The Discriminative Nature: It is the ability to recognize the type, location, and other details of the sensations and it is the function of cerebral cortex.
   2. The Affective Nature: The affective nature is the capacity to determine whether a sensation is pleasant or unpleasant and agreeable or disagreeable. Determining the affective nature of sensations is the function of thalamus.
4. Center For Sexual Sensations: Thalamus forms the center for perception of sexual sensations.
5. Role In Arousal And Alertness Reactions: Because of its connections with nuclei of the reticular formation, the thalamus plays an important role in arousal and alertness reactions.
6. Center For Reflex Activity: Since the sensory fibers relay here, the thalamus forms the center for many reflex activities.
7. Center For Integration Of Motor Activity: Through the connections with the cerebellum and basal ganglia, the thalamus serves as a center for the integration of motor functions.

Applied Physiology

Thalamic Lesion:

• Thalamic lesion occurs mainly mostly because of blockage (due to thrombosis) in the thalamogeniculate branch of the posterior cerebral artery.
• Mostly the posteroventral nuclei of the thalamus are affected because the thalamogeniculate branch of the posterior cerebral artery supplies this part of the thalamus. The lesion of the thalamus leads to a condition called thalamic syndrome.

Thalamic Syndrome: Thalamic syndrome is a neurological disease caused by infarction of the posteroventral part of thalamus. It is a rare disease and it has many names. The synonyms of this syndrome are:

1. Dejerine-Roussy syndrome
2. Thalamic hyperesthetic anesthesia
3. Thalamic pain syndrome
4. Central pain syndrome
5. Central-poststroke syndrome
6. Posterior thalamic syndrome
7. Retrolenticular syndrome.

In thalamic syndrome, whole body becomes hypersensitive to pain. The effects of thalamic lesions occur in the contralateral (opposite) side. The following are the symptoms of thalamic syndrome:

1. Loss of Sensations: Loss of all sensations (anesthesia) occurs as the sensory relay system in the thalamus is affected.
2. Astereognosis: Astereognosis is the loss of the ability to recognize a known object by touch with closed eyes. It is due to the loss of tactile and kinesthetic sensations in thalamic syndrome.
3. Ataxia: Ataxia refers to the incoordination of voluntary movements. It occurs due to loss of kinesthetic sensation. This type of ataxia due to loss of sensation is called sensory ataxia. It is very common in thalamic syndrome.
4. Thalamic Phantom Limb: Thalamic phantom limb is the inability to locate the position of a limb with closed eyes. The patient will search for the limb in air.
5. Amelognosia: It is the illusion felt by the patient that his limb is absent.
6. Spontaneous Pain and Thalamic Over-reaction: Spontaneous pain occurs often. The pain stimulus is felt more acutely than in normal conditions (hyperalgesia).
   • The pain may be so intense, that it even resists the action of powerful sedatives like morphine. The threshold for pain is very much reduced. Even the light touch may be unpleasant.
   • Sometimes, the patient feels pain even in the absence of pain stimulus. It becomes worst in conditions such as emotional disturbance and exposure to cold or heat. The pain is due to over activity of the medial mass of nuclei of thalamus, which escape the lesion.
   • Abnormal reaction to various stimuli is called thalamic over-reaction.
7. Involuntary Movements: Thalamic syndrome is always associated with some involuntary motor movements.
   1. Athetosis: Athetosis means slow writhing and twisting movements
   2. Chorea: Chorea is the quick jerky involuntary movements
   3. Intention tremor: Tremor is defined as rapid alternate rhythmic and involuntary movement of flexion and extension in the joints of fingers and wrist or elbow. Intention tremor is the tremor that develops while attempting to do any voluntary act. Intention tremor is the common feature of thalamic syndrome.
8. Thalamic Hand or Athetoid Hand: It is the abnormal attitude of the hand in a thalamic lesion. It is characterized by moderate flexion at the wrist and hyperextension of all fingers.