Date: 24. November 2017
Introduction to Thalamus
In this lecture educator explains the Thalamus and in the first section of lecture educator presents the Introduction to Thalamus. Thalamus is a Greek word meaning Inner Chamber, Meeting Point. It is Middle of Brain and is located between Mid brain and Cerebral Cortex. It’s a Part of forebrain and Corpus Callosum. Diencephalon contains Epithalamus, Thalamus, Hypothalamus and Infundibulum.
General Appearance of Thalamus is that it’s an Egg-shaped mass of Gray Matter, it’s a Major part of Diencephalon and 2 in number. Anterior end is narrow and rounded, forming posterior boundary of Interventricular Foramen and Posterior end expanded to form Pulvinar.
The thalamus is the large mass of gray matter in the dorsal part of the diencephalon of the brain with several functions such as relaying of sensory signals, including motor signals, to the cerebral cortex, and the regulation of consciousness, sleep, and alertness. It is a midline symmetrical structure of two halves, within the vertebrate brain, situated between the cerebral cortex and the midbrain.
Subdivisions of Thalamus
In the second section of lecture educator explains the Subdivisions of Thalamus. Thalamus is Covered superiorly by Stratum Zonale and Lateral surface- External Medullary Lamina, and Gray Matter is divided into lateral and medial halves. Anteosuperiorly, the internal medullary lamina splits, resembles a y shape. Thalamus is subdivided into three parts: 1. Anterior Part, 2. Medial Part, 3. Lateral Part
Anterior Part of Thalamus: Anterior part contains Anterior Thalamic Nuclei, it Receive the Mammilothalamic tract from the mammillary Nuclei and Reciprocal connections with Cingulate Gyrus and Hypothalamus. It is Concerned with Emotional tone and the mechanisms of recent memory
Middle Part of Thalamus: Contains Dorsomedial Nucleus and several smaller nuclei. It has two-way connections with Prefrontal Cortex of Frontal lobe of Cerebral Cortex. Its functions are Integration of a large variety of sensory information, including Somatic, Visceral Olfactory Information.
Lateral Part of Thalamus: The lateral region is subdivided into ventral and dorsal tiers each of which contain sub nuclei. The ventral tier of the lateral division contains the ventral posterior, the ventral lateral (VL) and ventral anterior (VA) nuclei. The ventral posterior is further divided into the ventral posteromedial (VPM) and ventral posterolateral nuclei (VPL). The dorsal tier contains, from caudal to rostral, the pulvinar, the lateral posterior (LP) and the lateral dorsal (LD) nuclei.
The metathalamus includes nuclei that protrude from the posterior aspect of the pulvinar of the thalamus. These include the medial geniculate body (an auditory relay nucleus) and the lateral geniculate body (the principal visual relay).
In the third section of lecture Thalamic Nuclei is explained in detail, Followings are explained then
Anterior Thalamic Nuclei: Association with Limbic system, emotional tone and mechanism of recent memory. DorsoMedial Nucleus: Integration of large variety of sensory information, emotional feelings and subjective states. Lateral Dorsal, Lateral Posterior and Pulvinar: Associated with other lobes Ventral Anterior Nucleus: Present between Corpus Strartium and Motor areas of Frontal cortex. It influences activity of Motor Cortex. Ventral lateral Nucleus: Input form Cerebellum and Red Nucleus and Pass Output to motor and premotor areas of Cerebral Cortex (Motor Activity).
Every thalamic nuclei send impulses (axons) to Cerebral Cortex and reciprocal fibers back to different parts of thalamic Nuclei. Thalamus is important relay station for two sensory motor axonal loops involving Cerebellum and basal Nuclei: i) Cerebellar-rubro-thalamic –cortical-ponto-cerebellar loop and ii) Corticalstriatal-pallidal-thalamic-cortical loop both involved in normal voluntary movement.
Blood Supply and Physiology of Thalamus
The thalamus derives its blood supply from a number of arteries: the polar artery (posterior communicating artery), paramedian thalamic-subthalamic arteries, inferolateral (thalamogeniculate) arteries, and posterior (medial and lateral) choroidal arteries. These are all branches of the posterior cerebral artery. Some people have the artery of Percheron, which is a rare anatomic variation in which a single arterial trunk arises from the posterior cerebral artery to supply both parts of the thalamus.
Thalamic Physiology: Primarily a relay station that modulates and coordinates the functions of various systems. Locus for integration, modulation, intercommunication for various systems and Has important sensory, motor, arousal, memory, behaviour, limbic and cognitive functions. Largest source of afferent fibers is cerebral cortex and cortex. Characteristically thalamic connections are reciprocal. However, thalamocortical projections are larger than their corticothalamic counterparts.
Functions and Clinical Consideration
Functions of Thalamus: Thalamus act as a relay station, or hub, relaying information between different subcortical areas and the cerebral cortex. For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the visual cortex in the occipital lobe. The thalamus is believed to both process sensory information as well as relay it. Each of the primary sensory relay areas receives strong feedback connections from the cerebral cortex. It is Important in regulating states of sleep and wakefulness. Thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming. Thalamo-cortico-thalamic circuits that are involved with consciousness. Major role in regulating arousal, the level of awareness, and activity. Damage to the thalamus can lead to permanent coma. Thalamus support motor and language systems, and much of the circuitry implicated for these systems is shared. The thalamus is functionally connected to the hippocampus that are crucial for human episodic memory and rodent event memory. Thalamic regions connection to particular parts of the mesio-temporal lobe provide differentiation of the functioning of recollective and familiarity memory
Clinical significance: A cerebrovascular accident (stroke) can lead to the thalamic syndrome, which involves a one-sided burning or aching sensation often accompanied by mood swings. Bilateral ischemia of the area supplied by the paramedian artery can cause serious problems including akinetic mutism, and be accompanied by oculomotor problems. A related concept is thalamocortical dysrhythmia. The occlusion of the artery of Percheron can lead to a bilateral thalamus infarction. Korsakoff's syndrome stems from damage to the mammillary body, the mammillothalamic fasciculus or the thalamus. Fatal familial insomnia is a hereditary prion disease in which degeneration of the thalamus occurs, causing the patient to gradually lose his ability to sleep and progressing to a state of total insomnia, which invariably leads to death. In contrast, damage to the thalamus can result in coma.
Thalamic syndrome (or thalamic pain syndrome) is a condition that can be associated with inadequate blood supply from the posterior cerebral artery. Rare neurological disorder in which the body becomes hypersensitive to pain as a result of damage to the thalamus. Primary symptoms are pain and loss of sensation, usually in the face, arms, and/or legs. Pain or discomfort may be felt after being mildly touched or even in the absence of a stimulus. The pain associated with thalamic syndrome may be made worse by exposure to heat or cold and by emotional distress. Sometimes, this may include even such emotions as those brought on by listening to music. It is also known as "Dejerine-Roussy disease".