The Thalamus: a Review of Its Functional Anatomy

The Thalamus: A Review of its Functional Anatomy

Author: Abstract

Rengin Kosif, Assoc. Prof. The diencephalon can be divided into four areas, Abant Izzet Baysal University, which are interposed between the brain stem and Faculty of Medicine, Department cerebral hemispheres. The four subdivisions of Anatomy include the hypothalamus to be discussed in a BOLU - TURKEY separate lecture, the ventral thalamus containing the subthalamic nucleus already discussed, the Correspondence Address: epithalamus which is made up mostly of the pineal Rengin Kosif, Assoc. Prof. body, and the dorsal thalamus (henceforth referred Abant Izzet Baysal University, to as the thalamus). Anatomically, the dorsal Faculty of Medicine, Department thalamus is subdivided into 50-60 nuclear groups. of Anatomy The focus of this review is its functional anatomy. BOLU - TURKEY In the spinal cord and brain stem portions of the Tel: (0374) 2534656-3043 course, certain relay nuclei of the thalamus transfer Fax: (0374) 2534559 information from sub-cortical structures to the E-mail: [email protected] cerebral cortex. By virtue of these relay functions that encompass the major senses and motor systems, the thalamus is often referred to as the

gateway to the cortex. The thalamus has numerous connections to other areas of the brain as well, and

these are thought to be important in the integration of cerebral, cerebellar, and brainstem activity.

Branches of the posterior cerebral artery supply much of the thalamus. Occlusion of these small branches results in a number of symptoms

characteristic of the thalamic syndrome.

Keywords: Thalamus, nuclei, functional anatomy

Medical Research Archives, Vol. 4, Issue 8, December 2016 The Thalamus: A Review of its Functional Anatomy

The Thalamic Nuclei: A the thalamus supplied by four arteries: Review of its Functional Anatomy tuberothalamic artery, paramedian artery, inferolateral artery, and posterior The human thalamus is a nuclear choroidal artery. Anatomically, the complex located in the diencephalon and thalamus can be subdivided in nuclear comprising of four parts (the groups with respect to the internal hypothalamus, the epithalamus, the medullary lamina, the anterior, medial, ventral thalamus, and the dorsal lateral, intralaminar, and posterior nuclei, thalamus). The dorsal thalamus is referred and to the external medullary lamina, the to as the thalamus and largest structure of reticular nucleus (RN). The thalamic the diencephalon. The thalamus is a relay nuclei can be grouped into six functional centre subserving both sensory and motor classes: the RN, the specific sensory mechanisms. Thalamic nuclei (50-60 nuclei, the effector nuclei, the limbic nuclei) project to one or a few well- nuclei, the intralaminar nuclei, and the defined cortical areas (1). There are four associative nuclei (2). thalamic vascular syndromes corresponding to the vascular territories of

1. Anterior nuclear complex: AD Anterodorsal nucleus, AM Anteromedial nucleus, AV Anteroventral nucleus, LD Laterodorsal nucleus.

2. Medial–midline group: MDmc Mediodorsal magnocellular nucleus, MDpc Mediodorsal parvocellular nucleus, Pt Paratenial nucleus, PV Paraventricular nucleus, Re Nucleus reuniens.

3. Lateral group: LP Lateral posterior nucleus (lateral dorsal group), VA Ventral anterior nucleus (lateral ventral group), VL Ventral lateral nucleus (lateral ventral group),VM Ventral medial nucleus (lateral ventral group), VP Ventral posterior nucleus (lateral ventral group), VPL Ventral posterolateral nucleus, VPM Ventral posteromedial nucleus, VPpo Ventral posterior nucleus, VPI Ventral posterior inferior nucleus.

4. Intralaminar group: CL Central lateral nucleus (intralaminar group – anterior/rostral), CM Centromedian nucleus (intralaminar group – posterior/caudal), PCn Paracentral nucleus (intralaminar group – anterior/rostral), Pf Parafascicular nucleus (intralaminar group – posterior/caudal)

5. Posterior group: Pul Pulvinar complex, PM Pulvinar inferior/medial nucleus

6. Metathalamus: LGN Lateral geniculate nucleus, MGN Medial geniculate nucleus, RN Reticular nucleus.

The thalamus is an important relay complex via the fornix and indirectly via center subserving sensory and motor the mammillary body and the mechanisms, arousal, cortical synchrony, mammillothalamic tract. Brainstem fibers emotion, cognition, and memory. from the dorsal raphe and However, not only thalamic neurons are pedunculopontine nuclei also reach the relays of information, but also they have a anterior nuclei. Efferent fibers project to role in controlling executive networks and all portions of the cingulate gyrus and to in regulating complex behaviors, such as other parts of the limbic cortex and behavioral flexibility and reward-directed orbitofrontal area (3). behavior (2). The anterior nucleus of the thalamus The thalamic anterior nuclei: The (ANT) is a key component of the nuclear ensemble is encapsulated by hippocampal system for episodic memory. myelinated fibers that appear to be an The ANT consist of 3 subnuclei with anterior extension of the intralaminar distinct connectivity with the subicular system. Afferent connections come largely cortex, retrosplenial cortex, and from the hippocampus and subiculum, mammillary bodies. Via its connections projecting directly onto the anterior with the anterior cingulate and

Copyright 2016 KEI Journals. All Rights Reserved Page │3 Medical Research Archives, Vol. 4, Issue 8, December 2016 The Thalamus: A Review of its Functional Anatomy orbitomedial prefrontal cortex, the ANT reticular activating system (ARAS), the may also contribute to reciprocal rostral continuation of the reticular hippocampal-prefrontal interactions formation (6). Intralaminar/midline- involved in emotional and executive induced cortical activation would lead to functions (4). greater vigilance, necessary for awareness

Clinical and experimental evidence of incoming information. It is important to indicate that damage of the ANT or its stress that the midline and intralaminar inputs from the mammillary bodies are nuclei do not ‘produce’ awareness, but primarily responsible for the episodic rather provide the necessary arousal of cortical and subcortical regions supporting memory deficit observed in Wernicke- Korsakoff syndrome and thalamic stroke. information processing that is correlated Experimental models also indicate that the with awareness (7). A role in the ANT may have a role in the propagation awareness of tactile and nociceptive of seizure activity both in absence and in information has been described as well for focal seizures. Because of its central the midline and intralaminar nuclei. This connectivity and possible role in derives from the anatomical evidence that propagation of seizure activity, the ANT the above nuclei receive nociceptive input has become an attractive target for deep from the spinothalamic and brain stimulation (DBS) for treatment of spinoreticulothalamic projections (8). The medically refractory epilepsy (4). intralaminar thalamic nuclei are a part of the thalamus, the relay station for visual, The thalamic midline and somatosensory, and auditory information intralaminar nuclei, long thought to be a to the cortex. These nuclei are also non-specific arousing system in the brain, involved in attention and arousal (9). have been shown to be involved in separate and specific brain functions, such The thalamic lateral nuclei: The as specific cognitive, sensory and motor VPL and VPM nuclei are part of the functions. Fundamental to the somatosensory system. The VPL relays medial lemniscal and spinothalamic participation of the midline and intralaminar nuclei in such diverse connections to the cerebral cortex. The functions seems to be a role in awareness VPM receives trigeminothalamic input (5). Because of their strong brainstem and relays to the inferior portion of the inputs, the intralaminar and midline nuclei postcentral gyrus. The VL receives input are considered as part of the ascending from the cerebellum, mainly from the

Copyright 2016 KEI Journals. All Rights Reserved Page │4 Medical Research Archives, Vol. 4, Issue 8, December 2016 The Thalamus: A Review of its Functional Anatomy dentate nucleus. There is a small input was demonstrated by antidromic from the basal ganglia to the rostral part of stimulation. The close topographical the VL, as well. The VL projects to the relationship between vestibular and primary motor area, area 4, of the somatic areas in the cortex is paralleled in precentral gyrus and also has a smaller the thalamus, the VPI being closely projection to premotor areas. The VL is related to VPL and VPM nuclei (13). thus involved in motor feedback from the The thalamic posterior nuclei: The cerebellum and basal ganglia to the pulvinar is the largest association nuclei, cerebral cortex. The VA nucleus receives occupying the posterior part of the dorsal most of its input from the basal ganglia, tier of the thalamus. This receives afferent especially the medial globus pallidus and projections from the superior colliculus as substantia nigra, parts reticulata. This well as from the association cortex. It projects to premotor cortex including the projects to secondary visual areas and to supplementary motor area of the frontal association areas in the parietotemporal lobes and is involved in planning and region. This contributes to visual initiating movements (10). VL projections perception and eye movements, probably target caudal motor areas (primary, relating to attention to these stimuli (10). supplementary, and caudal premotor The posterior nuclei were areas), whereas VA projections target complicated and varied considerably more rostral premotor areas (including among neurons: although almost all cingulate and presupplementary motor contained a single elliptical region near areas) (11). LP cells required visual input. the reticulothalamic border, in most cases, It was concluded that the LP may pass on they consisted of additional discontinuous to the hippocampal formation directional regions or relatively diffuse regions information that is not merely a reflection throughout the thickness of the thalamic of current sensory input. As such, the LP reticular nucleus. Our results suggest two may serve an important integrative sources of reticular inputs to the posterior function for limbic spatial learning nucleus neurons: one that is relatively systems (12). The VPI rather than the topographic from regions near the other nuclei with long latency responses is reticulothalamic border and one that is likely to be the thalamic relay in the relatively diffuse and convergent from vestibulo-cortical path. Projection of VPI most or all of the thickness of the thalamic neurons to the primary vestibular cortex reticular nucleus. We propose that the

Copyright 2016 KEI Journals. All Rights Reserved Page │5 Medical Research Archives, Vol. 4, Issue 8, December 2016 The Thalamus: A Review of its Functional Anatomy more topographic reticular input is the project to both amygdala and temporal basis of local inhibition seen in posterior cortex in the rat (17). nucleus neurons and that the more diffuse The thalamic metathalamic and convergent input may represent nuclei: Metathalamus is the part of the circuitry through which the ventral diencephalon inferior to the caudal end of posterior lateral and posterior nuclei the dorsal thalamus, comprising the lateral interact (14). and medial geniculate bodies. The lateral The pulvinar is the only nucleus of geniculate nucleus is visual, the medial this group to identify. This nucleus geniculate nucleus is auditory. The medial receives inputs from many diverse areas geniculate nucleus receives ascending of the major sensory systems and projects GABAergic afferents from the inferior to the all of the association areas of cortex colliculus, the lateral geniculate receives in the parietal, occipital and temporal such afferents from the pretectum, there lobes. One of the primary outputs of are GABAergic afferents from the zona pulvinar is to the secondary visual areas incerta to higher order thalamic relays and (18 and 19). There is evidence that this the globus pallidus and substantia nigra secondary pathway conveys information and zona incerta send GABAergic axons only about stimulus position and is not to the ventral anterior and the center involved in pattern recognition. For our median nucleus (18). The thalamic purposes you should think of the pulvinar reticular nucleus (TRN) controls the as an area, which can direct your attention internal attentional searchlight that to a new stimulus like a flash of light or a simultaneously highlights all the neural sound (15). circuits called on by the object of

Posterior thalamus seems to be attention. In other words, he submitted fundamentally involved in our control of that during either perception, or the upright body posture. Higher pressure, preparation and execution of any cognitive swelling, and other secondary pathologic and/or motor task, the TRN sets all the corresponding thalamocortical circuits in processes associated with posterior thalamic hemorrhage (vs thalamic motion. Over the last two decades, infarction) may provoke contraversive behavioural, electrophysiological, pushing in combination with additional anatomical and neurochemical findings neurologic symptoms (Pusher Syndrome) have been accumulating, supporting the (16). Cells in the posterior thalamus complex nature of the TRN and raising

Copyright 2016 KEI Journals. All Rights Reserved Page │6 Medical Research Archives, Vol. 4, Issue 8, December 2016 The Thalamus: A Review of its Functional Anatomy questions about the validity of this thalamic afferents and efferents is speculative hypothesis. Indeed, our contralateral, and the lateralisation of the knowledge of the actual functioning of the thalamic functions affects both sensory TRN is still sprinkled with unresolved and motoric aspects. Symptoms of lesions questions. Therefore, the time has come to located in the thalamus are closely related join forces and discuss some recent to the function of the areas involved. An cellular and network findings concerning infarction or haemorrhage thalamic lesion this diencephalic GABAergic structure, can develop somatosensory disturbances which plays important roles during and/or central pain in the opposite various states of consciousness. On the hemibody, analgesic or purely algesic whole, the present critical survey thalamic syndrome characterised by emphasizes the TRN's complexity, and contralateral anaesthesia (or provides arguments combining anatomy, hypaesthesia), contralateral weakness, physiology and cognitive psychology ataxia and, often, persistent spontaneous (19). The reticular thalamic nucleus pain (1). A case is presented of an adult receives afferents from the brain stem male with a thalamic infarct resulting in a reticular formation as well as from the paramedian thalamic syndrome, consisting cerebral cortex and thalamus. This makes of hypersomnolence, confabulatory a strongly inhibitory input to thalamic anterograde amnesia (including nuclei. This nucleus may be important in reduplicative paramnesia), vertical gaze sleep wake cycles and maybe an important deficits, and hypophonic speech (20). regulator of signals relaying through the It is now appreciated that the thalamus (10). thalamus plays critical roles in the most Multiple cortical areas receive fundamental aspects of brain function, afferents from a single thalamic nucleus including: (i) integration, processing, and and send back information to different cognition of sensory information; (ii) thalamic nuclei. The corticofugal regulation of consciousness, including projection provides positive feedback to arousal, awareness, and attention; and (iii) the "correct" input, while at the same time modulation of the pyramidal and suppressing irrelevant information. extrapyramidal motor systems (3). Topographical organization of the

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