The thalamus occupies approximately 80% of the medial diencephalic region. The term 'thalamus' derives from the Greek word thalamos, which means 'inner chamber' or 'marriage bed'. Galen (130-200 AD.) Gave this name to the 'chambers' located at the base of the brain. Embryologically, derived from the diencephalic vesicle, from turn of the prosencephalic vesicle: the two telencéfalos after training involves defining a central area which will create the future diencephalon. The thalamus has an oval shape, is a pair structure and more or
less symmetrical with respect to the midline. In humans, each thalamus is about 3 cm long (anteroposterior) and 1.5 cm wide at its widest point [1]. Is crossed by a band of myelinated fibers, inner core sheet, which runs along the rostrocaudal extension thereof, adopts a special distribution in the anterior pole Y-shaped and divides the thalamus into large blocks anatomic. This sheet contains intratalámicas fibers that connect the different thalamic nuclei together. Another band medullated, the external medullary lamina, form the lateral boundary of the thalamus, medial to the internal capsule. In the thalamus there are two types of neurons from a
functionally:
- Principal or projection neurons (transmit information outside the thalamus), representing about 75% of the
Total neuronal population.
- Local interneurons or local circuit, which can receive information from the same sources that neurons
major, but only in contact with thalamic cells involved in the same stage of processing.
Constitute around 25%. The principal neurons send their axons to the cerebral cortex, where they release an excitatory neurotransmitter (glutamate, usually) to activate cortical neurons. Glutamate and aspartate are excitatory neurotransmitters and are present in corticothalamic terminations and cerebellar and thalamocortical projection neurons. An exception is constituted afferents subcortical gray cores of the base, which are GABAergic, inhibitory. The local circuits of neurons release acid (GABA) in projection cells inhibit them. This inhibitory neurotransmitter is located at the ends that come from the globus pallidus neurons in local circuits and projection in the reticular nucleus and lateral geniculate body. GABAergic projections are major segment pallidal projections to the ventral anterior medial (parvocellular) and the lateral ventral (pars oralis) and projections of the reticular part of substantia nigra to the ventral anterior nucleus (magnocellular) and dorsomedial (paralaminar). These afferents play a key role in motor function [2]. GABAergic neurons have been identified in all laminae of the lateral geniculate body and are most abundant in laminae 1 and 2 (magnocellular). Afferents from subcortical regions and the cerebral cortex that are directed to the thalamic nuclei excited (depolarized) to projection neurons and local interneurons in these nuclei. In turn, neurons in local circuits inhibit (hyperpolarize) a projection neurons and the neurotransmitter GABA is used. Thus, afferents to the thalamus influence the projection neurons (thalamocortical) via two pathways: a direct excitatory and inhibitory indirect, through the local circuit neurons. The neurons in local circuits modulate the activity of projection neurons, which send their axons to targets extratalámicos. In addition, cells projection sent to a side branch reticular thalamic nucleus neurons, which contain the inhibitory neurotransmitter GABA neurons and act as local circuits. The cells of the thalamic reticular nucleus send axon branches to projection neurons and local circuits, so that both are inhibited.
The cerebral cortex, which received afferent projections excitatory cells thalamic axons projecting excitatory sent back to all cell types thalamic, so activate both cortical afferents to projection neurons as the inhibitory circuits
Local and reticular nucleus. Thus, the thalamus is not just a simple relay afferent information between centers and the cortex, but is in charge of information processing, and therefore influence on cortical function.
Thalamic nuclear groups.
The thalamus contains a very rich nuclear organization. Have been identified as 50 thalamic nuclei [4], several of which are microscopic subdivisions. The nomenclature of the thalamic nuclei is very complex, and in some cases are unaware of their connections and the functional significance of the smaller [2]. They have proposed various classifications of the different nuclei comprising the thalamus based on an evolutionary perspective [5], shared characteristics and fiber connectivity functions [6], citoarquitectónicos criteria [7,8] and anatomic criteria of the different thalamic nuclei ( Table).
Thalamocortical and corticothalamic connections.
The organization within the cerebral cortex of thalamocortical and corticothalamic projections and neurophysiological properties of the fibers that ascend or descend to or from the cerebral cortex are the basis of the complex relationships between different thalamic nuclei and cerebral cortex [9 ].
It was Lorente de No [10] who described the thalamocortical afferents and thalamocortical fibers and fiber specific thalamocortical nonspecific. The former have their origin in specific nuclei of the thalamus, forming synapses in layer IV of the cortex and are carriers of information of general and special sensation (except olfactory). The latter are ascending fibers with collateral primarily to layers I, II and VI. These pathways are nonspecific thalamocortical pathways related to diffuse from the nuclei of the midline and intralaminar into the cerebral cortex [11,12] and
related to the mechanisms of arousal (vigilance). We also note that there are reciprocal projections of all relay nuclei and some nuclei of association ranging from the thalamus to the cortex and from cortex to the thalamus through the internal capsule, called 'thalamic radiation'.
Although this radiation make connections to virtually all parts of the cortex, the richness of the connections varies between different cortical areas. The most abundant are directed toward the precentral gyrus and postcentral giri, the calcarine area of Heschl convolution, the posterior parietal region and adjacent parts of the temporal lobe [2]. To conclude this section on anatomophysiological aspects of the thalamus, we note that this brain structure is irrigated mainly by fine branches of the posterior cerebral artery (PCA), along with branches of the internal carotid artery and posterior communicating artery . Talamoperforantes arteries (arteries posteromedial or paramedian), which originate in the medial parts of the ACP and the terminal part of the basilar artery, supplying the medial thalamus (medial thalamic territory), particularly the intralaminar nuclei (centromedian nucleus and parafascicular), dorsomedial (dorsal), ventral lateral, ventral anterior, and ventroposteromedial ventroposterolateral [13,14]. Talamogeniculadas branch (posterolateral artery) of the ACP supplying the caudal half of the thalamus (posterolateral thalamic territory), including the following centers: ventroposterolateral, ventroposteromedial, geniculate bodies (lateral and medial) pulvinar, dorsomedial, lateral and reticular. The posterior communicating artery supplies the anterolateral thalamic territory through the branch tuberotalámica (polar optical): ventral anterior, ventral lateral, dorsomedial and anteroventral. The internal carotid artery supplies the lateral thalamic territory through its anterior choroidal artery: the lateral geniculate body, ventroposterolateral, pulvinar and reticular. The posterior thalamic territory is supplied by the posterior choroidal artery, provides nourishment to the lateral geniculate body, pulvinar, dorsolateral, dorsomedial and anteroventral. The cerebral venous drainage depends on two systems, the superficial and deep. The first drains the cerebral cortex and subcortical white matter, and the second drain the choroid plexus, periventricular regions, the diencephalon and the num-Table. Classification of thalamic nuclei. Main Classification criteria thalamic nuclei. Arquitálamo perspective: the midline nuclei, evolutionary [5] intralaminar and reticular.
Paleotálamo: bodies geniculate nuclei, ventral posterior and anterior cerebellar relay Neotálamo: medial nuclei, laterodorsal, lateral and ventral anterior connections [6] specific mode: ventroposterolateral nuclei,
ventroposteromedial, geniculate bodies, ventral lateral, ventral anterior, anterior dorsolateral and multimodal associative: dorsomedial nucleus and pulvinar-lateral posterior complex and reticular Nonspecific: intralaminar nuclei of the midline and reticular Motor Function: ventral anterior and ventral nuclei play [6] side) Sensitive: ventroposterolateral nuclei and geniculate bodies ventroposteromedial and Associative: medial dorsal nucleus and pulvinar-lateral posterior complex and reticular Nonspecific: intralaminar nuclei of the midline and reticular cytoarchitecture [7.8] lateral nuclear group: ventroposterior complex, ventral nuclei lateral, anterior and ventral medial ventral
Medial nuclear group: intralaminar nuclei and dorsomedial nucleus.
Posterior nuclear group: complex posterior, posterior lateral nuclei, pulvinar and geniculate anterior nuclear group: anteroventral nuclei.
Anteromedial, anterodorsal and dorsal lateral reticular nuclei.
Anatomic anterior nuclear group: anteroventral nucleus, dorsomedial nucleus anterodorsal and anteromedial lateral nuclear group: dorsolateral nucleus, lateral posterior, ventral anterior, ventral lateral nuclear group ventroposteromedial ventroposterolateral and later: pulvinar, geniculate bodies of the midline nuclei: paratenial, paraventricular, reuniens, rhomboid intralaminar nuclei: centromedian, parafascicular, paracentral, central medial and central lateral reticular nuclei.
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The core serves ventroposteromedial sensitivotalámico relay center of the head and face. The efferents of this nucleus are directed through the internal capsule to the primary somesthetic cortex of the parietal lobe.
Through this area thalamic projections to frontal areas (areas 4, 8, 6, 44 and 45), the thalamus is involved in sensory perception of movements.
The thalamus is also involved in pain mechanisms. The main target nuclei axons ascendentesn for pain and temperature are in the ventral posterior nucleus. The ventroposterolateral ventroposteromedial and receive most of these afferents. The ventroposteromedial receives nociceptive information from the face, and ventroposterolateral, the rest of the body. The similar arrangement of mechanosensitive and harmful stimuli is responsible for discriminating mechanisms of pain [15]. The intralaminar thalamic nuclei, in terms of pain is concerned, involved in evoking the response triggered by a noxious stimulus through projections from these nuclei reach the reticular formation. Some sensory modalities are perceived in the thalamus, a fact that becomes apparent when lesions or ablations.
of the cerebral cortex. In these cases, after the lesion is lost all sensitivity contralateral to the lesion, and recovered the pain, temperature and sensitivity epicritic (gross). The clinic is well described this painting, known as thalamic syndrome. In these cases, the threshold of stimulation that produce these feelings are high and the sensory modalities are exaggerated and unpleasant, moreover, are usually accompanied
of a strong emotional response, usually attributable to an intact dorsomedial nucleus (common in vascular lesions). Vascular lesions affecting the posterolateral thalamic territory (cores ventroposterolateral, ventroposteromedial, medial geniculate body, pulvinar and centromedian) can
result in a contralateral sensory loss, paresthesias, and thalamic pain. It has been well described Dejerine and Roussy syndrome, characterized by severe pain, persistent and paroxysmal, often intolerable, usually present at the time of injury or after a period of transient hemiparesis, sensory loss hemiataxia and hemibody. The involvement of the thalamus in motor control is reflected by the afferents from the base gray nuclei, cerebellum and motor cortex that come to him and that he depart efferents to the motor and premotor cortex. In the motor system nuclei involved mainly the following:
ventral anterior and lateral, intralaminar and reticular, we are two main systems: pallidal and cerebellar. The separation between the two circuits is that afferents are distinct and their efferent toward the cortical areas that project. Alterations in the ventral lateral projections can lead to movement disorders (dyskinesias). Lesions in this nucleus decreases the abnormal movements and cerebellar gray nuclei of the base [6].
Lesions on the ventral intermediate nucleus (Vim), ventral caudal nuclei, the centromedian, sensory and pulvinar nuclei can cause a wide variety of disorders
Movement, including dystonia, tremor, and chorea ballism [16-18].
Vascular lesions affecting the ventral nuclei above, lateral, dorsomedial and anterior nuclei can cause contralateral hemiparesis and visual field disorders. gray cores of the base. The deep cerebral veins of interest are the internal cerebral vein, basal vein (of Rosenthal) and the great cerebral vein of Galen. The internal cerebral veins are the superior choroidal veins (lateral drainage of the choroid plexus), the roof of the lateral ventricle (deep white matter of the frontal lobes anterior and posterior parietal), the dorsal horn of the lateral ventricle (white matter of the lobes occipital and temporal posterior fornix) and the thalamus.
They drain into the internal cerebral vein through small veins talamoestriadas, which are responsible for drainage of the thalamus.
There is evidence that the intralaminar nuclei are also involved in controlling movement. These nuclei receive afferents mainly from the reticular formation,
the pale, putamen, subthalamic nuclei and cortical areas 6 and 4. The connections you have with these nuclei and the caudate putamen contribute to subcortical motor control.
The centromedian nucleus receives input from the pale, the substantia nigra (zona reticularis), the gray zone, the deep cerebellar nuclei, the primary motor cortex and the reticular nuclei [19,20]. Sends excitatory glutamatergic projections broad and diffuse projections to the putamen dorsolateral brink
caudate and subthalamic nuclei [21,22]. Reticular thalamic nuclei end so
diffuse cerebral cortex and allow activation necessary for correct operation of the motor system. There are studies that show some involvement of the cores
midline of the motor system. Lee and Marsden [17] indicate that the lesions of the thalamic dystonia should not place them in the anterior and lateral ventral nuclei, but in later areas or in the nuclei of the midline. We describe a motor semiology that characterize
to thalamic lesions:
- Voluntary motor system disorders: lack of coordination
contralateral cerebellar, faux synkinesias ipsilateral and contractures.
- Involuntary motor system disorders.
- Disturbances global movement: hand thalamic
characterized by incessant movements of the fingers, both
in the horizontal and vertical.
- Alterations of gait [23].
Impact of the Thalamus in Higher psychofunctional processes: Attention, Emotion, Language, Memory
and Executive Function
The thalamus regulates the functions of the association cortex and is important in functions such as language, speech and cognitive functions mediated by the cortex [24].
There are three major regions of association cortex, parietotemporooccipital, prefrontal and limbic toward which project different thalamic nuclei. So parietotemporo occipital cortex (areas 39 and 40) is related to the perceptual functions, vision and reading and receives information from the pulvinar.
The prefrontal association cortex is important for planning movements and behavior, cognition, learning, memory and thought. The dorsomedial nucleus project their fibers to this cortical area. A recent study performed in monkeys, which were asked ablation of the dorsomedial nucleus, magnocellular region has shown that the thalamic lesions in this area cause memory disorders mainly due to the disruption of function between this nucleus and the prefrontal cortex [25]. The limbic cortex associated with learning, memory and emotion, mainly receives input from the anterior nucleus of thalamus.
Attention thalamus and Scope.
The involvement of the thalamus and reticular formation in regulating level of arousal was evident already in the
first half of the twentieth century with the pioneering work that made Morison and Dempsey [26], Jasper [27] Moruzzi and Magoun and
[28].
The intralaminar nuclei are related to the general excitability of the cerebral cortex, to transmit information from the midbrain reticular formation to multiple cortical areas and striatum, and play an important role in controlling sleep and wakefulness. Stimulation
Electrical these nuclei causes widespread activation of the cerebral cortex (recruiting response), part of the anatomic substrate of the ascending reticular activating system and therefore the mechanisms of sleep and wakefulness.
The nuclei of the midline appear to be the place where the thalamus, together with the reticular formation, control signals accessing the cerebral cortex. The work done
in this field indicates that the thalamus regulating the degree of cortical arousal through thalamocortical connections originating from dorsomedial nuclei, intralaminar and the midline, and through interactions with the reticular nuclei intratalámicas [19.29].
Studies conducted in various animal species have provided evidence that the reticular nuclei are related to the sleep-wake cycle [19.29]. It has been found that GABAergic neurons of the reticular nuclei control the activity of thalamocortical neurons and thus modulate the cortical activity [29,30].
Studies in humans with functional neuroimaging techniques have shown that there are variations in thalamic blood flow depending on the level of consciousness [31,32]. Kinomura et al [33] have shown changes in blood flow to the intralaminar nuclei of the thalamus and reticular formation depending on the level of arousal of the subject.
In research conducted by Fiset et al [34], which manipulated the level of awareness of the subjects using Propofol - drug with anesthetic properties that decreases cerebral blood flow, which is accompanied by a
reduction of cerebral metabolic oxygen demand and decreased intracranial pressure, they found a negative relationship between thalamic blood flow (PET) and the concentration of propofol was used. The effects of this anesthetic are more pronounced in the medial thalamus, cingulate gyrus, orbitofrontal spin and angular gyrus. It appears that the variations observed in the thalamus (especially
medial area) are significantly related to the activity of the reticular formation. These authors suggest
reticulotalámico the system plays a fundamental role in the modulation of consciousness.
At the clinic has been observed that vascular lesions in intralaminar and dorsomedial nuclei can cause akinetic mutism and Kleine-Levin syndrome (hypersomnia syndrome and bulimia). This syndrome is characterized by recurring periods of excessive somnolence, hyperphagia, hypersexuality, and changes in recent memory. Different aspects of care may be attributable to prelímbico cortex and dorsomedial nucleus [35]. Thalamic infarcts can cause attentional deficits neglect and extrapersonal space contralateral to the lesion [36-38].
Thalamus and emotion
The main nuclei involved are the ventral anterior, the dorsomedial and anterior nuclear group. The anterior ventral receives input from the mammillary body and projecting fibers to the
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Girdle. The dorsomedial nucleus receives from the hypothalamus and amygdala and sends its fibers to the prefrontal lobe. The dorsomedial, with its projections to the prefrontal cortex and the limbic structures involved in the integration of visceral information with affection, emotions and thought. Previous half visual and emotional information. Electrical stimulation and ablation of this nucleus induce changes in the blood tesión and motivational drives.
Thalamus and language
Penfield and Roberts in 1959 [39] were the first to note that the thalamus, with its extensive cortical projections is related to language functions.
In the language, mainly the pulvinar, the lateral nuclear group (primarily ventroposterolateral and ventroposteromedial) and the anterior nuclear group. There are reciprocal connections between the pulvinar and the cortex important for language and symbolic thought (to the functional crossroads parietotemporooccipital). The ventroposteromedial ventroposterolateral and participate in language through their relations with somesthetic areas and specific integration occurs in them.
Electrophysiological Evidence of the involvement of the thalamus in the motor aspects of language. Mateer [40] found an increase in the duration of the verbal response after stimulating the left thalamus, resulting in a mispronunciation of words and articulatory changes.
Later, Andy and Bhatnagar [41] observed spasms articulatory motor nucleus after stimulation of centromedianoizquierdo.
Johnson and Ojemann [42] indicate that the ventrolateral area of the left thalamus (especially the center) participates in the integration of speech motor mechanisms, including breathing, as after stimulation of the thalamic area observed an inhibition of breathing, slowing of speech and the presence of perseverations.
The pulvinar is not only sandwiched between optical and acoustic tracks, but projects to cortical areas important for language and symbolic thought (parietotemporooccipital crossroads).
Injuries to the anterior nucleus or the pulvinar may cause anomia, semantic paraphasias and syntactic errors [43]. Ojemann [44] found that, after stimulation of the anterior (lateral part) of the thalamus, are repetitions of
words which have previously been called correctly. If the stimulation was performed in the central part of the ventrolateral region, perseverations appeared. Stimulation of the back of the ventrolateral region and anterior pulvinar resulted in the appearance of errors and omissions in the description
objects.
Thalamus and memory
It seems that the anterior thalamic nuclei are those of the midline, the dorsomedial and intralaminar thalamic nuclei involved in memory processes, although no conclusive evidence to indicate which of these structures is crucial for the proper functioning of the anterograde memory [45].
Weiskrantz [46] indicates that the memory deficits that often occur in patients with thalamic lesions are similar to those seen after lesions in the medial temporal lobe: a deficit in encoding new information resulting in an impaired memory anterograde, while memory remains intact in the short term. There is evidence of impairment of specific memory after thalamic lesions, especially in the dorsomedial nucleus [47], the former [48, 49] and the intralaminar nuclei [50]. It seems that the anterior nucleus is related to the consolidation of information, allows the formation of memory tracing, and working memory [51].
Recently, Celerier et al [52] have shown in mice that lesions in the anterior nucleus cause alterations in the performance of memory tasks. According to these authors, this group is linked to the nuclear maintenance of information in time, irrespective of the nature of the information, and processes the information associative unimodal and polymodal.
The anterior nuclei of the thalamus are involved in the process of temporal organization of memory [53]. Intralaminar nuclei output enable memory tracing already stored, ie, the activation process. In the process of temporal organization of recent and old memories involving the dorsomedial nucleus. The lesions in these nuclei can result in temporary disruption of memory that would affect not only new information but also to the former. Confabulations may occur, as described in Korsakoff syndrome. Victor et al [54] believe in 100% of patients with Korsakoff syndrome, the dorsomedial nucleus is affected, along with the mammillary bodies. The deficit is more severe if they involved the dorsomedial nucleus of the thalamus and midline nuclei [55]. Furthermore, Korsakoff's syndrome [56] has found a relationship between amnesia and the degree of atrophy in the nuclei of the midline, without any evidence unrelated to the atrophy of the mammillary bodies, hippocampus
or parahippocampal gyrus. Gaffan and Parker [25], in a study with monkeys have found that the dorsomedial magnocellular nucleus plays an important role in memory. An injury to this area leads to a change in this cognitive function attributable to disconnect the prefrontal cortex. However, despite these results, there is still controversy over whether the lesions in the dorsomedial can cause memory deficits. In an extensive review that made van der Werf et al [57] on the neuropsychological deficits that may occur after thalamic infarcts, note that there is insufficient evidence to establish the relationship of the dorsomedial
with memory problems that occur after diencephalic lesions. They conclude that the deficit
memory that can occur and that are compatible with an 'amnesic syndrome' depend on the integrity mamilotalámico tract. The involvement of the thalamus in memory processing is also shown by electrophysiological studies. Ojemann [44] found that ventrolateral thalamic stimulation affects the short-term verbal memory. Stimulation of this area during the presentation of material which will later be evoked reduces the number of errors. The left pulvinar stimulation alters rote verbal processing, while the right pulvinar stimulation alters rote nonverbal processing [42].
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Thalamus and executive function
The lesions in the thalamus may also cause alterations in executive functions, attention, initiative, inhibition and temporal organization of behavior, functions related to the prefrontal cortex. It is proposed that between the thalamic nuclei involved in executive function are the dorsomedial, the intralaminar nuclei and the midline.
Some patients show an impairment in executive functioning after selective dorsomedial infarction [48, 58]. Mennemeier et al [59] have indicated that patients with thalamic lesions may have difficulty using memory strategies, rather than having a default encoding of information. It is proposed that a gap between the core and dorsomedial prefrontal cortex may be responsible for the emergence of these deficits. However, there are data that show the occurrence of a similar impairment in executive function after thalamic infarcts not involving the dorsomedial nucleus. It has been reported that lesions in the intralaminar nuclei and the adjacent parts of the nuclei of the midline can cause deficits in executive function [48.59].
Van der Werf et al [57] indicate that lesions involving a single thalamic nucleus are not sufficient by themselves to display impaired executive function, it is necessary involvement of two or more nuclei (dorsomedial, intralaminar and midline).
The hypothalamus receives afferent connections multiple functions related to visceral, olfactory and limbic system.
Among these are:
Visceral and somatic afferents that reach the hypothalamus as collateral lemniscales systems via reticular formation.
- Cortical afferents that reach the hypothalamus directly from the frontal cortex.
- The afferents from the hippocampus via fornix-mammillary nuclei.
The limbic system consists of a series of complex structures, which are located around the thalamus and below the cerebral cortex. It is primarily responsible for the emotional life, and is a participant in the formation of [memory (process)], involving the hypothalamus, hippocampus, amygdala and four related areas. The main functions of the limbic system are the motivation for the preservation of the organism and the species, the integration of genetic and environmental information through learning, and the task of integrating our internal environment with the external behavior before.
The hippocampus ([TA]: hippocampus, which in turn comes from the Greek: ιππος, hippos = horse, and καμπος, kampos = sea monster Campe) is one of the main structures of the human brain and other mammals. The name was given by the sixteenth century anatomist Giulio Cesare Aranzio, who noticed a strong resemblance to the shape of the sea horse or hippocampus.
It is a marginal and less complex structure in terms of layers of the cortical gray matter of the temporal lobe. Therefore it belongs, on the one hand to the limbic system and the other to the archicortex, composing by the subiculum and dentate gyrus hippocampal formation call. Like the rest of the cerebral cortex is a paired, with two halves that are mirror images in both hemispheres. In both humans and other primates, the hippocampus is located inside the medial temporal lobe or internal, under the cortical surface. The seahorse shape is typical of primates, but in other mammals have various forms, such as bananas.
While rooted in a brain structure called the pallium of vertebrates, comprising olfactory functions in its current form in mammals mainly plays important roles in memory and space management. Studies on its function in humans are scarce, but it has extensively investigated in rodents as part of the brain responsible for spatial memory and navigation. Many neurons in the hippocampus of rats and mice respond as "place cells" or cell position: namely, that fire action potentials when the animal goes through a specific area of your environment. The "place cells" of the hippocampus interact extensively with the "cell orientation" of the head, which act as inertial compass, and also with the "grid cells" or cell network in the vicinity of the entorhinal cortex.
Because of its densely packed layers of neurons, the hippocampus has often been used as a model system for studying neurophysiology. The form of neuronal plasticity known as long-term potentiation (LTP) was first discovered in the hippocampus, and is still studied in this structure. It is widely hypothesized that LTP is one of the main neural mechanisms by which memory is stored in the brain.
In Alzheimer's disease the hippocampus is one of the first brain regions to suffer damage. Memory problems and disorientation appear among the first symptoms. Damage to the hippocampus can also come from situations of hypoxia, encephalitis or temporal lobe epilepsy. People who have suffered extensive damage in the hippocampus may experience amnesia, that is, the inability to acquire or retain new memories.
The term to refer to a limbic brain area was coined in 1878 by French physician Paul Broca, he spoke of le grand lobe limbique (the great limbic lobe) to refer to the area located to the lower edge of the pineal gland (limbus Latin MEANS very edge). The initial description Broca made the "great limbic lobe" was the one that is formed by three molecules in the form of bats, the "corozo" of this "racket" would be the nerve and-especially-the olfactory bulb, the top correspond to the or cingulate gyrus CINGULI (cingulus Latin for belt) and the bottom to the parahippocampal gyrus, for further annotation using the word "limbic" by Broca time provided by the bottom of the cerebral cortex. Henry Turner in 1890 called rhinencephalon (rinoencéfalo, brain nose) to most of the limbic areas of the importance that they seemed to charge the olfactory bulb and olfactory responses to stimuli (evolutionarily older than the areas corresponding to visual stimuli and hearing). James Papez circuit discovered in 1937 that bears his name. Paul MacLean (1949)-as-Jakob Christofredo spoke of "visceral brain" and extended these ideas to include more structures in a more diffuse in 1952 comes the name "limbic brain" and limbic system (as well as alongside the reptilian brain or reptilian brain that MacLean's limbic hypothesized as a precedent, and even "brain paleomamífero"). The limbic system concept has been extended by Goldar, Heimer, Nauta, Yakovlev and others.
However, it maintains a strong controversy about the definition of the limbic because if initially, when he coined the word, it was postulated that the limbic area was only the instinctive and emotional center of the brain being the cognitive, intellectual and rational as an activity typical of the neocortex, it was soon discovered that such a distinction as exhaustive is more diffuse: for example an injury to the hippocampus leads to severe cognitive impairment.
Cortical rim areas corresponding to the limbic system are generally less than the typical neuronal layers 6 layers of the bulk of the neocortex and are classified as being arqueocórtex alocórtex and phylogenetically primitive.
In various schools of psychology have been considered during the twentieth century that the limbic system corresponded to the location of the called subconscious while phylogenetically newer areas of the cortex or cerebral cortex were related to consciousness, although this is partially true localizacionanismo more truth is that the activities of human thought or perhaps always almost always involve the entire central nervous system activity, though certainly more elaborate processing (the intellectual-cognitive-reflective) can only be carried out in modern cortical areas located in front prefrontal cortex, while emotions or instincts (usually processed mainly by neocortical areas in instincts in man) have a "relay" or main processing area in the limbic system.
In the hippocampus is predominantly found two "modes" of activity, each associated with a different pattern of neuronal population activity and waves of electrical activity as measured by electroencephalography (EEG). These modes are named corresponding to EEG patterns associated with them: theta waves and patterns over irregular activity (LIA). The main characteristics described below were observed in rats, which is the animal most widely estudiado.40
Theta mode appears during states of alert and activity (especially in locomotion) and also during sleep REM.41 In this mode, the EEG is dominated by long and irregular waves with a frequency range between 6-9 Hz, and major groups of hippocampal cells (pyramidal cells and granule cells) are a population activity low, which means that in a short time interval, the vast majority of cells are silent, while the remaining fraction trips at relatively high rates, higher than 50 spikes per second in the case of the most active. An active cell typically remains in this state for half a second a few seconds. As the rat conducts its business, active cells are silent and new cells become active, but the overall percentage of active cells remains more or less constant. In many situations, cell activity is largely determined by the spatial location of the animal but also other behavioral variables clearly influence it.
LIA mode appears for short-wave sleep without dreams, and also during states of immobility to walk, as when at rest or comiendo.41 In the LIA mode, the EEG is dominated by sharp waves, which are set to times random for EEG signals lasting around 200-300 ms. These sharp waves also determine the patterns of neural population activity. These include pyramidal and granular cells are very active (but not silent). During a sharp wave, an amount as large as 5-10% of the population can emit a neural action potential during a period of 50 ms, many of these cells emit several outbreaks of action potentials.
These modes of hippocampal activity can be tested in primates as in rats, except that it has been difficult to see a robust theta rhythmicity in the hippocampus of primates. There are, however, qualitatively similar sharp waves, and similar changes in the state-dependent activity of the population neural.42
Stress.
The hippocampus contains high levels of mineralocorticoid receptors which makes it more vulnerable to biological stress long term than other areas cerebrales64 The stress-related steroids affect the hippocampus in at least three ways: first, reducing the excitability of some neurons of the hippocampus. Second, inhibiting the genesis of new neurons in the dentate gyrus, and third, resulting in atrophy of dendrites of pyramidal cells of the CA3 region. There is evidence that humans who have experienced severe traumatic stress and long term (for example, Holocaust survivors) show atrophy of the hippocampus to a greater extent than other brain areas. These effects are observed in posttraumatic stress disorder and may contribute to hippocampal atrophy seen in schizophrenia and depression mayor.65 hippocampal atrophy is also frequently seen in Cushing's syndrome, a disorder caused by high levels of cortisol in the bloodstream. At least some of these effects appear reversible if the stress continues. There are, however, evidence derived mainly from studies using rats that stress may affect shortly after birth to hippocampal function so that the damage persists throughout life.
Long-term potentiation
Main article: long-term potentiation.
Since at least the time of Ramon y Cajal, psychologists have speculated that the brain stores memory by altering the connections between neurons that are active simultáneamente.53 This idea was formalized by Donald Hebb in 1948.54 but many years later, attempts to find a brain mechanism for these changes were in vain. In 1973, Tim Bliss and Terje Lomo describe a phenomenon in the rabbit hippocampus seemed to fit the specifications of Hebb: a change in synaptic responsiveness activation induced by short, sharp and lasting for hours, days or más.55 Soon scientists referred to this phenomenon as "LTP" English Abridged LTP long-term potentiation. As a candidate mechanism for memory, the LPT has been studied intensively in the coming years, of which much has been learned.
The hippocampus is a particularly favorable site for studying the LTP for its densely packed layers of neurons and clearly defined, but now have found similar changes dependent synaptic activity in many other areas cerebrales.56 The best studied form of LTP occurs in synapses ending on dendritic spines and use the neurotransmitter glutamate. Several of the major pathways of the hippocampus fit this description, and present LTP.57 synaptic changes depend on a special type of glutamate receptor or NMDA receptor has the special property of allowing the calcium entry into the postsynaptic spine only when presynaptic activation and postsynaptic depolarization occur at the same tiempo.58 Drugs that interfere with NMDA receptors block LTP and also have important effects on some types of memory, particularly spatial memory. Transgenic mice genetically modified to disable the LPT mechanism also generally show severe deficits memoria.58
Role in Memory
See also: Amnesia
Psychologists and neuroscientists generally agree that the hippocampus plays an important role in the formation of new memories of events experienced both episodic autobiográficos.3 15 Part of this role that the hippocampus is involved is the detection of events , places and stimuli novedosos.16 Some researchers conceive the hippocampus as part of a larger system memory of the medial temporal lobe declarative memory responsible for general. The reason, for example, memories can be verbalized explicitly, which affects, for example, to the memory of the memory also made episódica.11
Severe hippocampal lesions produce profound difficulties in forming new memories (anterograde amnesia), and often also affects memories formed before the injury (retrograde amnesia). Although the retrograde effect normally extends some years before brain damage, in some cases older memories remain. This preservation of memories former led to the idea that consolidation over time implies the transfer of memories from the hippocampus to other parts of cerebro.15
Damage to the hippocampus does not affect some types of memory such as the ability to acquire new motor or cognitive skills (playing a musical instrument or to solve certain types of logic puzzles, for example). This suggests that such abilities depend on different types of memory (procedural memory, for example) and different brain regions. Furthermore, amnesic patients often show memory "implicit" to the experiences, even in the absence of conscious awareness. For example, when you ask a patient to say which of two faces is one that has seen more recently, can almost always hit the right answer, despite claiming he had never seen any of them. Some researchers distinguish between conscious recollection, which depends on the hippocampus, and familiarity, which depends on parts of the temporal cortex medial.17
Role in spatial memory and orientation
Main article: Cell site.
Spatial firing patterns seven registered place cells in CA1 of rat layer. The rat traverses several hundred turns in the direction of clockwise around a triangular track, stopping in the middle of each side to eat a small portion of food. Black dots indicate the positions of the head of the rat. The colored dots indicate action potentials, using a different color for each célula.18
Studies have been conducted in rats and freely moving in mice have shown that hippocampal neurons have "place fields", ie are neurons which trigger action potentials when a rat passes a particular location of their environment. Evidence of such neurons in primates are limited, perhaps partly because it is difficult to record brain activity in freely moving monkeys. Neuronal activity was reported at hippocampus relates to the place where experimental monkeys traveled within a room as they sat in a chair somewhat limiting their movimientos.19 On the other hand, Edmund Rolls and colleagues reported that instead of hippocampal cells that fire in relation to the place where the primate turns his gaze, instead of where it places its cuerpo.20 In humans, place cells found in a study of patients with drug resistant epilepsy and subjects an invasive procedure to locate the source of their attacks. This allowed the exploration through the area subject to surgical resection. Patients were diagnosed electrodes implanted in hippocampus. Later they were asked to employ a computer to navigate in a virtual environment representing a ciudad.21
We have studied the location-dependent responses in hundreds of experiments on four decades, resulting in a large amount of information.14 The responses of the cells in place are shown by pyramidal cells in the same hippocampus, and in the granulosa cells of the dentate gyrus. These constitute the majority of neurons in the densely packed layer of the hypothalamus. Inhibitory interneurons, which constitute the majority of the remaining cell population, often show significant variations dependent on where the firing rate, but much weaker than previously shown by pyramidal cells or granule. There is little, if any, topography spatial representation: the cells that are located close together in the hippocampus usually have spatial patterns of uncorrelated firing. The place cells are typically almost silent when the rat moves outside the field of place, but reach sustained rates shooting up to 40 Hz when the rat is near the center. The neural activity of 30-40 cells sampled randomly taken place have enough information to allow the location of the rat is reconstructed with sufficient accuracy. The size of the fields of a gradient rather varies along the length of the hippocampus, with cells of the dorsal showing smaller fields, the cells near the center showing larger fields, and ventral cells covering the apex completo.14 environment In some cases, the firing rate of rat hippocampal cells depend not only on the site, but also the direction in which the animal moves, the destination addresses to which other variables related to task realiza.22
The discovery of place cells in 1970 led to the theory that the hippocampus may act as a cognitive map, ie, a schematic representation of the spatial neural entorno.13 Some lines of evidence support this hypothesis. It is a common observation that without a fully functional hippocampus, humans do not remember where they've been and how to get to the place where they go: the sense of loss is one of the most common symptoms of amnesia.23 Animal studies have shown that requires an intact hippocampus for some spatial memory tasks, particularly those who need to find a path to a target oculto.24 The "cognitive map hypothesis" has received further impetus by the recent discovery of the "cells of head direction "network cells" and "boundary cells" in different parts of the rodent brain that are strongly connected to hipocampo.14 25
Neuroimaging techniques applied to the brain show that people have a more active hippocampus when directed correctly, as verified by guidance tasks in an environment virtual.26 However there is also evidence that the hippocampus plays a role in the activity to find shortcuts and new routes between familiar places. For example, London taxi drivers must learn a large number of places and the most direct routes between them (and have to prove it by passing a strict examination, known as "The Knowledge" before becoming licensed to drive the famous black cabs). A study by University College London by Maguire, et al. (2000) 27 showed that a part of the hippocampus is larger in taxi drivers than in the general public, and more experienced drivers had a bulkier hippocampus. Remains to be elucidated whether having a larger hippocampus helps an individual to become a taxi driver, or find shortcuts in life do you grow your hippocampus. However, the study of Maguire et al. examined the correlation between gray matter and the time taken by a taxi driver in their careers, finding a positive correlation between them. It was obvious that the total volume of the hippocampus remained constant between the control and the taxi drivers. This is the same as saying that the rear portion of the hippocampus of the taxi driver actually increased, but at the expense of the anterior portion. No harmful effects have been found in the disparity in the proportions of the hippocampus.
- The input from the amygdaloid nucleus stria terminalis pathway.
Basal Ganglia (ganglion Greek, "conglomerate", "knot", "tumor") are collections of nerve cell bodies are found near the base of the brain, within the telencephalon. This gray nerve tissue is interconnected with the cerebral cortex, thalamus and brain stem. In mammals are associated mainly with the movements (which have their origin in the motor cortex): its fibers, which are not addressed directly to the spinal column, link to the supraspinal motor center of the brainstem, groups of neurons send nerve fibers to the spinal cord. The basal ganglia are associated with voluntary movements performed in a mainly unconscious, that is, those involving the whole body or daily routine. The basal ganglia are located on an area called the striatum, two gray bodies separated by a fiber bundle, called the internal capsule. On this are putting the basal ganglia: the caudate nucleus, putamen, globus pallidus, subthalamic nucleus and substantia nigra. On the inner side of the internal capsule is the caudate nucleus (nucleus of the tail) and putamen on the outer side (shell-shaped core), which is situated next to the globus pallidus (a triangular structure in light gray with a thin layer of white matter in half sometimes joins the putamen to form the lentiform nucleus). Located next to the globus pallidus, but further inland, is the subthalamic nucleus, and below this, the substantia nigra. We discuss whether a thin strip of gray matter next to the putamen, the cloister, of unknown function, belongs in the basal ganglia: the damage of the basal ganglia involves a failure of coordination involves the appearance of the characteristic symptoms of a global motor disorder, especially the movements characteristic of diseases such as Parkinson's, the tribalism and Huntington's chorea.
- The inputs from the thalamus nuclei from dorsomediano and the midline.
- The afferents from the mesencephalic tegmentum.
Efferent connections of the Hypothalamus
- These are also very numerous and complex. Among them are:
- Mamilotalámicas efferents to the anterior nucleus of the thalamus, and then projecting to the cingulate cortex.
- Mamilo-tegmentales efferent connections that allow the mesencephalic tegmental reticular formation.
- Descending efferents to the brainstem and spinal cord.
- These allow the hypothalamus to influence the segmental sympathetic and parasympathetic centers such as accessory oculomotor nucleus, nucleus salivatorios upper and lower dorsal vagal nucleus, nucleus of the lateral horn sympathetic, parasympathetic nucleus of the lateral region of the spinal intermediate sacred.
- The hypothalamus also establishes connections with the pituitary gland in two different ways. One is through the hypothalamic-hypophyseal tract, and the other is through a portal system of capillaries.
Hypothalamic-pituitary tract
- Allows the hormones vasopressin and oxytocin, which are synthesized by neurons of supraoptic and paraventricular nuclei, respectively, are released into the axon terminals that contact with the neurohypophysis. These hormones act to produce vasoconstriction and antidiuresis (vasopressin) or uterine muscle contraction and myoepithelial cells surrounding the alveoli of the mammary gland (oxytocin) in women.
Hypophyseal portal system
- It consists of capillaries that form a network that descends to the anterior lobe of the pituitary.
- The portal system carries hormone releasing factors are synthesized in the hypothalamus and whose action in the posterior lobe of the pituitary induce the production and release of hormones such as:
Adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH), luteinizing hormone (LH), thyrotrophic hormone (TSH), growth hormone (GH), etc..
Functions of the hypothalamus
One of the vital functions that have the hypothalamus is the management of our internal system, homeostasis or internal balance. This control is done through two ways: Via Via endocrine and SNA.
VIA NERVE
- The hypothalamus also controls the autonomic nervous system. Various centers of the hypothalamus adjust and coordinate
activities visceromotor centers of the brainstem and spinal cord, to regulate heart function
(Frequency), blood pressure, respiration, digestive activity, etc.
- For example, if we stimulate the anterior hypothalamus is like estimuláramos the parasympathetic system and if we stimulate the
posterior hypothalamus to stimulate the sympathetic system.
- Therefore, the hypothalamus is related to the coordination between voluntary and autonomic functions. When an individual
facing stressful situations, the heart beats at a faster rate, respiratory rate is altered, it can
produce sweating, redistribution of blood flow, etc..
- It also has regulatory function of temperature, sleep and wakefulness, ie circadian rhythm.
- An injury of the posterior hypothalamus causes sleepiness.
- The ventromedial nucleus is that of satiety.
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