Hipotálamo

NEUROTRANSMISORES DEL SISTEMA LÍMBICO. HIPOCAMPO, GABA Y MEMORIA. PRIMERA PARTE
Eduardo Castro-Sierra1,Fernando Chico Ponce de León2, 3, Luis Felipe Gordillo Domínguez2, Alison Portugal Rivera2

SUMMARY
Introduction. The entire hippocampus is derived from the telencephalon. Embryologically, it is made up of the most archaic cortices. Through special phylogenetic and ontogenetictelencephalization processes, it will arrive at its particular mesial basal position. This structure has three components: a) Retrocommisural hippocampus, or hippocampus proper (RH). b) Supracommisural hippocampus (SH). c) Precommisural hippocampus (PH). The RH is situated in the most medial part of the 5th temporal gyrus (5 TG). The outer/upper face of the RH is to be found in the temporal recess of thelateral ventricle. It is called pes hippocampi or albeus. Inwards, it is limited by the choroid fissure, outwards and downwards by the parenchyma of the 5th TG, forwards, by the amygdala of the striatal body and, backwards, by the isthmus. The fornix is a continuation of efferent pathways from CA3, CA1 and the subiculum. By means of a circular course, it ascends over the thalamus and, descending infront of Monro’s foramina and traversing the hypothalamus, reaches the mammillary bodies. It consists of fimbria, posterior pillars and a body and anterior pillars. The latter pass behind the anterior white commisure (AWC), and make up the anterior portion of Monro’s foramina. The SH originates in the RH. At the level of the splenium of the corpus callosum (CC), the fornix produces two striae, medialand lateral, and the dentate gyrus turns from fasciola cineria into induceum griseum. These structures are to be found in both hemispheres and, traveling over the CC, will reach the preoptic and hypothalamic septal areas, as well as the PH. The PH is a small fiber contingent which stems from the fornix at the level and in front of the AWC. Memory. General aspects. There is general agreement thatthe main role of the hippocampus is that of creating new memories relative to experienced events (episodic or autobiographic memory). Some researchers, however, prefer to think of the hippocampus as part of a major medial temporal lobe memory system responsible for declarative memory. This memory would include, besides episodic memory, memory of events. Another very important hippocampal functionwould relate to storage of semantic (conceptual) memories. Engrams. Memory and synaptic plasticity. Engrams are hypothetical means whereby memory traces are stored as physical or chemical

changes in the brain in response to external stimuli. The existence of engrams has been proposed by diverse scientific theories which try to explain the persistence of memory and how some memories are stored inthe brain. The term engram was coined by Sermon and explored by Pavlov. Lashley tried to locate the engram and failed in finding a sole biological locus for the same which made him think that memories were not localized in any particular part of the brain, but distributed throughout the cerebral cortex. Afterwards, in 1949, Hebb, a student of Lashley’s, published his empiricist theories in TheOrganization of Behavior. Hebb referred to Lorente de Nó’s reverberating circuits to propose a mechanism for maintaining activity in the cerebral cortex after the external stimulus had ceased: the so called central autonomous process. This led him to consider the cellular assembly, a complex reverberating circuit which could be assembled by experience. Changes in synaptic resistance with experiencewere eventually named Hebb’s, or the Hebbian, synapse. Hebbian theory describes a basic mechanism for synaptic plasticity by means of which an increment in synaptic efficacy stems from repetitive and persistent stimulation of the post-synaptic cell. This theory receives the name of Hebb’s rule. The fact that memory is persistent stresses the relevance of understanding those factors which...