Kortiko-hipokampal Devre: Beynin Haritalama ve Deklaratif Bellek Merkezi

Dış dünyadan duyu sistemleri aracılığıyla algılanan uyaranlar, ilgili birincil kortekse ulaştıktan sonra çeşitli asosiyasyon kortekslerine iletilir. Uzunsüreli bellek olarak kodlanacak veya süregelen hipokampal hesaplamalarda kullanılacak bilgiler, buradan sırasıyla perirhinal/parahipokampal(postrhinal) korteks ve entorhinal korteks üzerinden hipokampusun dentat girus bölgesine aktarılır. Asosiyasyon kortekslerini ortalama 3-4 sinapsile hipokampuse bağlayan bu kortiko-hipokampal devre, deklaratif (eksplisit) belleğin temel nöronal altyapısını oluşturur. Bu nöronal sistemindış dünyadan en uzak yapısı olan hipokampus, uzun süreli bellek edinimi ve konsolidasyonunu gerçekleştirir. Hipokampus esasen çeşitli kortikalbilgileri birleştirip, ortaya anlamlı bütünler çıkaran bir haritalama merkezidir. Uzun süreli belleğe dönüştürülecek semantik bilgileri bir araya getirenhipokampus, bir belleksel (mnemonic) navigasyon sistemi gibi çalışır. Hipokampusun geniş kapsamlı haritalama ve navigasyon yetisini oluşturannöronal altyapı, aslında mekansal bellek için evrilmiştir. Hipokampusun CA3 ve CA1 alt bölgelerinde bulunan ana tip glutamaterjik hücrelerin, belirlibir alan içinde sabit konumlarını kodlayan yer (belirleme) hücreleri (place cells) oldukları keşfedilmiştir. Görece yakın zamanda, hipokampuse enyoğun sinaptik girdiyi sağlayan entorhinal kortekste de benzer konum hücreleri (grid cells) bulunmuştur. Bu bakımdan, dış dünyadan farklı duyusistemleri aracılığıyla, beynin sinaptik bakımdan iç kısımlarına aktarılan girdilerin, entorhinal korteksten itibaren anlamlı bir bütün oluşturmayabaşladığı düşünülmektedir. Buna karşın, entorhinal bölgeye en yoğun girdiyi sağlayan perirhinal ve parahipokampal korteksin işlevleri yeterincearaştırılmamıştır. Bunun önemli bir nedeni bu bölgelerde, yer belirleme hücreleri gibi, dış dünya ile ilişkisi birebir gözlemlenen nöron tiplerininkeşfedilmemiş olmasıdır. Bununla beraber, in vivo elektrofizyolojik kayıtlarda oldukça sessiz (inaktif) olan perirhinal korteksin hangi girdilerin uzunsüreli belleğe dönüştürülmek üzere hipokampuse aktarılacağını belirleyen kritik bir filtreleme istasyonu olduğu anlaşılmaktadır. Bu derleme, beyninharitalama ve deklaratif bellek merkezi olan kortikal-hipokampal devreyi oluşturan yapıların nöroanatomik ilişkilerini, monosinaptik bağlantılarını,kendilerine özgü elektrofizyolojik özelliklerini ve bellek süreçlerindeki işlevlerini incelemektedir.

The Cortico-hippocampal Circuit: The Brain’s Center for Mapping and Declarative Memory

Perceived stimuli from the external world are relayed to various association cortices after they reach their respective primary cortices. Information to be encoded as long-term memory is relayed to the dentate gyrus via perirhinal/parahippocampal and entorhinal cortices. This cortico-hippocampal circuit connecting the association cortices to the hippocampus via 3-4 synapses, forms the neuronal basis of declarative memory. Hippocampus, the most distant structure of this neuronal system from the external world, carries out the acquisition and consolidation of long-term memories. In fact, hippocampus is a mapping station that associates various cortical information to produce meaningful wholes. Putting together the semantic information to be encoded as long-term memory, hippocampus works as a mnemonic navigation system. The neuronal mechanism underlying this wide-ranging mapping capacity has evolved for spatial navigation. Principal glutamatergic cells of the CA3 and CA1 regions of hippocampus have been identified to be place cells. Similar location-encoding cells have been found in the entorhinal cortex, which provides the bulk of input to the hippocampus. Accordingly, it is theorized that inputs relayed to the central parts of the brain from the external world through different sensory systems, start forming meaningful gestalts once at the entorhinal cortex. In contrary, the perirhinal/parahippocampal cortex, providing the densest input to the entorhinal region, has not been sufficiently investigated. An important reason is that in these regions, no neuronal types showing a direct relationship with the external world, such as location-encoding cells, have been discovered. However, it is understood that the perirhinal cortex, remaining very silent in in vivo recordings, is a critical filtering station determining the inputs to be relayed to the hippocampus. This review focuses on the neuroanatomical correlates, monosynaptic connections, unique electrophysiological properties and differential roles in memory processes of the structures that make up the cortico-hippocampal circuit-brain’s center for mapping and declarative memory.

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