116 Lecture 13.S10 - CBNS 116 Lecture 13 The Hippocampus...

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CBNS 116 Lecture 13 - The Hippocampus: Intrinsic Circuitry and Physiology Lecture notes by Todd Fiacco ASSIGNMENT: In Nolte read Chapter 24: “Formation, modification, and repair of neuronal connections”, pp. 616-626. FOR NEXT LECTURE: Read Nolte Chapter 16: “The thalamus and internal capsule: Getting to and from the cerebral cortex”. TODAY'S LECTURE: The big picture: The hippocampus is one of the most intensely studied structures in all of neuroscience. The reasons for this are mainly because: 1) The simple 3-layered cytoarchitecture of this cortical area is arranged into very distinct laminae, or layers, making it very amenable to electrophysiological analysis; 2) Excitatory synapses here exhibit long-term potentiation (LTP) and long-term depression (LTD), synaptic phenomena that may underlie learning and memory in the brain (lesion studies have already told us the importance of the hippocampus in learning and memory); and 3) The hippocampus is particularly sensitive to pathological changes occurring in the brain, being the primary target of neurodegeneration seen in Alzheimer’s disease and is the site of initiation of seizures in one of the most common forms of epilepsy (called “medial temporal lobe epilepsy”). Epileptic seizures were the reason behind the operation to bilaterally remove patient H.M.’s hippocampus. I. Anatomy and intrinsic circuitry of the hippocampus [slides 24-28]. A. Looking at a cross section through the hippocampal formation reveals 2 interlocking cortical structures (like 2 interlocking “C”s): One interlocking “C” is called the dentate gyrus , while the second “C” is made up of the “hippocampus proper” or cornu ammonis [slide 24]. Each of these structures contains a different “principal” cell type. (A “principal cell” refers to the main projecting cell type that uses the excitatory neurotransmitter glutamate; as opposed to a locally projecting interneuron that uses GABA as a neurotransmitter). The principal cell type of the dentate gyrus is the dentate gyrus granule cell , while pyramidal neurons are the main cell type of the cornu ammonis. The pyramidal neurons of cornu ammonis are separated into 4 fields based on differences in cytoarchitecture (cell layering) and connections. These are, running from the end of the
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“C” emerging from the dentate gyrus, CA3 (CA for c ornu a mmonis), CA2, CA1 , and finally, the subiculum . Together the pyramidal cells of the CA fields and subiculum constitute the pyramidal cell layer of the hippocampus. The pyramidal cells of the CA fields are very tightly packed together, as can be seen in the Nissl stain of slide 26. The 3 layers of the cornu ammonis are [slide 26]: 1) The pyramidal cell layer (called stratum pyramidale ; stratum is from the Latin meaning “layer”), containing the tightly packed cell bodies of the pyramidal cells; 2) The molecular layer , the primary synaptic layer in the hippocampus where the large apical dendrites (the dendrites coming off of the apex of the pyramidal cells) receives synaptic input. The molecular layer is further subdivided into 2 layers:
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