Hippocampal organization Anatomy

The hippocampus resembles the neocortex in containing a majority of excitatory neurones, the pyramidal cells and granule cells, which use glutamate as their neurotransmitter (E in Fig 1).(12) Most of0 the remaining neurones in the hippocampus are inhibitory, and use g-aminobutyric acid (GABA) as their neurotransmitter (I

in Fig 1). The inhibitory neurones fall into several distinct subtypes according to where their axons go (and hence which cells they inhibit), where their cell bodies are, the shapes of their dendrites, whether they contain more than one transmitter, and whether they contain particular calcium-binding proteins. This chapter will ignore most of the diversity of interneurones.

Fig. 1 Schematic illustration of hippocampal neuronal network. There are many more excitatory pyramidal cells (E and triangles) than inhibitory interneurones (I and circles). As with most neurones, they receive inputs onto their dendrites and somata (the latter contain the nucleus and are represented by a triangle or circle). The level of simplification is clear from the observation that each pyramidal cell receives tens of thousands of synapses. Axons from other regions, known as afferents, make excitatory synapses (e) with both pyramidal cells and interneurones. Most of the interneurones make inhibitory synapses (i) onto pyramidal cells. The inhibition of the pyramidal cell is called 'feed-forward' (f.f.) when the interneurones were excited by afferent axons and 'feed-back' (f.b.) when they were excited by pyramidal cells. Interneurones also inhibit each other forming a mutually inhibitory network (m.i.); this network is important in some kinds of physiological network oscillation (see text). Finally, pyramidal cells make excitatory synapses onto each other (r.e.) which can lead to epileptic discharges if not held in check by inhibitory mechanisms.

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