THE BRAIN FROM TOP TO BOTTOM (original) (raw)

PLASTICITY IN NEURAL NETWORKS

No one neuron alone contains all the information needed to reconstruct a memory. Rather, the trace of that memory is latent or virtual. Its existence can be manifested only when a network of many interconnected neurons is activated.

Multiple memories can be encoded within a single neural network, by different patterns of synaptic connections. Conversely, a single memory may involve simultaneously activating several different groups of neurons in different parts of the brain.

This association of groups of cortical neurons distributed across different parts of the brain is made possible by certain networks of neurons that are pre-wired for this task. Certainly the best known of these networks are the circuits of the hippocampal formation, which are involved in establishing explicit long-term memories.

The information from the visual, auditory, and somatic associative cortexes arrives first at the parahippocampal region of the cortex, then passes through the enthorinal cortex and on to the hippocampus proper. Within the hippocampus, the information passes through three distinct regions in succession.

The hippocampus proper is composed of regions with tightly packed pyramidal neurons, mainly areas CA1, CA2, and CA3. (“CA” stands for Cornu Ammonis, or Horn of Ammon. The reference is to the ram’s horns of the Egyptian god Ammon, whose shape these three areas together roughly resemble.) This is what is called the trisynaptic circuit or trisynaptic loop of the hippocampus.

Information enters this one-way loop via the axons of the entorhinal cortex, known as perforant fibres (or the perforant path, because it penetrates through the subiculum and the space that separates it from the dentate gyrus). These axons make the loop’s first connection, with the granule cells of the dentate gyrus. From these cells, the mossy fibres in turn project to make the loop’s second connection, with the dendrites of the pyramidal cells in area CA3. The axons of these cells divide into two branches. One branch forms the commissural fibres that project to the controlateral hippocampus via the corpus callosum. The other branch forms the Schaffer collateral pathways that make the third connection in the loop, with the cells in area CA1.

It is in these synapses that the spatial memory associated with the hippocampus seems to be encoded (see sidebar). This region also displays a high propensity for long-term potentiation (LTP), though this same phenomenon is also observed in many other parts of the hippocampus as well as in the cortex.

Lastly, the axons of the cells in CA1 project to the neurons of the subiculum and of the entorhinal cortex. The receiving portion of the hippocampal formation thus consists of the dentate gyrus, while the sending portion consists of the subiculum. The axons of the large pyramidal neurons of the subiculum then project to the subcortical nuclei via the fimbria, a thin tract of white matter at the inner edge of the hippocampus. Lastly, the information returns to the sensory cortical areas from which it came before it was processed by the hippocampus.