The Hippocampal Slice
Objective 2: Explain the importance of the hippocampal slice preparation to studies of learning and memory.
The name hippocampus derives from the Greek word for “seahorse”. It is easy to see how the 16th century Italian anatomist Giulio Cesare Aranzio coined the term: this part of the brain looks like a seahorse.
It is an important part of the limbic lobe, a set of interconnected structures that make up the bulk of the brain structures both creating and regulating emotional responses. Its inputs come from olfactory and emotional parts of the brain. The cingulate cortex, temporal lobe cortex, amygdala, orbital cortex, and olfactory bulb all feed into the entorhinal cortex, adjacent to the hippocampus. Sensory association areas also connect to the entorhinal cortex. Sensory association areas are the regions of the brain where sensory information is integrated between senses and subjected to higher-order processing which forms a complete, conscious perception of the world. The medial septum, which has been called “an information hub for social memory”, also provides significant input to the hippocampus. A region called the subiculum lies between the entorhinal cortex and the hippocampus.
The hippocampus, in turn, sends efferents to the entorhinal cortex and amygdala, and via a major fiber bundle called the fornix to the mammillary bodies of the thalamus as well as many areas of cortex. This wiring supports its primordial function: to add an emotional “tag” onto memories of a particular place. One can easily imagine that an animal needs to remember where one encounters dangerous stimuli; I have a clear memory of a specific location on a running trail where I encountered a skunk almost 10 years ago.
The hippocampus is made up of two interlocking letter Cs: one C is called the dentate gyrus. The dentate gyrus, which can be seen in the photograph of the dissected hippocampus, looks like a set of teeth from the outside of the hippocampus, hence the name.
The other letter C has four regions. The entire area of the C has been named cornu Ammon, or “Ammon’s horn” in English, after an Egyptian/Greek deity, named Amun or Amun-Ra in Egypt who became Zeus in Greek mythology. Amun/Ammon was often represented as having a man’s head with ram horns, and those horns are the cornu Ammon. In the hippocampus, cornu Ammon (CA) is divided into four numbered areas: CA1, CA2, CA3, and CA4. CA1 is closest to the subiculum while CA4 is closest to the dentate gyrus.
Of course, Scoville’s inadvertant experiment on Henry Molaison also provided us with important clues as to the function of the human hippocampus.
Some 20 years after Henry’s surgery, and 24 years after Hebb’s seminal monograph on modifiable synapses, Timothy Bliss and Terje Lømo published a key paper in the Journal of Neurophysiology describing a new way of studying the hippocampus in experimental animals.
Oxygen, needed for neuronal function, can only penetrate about 1/5 of a millimeter (200