Septal area · Grey Matter

The septal area is a small group of nuclei on the midline of the forebrain, under the front of the corpus callosum, that adjusts the intensity of emotional and motivated behaviour and sets the rhythm of the hippocampus.

Septal area. The septal area is a small group of nuclei on the midline of the forebrain, under the front of the corpus callosum, that adjusts the intensity of emotional and motivated behaviour and sets the rhythm of the hippocampus.

It lies just below the genu of the corpus callosum and in front of the third ventricle, beneath the frontal lobe. Its two main parts do different jobs. The medial septum sends cholinergic and GABAergic fibres to the hippocampus and paces its theta rhythm, which is needed for spatial memory. The lateral septum receives from the hippocampus and projects to the hypothalamus, tuning aggression, anxiety, feeding and social behaviour.

Lesions in rats make them hyper-reactive. Brady and Nauta described in 1953 a septal rage: animals with septal lesions attacked or startled at harmless handling, an effect that fades over weeks.

Stimulation can be rewarding. In 1954 Olds and Milner found that rats would press a lever again and again to stimulate electrodes placed in or near the septal area, the first demonstration of reward sites in the brain; later work traced the strongest effects to the fibres of the medial forebrain bundle nearby.

It is part of the limbic system in every classic list, and one of its least studied members in humans.

Questions: Why did destroying the septal area make rats furious, while stimulating it made them seek more? The lateral septum sends largely inhibitory projections to the hypothalamus and other regions that generate defensive and aggressive behaviour, so it acts as a brake on how strongly an animal reacts. Brady and Nauta found in 1953 that rats with septal lesions attacked or startled at harmless handling, the brake gone, an effect that fades over weeks. In 1954 Olds and Milner found that rats would press a lever repeatedly to stimulate electrodes in or near the septal area, the first evidence of reward sites in the brain, later traced mostly to the medial forebrain bundle that runs nearby toward the hypothalamus. What does the medial septum do for the hippocampus? The medial septum sends cholinergic and GABAergic fibres through the fornix to the hippocampus and acts as the pacemaker of its theta rhythm, a 4 to 10 Hz oscillation present when an animal explores or attends. Theta organises the timing of hippocampal firing, including the sequences of place cells, and lesions or inactivation of the medial septum abolish it and impair spatial memory. The septum thus gives the memory system a clock, while the lateral septum carries the hippocampus's output toward the hypothalamus.