First recordings from medial mammillary body cells in awake animals

It's often said that the key to Bill Bradley's basketball success was summed up in the title of the 1965 book by John McPhee, "A sense of where you are." Bradley always seemed to know where all nine other players were, where Bradley himself was in relation to the basket – and where the open spot was to be found for his stylized jump shot.

"Navigation is a very interesting problem: It's very abstract and involves a high level of higher integrative, cognitive skills," noted Patricia E. Sharp, of Bowling Green State University. "And it turns out that the humble laboratory rat probably solves navigational problems about as well as we do," she adds.

Sharp and her collaborator Shawnda Turner-Williams measured the electrical firing of 51 individual cells in the medial mammillary nucleus of five rats' brains – "to our knowledge…the first recordings from medial mammillary body cells in awake animals," according to their research paper.

The paper "Movement-related correlates of single cell activity in the medial mammillary nucleus of the rat during a pellet-chasing task," appears in the Journal of Neurophysiology, published by the American Physiological Society.

Sharp and Turner-Williams found that about one-third (17 of 51) of the cells "showed a significant, closely timed relationship to angular head motion" meaning that "the cell population rate vector provides an unambiguous indication of whether the movement trajectory is to the left or to the right," according to the paper. "In addition to these angular head velocity correlates, many (nearly 60%) of the cells were correlated with running speed. The majority of these showed a positive correlation, so that faster running was accompanied by higher firing rates," the paper noted.

What the brain needs to figure out where it's headed is still pretty theoretical. One model figures three types of information are needed: (1) current position (spatial location), (2) current directional heading, and (3) current movement state. The Sharp paper said that the "necessary information for current position is assumed to arise from the place cells themselves" – cells that have been identified in rats and mice. "The directional information is assumed to come from the limbic system head direction signal, which…is postulated to arise from the lateral mammillary nucleus," the paper added. These signals come from head direction cells, also found in rats and mice.

"The data presented here suggest that the necessary movement trajectory information may be provided, at least in part, by the medial mammillary nucleus," the paper reported. "Thus it could be that the lateral and medial mammillary nuclei together provide necessary building blocks for construction of the limbic system place cell activity. This could, in turn, explain why mammillary body damage seems to preferentially affect memory on spatial tasks," the paper concluded.

In addition to finding "place" and "head direction" cells in rats and mice, "we know for sure that monkeys have head direction cells and a kind of place cells, which are different from those in rats," Sharp said, adding: "This strongly suggests that humans also probably have both these kinds of cells, with human place cells probably more like those in the primates."

Sharp and Turner-Williams measured the brain cell firing as rats foraged for food pellets dispensed randomly in a chamber at 15-second intervals. After three training sessions the rats had developed a "a pattern of diverse, seemingly random trajectories" around the area.

"What we're essentially doing in this method of single-cell recording is 'eavesdropping' on once cell at a time to get a kind of sense of when each cell fires an electrical signal in relation to what the animals are doing at the time," Sharp said. "In this particular experiment we found that the cells signal when the head is turned. This is important because think about what happens when you turn your head with your eyes closed. You still keep track of what direction you're facing, and there must be information that gets passed along to help you 'know' that," she added.

One interesting earlier finding, Sharp noted, is that "when a person is usually right about what direction they're facing, the same set of cells will always fire when you're facing south – and you'll know that's south. But if you're a person who's confused about direction, you'll always be confused, because it's probably the cells themselves that are confused." She also quashed a popularly held belief that people who are "good at compass directions" somehow get cues from the earth's gravity. "We know that this ability isn't 'strictly tied' to the earth's magnetic field," she said, "though we think there could be some influence."

Sharp added that "place" cells work in a manner complimentary to the "head direction" cells, and are specific for each "place." Depending on where a rat is, a specific "set" of such cells will fire when the rat moves in an ever-changing series of cells sets continuously fire.

The current paper reports an important aspect in how the brain integrates the various signal pathways, Sharp said, and "though we didn't find the origin of the place cells, we believe these medial mammillary body cells may be one piece of the puzzle."