Friday, January 18, 2008

Hippocampus, theta waves and movement IV

In part III I brought some of the info from Vanderwolf's book to do with movement, types of movement, and their association with hippocampal wave function. On p. 16 he says:
".. in 1962 (we) had shown that rhythmical waves could occur in the thalamus and hippocampus slightly in advance of overt motor activity. ...I had no accurate means of determining the precise instant of movement onset and.. spontaneous movements do not have an abrupt onset. Spontaneous walking, for example, is usually preceded by small head movements and adjustments in posture (intention movements). What was need was an abrupt transition from complete immobility to vigorous gross movement of the type that is consistently accompanied by hippocampal waves."
To solve this problem he designed a box with a metal floor into which a rat could be placed and a small shock delivered to its feet, part of a training process teaching them to jump out of the box; i.e., shock was not part of the experimental design, just the jump-from-inside-the-box training.
"..a trained rat could be placed gently on the floor, standing on its hind legs. After a delay of several seconds during which the rat stood motionless, the hind legs would extend suddenly, propelling the rat to the top of the box. A movement-sensing device mounted on the box recorded the onset of this jump with an accuracy of a few milliseconds."
Vanderwolf found that rhythmical waves of 6-7 Hz could occur several seconds prior to the jump. Beginning about a second before the jump the frequency increased to 8-12 Hz, peaking at jump initiation, continuing until the rat landed on the "safe" shelf.
"The data from this experiment suggested that the hippocampus might have some role in both planning and the performance of a motor pattern. It also suggested a problem which subsequently became a major focus of my research. If the rhythmical waves of the hippocampus are related to motor activity, how is it possible that these waves can be present during relatively long intervals (several seconds) when a rat is absolutely motionless?"
Meanwhile, in a nearby lab the hippocampus of New Zealand white rabbits showed rhythmical activity being elicited with visual and auditory input only, no visible motor activity, something not noted in rats.

REM sleep presented another exception to the idea that hippocampal rhythmical activity was associated with movement, and was noted by Vanderwolf in the late 60's.
"The onset of REM sleep in a rat is always associated with an utter collapse of any pre-existing muscle tone. Thus, if a rat falls asleep in a crouched sitting posture, as they sometimes do, the onset of REM sleep is associated with the body slumping down limply on the floor. Despite this, bursts of muscular twitches occur periodically in the limbs, trunk, and especially in the vibrissae. Rhythmical slow waves occurred in the hippocampus throughout an episode of REM sleep, with higher frequency waves occurring during the muscular twitches than during the inter-twitch intervals. An interpretation of this curious phenomenon was suggested by research originating with Otto Pompeiano of the University of Pisa.It appears that brain motor systems generally are in a state of high activity during REM sleep but that overt expression of this activity is blocked by a powerful inhibition of spinal motor neurons and of reflex afferents to those neurons. Consequently, instead of running, jumping, etc. the animal lies limply on the floor, twitching slightly. The hippocampal record then is related to motor activity during REM sleep as well as during waking."
At this point Vanderwolf teamed with Bob Sainsbury and two students, Brian Bland and Ian Whishaw, to continue the work at U. of Western Ont.

Next: hippocampal slow waves, learning and instinctive behavior.

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