Biomedical Engineering Reference
In-Depth Information
q
3
q
0
q
1
q
2
0
V
Oculomotor Nucleus
Action Potential
q
0
q
3
q
1
q
2
5 ms
Time
FIGURE 13.10
Recording of a series of saccades using a micropipette and the resultant electrical activity in
a single neuron. Spikes in the membrane potential indicate an action potential occurred. Saccade neural activity
initiates with a burst of neural firing approximately 5 ms before the eye begins to move and continues until the
eye has almost reached its destination. Relative position of the eye is shown at the top with angles y
0
through y
3
.
Initially, the eye starts in position y
0
, a position in the extremity in which the muscle is completely stretched with
zero input. To move the eye from y
0
to y
1
, neural burst firing occurs. To maintain the eye at y
1
, a steady firing
occurs in the neuron. The firing rate for fixation is in proportion to the shortness of the muscle. Next, the eye moves
from y
1
to y
2
. This saccade moves much more slowly than the first saccade with approximately the same duration
as the first. The firing level is also approximately at the same level as the first. The difference in input corresponds
to fewer neurons firing to drive the eye to its destination, which means a smaller input than the first saccade.
Because the muscle is shorter after completing this saccade, the fixation firing rate is higher than the previous
position at y
1.
Next, the eye moves in the opposite direction to y
3
. Since the muscle is lengthening, the input to
the muscle is zero—that is, no action potentials are used to stimulate the muscle. The fixation firing level y
3
is less
than that for y
1
because the muscle is longer.
