Biology Reference
In-Depth Information
causes hyperpolarization and the following rebound depolarization, spike bursts and
consequent Ca
2+
influx in a DCN neuron. LTP is induced depending on the strong Ca
2+
influx, whereas LTD is induced with a moderate Ca
2+
influx [100]. This direction control
mechanism of synaptic plasticity is similar to the BCM model at excitatory synapses
[105,106].
LTP and LTD have also been reported at the mossy fiber - DCN neuron excitatory
synapses [107,108]. Both AMPA and NMDA receptors are expressed at the synapse.
However, unlike other synapses, NMDA receptor shows low Mg
2+
sensitivity and weak
voltage dependence. Therefore, NMDA receptor can be activated at a basal condition [109].
LTP is induced by pairing the high-frequency mossy fiber stimulation with the postinhibitory
rebound depolarization of a postsynaptic DCN neuron. NMDA receptor activation and the
increase in postsynaptic Ca
2+
concentration are necessary for the induction. It was also shown
that mossy fiber activity has to precede the postinhibitory rebound depolarization, implying
that the LTP induction is controlled by relative timing of the mossy fiber input and the
Purkinje cell input [107]. On the other hand, LTD is induced by the high-frequency burst
stimulation of mossy fibers, either alone or paired with the postsynaptic depolarization [108].
The LTD induction depends on the postsynaptic Ca
2+
increase, mGluR1 activation and
protein translation.
R
OLE OF
S
YNAPTIC
P
LASTICITY IN
M
OTOR
L
EARNING
A
DAPTATION OF
VOR
AND
OKR
Adaptation of vestibulo-ocular reflex and opto-kinetic response (VOR and OKR,
respectively) are well-studied models of motor learning which depend on the cerebellum
[110-113]. Both VOR and OKR are reflex eye movement that stabilizes image on the retina
during head motion. Eyes move in the opposite direction to the head movement or the same
direction of visual field movement. In OKR, eye balls follow the slow visual field movement
using the visual signal. In VOR, the vestibular organs in inner ears sense the head movement,
and the vestibular signal drives the eye ball movement. OKR dominates in the relatively slow
eye movement, and VOR plays a more important role in the relatively quick movement. They
work cooperatively in a daily life. The efficacy of these reflexes during the sinusoidal rotation
of an animal or visual field can be quantitatively evaluated by two parameters, the gain and
the phase. The gain is the amplitude of eye movement divided by the amplitude of stimulus,
the head movement in VOR and the visual field movement in OKR. The phase indicates the
delay or lead of eye movement relative to the stimulus.
Adaptive changes of both VOR and OKR have been regarded as models of motor
learning. Sustained stimulation of an animal with sinusoidal rotation of the visual field
gradually increases the OKR gain toward one and decreases the phase toward zero. Thus,
OKR adaptation minimizes the retinal slip. On the other hand, sustained sinusoidal rotation of
an animal coupled with the visual field rotation changes the gain and phase of VOR. When
the direction of head and visual field movement are opposite, the VOR gain increases and the
phase difference decreases, which is called the gain-up VOR adaptation. When the direction
of the two stimuli is the same, the VOR gain decreases, which is called the gain-down VOR
adaptation. For example, when we start to wear new eyeglasses, VOR adaptation minimizes
