Biology Reference
In-Depth Information
3.
E
XPERIENCE
-D
EPENDENT
A
LTERATIONS OF
H
IPPOCAMPAL
P
LACE
F
IELDS
Place cells are complex-spiking cells
that fire in response to a rodent's spatial location
(O'Keefe, 1976) and these cells are recorded in all areas of the hippocampus proper (Barnes
et
al.
, 1990). Place-dependent complex spiking cells are
found
also
in regions afferent to and
efferent from hippocampus. Single
units coding for spatial information are present in
subiculum (Sharp & Green, 1994)
, entorhinal cortex
(Quirk
et al.
, 1992; Hafting
et al.
,
2005)
, parasubiculum (Taube, 1995b) and postrhinal cortex (Burwell & Hafeman,
2003). Neurons with similar patterns are described in
primate hippocampal region. The
spatial cells there respond to a certain part of space - “view” neurons (Rolls & O'Mara, 1995).
Cells in the human hippocampus are also shown to fire in correlation with spatial orientation
tasks (Ekstrom
et al.
, 2003).
3.1. Place field plasticity for stable environment
Naturalistic studies demonstrate of how the place representation is modified by
behavioral experience and the properties of such place cell plasticity share the principles of
dynamic connectivity of synchronously active neurons. A substantial number of reports
demonstrate systematic alterations in place fields in response to experiences that the animal
has in an environment.
The simplest kind of experience is repeated entry into the same environment, and small
but pronounced short-lasting changes in place fields have been observed when rats repeatedly
run in the same direction along a linear track (Mehta
et al.
, 1997). Place fields undergo with
experience asymmetrical expansion such that cells recorded over multiple laps around the
same track displayed place fields that shifted backwards relative to the direction of motion
and increased their both their firing rate and firing field size (Mehta
et al.
, 1997; Mehta
et al.
,
2000) (Fig 2). Hippocampal CA3 fields shift backward immediately after the environmental
exposure and maintain these changes for several days, while CA1 fields shift backwards from
the second day of exposure and fail to maintain the changes (Lee
et al.
, 2004). This feature
favors CA3 region as a network that can store long-term sequential representations. The
asymmetrical development of place fields is in accordance with the models of long-term
potentiation which is suggested to occur only when the postsynaptic neuron is depolarised
shortly after the depolarisation of the presynaptic neuron (Levy & Steward, 1983; Bi & Poo,
1998). As the cells with place fields are always activated in a particular temporal order, it is
assumed that the synapses from early firing cells to later firing cells become selectively
potentiated. Therefore each place cell will be driven to firing threshold progressively earlier
with each lap around the track, resulting in a backwards shift along the track and increase of
the field size. The development of place field expansion and backward shift is also dependent
on NMDA receptors (Mehta
et al.
, 2000; Ekstrom
et al.
, 2001) in accordance with
hippocampal models of NMDA-dependent synaptic plasticity (Bliss & Collingridge, 1993;
Abraham & Bear, 1996; Jensen & Lisman, 1996).
