Spatial distribution of net input and intrinsic cellular properties potentially underlying the difference between place and silent cells in a novel environment.
AP firing rate (red) and underlying membrane potential (V_m) at the soma due to input into cell (black) as a function of animal’s location along a linear environment for place (A) and silent (B–E) cells. Baseline V_m (bottom of each V_m plot), baseline V_m of place cell (blue), and AP threshold (dashed).
(A) For a neuron to have a place field, V_m by definition must consistently reach spike threshold in a spatially-selective manner.
(B)–(E) Conversely, V_m must generally stay below threshold for silent cells, but this could be achieved in many different ways.
(B) Spatial tuning of input and amplitude of change in V_m due to input could both be same for place and silent cells but start from a lower baseline V_m in silent cells.
(C) Baseline V_m and threshold could both be same for place and silent cells but the amplitude of change in V_m due to input could be smaller for silent cells. Note that smaller V_m change could occur in various ways such as a smaller net input or the same input acting on a neuron with lower input resistance (R_N).
(D) Spatial tuning of input and amplitude of change in V_m due to input could both be same for place and silent cells but threshold could be lower in place cells.
Other combinations of input pattern and intrinsic properties are possible. For instance, in (E), threshold is higher, baseline V_m is somewhat higher, and amplitude of change in V_m due to input and possibly spatial tuning of input are much lower in silent versus place cells.

Whole-cell recording of a hippocampal CA1 place cell in a freely moving rat exploring a novel maze.
(A) Trajectory of animal (blue) and locations where APs occurred (red) during entire 16 min awake recording in “O”-shaped maze (inner wall not shown).
(B) AP firing rate map during periods when animal faced CCW direction.
(C) Linearized location (measured around oval maze path) of animal (blue) and APs (red) as a function of time.
(D) Linearized AP firing rate map for CCW direction.
(E) V_m (black, lowest trace expands trace above it around narrower V_m range) and head speed (blue) during 3 successive CCW laps (corresponding to black vertical bar in (C)). Gray bars correspond to shaded place field region in (A). Above: 2 passes through the place field. *’s mark brief hyperpolarizing current steps used to probe R_N and membrane time constant (τ_m).

Whole-cell recording of a hippocampal CA1 silent cell in a freely moving rat exploring a novel maze.
(A) Trajectory of animal (blue) and locations where APs occurred (red) during entire 16 min awake recording.
(B) AP firing rate map during periods when animal faced CW direction.
(C) Linearized location of animal (blue) and APs (red) as a function of time.
(D) Linearized AP firing rate map for CW direction.
(E) V_m (black) and head speed (blue) during ~1.5 CW laps (corresponding to black vertical bar in (C)). Above: V_m dynamics leading to AP (left), V_m theta frequency oscillations (right).

Classical spiking place fields and subthreshold fields of hippocampal place and silent cells in a novel environment.
(A) Mean AP rate (red) and mean subthreshold V_m (black) as a function of animal location, averaged over all periods in that direction. AP plots: uniform 0–15 Hz scale, peak rate (left). Subthreshold plots: individual scales, AP threshold (dotted), minimum V_m (bottom).
(B)–(G) Intracellular features of place versus silent cells or directions (mean±SEM, red). Horizontal jitter added to individual values for visibility. Place cells (PC): cells 1–4, silent cells (SC): 5–9. PD = place field directions, SD = silent directions. *,**,*** correspond to P < 0.05, 0.005, 0.0005 differences. See text.
(H) AP and subthreshold fields during first experience of each location for same cells and directions in (A). AP plots: uniform 0–33 Hz scale.
See also Figure S1.

Pre-exploration firing pattern in response to depolarizing current injection for each eventual place and silent cell in Figure 4A. Current step amplitude (right).

Complex-spike (CS) events at the soma of CA1 pyramidal cells in freely moving rats exploring a novel maze.
(A) Examples of similarly-shaped CSs from 2 cells. Shaded area marks large, slow depolarization used in CS classification (left).
(B) Examples of CS shape variety from same 2 cells in (A). “Plateau-like” event occurring within place field (right).
(C) Series of CSs occurring within a place field. Arrows mark spikelets, and 55.7% of APs in this trace had shoulders (i.e. were immediately preceded by spikelet-like events).
(D) Statistics of APs within CSs for Figure 2 place cell (mean±SEM). No “relative threshold” value for AP #8 (right) because all were shoulder APs (Experimental Procedures).
(E) AP (red), CS (green), and extracellularly-defined complex spike (blue) rates as a function of animal position for Figure 4A place fields. Peak rates (left).
See also Figure S2.