Figure 5.

Characteristics of CA3c “pyramidal cells” and mossy cells. (A,B) Pyramidal cells of CA3c often have morphology that is not pyramidal-like in that the cell body and proximal dendrites are not pyramidal. They are not interneurons because of their dense spines and physiology. The example shown is from a neuron that was physiologically-identified as a pyramidal cell because of its intrinsic burst firing (D). The merged images through multiple focal planes are shown. Calibration [located in (A)] = 100 μm for (A) and 20 μm for (B). (C) A drawing of the cell in (A,B) is shown. Arrowheads indicate the part of the axon that terminated in area CA3. The arrows point to the part of the axon that gave rise to collaterals in the hilus. GCL, granule cell layer; HIL, hilus; PCL, pyramidal cell layer of CA3c. (D,E) Physiological discrimination of CA3c neurons from hilar mossy cells. (D) A continuous recording from a neuron in CA3c (top) that exhibited firing behavior of a pyramidal cell rather than a mossy cell. During the record, a series of increasing currents are triggered (middle) to elicit firing and the firing behavior is expanded below (arrows). Weak currents (#1–3) did not elicit burst firing but the strongest current command did (#4). The burst in #4 has a characteristic decrement in action potential amplitude, and rides on a triangular depolarization, followed by a large afterhyperpolarization. In contrast, these types of bursts are not found in mossy cells under these recording conditions, and the afterhyperpolarizations are not either. (E) CA3c pyramidal cells can also be discriminated from mossy cells by a large IPSP triggered by perforant path or molecular layer stimulation. Top: A continuous record from a CA3c cell showing an IPSP evoked in response to molecular layer stimulation (at the dot) and an afterhyperpolarization following directly-evoked action potentials (arrow). An arrowhead marks a spontaneous burst of action potentials followed by an afterhyperpolarization. Bottom: A response of the same cell to stronger stimuli (at the dots). On the right, the response to the strongest stimulus is shown, which elicited an action potential followed by hyperpolarization, characteristic of pyramidal cells under these recording conditions, but not mossy cells. (A–E) are from Scharfman (1993b).