Table 1.
Summary of properties of pyramidal cells in cortical layers 3–6, for references, see text.
| Layer 6 spiny cell type | Structural features | Excitatory inputs | Outputs | Firing characteristics | |
|---|---|---|---|---|---|
| Dendrites | Axons | ||||
| L6 Cortico-thalamic pyramidal cells | Small-medium, upright pyramidal cells. Apical dendritic tuft in L4 | Axon ascending to L4 (some also to lower L3). Drumstick-like branches in L4 | Reciprocal from specific thalamic nuclei. From L6 cortico-cortical pyramids | To specific thalamic nuclei and nRT. Local outputs predominantly to L6-L4 GABAergic interneurones (synapses on shafts of aspiny dendrites). Facilitating EPSPs to all targets | Modest Accommodation and Adaptation. Almost tonic discharge in response to maintained depolarization. |
| L6 Cortico-thalamic pyramidal cells | Short, small-medium, upright pyramidal cells. Apical dendritic tuft in L5 | Ascending to L5. Some with drumstick-like branches | Thalamus. L6 cortico-cortical pyramids | To specific and non-specific thalamus and local L5/6 interneurones. Facilitating EPSPs to all targets | Modest Accommodation and Adaptation. Almost tonic discharge in response to maintained depolarization. |
| L6 Cortico-cortical pyramidal cells (latexin positive) | Small-medium “pyramids.”Dendrites confined to L5/6. Several structural classes: short upright pyramids, bipolar, inverted and multipolar “pyramids” | Long horizontal branches confined to L5/6 | Other local and distant cortical neurones | Preferentially innervate cortical pyramids with depressing EPSPs | Rapidly and powerfully adapting. Spike inactivation can be “rescued” with ramp-shaped current |
| L6 Claustrum-projecting pyramidal cells | Tall, upright, long thin apiical dendrite to L1-no tuft | Long horizontal branches confined to L5/6 | Other local and distant cortical neurones | Claustrum, L5/6 pyramids with depressing EPSPs | Near tonic firing |
| Layer 5 spiny cell type | Structural features | Excitatory inputs | Outputs | Electrophysiology | |
| Dendrites | Axons | ||||
| L5 Large burst-firing pyramids, upper L5 | Thick basal dendrites L5, Thick apical with tuft L3-L1 | Largely confined to deep layers, short branches | Local inputs include other large and small L5 cells and a powerful focused input from deep L3 as well as distant cortical and subcortical. Most inputs accocunted for | To non-specific thalamic nuclei, superior colliculus, pons, spinal cord (targets depending on cortical region). Depressing EPSPs to most targets | Intrinsic burst-firing superimposed on a depolarizing envelope. Resting Potential in vitro near firing threshold. |
| L5 smaller cortico-thalamic pyramids | Smaller upright pyramids. Slender apical dendrites terminating in L2/3 with little/no tuft | Ascending to L2/3 and horizontal branches | No reciprocal input from thalamus | Large boutons to non-specific thalamus. Depressing EPSPs | Adapting and accommodating firing pattern |
| L5 smaller cortico-cortical pyramids, incl. transcallosally projecting cells | Smaller upright pyramids. Slender apical dendrites terminating in L2/3 with little/no tuft | Long horizontally oriented | Other cortical pyramidal cells, local and distant | Local and distant cortical neurones with largely depressing EPSPs | Radidly adapting and accommodating firing pattern |
| Layer 4 spiny cell type | Structural features | Excitatory inputs | Outputs | Electrophysiology | |
| Dendrites | Axons | ||||
| L4 pyramidal cells predominantly innervating L4 cells | Often small, simple cells. A modest number of slender dendrites, basals in L4, apical obliques in L3, with a tuft in L1 | Local axonal arbor and a descending arbor with sparse branching in L5 and/or L6 | From local L4 cells (28% of input), 6% from specific thalamus (large, potent en-passant boutons on dendritic shafts), 45% from L6 corticothalamic (small boutons, on spines). Almost none from L3. Remainder currently unaccounted for | Predominantly other L4 cells. Strength and probability falling off rapidly with separation. Proximal, basal dendritic inputs. Brief, depressing EPSPs | Rapidly adapting and accommodating |
| L4 pyramidal cells preferentially innervating L3 cells | Often small, simple cells. A modest number of slender dendrites, basals in L4, apical obliques in L3, with a tuft in L1 | Strong, ascending, topographically precise input to L3 and descending projection with sparse branching in L5 and/or L6 | Predominantly to L3 cells. Pyramids more than interneurones. Proximal, basal dendritic inputs. Brief, depressing EPSPs | Brief, short interspike interval spike train followed by brief afterdepolartization, slow hyperpolarization then tonic firing | |
| L4 Spiny stellate cells | Often small, simple cells, with slender dendrites largely confined to L4. No apical dendrite | Ascending topographically precise input to L3, descending projection with sparse branching in L5 | Probably similar to the above | ||
| Layer 3 spiny cell type | Structural features | Excitatory inputs | Outputs | Electrophysiology | |
| Dendrites | Axons | ||||
| L3 pyramidal cells | Well developed basal and apical oblique dendrites and a tuft in L1. Largest cells close to L4 border | Dense, fairly narrow ramifications in L3 and L5, not in L4 (but see text for mouse) | Inputs from L4 and thalamus to deep L3 proximal basal dendrites. Tall, brief, depressing EPSPs. High hit-rate inputs from other local L3 pyramids. Cortical and thalamic inputs account for most synapses. 97% of L3 pyramid-pyramid inputs onto spines of less proximal basal and apical oblique dendrites | Dense local innervation of L3 pyramids and interneurones and patchy, long distance terminal axonal arbors. Dense, very high probability innervation of large (not small) L5 pyramids sharing the same vertical axis. To interneurones in L4 that have dendrites in L3, but not to spiny L4 cells. Transcallosal projections | Very negative resting potentials −80mV (in vitro). “Typical” adapting/accommodating pyramidal cells |
With the exception of presynaptic L6 cortico-thalamic pyramids, all pyramidal inputs to FS, parvalbumin-immunopositive interneurones recorded were depressing and all excitatory inputs to SOM cells were facilitating.
Few studies in L4 have systematically correlated anatomy with electrophysiology and connectivity. Some characteristics, like their inputs and the descending projections may, or may not be common to 2 or more subclasses.