An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. Apical dendrites are one of two primary categories of dendrites, and they distinguish the pyramidal cells from spiny stellate cells in the cortices. Pyramidal cells are found in the prefrontal cortex, the hippocampus, the entorhinal cortex, the olfactory cortex, and other areas. Dendrite arbors formed by apical dendrites are the means by which synaptic inputs into a cell are integrated.Cline HT. Dendritic arbor development and synaptogenesis. Current Opinion in Neurobiology 2001; 11: 118–126 The apical dendrites in these regions contribute significantly to memory, learning, and sensory associations by modulating the excitatory and inhibitory signals received by the pyramidal cells.
Background Two types of dendrites present on pyramidal cells are apical and basal dendrites. Apical dendrites are the most distal along the ascending trunk, and reside in layer 1. These distal apical dendrites receive synaptic input from related cortical as well as globally modulatory subcortical projections. Basal dendrites include shorter radially distributed dendrites which receive input from local pyramidal cells and interneurons.Henze DA CW, Barrioneuvo G. Dendritic Morphology and its effects on the amplitude and rise-time of synaptic signals in hippocampal CA3 pyramidal cells. Journal of Comparative Neurology. 1996;369:331–344. Pyramidal neurons segregate their inputs using proximal and apical dendrites.CL W. Dendritic Reorganization in Pyramidal Neurons in Medial Prefrontal Cortex after Chronic Corticosterone Administration. Journal of Neurobiology. 2001;49:245–253.
Apical dendrites are studied in many ways. In cellular analysis, the electrical properties of the dendrite are studied using stimulus responses. A single surface shock of the cerebral cortex induces a 10–20 ms negative potential, a manifestation of the summed excitatory post-synaptic potentials (EPSPs) evoked in the distal portions of the apical dendrite.Jibiki I MK, Ohtani T, et al. Dendritic Potential in Direct Cortical Responses and Seizure Activity. Folia Psychiatrica et Neurologica. 1978;32(3):329–337 This has been called the dendritic spike.
Areas of interest
Hippocampus
The hippocampus contains pyramidal neurons in three areas: CA1, CA2, and CA3. The pyramidal neurons of each area have different properties. However, in all areas, dendritic synthesis of proteins is necessary for late long-term potentials in the hippocampal neurons.Bradshaw KD EN, Bliss TVP. SHORT COMMUNICATION: A role for dendritic protein synthesis in hippocampal late LTP. European Journal of Neuroscience. 2003;18:3150–3152 Neurons throughout the limbic system are known to have "burst" properties. These cells undergo synchronous and paroxysmal depolarizations, firing short sequences of action potentials called bursts.
The stratum oriens is the location between layers containing basal dendrites. The stratum lucidum, stratum radiatum, and the stratum moleculare-lacunosum are layers of apical dendrites and are ordered from least distant to most distant from the soma of the neuron.
Pyramidal cells in CA3 CA3 projects Schaffer collaterals to apical dendrites in CA1. Individual pyramidal cells in the CA3 region have burst properties due to high densities of calcium channels in their proximal dendrites.Jerome Engel TAP, ed. Epilepsy: A Comprehensive Textbook in Three Volumes. Philadelphia, PA: Lippincott Williams & Wilkins; 2008 Depolarization of the membrane may also trigger these bursts. Calcium entry into the cell causes more prolonged depolarization and increased action potentials. Usually curtailed by the hyperpolarizing local inhibition (due to the excitatory collateral system), this can lead to gradual recruitment of CA3 neurons and result in synchronized burst discharges. After hyperpolarization by calcium-dependent potassium conductance is also used as a method of controlling these bursts.
Hippocampal CA3 pyramidal cells have complex dendritic arbors which receive a stratified pattern of synaptic input from a variety of sources, including: the commissural/associational fibers from ipsi- and contra-lateral CA3 pyramidal neurons which synapse on both the basal and mid-apical dendrites in the stratum oriens and stratum radiatum the mossy fibers from the granule cells of the dentate gyrus which synapse on the most proximal apical region, the stratum lucidum the preforant path fibers from the entorhinal cortical pyramidal cells which synapse in the region of the most distal apical dendrites, the stratum lacunosum-moleculare.
Distal apical dendrites extend upwards from the soma. The shorter proximal apical dendrites extend outward and below. Shape of majority of 2d section is approximately a cylinder with a pointed base for the apical arbor. The apical dendrites and basal dendrites possess a radial organization pattern as they extend from the soma. Proximal apical dendrites and basal dendrites have approximately the same density. Apical dendrites possess a larger average total dendritic length (6332 vs 5062 micrometres) and surface area (12629 vs 9404 square micrometres; neither includes spines). However, the number of terminal branches for both apical and basal dendrites appear to be similar. Distances between successive branch points are shorter for basal dendrites. The basal dendrite however has approximately 3 fold fewer endings per primary dendrite. This and the lower maximum branch order suggest lower complexity than apical dendritic trees. Basal dendrites have a shorter distance to the tips and a more restricted range than apical dendrites. Data suggests that proximal apical and basal dendrites are more compressed but offer a wider local range of activity than distal apical dendrites.
In CA3 neurons the inputs are stratified and run in bands parallel to the cell body layer. Dendritic attenuation of synaptic current is described by an exponential relationship. The closer to the body the dendrite, the higher the EPSP amplitude. Electrical measurements and predictions validate the cylinder cross-section model. In the CA3, the temporoammonic (TA), commissural (COM), associational (ASSOC), and mossy fiber (MF) afferents all make excitatory glutamatergic (Glu) synapses on pyramidal cell dendrites (both apical and basal).
Since fast signals occurring in the basilar and proximal apical dendrites are transferred to the soma with at least a 20–25% efficiency, synapses in these dendrites each contribute more to the neuronal activation than distal apical synapses. In contrast, only slow signals from the distal dendrites are efficiently transferred to the soma, suggesting a modulatory role on the resting potential of the cell. It is hypothesized in several studies that this could be accomplished by varying the overall frequency of synaptic activity in the distal apical dendrite. Since a constant barrage of synaptic activity would approximate a constant current injection, the overall level of synaptic activity in the distal apical dendrite could set the depolarization level of the entire neuron. When a more efficient proximal synaptic activity is superimposed upon a sub-threshold depolarization due to distal activity, the cell has a high probability of firing an AP. In CA3, it is the perforant path projection from the entorhinal cortical cells that provides synaptic input to the most distal dendrites of the pyramidal cells. Assuming a frequency average of 7 spikes/sec, as few as five randomly firing entorhinal cortical cells would cause a steady level of depolarization in the distal dendrites of CA3b pyramidal cells. Amplitude and kinetics of the electrical signal vary as a function of position within the dendrite and signal frequency.
The major trigger for CA3 discharge is the afferent input from the dentate gyrus granule cells, from which mossy fiber terminals create very complex synapses on the proximal part of the CA3 apical dendrite in the stratum lucidum. Here they contact very complex dendritic spines. Glutamate release from single terminals evokes a large non-NMDA mediated EPSP. The most proximal regions of CA3 pyramidal dendrites receive mossy fiber input exclusively, mid-dendritic regions (strata radiatum on the apical side and the oriens on the basal side) receive principally associational and Commissural fibers (from other CA3 cells), and the distal apical dendrites (stratum lacunosum-moleculare) receive input from the temporoammonic afferents (from the entorhinal cortex). Mossy fiber input to CA3 exhibits different plasticity than that of typical long term potentiation because it is dependent on (or at least sensitive to) monoaminergic (see monoamine) activation of the cAMP 2nd messenger system.
Interneurons in CA3 These are similar to dentate cells. Interneuron cell types show unique dendritic arborization patterns and region specific targeting by axon collaterals. Investigators have shown that different morphologically defined interneurons show different electrical properties. These include both fast-spiking cells whose inhibitory post-synaptic potentials (IPSPs) sum to create small, smooth IPSPs in pyramidal cells and slow spiking cells (these produce large, fast-rising IPSPs in the pyramidal cell target). The dendritic region of CA3 is laminated.
For the input to the hippocampus proper, the temporoammonic pathway arises in layer III cells of the entorhinal cortex but separates from the perforant pathway to contact the most distal branches of the pyramidal cells in the stratum lacunosum-moleculare of CA1-CA3. The excitatory (glutaminergic) influence of this path has been questioned because influence on the pyramidal cells has been difficult to demonstrate. Recent experiments show that this modulation of pyramidal cells may differentially activate an interneuron subpopulation located in the distal reaches of the apical dendrites.
The study of inhibitory transmission is limited in the pyramidal neurons and their modulators because the large number of excitatory synapses has overshadowed physiological studies of the inhibitory neurons.Mathews, Gregory. Telephone Interview.11/19/08. The structure of inhibitory synapses on apical dendrites may not be as plastic as the excitatory synapses on these neurons. There is difficulty in differentiating the excitatory and inhibitory synapses using the electrophysiological recordings in many experiments. The excitatory synapses and their patterns are by comparison to the inhibitory system rather uniform in type and properties. The inhibitory system, by contrast, possess several (10) different types of synapses originating from specifically differentiated cells and are much more difficult to track. There is insufficient information to precisely distinguish between excitatory and inhibitory pathways contributing to the alterations in neurotransmitter expression and cell structure changes.
CA1 CA1 pyramidal cells make up a homogeneous population which together with relatives in subiculum comprise the primary output cells of the hippocampal formation. Primary excitatory inputs are via glutamatergic CA3 Schaffer collaterals (both ipsi- and contralateral), which contact dendritic spines on the apical and basal dendrites in strata radiatum and oriens. Additional excitatory input is via the temporoammonic system which synapses on distal apical dendrites in the stratum lacunosum-moleculare.
Imaging studies following localized changes intracellular calcium from discrete synaptic inputs have shown a role for these currents in synaptic plasticity. There is disagreement, however, as to how activity-dependent changes in synaptic inhibition might occur. Studies do agree that plasticity is enhanced when inhibition is reduced.
CA2