The cortex is filled with pyramidal cells, a type of neuron with a large cell body and very long dendrites extending in the basal and apical directions. The apical dendrites of pyramidal cells are oriented towards the pia of the brain, as shown in the figure below. Cell bodies in Layer 6 have dendrites that form a bundle extending to Layer 1.
A cross-section of apical dendrites is shown in the box (figure above), with an arrow pointing to the region where dendritic membranes are closely opposed. Microtubule, mitochondria, and other proteins are stained at the site of communication between two dendrites.
The next figure shows apical dendrite bundles in cat visual cortex, cut in cross-section (the lower half of the figure).
Each white spot represents a dendrite; where two white spots touch is the place of dendritic membrane contact. These constellations of dots are sometimes called puncta or puncta adherentia by histologists to refer to an ambiguous structure, but in many cases puncta resemble dendrites in cross-section.
The basilar dendrites of pyramidal neurons are very long. Scheibel and Scheibel were impressed with the length of the Betz cell basilar dendrites, and suggested that the great length of the dendrites was a unique feature for which these cells evolved. In mice, rats, and cats, giant pyramidal cells of Betz and large solitary cells of Meynert in visual cortex have long basilar dendrites organized into bundles. As shown in the figure below, cat cortex had thick patches of basilar dendrites in layer 5 and layer 2/3. In human cortex, basilar dendrite bundles can reach a length of 2000-3000 um, with diameters of 12-40 um.
Part C of Figure 2 (below) shows a close-up view of a dendrite bundle, which contains 6-10 basilar dendrite shafts and varies in total diameter from 7-8 um to 1-3 um. It appears to change as individual dendrites are added or subtracted along its length.
The length of basilar dendrites permits Mountcastle's columns to link to each other, as shown in the figure below. Gap junction coupling at dendrite bundles could serve as an averaging mechanism, uniting the neurons involved into an assembly.
Studies on ventral horn motoneurons had led to a presumptive relationship between onset of reciprocal flexor-extensor activity in the muscle masses of the leg and the appearance of bundling in motoneuron dendrites. The onset of dendritic bundles with the development of discrete items of output performance could have implications for non-motor memory as well. Scheibel and Scheibel surmised that age-related problems with cognitive association skills could be due, as shown in the figure below, to a loss of the large dendrite bundles within basilar shafts of giant pyramidal neurons.
Senility may be associated with dendrite retraction and less thickness of basilar dendrite bundles. The figures (above, below) are from Golgi-impregnated sections of human cortex at different stages of aging. In particular, there is a loss of horizontal lengths (basilar dendrites) during aging rather than vertical (apical) dendrites. The decline of effective dendrite connectivity may be an important contribution for the fading of human consciousness.
Like the dendrite bundles in spinal cord, thalamic reticular nucleus, and raphe, dendrite bundles formed by the apical and basilar dendrites of pyramidal neurons appear to make direct dendrodendritic connections. It has been estimated that the dendritic bundles of the rabbit neocortex are characterized by such a close packing that about 20% of the surface of every dendrite is common to adjacent dendrite surfaces, separated by the extracellular space only. There are no intervening glia between the dendrites, and no post-synaptic specializations that resemble axon-synapses machinery, thus gap junctions appear to be the primary means of communication between two or more dendrites in a bundle, although it is controversial whether adult pyramidal neurons have significant numbers of gap junctions.
Chemical material not exceeding a molecular weight of ~1 kDa can fit through an open gap junction. There is a slight time lag as the chemical moves from one cell to another, but this is how low molecular weight messengers such as cyclic AMP move through gap junction-connected cells, allowing two or more cells to synchronize their metabolic state. If many gap junctions are open, millions of cells in the network can be coupled at once. This may contribute to an ancient way of communicating between cells that does not depend on synaptic activity. While axons are busy firing, dendrites are synchronizing subthreshold activities and debris with neighboring neurons.
Dendrite bundles in particular seem to serve as one collecting system for many afferent influences. Dendrites contain a large surface area for the interplay of fractional changes in membrane potentials. It is generally believed that the current paths of individual neurons that summate in the extracellular space in and around the dendrites gives rise to summed extracellular potentials, the EEG, and rhythmic EEG potentials.
Roney K. J., A. B. Scheibel and G. L. Shaw. 1979. Dendritic bundles: survey of anatomical experiments and physiological theories. Brain Res. 180, 225-271.