Dendrite bundles in spinal cord

Virtually all neurons within the central nervous system and certainly the pyramidal neurons in cortex have extensive dendritic trees, which in fact contain most of the membrane of the cells.

Dendrites participate in very old forms of cell-to-cell communication, which pre-date axon potentials. Suppose two cells want to communicate for the first time. Whereas hundreds of proteins are needed to form a chemical synapse between two cells, just one protein, connexin, must be expressed in the dendrite region of each cell in order to form a gap junction synapse, then information can be transferred from dendrite to dendrite (G.D. Pappas, 1972).

The tendency of neuroscientists to explain what is happening in the brain in terms of action potentials fired and received is due partly to work on the axon by Hodgkin and Huxley. Dendrites are often regarded as receiving stations for axons, or structural components of the cell that support axons. However, dendrites have primary sensory functions, for example, the dendrites of internal retinal ganglion cells are sites of phototransduction. Dendrites typically represent regions of excitable cell membrane, which initially served an important function in gathering sensory information.

“We thus find a type of response of excitable tissue occurring over a range from the protozoan slime mold to the mammalian cortex which in essential respects is distinct from the all-or-none response of nerve, and probably is the more primitive process.” (M.H. Clare, 1955)

Dendrites in spinal cord

Enormously long dendrites of motoneurons were first identified in the ventral spinal cord of reptiles and amphibians. Ramon y Cajal illustrated these dendrodendritic bundles in the ventral commissure of spinal cord (below).

From dendrites

The next figure shows a motoneuron dendrite bundle in the cat spinal cord. There are electrotonic synapses, or gap junctions, between motoneuron dendrites.

From dendrites

Electrophysiological demonstration of gap junctions between motoneuron dendrites occurred first in the spinal cord of toadfish (Pappas et al, 1966) and then the supramedullary neurons of Atlantic puffer fish (Bennett et al, 1967), although as early as 1948 Eccles and colleagues had begun to suspect that the spinal cord and other areas of brain might exhibit subthreshold responses.

What do you mean by subthreshold responses?

When large currents are injected in motorneurons, the motoneurons show action potentials, as expected. However, when the current injection is small enough that it does not cause a full action potential, the cell still responds with small graded potentials (bottom traces of A and B, figure below). The graded potentials are referred to as subthreshold responses.

From Gap junction

A coupling coefficient of 0.13 was determined for gap junction transmission in toadfish motoneurons. These experiments showed that electronic coupling can take place between motoneurons; even if there is no spike, the areas of close apposition of dendrites give a graded response to current injection, and nearby neurons are somewhat "connected" in a way that doesn't rely on neurotransmitter release.

What is the function of dendrite bundles?

In early 1970s, Scheibel and Scheibel began to study dendrite bundles with the Golgi staining technique. Investigations in the ventral spinal cord of cat revealed dendrites of motoneuron pools innervating muscles of antagonistic function. In kittens, bundles are not present at birth but develop near the end of the second week, when kittens start using hind limbs for weight bearing and walking.

From dendrites

A gain in dendrite bundle thickness coincided with the kittens' use of coordinated limb movements, so the authors suggested that dendrite bundles may serve as reservoirs harboring central programs necessary to the inception of movement in the antigravity (weight-bearing) muscles of the lower extremities and back. At birth in kittens, motoneuron dendrites are poorly developed, and motor behavior is minimal, but at 12 days, lengthening of dendrites is apparent, and at 4-5 months, dendrite bundles are dense and well-developed, at which time the animals can make coordinated limb movements for jumping and running. It was also found that a loss in motor strength with aging was associated with a loss of dendritic bundles, and re-exertion of effort.

In addition to spinal cord, bundled dendrites are found in the thalamus, cortex, hypothalamus, and reticular formation.


References

Pappas, G. D. and M.V. Bennett (1966). Specialized junctions involved in electrical transmission between neurons. Ann. N. Y. Acad. Sci. 137, 495-508.

Bennett, M. V., Y. Nakajima and G.D. Pappas (1967). Physiology and ultrastructure of electrotonic junctions. I. supramedullary neurons. J. Neurophysiol. 30, 161-179.

Scheibel M. E. and A. B. Scheibel (1970). Developmental relationship between spinal motoneuron dendrite bundles and patterned activity in the hind limb of cats. Exp. Neurol. 29, 328-335. 10.1016/0014-4886(70)90062-2

Scheibel M. E. and A. B. Scheibel (1971). Developmental relationship between spinal motoneuron dendrite bundles and patterned activity in the forelimb of cats. Exp. Neurol. 30, 367-373. 10.1016/S0014-4886(71)80015-8

Scheibel M. E. and A. B. Scheibel (1973). Dendrite bundles in the ventral commissure of cat spinal cord. Exp. Neurol. 39, 482-488. 10.1016/0014-4886(73)90032-0