Properties of dendrites; apical dendrites of the cat cortex

The all-or-none spike characteristic of the nerve axon may be considered a special case of excitable tissue response, of which a more general and fundamental activity is a decremental and nonrefractory response such as dendrites exhibit. Intercortical paths are found which end only on apical dendrites of cortical pyramidal cells, and may activate only their terminal portions near the cortical surface. When the dendrites are stimulated indirectly via such paths, or directly by local electric shocks, conduction is not all-or-none, occurs more readily away from the cell body than toward it, and has a value of a small fraction of a meter per second. Following indirect activation a second stimulus finds the dendrites excitable at any time later than the absolutely refractory period of the axons exciting them. After a 20 msec. facilitation period, a depression phase ensues accompanied by positivity. It is inferred that the depression phase which in general has been found to follow activation of neurones is chiefly assignable to the properties of their dendrites. Since apical dendrites exhibit no absolutely refractory period, a second response initiated during the first sums with it. By repetitive stimulation a continuous negativity can be maintained. Thus modulation of stimulation afferent to dendrites alone could induce potential wave forms of any duration, and the activity of dendrites appears to be such as to appropriately account for the potentials of the electrocorticogram. Activation of dendrites alone at or near the cortical surface facilitates the discharge of cell body spikes in the responses to afferent radiation volleys if the latter fall during dendritic negativity. Such activation depresses the response to a radiation volley falling during the phase of depression, and more severely depresses the negative phase of that response. It also depresses the response to a second stimulus to dendrites. The different types of responses of excitable tissues are discussed in relation to the type of activity exhibited by dendrites. Patterns of cell-axon spike discharge are inferred to differ depending on the points of activation of the neurone, at the cell body, at dendritic terminals, or at cell body and basal portions of dendrites simultaneously. Examples are cited of graded responses similar to those of dendrites in a wide variety of tissues, ranging from the protozoan slime mold to the mammalian cortex. This type of response is presumed to be the more primiti