Margaret H. Clare

Properties of dendrites; apical dendrites of the cat cortex

Abstract 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 primitive one, from which the all-or-none spike with an absolutely refractory phase is probably a highly specialized derivative.

Potential wave mechanisms in cat cortex

Abstract Evidence is presented for two types of potential-wave responses of apical dendrites of cortex. The shorter-latency of these responses decreases when its afferents are stimulated at 6 per sec. or faster, and the longer-latency process increases in amplitude. The incremental sequences include the classical recruiting waves following thalamic stimulation, and similar responses from cortex to stimulation of other regions of cortex, or from stimulating locally at the lead position. Four groups or size ranges of fibers activating cortex can be differentiated. The largest fibers are those of the afferent radiations to sensory primary projection areas. The next smaller activate somewhat similar responses in association area cortex when projection cortex is stimulated. A third group or division activates the decremental short-latency responses of cortex. The fourth group activates incremental or recruiting waves. From its conduction time, the latter group is inferred to consist of unmyelinated fibers. The recruiting property appears to be a function of the excitability cycle of one type of dendritic synapse, which shows two periods of facilitation. After a first stimulus, a second is effective during the 15 msec. duration of response to the first, after which a period of depression or relative refractoriness follows. At one sixth to one tenth sec. this depression passes over into supernormal excitability. That is, the repetition of an initial stimulus causes an increased response during the supernormal phase. Spontaneously arising recruiting spindles in nembutalized or decerebrate preparations show close resemblances to stimulated recruiting trains, and the interaction of these two sequences of waves indicates that they may occur in the same elements. In fact what are apparently waves of the spontaneous spindle type may appear during recruiting responses to repetitive stimulation if the frequency of stimulation is less than the frequency of spontaneous spindle discharge. This phenomenon seems related to that of “doubling” of response to each stimulus during repetitive stimulation that finally passes over into paroxysmal afterdischarge. Certain relations between incremental and decremental sequences of waves, when stimulated at different loci but recorded at the same leads across cortex, indicate that the two types of response may be those initiated at synapses via two afferent paths of different fiber types, impinging on the same dendrites. Inferences that seem reasonable will account for the alpha rhythm and recruiting and spindle responses as processes in the same or similar elements, activated in somewhat different patterns.

Further analysis of fiber groups in the optic tract of the cat

Abstract In adult cats the pattern of conducted compound action potential in the retrochiasmic optic tract after contralateral optic nerve stimulation is described. The usual pattern consists of two early spikes, T1 and T2, followed by a diminishing “tail,” T3. The later components have increasing thresholds to electric stimulation. Optic nerve fiber counts by electron microscopy show no conspicuous humps in a diameter histogram peaking at 1 μ. Using a new arithmetic technique for relating diameter spectrum to recorded action potential, good approximation of morphologic and physiological data was obtained. Physiological studies also show the projection of only T1 and T3 to superior colliculus, T1 being projected in attenuated collaterals of the largest tract fibers. Action potential reconstruction from fiber counts of the colliculus projection confirms this interpretation.

A note on the characteristic response pattern in primary sensory projection cortex of the cat following a synchronous afferent volley.

Abstract Electrical stimulation of the thalamic relay nuclei (ventralis posterior, medial geniculate, lateral geniculate) or their immediate prethalamic projection tracts, elicits in the appropriate sensory cortices a characteristic polyspike positive and negative wave. This response may also be elicited by direct cortical stimulation in striate cortex, but not in suprasylvian association cortex. The polyspike patterns are not elicited via the corpus callosum or other pathways. The cortical excitability cycle and the variation with stimulus intensity are described for these responses. Their mechanisms and nature are discussed.

The cortical response to direct stimulation of the corpus callosum in the cat.

Abstract The cortical response to direct stimulation of the corpus callosum consists of a brief surface-positive spike which originates deep in the cortex, followed by a slower positive-negative wave sequence which derives from structures extending to varying depths of cortex. The spike is assigned to dromic and antidromic axonal and possibly cell body discharge. The wave sequence is assumed to represent predominantly a response propagated upward along apical dendrites. The excitability cycle, strychnine spike driving, and post-tetanic potentiation phenomena are described, and the direct callosal response is compared with that elicited by homatotopic cortical stimulation. The antidromic callosal response is described in preparations in which dromic fibers had previously been allowed to degenerate. The response is remarkably similar to the normal mixed dromic and antidromic response, and it is inferred that the latter may be mediated synaptically via retrograde axonal collaterals. The interpretation of potential reversal in the cortical volume conductor is considered in relation to the probable anatomical substrate of the callosal response.

The Relation of Axon Sheath Thickness To Fiber Size In The Central Nervous System of Vertebrates

In a series of land vertebrates sheath thicknesses of myelinated nerve fibers in a variety of central tracts have been measured, and plotted against axon diameters. Preparations include monkey, cat, rat, mouse, mole, hedgehog, bullfrog, and green frog. Tracts examined include dorsal column cuneate and gracilis bundles, pyramidal tract, optic nerve and tract, trigeminal and auditory nerves, and for comparison, the saphenous nerve. Maximal sizes of axons (inside sheath diameter) in different preparations vary widely, from 12 μ or more to 2 μ, and most tracts have axon diameter ranges down to 0.3 μ. In the cat auditory nerve the axon size range is restricted (80% between 2 and 3 μ). All fibers in the mammalian central tracts are myelinated, with a wide scatter of values of the ratio of sheath thickness to axon diameter, as measured in a single cross section. The ratio varies with fiber diameter, becoming larger with decreasing diameter. Average ratios attain the value of 1/5 in different preparations at diff...

RECONSTRUCTION OF MYELINATED NERVE TRACT ACTION POTENTIALS: AN ARITHMETIC METHOD

Abstract Compound action potentials from central nerve tracts or peripheral nerves may be constructed from corresponding fiber diameter histograms by a new direct method. The ordinate for each fiber diameter group is determined by the product of the number of fibers and the cube of the mean diameter (ND 3 ), and the abscissa, by the position of the mean group diameter on a scale proportional to the reciprocal of fiber diameters ( 1 D ). The time scale is derived from a prominent component, conveniently the first spike peak, of the corresponding recorded compound action potential. Direct comparison with published reconstructions from peripheral nerves is made, using the original morphologic data. Reconstruction for a central tract, the cat optic nerve, is also shown. The method avoids the cumbersome geometry of the classical approach, and is especially convenient for the correlated anatomic and physiologic analysis of central tracts where accurate measurement of conduction distances is difficult, and where postsynaptic activity may distort the record of tract potential.

The interactions of several varieties of evoked response in visual and association cortex of the cat

Abstract Interactions between responses at one locus of cortex, suprasylvian or visual area of the cat, led from surface to white matter, involve the specific sensory response, callosal response, recruiting response, and that to direct stimulation of the cortical surface. In general, all other responses facilitate the specific response to geniculate radiation stimulation either when single responses precede it, or during tetanus of the conditioning path. Responses other than the specific response may show no interaction, depression of response, or occasional facilitation, to a single test stimulus of one path when the conditioning stimulus to another path precedes this. Tetanization of the conditioning path tends to accentuate this depression, both during and immediately after the tetanus. All these responses are those which occupy primarily or significantly the surface layers of cortex, and presumably the dentritic terminals of pyramid cells. It seems probable that the degree of effect depends on the number of elements which actually receive synaptic contacts from each of two paths, among a population of elements which are to a variable degree separately innervated.

The relationship of optic nerve fiber groups activated by electrical stimulation to the consequent central postsynaptic events.

Abstract A physiological analysis is described for the major central projections in the cat of the three fiber-size components of the optic tract compound action potential produced by electrical stimulation of the contralateral optic nerve. The first two waves, T1 and T2, both project to layers A and B of the lateral geniculate body, and thence to visual cortex. The superior colliculus response has three components: C1 is a response to direct projection of T1 axons; C2 is relayed via geniculate and striate cortex, and accordingly is influenced by both T1 and T2 tract fibers; C3, the largest response, is fired directly by the smallest diameter tract fibers, T3. The region of the nucleus of the optic tract and the lateral posterior nucleus of the thalamus can be activated by T1 and T2 tract fibers relayed by both cortical and subcortical mechanisms. Phylogenetic and functional considerations of the data are discussed.

The intracortical excitability cycle following stimulation of the optic pathway of the cat

Abstract Like many neurones of the central nervous system, those of the optic cortex of the cat exhibit a brief phase of facilitation, followed by a more prolonged phase of depression, as tested by a shock to the optic radiation following a conditioning shock. Recovery toward normal occurs gradually, and is complete in about 200 msec. The negative phase of the specific response returns only much later than the preceding portions of the response. This cortical depression is not complete, and varies in intensity in different preparations treated similarly. A weak first shock depresses the response to a second to some degree, and as the first shock is made stronger this effect is greater, in some cases to complete occlusion of the second response. The depression is accompanied by a surface-negative swing of the baseline. A more severe depression occurs in the dorsal nucleus of the lateral geniculate body, as previously described by Marshall. Its recovery of responsiveness is slower than is that of the cortex. Thus to optic nerve stimulation the cortical activity is affected by two conditions of depression operating serially. The depressive effect at the geniculate can be recognized in responses from the cortex in terms of the amplitude of the first spike representing responses of radiation axons. Two shocks, the second within the facilitatory period of the first, can break through the depression left by a previous response, when neither can do so alone. Similarly two such shocks may cause a larger single response than can any one stimulus. From these superimposed effects of facilitation and depression exhibited by arbitrary volley responses, certain speculations are offered as to the normal manner of cortical functioning.