George H. Bishop

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.

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.

Potential phenomena in thalamus and cortex

1. The cortical response to stimulation of an afferent path consists of two parts, a specific response indicating projection of the afferent and a response of the alpha mechanism. The former of these under the influence of strychnine can be identified with the cortical spikes of paroxysmal activity. 2. The response of the geniculate and other simply arranged nuclei are monophasic spikes. The diphasic spikes of the cortex and elsewhere are inferred to be assignable to two different groups of cells related by synaptic passage. 3. The unit cell potential may be interpreted as the resultant of unequal depolarization during activity of the two ends of the cell dendritic and axonal. If this is so, polarization by external currents should modify or even reverse the potentials and this proves to be the case. The phenomena are comparable to the effects of polarization on nerve axons. 4. Three types of potential fields can be visualized of which different cell structures of the brain may be considered as variants. Plots of the potentials of these fields indicate what relation of potential form to structure may be expected to obtain. 5. The character of the relation between thalamus and cortex appears to be different for the specific response apparatus which is not spontaneously active expect in paroxysmal states and the regulatory apparatus which is normally spontaneously active but suppressed in paroxysm. It is not necessary to infer reverberating closed circuits to account for the activity in either system.

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.

L The Transmission of Pain Impulses via the Chorda Tympani Nerve

The chorda tympani nerve, once described as the sensory part of the seventh cranial nerve, carries preganglionic fibers from the superior salivatory nucleus to the submaxillary ganglion, and thus contributes largely to the secretory activity of the gland. Its terminal part fuses with the lingual nerve and supplies taste sensation to the anterior two-thirds of the tongue. Kuntz! reviewed the proof of secretory fibers to the salivary glands by Ludwig in 1851, and by Heidenhain in 1878; they stimulated the chorda by weak induction shocks, causing the submaxillary and sublingual glands in dogs to secrete actively; they were able to vary and regulate the degree of secretory function by stimulation of the sympathetic or the parasympathetic nerves to the glands, alternately. Contemporary investigators confirmed the findings.

The equivalence of recruiting and augmenting phenomena in the visual cortex of the cat

Abstract The augmenting wave phenomenon in striate cortex of the cat could be elicited only by stimulation within the cerebral hemisphere; even supramaximal stimulation of the optic nerve did not produce it. Augmenting waves showed mutual occlusion with recruiting waves elicited from medial thalamus, and both showed some interaction with the primary specific sensory response. Augmenting and recruiting phenomena had similar excitability characteristics and distribution with respect to depth in the cortex. It is concluded that they differ operationally only in that augmenting waves are conventionally elicited together with the primary sensory projection responses, when both are aroused by stimulation of thalamic relay nuclei or their radiation paths to cortex.