Showing papers in "Journal of Neurophysiology in 1965"

Receptive fields and functional architecture in two nonstriate visual areas (18 and 19) of the cat.

Abstract: To UNDERSTAND VISION in physiological terms represents a formidable problem for the biologist. I t am0 unts to learning how the nervous system handles incoming messages so that form, color, movement, and depth can be perceived and interpreted. One approach, perhaps the most direct, is to stimulate the retina with patterns of light while recording from single cells or fibers at various points along the visual pa thway. For each cell the optimum stimulus can be determined, and one can note the charac teristics common to cells at the next. each level in the visual pathway, and compare a given level with

Functional significance of cell size in spinal motoneurons

Abstract: SINCE THE BEGINNINGS OF NEUROHISTOLOGY it has been recognized that neurons within the central nervous system vary widely in size, but the functional significance of this basic observation has never emerged from the realm of speculation. The largest cells have surface areas which are at least 100, perhaps 1,000, times greater than those of the smallest cells. Correspondingly, the diameters of axons in the central and peripheral portions of the nervous system range from less than .25 p. to more than 20 c-c. This broad spectrum of physical dimensions invites a search for functional correlates. This is one of a series of studies on the problem of size as it relates to spinal motoneurons. The preceding papers (21, 25) were concerned chiefly with the peripheral part of the motoneuron and the muscle fibers it innervates. They provided experimental evidence that the diameter of a motor nerve fiber is related to the number of muscle fibers it supplies. This finding seemed to make good sense: if a motor fiber innervates many muscle fibers and forms a large motor unit, it must have sufficient axonal substance to give off a large number of terminals. The present paper is concerned with the central part of the motoneuron and the significance of its size in synaptic transmission. It asks whether the cell bodies (and dendrites) connected with large and small motor fibers have different functional properties which can be recognized by their discharge characteristics. In order to investigate this problem one must be able to distinguish the signals of a large motoneuron from those of a small one. This may be done by recording their action potentials from thin filaments of lumbar ventral roots. As Gasser (8) demonstrated, the amplitudes of nerve impulses recorded externally from peripheral nerves are directly related to the diameters of their fibers. If it may be assumed that the diameters of axons are also related to the sizes of their cell bodies, as scattered histo-

Comparison of the effects of unilateral and bilateral eye closure on cortical unit responses in kittens

Abstract: IN THE NORMAL CAT OR KITTEN about four-fifths of cells in the striate cortex can be driven by both eyes (3, 4). If, however, one eye of a newborn kitten is sewn shut and the visual cortex recorded from 3 months later, only a small fraction of cells can be driven from the deprived eye (8) . In contrast, many cells in the latera .I geniculate are driven normally from the d ,eprived eye (7 ), suggesting that the abnormality occurs somewhere between geniculate cells and cortex. Since clear receptive-field orientations and directional preferences to movement are seen in cortical cells of newborn visually inexperienced kittens, the deprivation effects presumably represent some sort of disruption of innately determined connections, rather than a failure of postnatal development related to lack of experience. In these experiments the use of monocular deprivation made it possible to compare adjacent geniculate layers, and also to compare the two eyes in their ability to influence cortical cells, so that each animal acted, in a sense, as its own control. The results led us to expect that depriving both eyes for similar periods would lead to an almost total unresponsiveness of cortical cells to stimulation of either eye. That should be so, provided the effects of depriving one eye were independent of whether or not the other eye was simultaneously deprived. It seemed worthwhile to test such an assumption, since any interdependence of the two pathways would be of considerable interest. We accordingly raised kittens with both eyes covered by lid suture, and recorded from the striate cortex when the animals had reached an age of 23-43 months.

Binocular interaction in striate cortex of kittens reared with artificial squint.

Abstract: BEFORE A KITTEN OPENS ITS EYES, and long before the eyes are used in visual exploration, single cells of the primary visual cortex respond to natural stimulation with the same specificity as is found in the adult (5). This suggests that the anatomical connections between retina and striate cortex are for the most part innate. During the first 3 months of life the connections are highly susceptible to the effects of visual deprivation, to the extent that exclusion of all form and some light from one eye leads to a severe decline in the ability of that eye to influence cortical cells. Anatomical and physiological evidence suggests that the defect is chiefly, though not entirely, a cortical one (7-9). The object of the present study was to influence cortical connections by some means less drastic than covering one or both eyes. We wished if possible to alter the input in such a way that there would be no question of effects on the visual pathway below the level of the striate cortex. A method was suggested by the well-known clinical observation that a child with a squint (strabismus or nonparallel visual axes) may suffer a deterioration of vision in one eye (amblyopia ex anopsia). Since the visual pathways from the two eyes are for practical purposes separate up to the level of the striate cortex, it is unlikely that in these children the defect is in the retina or geniculate. An artificial squint therefore seemed to provide a possible means of obtaining a cortical defect while sparing the retina and lateral geniculate body. Accordingly , we produced a divergent strabismus by cutting one of the extraocular muscles in each of four newborn kittens, with the plan of testing vision and recording from single cortical cells after several months to a year. When at length each eye was tested in these kittens by observing the ani-mal's behavior with the other eye covered the results were disappointing: there was not the slightest suggestion of any defect in vision in either eye. This was not entirely unexpected, since with both eyes uncovered the animals had appeared to fix at times with one eye and at times with the other. At this stage there seemed to be little point in proceeding further, for there- .

Excitability and inhibitibility of motoneurons of different sizes

Abstract: IN A RECENT PAPER (7) data were presented which indicate that during a gradually increasing stretch of an extensor muscle in a decerebrate cat the smallest alpha motoneurons of the muscle are the first to be discharged and that larger cells are recruited in order of increasing size. Some of the possible explanations for this relationship between size and order of recruitment were discussed briefly. It was concluded tentatively that the size or surface area of a motoneuron determines its excitability and hence its responsiveness to stretch-evoked impulses. If this conclusion is correct, we may infer that size is a prime determinant of excitability throughout the nervous system; if not, the size principle may apply only to motoneurons discharging in response to stretch. Fortunately, it is possible to test the proposed explanation experimentally. If size dictates excitability, the order in which a group of cells are recruited should be the same regardless of the source of the excitation and the particular circuits involved in mediating it. On the other hand, if the organization of the recruitment in input from , the stretch stretch receptors is responsible for the order of reflex, the order shoul .d change wi th d .ifferent types of excitatory input. Experiments were therefore designed in which the relative excitabilities of motoneurons were first determined by means of stretch reflexes and then retested by * means of crossed-extension reflexes, flexor reflexes, monosynaptic reflexes, and electrical “driving” from the muscle ner ve. The relative susceptibilities of neurons of different sizes to several varieties of inhibition were also investigated to ascertain whether inhibitibility bears some regular relation to cell size.

Relations between structure and function in the design of skeletal muscles.

Abstract: INTWORECENTLYPUBLISHED PAPERS (11,18)a detailedstudy wasmade of the properties of motor units in the soleus and medial gastrocenmius muscles of the cat. In particular, it was shown that there were great variations in the size of individual motor units and, paralleling them, important differences in their contractile characteristics. The full significance of these findings did not become apparent, however, until the observations reported in the preceding paper (7) had been made, indicating that the excitability of motoneurons is an inverse function of their size. From this it follows that the participation of a motor unit in graded motor activity is dictated by the size of its neuron. A corollary of this conclusion is that the total amount of contractile activity of a unit decreases as its size increases. These new findings on motor units have prompted a histochemical study of the soleus and m. gastrocnemius (MG), using recently developed methods (14) of staining muscle fibers for adenosine triphosphatase (ATPase). The results of this study are presented here. They provide a background for a functional analysis of the data on motor units. The hypotheses emerging from this analysis, in turn, offer a rationale for the histological findings. The conjunction of these different findings, all concerned with the same muscles, permits us to point out some previously unrecognized principles which govern the design of skeletal muscles.

Extent of recovery from the effects of visual deprivation in kittens.

Abstract: Seven kittens were used, and the various procedures of deprivation and subsequent studies are summarized in Table 1. In six animals the Iids of one eye were closed for the first 3 months of life. In the recovery period two of these kittens had the deprived eye opened. The other four had the deprived eye opened and the other (previously open) eye was closed. The seventh animal had both eyes closed for 3 months; the right eye was then opened. Recovery periods

Properties of motor units in a homogeneous red muscle (soleus) of the cat.

Abstract: THE INFORMATION WHICH IS AVAILABLE on the properties of motor units in skeletal muscles is chiefly of an indirect nature, having been inferred from observations on whole muscles. Although a muscle is no more than an assembly of motor units, the collective properties of the whole do not convey much information about its parts. The innervation ratios, contraction times, and tensions of motor units, reported by Eccles and Sherrington (5) and others, are average values calculated from data on whole muscles. Inevitably, these averages conceal the normal range of values for individual units. AS this study will illustrate, the properties of motor units vary over a wide range even within a homogeneous muscle. The distribution of these properties confers on the muscle functional characteristics it would not otherwise possess. The soleus and m. gastrocnemius muscles of the cat were selected for study because they seemed to be ideally suited for comparison. Although their origins are slightly different, they insert on a common tendon and both serve to extend the ankle. They differ considerably, however, in their gross and microscopic appearance and their functional capacities. M. gastrocnemius, which is considered in the paper following this one, is a typical pale muscle, consisting of three types of muscle fibers (10). In histological studies carried out in this laboratory it has been found that the soleus, which is the object of this first study, is a homogeneous muscle, consisting wholly of one type of fiber. As will become apparent, the properties of soleus motoneurons clearly reflect its more uniform structure. In order to investigate single motor units, it was necessary to activate them one at a time, leaving the rest of the muscle completely quiescent. This was done by isolating and stimulating filaments of ventral roots which contained a single alpha fiber to the soleus muscle. Data were obtained on the following characteristics of 97 soleus units in the cat: conduction velocity of the axon, contraction time, twitch tension, and maximum tetanic tensions at several frequencies of stimulation.

Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of the cat.

Abstract: THE PRECEDING PAPER (7) is an account of motor units in the soleus muscle of the cat, which is an example of the red or slow variety of skeletal muscles. For comparison with it, we present here the results of a similar study on the medial gastrocnemius (MG), a pale or fast muscle. The two muscles are separate heads of the triceps surae, which is an extensor of the foot. Although they are closely related anatomically they differ strikingly in their microscopic appearance and in their contractile properties. Ranvier (8) correlated the pale color of muscles such as the MG with their phasic mechanical properties (fast contraction, higher fusion rates, easy fatigability), but later authors have cautioned against such simplification. For many years it has been known that pale muscles consist of a mixture of dark and pale fibers. Recent studies (9) show that they contain three kinds of fibers which differ in their mitochondrial enzymes and in their appearance under the electron microscope. We shall attempt to correlate these observations with the differing properties of individual motor units. We shall also try to reconcile our findings with the striking differences in functional capacity of the soleus and MG, notably their relative speed and power.

Rat retinal ganglion cells: receptive field organization and maintained activity

Abstract: Receptive field organization of rat retinal ganglion cells, examining maintained impulse activity with stationary spots of light