Dale Purves

Functional and structural changes in mammalian sympathetic neurones following interruption of their axons.

The effects of interrupting the axons of principal neurones in the superior cervical ganglion of adult guinea‐pigs were studied by means of intracellular recording, and light and electron microscopy. 1. Within 72 hr of axon interruption, the amplitude of exitatory postsynaptic potentials potentials (e.p.s.p.s) recorded in principal neurons in response to maximal preganglionic stimulation declined. E.p.s.p.s were maximally reduced (by more than 70% on average) 4‐7 days following interruption, and failed to bring many cells to threshold. E.p.s.p.s. recorded in nearby neurones whose axons remained intact were unaffected. 2. In ganglia in which axon interruption was achieved by means of nerve crush (thus allowing prompt regeneration), mean e.p.s.p. amplitudes began to increase again after about 1‐2 weeks. One month after the initial injury many neurones had e.p.s.p.s of normal amplitude, and by 2 months affected neurones were indistinguishable from control cells. Functional peripheral connexions were re‐established during the period of synaptic recovery. 3. The mean number of synapses identified electron microscopically in ganglia in which all the major efferent branches had been crushed decreased by 65‐70% in parallel with synaptic depression measured by intracellular recording. However synapse counts did not return to normal levels even after 3 months. 4. During the period of maximum synaptic depression, numerous abnormal profiles which contained accumulations of vesicular and tubular organelles, vesicles, and mitochondria were observed in electron microscopic sections. Injection of horseradish peroxidase into affected neurones demonstrated dendritic swelling which probably correspond to these profiles. 5. Little or no difference was found in the electrical properties of normal neurones and neurones whose axons had been interrupted 4‐7 days previously. However, the mean amplitude of spontaneously occurring synaptic potentials was reduced, and the amplitude distribution was shifted. This abnormality of the synapses which remain on affected neurones also contributes to synaptic depression. 6. Counts of neurones in normal and experimental ganglia showed that approximately half the principal cells died 1‐5 weeks after crushing the major efferent brances. This finding presumably explains the failure of synapse counts to return to control levels after recovery. 7. If axons were prevented from growing back to their target organ by chronic ligation, surviving neurones whose axons were enclosed by the ligature did not generally recover normal synaptic function. Following ligation, most affected cells died within a month. 8. Thus the integrity of a principal cells axon is necessary for the maintenance of preganglionic synaptic contacts, and ultimately for neuronal survival. The basis of neuronal recovery from the effects of axon interruption appears to be some aspect of regeneration to the peripheral target.

Correlated Size Variations in Human Visual Cortex, Lateral Geniculate Nucleus, and Optic Tract

We have examined several components of the human visual system to determine how the dimensions of the optic tract, lateral geniculate nucleus (LGN), and primary visual cortex (V1) vary within the same brain. Measurements were made of the cross-sectional area of the optic tract, the volumes of the magnocellular and parvocellular layers of the LGN, and the surface area and volume of V1 in one or both cerebral hemispheres of 15 neurologically normal human brains obtained at autopsy. Consistent with previous observations, there was a two- to threefold variation in the size of each of these visual components among the individuals studied. Importantly, this variation was coordinated within the visual system of any one individual. That is, a relatively large V1 was associated with a commensurately large LGN and optic tract, whereas a relatively small V1 was associated with a commensurately smaller LGN and optic tract. This relationship among the components of the human visual system indicates that the development of its different parts is interdependent. Such coordinated variation should generate substantial differences in visual ability among humans.

Monosynaptic chemical and electrical connexions between sensory and motor cells in the central nervous system of the leech

The synaptic connexions that underlie three different segmental shortening reflexes have been traced by recording intracellularly from individual sensory and motor nerve cells in the C.N.S. of the leech. The fourteen sensory cells involved in these reflexes respond specifically to one of three modalities: touch, pressure, or noxious stimuli applied to the skin. All three types of sensory neurone give rise to excitatory synaptic potentials in two large motoneurones. Each of these motor cells provides excitatory innervation to the longitudinal muscle fibres of the opposite side of the segment. The mechanism of synaptic transmission is, however, different for each type of sensory cell.

Iterated patterns of brain circuitry (or how the cortex gets its spots)

The prominence of repeating patterns of circuitry in the mammalian brain has led to the general view that iterated modular units reflect a fundamental principle of cortical function. Here we argue that these intriguing patterns arise not because the functional organization of the brain demands them, but as an incidental consequence of the rules of synapse formation.

Re-innervation of guinea-pig superior cervical ganglion cells by preganglionic fibres arising from different levels of the spinal cord.

1. The ability of preganglionic axons to re‐establish their normal pattern of synaptic connexions with superior cervical ganglion cells has been studied after section of the cervical sympathetic trunk.

A statistical explanation of visual space

The subjective visual space perceived by humans does not reflect a simple transformation of objective physical space; rather, perceived space has an idiosyncratic relationship with the real world. To date, there is no consensus about either the genesis of perceived visual space or the implications of its peculiar characteristics for visually guided behavior. Here we used laser range scanning to measure the actual distances from the image plane of all unoccluded points in a series of natural scenes. We then asked whether the differences between real and apparent distances could be explained by the statistical relationship of scene geometry and the observer. We were able to predict perceived distances in a variety of circumstances from the probability distribution of physical distances. This finding lends support to the idea that the characteristics of human visual space are determined probabilistically.

The effect of contractile activity on fibrillation and extrajunctional acetylcholine−sensitivity in rat muscle maintained in organ culture

1. The effect of contractile activity on the initiation of spontaneous action potentials (fibrillation) and on extrajunctional acetylcholine‐sensitivity has been studied in single fibres in strips of previously denervated rat diaphragm maintained in organ culture for up to 10 days.

The elimination of redundant preganglionic innervation to hamster sympathetic ganglion cells in early post‐natal life.

The superior cervical ganglion of adult and neonated hamsters has been studied with intracellular recording. 1. Neurones in adult hamster ganglia are innervated by an average of 6‐7 preganglionic axons. During the first week of post‐natal life, however, these cells are innervated by at least eleven to twelve axons. Ganglion cells in animals 2‐3 weeks old are innervated to an intermediate degree, indicating that these neurones lose a substantial portion of their initial synaptic contacts during the first weeks after birth. 2. The over‐all innervation of the superior cervical ganglion in adult hamsters arises from thoracic segments T1‐T5; no additional segments contribute significantly to the innervation of neonatal ganglia. 3. The average number of segments innervating each adult ganglion cell is 2 . 8 compared to 3 . 7 segments innervating neonatal neurones. Throughout post‐natal development the innervation of individual neurones arises from a contiguous subset of the spinal segments that innervate the ganglion as a whole. 4. We conclude that the elimination of redundant innervatin in early life is not limited to those nerve and muscle cells contacted by a sigle axon in maturity, but also occurs in sympathetic ganglia where adult neurones remain multiply innervated. Moreover, the loss of some synaptic contacts during development refines the selective innervation of individual neurones.

Membrane properties underlying spontaneous activity of denervated muscle fibres

We have examined the events underlying the initiation of spontaneous action potentials (fibrillation) in fibres of previously denervated rat diaphragm maintained in organ culture for up to 10 days.

Post‐natal reduction of neural unit size in the rabbit ciliary ganglion.

We have studied the innervation of adult and neonatal ciliary ganglia in the rabbit to determine the average number of ganglion cells innervated by each preganglionic neurone at different stages of development. 1. The adult ciliary ganglion comprises about 380 ganglion cells which are innervated by about forty preganglionic neurones. 2. Ciliary ganglion cells in adult rabbits are on average innervated by 2.2 different axons; in contrast, neonatal ganglion cells are on average innervated by 4.6 different axons. The transition to the adult pattern of innervation occurs gradually during the first few post‐natal weeks. 3. The numbers of ganglion cells and preganglionic neurones do not change appreciably after birth. Accordingly, the loss of some innervation to individual neurones during post‐natal development indicates that each preganglionic axon innervates progressively fewer ciliary ganglion cells. 4. The number of synaptic boutons found in ganglia at birth, however, is less than the number of synaptic boutons found in adult ganglia. 5. We conclude that synaptic connexions in this ganglion age gradually rearranged in early post‐natal life such that each preganglionic neurone focuses an increasing number of synaptic contacts on a progressively smaller subset of the ganglion cell population.