Effects of sleep injuries

The word 'experiment' will be used in a restricted sense, in order to make a clear conceptual distinction between the phenomenology of sleep and the work on the neurophysiological mechanisms of the sleep-wake cycle. ‟The name of the experimenter is given to someone who uses simple or complex investigation procedures to vary or modify, for any purpose, natural phenomena and make them appear in circumstances or conditions in which nature does not present them to him. In this sense, 'observation' is the examination of a natural phenomenon and 'experiment' is the examination of a phenomenon modified by the examiner" (see Bernard, 1865, p. 29). In this chapter, we will summarize the results of experiments in which coma, lethargy or insomnia, or any change in the sleep-wake cycle,

a) The ascending reticular formation

This line of research is closely related to the physiology of the ascending reticular system. We therefore refer to Bremer's article of the same name, whose main conclusions we will briefly summarize as an introduction to this part.

It was later seen that even the behavioral aspects of the awakening reaction can be reproduced with the stimulation of the reticular formation, in animals without narcosis and free in their movements (see Moruzzi, 1972, for the literature). These are phasic, short-lived effects.

The next step taken by Moruzzi and Magoun (v., 1949) was to suggest the hypothesis that the ascending reticular system was continuously, i.e. tonically, active and that its influence on the brain must be above a certain critical level to maintain vigil. The interruption of this ascending influence would be the cause of the coma that appears in the cat after the section of the midbrain, in the cerveau isolé preparation by Bremer (see is c). This syndrome would also be observed in humans after a lesion of the midbrain produced by trauma. Less complete disruptions of the midbrain, combined with hypothalamic lesions, would produce lethargy, thus explaining von Economo's observations. The demonstration that a continuous, 'tonic' activity was present in the reticular system was obtained following two different lines of research: a) reproduction of the syndrome of Corna del cerveau isolé with interruption of the ascending reticular projections (see Magoun, 19632; see Moruzzi, 1972, for literature); b) demonstration by microelectrode recording of the existence of a continuous, irregular discharge in reticular neurons.

A third step forward was made by C. Batini and others (see, 1959), when they demonstrated that a behavioral and electroencephalographic syndrome of insomnia, thus opposed to the coma syndrome of cerveau isolé, could be obtained by dissecting the brainstem a few millimeters backward, at the pontine level. This 'trigeminal mid pontine preparation' is characterized by desynchronized EEG and alert eye behavior. This observation and many others made following different research paths (see Moruzzi, 1963, 1972; see Bonvallet, 1966, for the literature) led to the conclusion that in the brainstem there is also a system that can be called ‛ deactivating', because it is antagonistic to the ascending reticular system which we have seen instead to be activating. These are populations of neurons with EEG synchronizing and hypnogenic effects.

In summary, at the end of the 1940s a unitary explanation of apparently unrelated observations, such as those of von Economo and Berger, appeared possible. This result was due to the demonstration of an ascending reticular system with an activating, tonic and phasic influence on the brain. Finally, at the end of the 1950s, the classic hypnogenic effects obtained with electrical stimulation (see Chapter 4) could be related in some way to the deactivating influences exerted by other structures of the brainstem.

b) The deactivating regions of the brainstem

All of these results were obtained in acute experiments and the chronic effects of brainstem sections were used only as a control. Chronic experimentation began to be used especially during the sixties. These experiments led to important results for the problem of the origin of the sleep-wake cycle. The demonstration that two opposing influences are exerted on the brain led to the hypothesis that the cycle itself, i.e. the alternation of sleep and wakefulness, could originate in the brainstem.

The history of chronic decerebration experiments is long (see Moruzzi, 1972, for the literature). However, we will only discuss the results obtained by J. Villablanca (v., 1966) on cats in which the brainstem had been separated from the brain with a section made at a higher level, i.e. just in front of the superior colliculi (decerebration collicular). These tall midbrain cats were followed for an extended period of time. It is, of course, impossible to define any state observed in brainless animals as sleep or wakefulness. All we can say is that, after chronic decerebration, it is possible to observe behaviors resembling those of the normal animal during sleep and wakefulness. After 15-20 days the cats were found crawling or sitting or even attempting to walk; the lids were open and the pupils dilated. These were manifestly symptoms of wakefulness and this impression was reinforced by the fact that these periods alternated with states characterized by bodily manifestations of sleep. Villablanca (ibid .) made a distinction between a state characterized by the closing of the eyelids, by the lifting of the nictitating membrane, and, above all, by the fluctuation of the pupil diameter (fluctuating miosis) - which corresponds to the synchronized sleep of the intact animal - and a state characterized by the narrowing extreme of the pupils (fissured myosis) and by the generalized collapse of the postural tone, which corresponds to the desynchronized sleep of the intact animal. The periods characterized by the reversible disappearance of decerebrate rigidity are usually called 'cataplexic episodes', from the name of a clinical syndrome that we will examine later.

The main result obtained by Villablanca with his experiments is not the demonstration that fragments of sleeping or waking behavior can be observed in the absence of the brain - this had already been seen by others before him - but rather the demonstration that it is both the sleep-wake cycle and the rhythmic alternation of the two stages of sleep can arise when the brainstem is separated from the brain. Of course, only the cranial nerves and spinal cord are available for sleep and waking manifestations when the brain is absent. But it is a fundamental achievement to have demonstrated that rhythms of this type can arise, in a brainless animal, in the brainstem.

The obvious explanation for these results, if we overlook the problem of paradoxical sleep, is that there is an alternation of activity between two systems: the ascending or activating reticular system and the deactivating regions of the caudal part of the brainstem. Recent research has allowed us to locate at least two of these deactivating regions: 1) the region of the solitary tract, which is endowed with phasic activities, as demonstrated by lesion and stimulation experiments (see Moruzzi, 1963; see Bonvallet, 1966, for literature); 2) the nuclei of the raphe, which are tonically active, as demonstrated by prolonged insomnia produced by their lesion (see Jouvet and Renault, 1966). These are two independent systems, and this is demonstrated by the fact that it is still possible to produce synchronization of PHEO and miosis by stimulation of vagoaortic afferent fibers leading to the region of the solitary bundle. This deactivating effect is also present when the raphe crossing has been interrupted by a sagittal section (see Puizillout and Ternaux, 1974).

c) Alternation of activities and reciprocal connections between antagonistic systems

Chronic experiments on the cerveau isolé show, on the other hand, that a sleep-wake cycle can also arise in an isolated brain, after a complete section of the midbrain. Bremer's classic experiments were clever and the main result was the discovery that the sleep-wake cycle was present after the section of the cervical cord at C 1 , i.e. when the brain was still connected to the brainstem ( encéphale isolé ), while the period disappeared, and was replaced by a 'permanent' coma after the section of the midbrain ( cerveau isolé). Manifestly either the abolition of the flow of sensitive and sensory impulses through the cranial nerves, as Bremer had originally suggested (see, 1937 and 1938), or the suppression of an ascending influence arising between the two sections could explain such differences evident between these two acute preparations. We now know that the elimination of the tonic influence of the ascending reticular system is responsible for the acute cerveau isolé coma.

Sleep

1. Historical introduction

A little more than half a century has passed since C. von Economo (v., 1918) reported that in epidemic lethargic encephalitis, or sleeping sickness, the lesions are localized above all in the region of the midbrain that surrounds the Silvio aqueduct and in the 'hypothalamus. The interpretation he gave of his observations (see Von Economo, 1929) was wrong, but he was certainly right when he thought that the nervous structures damaged by the epidemic disease had critical importance in regulating the sleep-wake cycle.

Not many years later WR Hess (v., 1927) found that sleep could be obtained, in the cat without narcosis and free in its movements, with appropriate electrical stimulations of the medial region of the thalamus. Despite the possibility of errors (see chapter 4), this line of research had a profound influence on the development of sleep physiology. It was the first demonstration that physiological sleep can be achieved with localized electrical stimulation of the brain, and the results needed to be repeatedly confirmed by stimulating other regions of the diencephalon and brainstem. Furthermore, Hess's experiments paved the way for a fruitful line of research in which the study of behavior was associated with different types of electrophysiological stimulation and recording. Of course, it was impossible to locate, in those times, any relationship between von Economo's observations and Hess's experiments. Everything felt so different: case reports and animal experiments, injuries and stimulations, lethargy, and physiological sleep.

The discovery by H. Berger (v., 1929) that the electrical activity of the cerebral cortex could be recorded, in man, through cranial integuments (electroencephalogram, EEG) opened the era of electrophysiological studies in man and animals in the absence of narcosis. Berger himself made a major contribution to sleep studies when he showed that the human EEG exhibits slow, large voltage oscillations during this state, thus eliminating from discussion the naïve assumption that higher centers should be silent when there is no consciousness. He also demonstrated that this type of electrical activity is markedly different from the rapid low-voltage waves (β-waves) that characterize active wakefulness; and also by the slow oscillations of potential at 10/s (α waves) that appear during the relaxed vigil. Finally, Berger found that both α-waves and sleep rhythms could be abolished by sensitive or sensory stimulation and replaced by β-waves: two phenomena designated, respectively, Berger's arrest reaction and electroencephalographic awakening. All these effects were generalized and therefore not limited to the cortical area corresponding to the stimulated sensory modality. Their abrupt onset then prevented the recognition of links with the tonic phenomena responsible for maintaining vigil. It will be up to ED Adrian (v., 1934) to explain Berger's observations on the basis of models of activity of cortical neurons. He suggested the

Two lines of research arose from these early works. The line of 'phenomenological investigation', essentially based on observations, became extremely fruitful with the introduction of electroencephalographic and microelectrode recording techniques in Mammals without narcosis, free in their movements. It led to the demonstration that during sleep, in cats (see Klaue, 1937) and in humans (see Dement and Kleitman, 1957), periods characterized by EEG desynchronization are present and to studies on the relationships between this phase of sleep, called desynchronized sleep or paradox, and dreams ( ibid.). The investigations on the behavior of single units of the cerebral cortex must be situated along the same lines of phenomenological research (see Evarts, 1962 and 1964); and also the electrophysiological works on postural tone (see Jouvet, 1962), on spinal reflexes (see Pompeiano, 1965, 1966 and The neurophysiological ..., 1967) and on sensory transmission during desynchronized sleep (see Pompeiano, Sensory inhibition ..., 1967).

The second line of research, 'experimental investigation', is based on the study of changes in the wake-sleep cycle produced by lesions or by stimulation. Many aspects of this field of investigation have been examined by F. Bremer in the article ascending reticular system. Suffice it to recall here the research on the encéphale isolé and on the cerveau isolé(see Bremer, 1935, 1937, and 1938), the work of G. Moruzzi and HW Magoun (see, 1949) on the ascending lattice system, and the experiments of DB Lindsley et al. (see, 1949 and 1950) on the effects of reticular lesions; and finally the demonstration of the existence in the lower part of the brainstem of a deactivating system, probably hypnogenic (see Moruzzi, 1963, for the literature). The ascending reticular system has made it possible to give a unitary explanation of observations that are apparently unrelated to each other, such as those we referred to in the first part of this introduction.

The most recent experimental developments concern the neurochemical study of the brainstem structures that control the wake-sleep cycle and the electrophysiological investigation of the behavior of single nerve cells of the pons and cerebral cortex.

2. Sleep phenomenology

The study of the neurophysiological and neurochemical mechanisms of the sleep-wake cycle, which will be treated in the following chapters, must be preceded by the description of the phenomena that are observed when no attempt is made to modify sleep or wakefulness or their rhythmic alternation. ‟The name of observer is given to one who applies simple or complex investigation procedures to the study of phenomena which he does not modify and who, consequently, collect them as nature offers him" (see Bernard, 1865, p. 29 This is always the first approach in any field of natural sciences.

Half a century ago the only avenue open to researchers was the study of animal behavior and this line of attack is still followed in much research in comparative physiology and in a field of the natural sciences, ethology. Animal behavior is mainly based on movements or positions, therefore on the phasic and tonic activity of skeletal muscles and the corresponding motor neurons. Modern developments in electrophysiological techniques have made it possible to study the behavior not only of large populations of cerebral neurons (electroencephalography), but also of the activity of single nerve cells in the animal without narcosis, free in its movements. However, the aim of the research is always the same, whether we study the behavior of muscle fibers or that of brain neurons. We simply observe what is happening, making no attempt to influence the physiological sleep-wake cycle. Naturally some experimentation, in the strict sense of the word (see chapter 3), is also inevitable in this type of research. Indeed, it can be useful to specifically ask nature a few questions, as we do, for example, when we observe the effects of sensitive or sensory stimuli. The fundamental point, however, is that there is no attempt to modify the physiological rhythm of the alternation between sleep and wakefulness. Our aim is only to carefully describe natural phenomena and their relationships over time. it is unavoidable even in this type of research. Indeed, it can be useful to specifically ask nature a few questions, as we do, for example, when we observe the effects of sensitive or sensory stimuli. The fundamental point, however, is that there is no attempt to modify the physiological rhythm of the alternation between sleep and wakefulness. Our aim is only to carefully describe natural phenomena and their relationships over time. it is unavoidable even in this type of research. Indeed, it can be useful to specifically ask nature a few questions, as we do, for example, when we observe the effects of sensitive or sensory stimuli. The fundamental point, however, is that there is no attempt to modify the physiological rhythm of the alternation between sleep and wakefulness. Our aim is only to carefully describe natural phenomena and their relationships over time.

NEURODEGENERATIVE DISEASES.

The term degenerative is generic and indicates a pejorative change from a previous level of normality. The mn can strictly depend on genetic factors and affect several members of the same family, or occur in isolated cases. However, they can be characterized by an almost identical clinical phenotype in terms of onset and course (such as amyotrophic lateral sclerosis, ALS) or dissimilar and well differentiable (e.g., a picture of dementia associated early with spastic paraparesis is very rare in the forms sporadic cases of Alzheimer's disease, MdA, but far from infrequent in familial forms). While the cause of many MNs remains unknown to this day, for some of them much information has recently been acquired regarding the pathogenesis (ie the sequence of events and biological alterations that determine the dysfunction and finally the neuronal death). Furthermore, if it may happen that sporadic and hereditary forms of single mn do not have a completely superimposable clinical picture, common pathogenetic and biomolecular characteristics have often been identified, an aspect that heralds potential future therapeutic developments. 

For some NMs, such as sporadic forms of AD, a recurrence of late-onset cases (over 60 years of age) is documented within the same family without the identification of a specific pattern of inheritance, which may in any case suggest the intervention of genetic factors,ε4 (the other two are the ε3 and ε2 alleles, respectively the most common and the rarest in the general population), especially if present in duplicate (homozygous), increases the risk and reduces the age of onset of the disease, without confer substantial peculiarities to the clinical phenotype. On the other hand, in other mns such as sporadic Creutzfeldt-Jakob disease (CJD), the polymorphism of the prion protein gene at codon 129, of which there are two allelic variants encoding respectively the amino acids methionine and valine, can modify more significantly the phenotype with regards not only the age of onset but also the clinical presentation and the rapidity of progression.

Although the knowledge of the genetic and molecular basis of many mns has considerably progressed in recent times, their classification based on genetic and/or biomolecular anomalies may not be immediately useful for the clinician, since a single genetic anomaly can be associated with different phenotypes clinical and also to different biomolecular alterations, while, conversely, a single clinical phenotype can be associated with different biomolecular alterations (generally, dysproteinopathies originating from abnormal intra or extracellular accumulation of proteins due to overproduction or decreased degradation) and to different gene anomalies. Furthermore, the possible underlying genetic anomalies of some mn are still unknown and the biomolecular alterations are not clearly known. However,disease-modifying (interfering on the pathophysiological mechanisms of the disease), that individual clinical phenotypes are associated with prevalent biomolecular alterations (e.g., excess deposition of tau protein for corticobasal degeneration and progressive supranuclear palsy, excess deposition of TDP- 43, Transactive response DNA-binding Protein 43, for semantic dementia) and, if present in multiple members of the same family, to prevalent gene abnormalities (e.g., an association of ALS and frontotemporal dementia, DFT, with mutations of the C9ORF72 gene, Chromosome 9 Open Reading Frame 72 ).

In general, the men's are characterized by having a sneaky, insidious onset, of often uncertain dating, after a long period of normal functionality. An inexorable progression follows which, apart from rare exceptions (CJD), is mostly slow and gradual, even over many years. Only a few mn (such as Parkinson's disease, PD) are favorably influenced by pharmacological treatment, moreover with a mostly symptomatic effect. In many of them, there is a selective or preferential involvement of particular neurons, e.g. Purkinje cells of the cerebellum in spinocerebellar ataxias, or of functionally related neurons, e.g. the motor neurons of the cerebral cortex, brainstem, and spinal cord in ALS. Since the slow degeneration and eventually

From a clinical point of view, mn can be classified in pleomorphic clinical syndromes characterized by sensorineural alterations (as in retinitis pigmentosa, Leber's hereditary optic neuropathy, and sensorineural deafness, either pure or associated with retinal alterations), or by alterations of the peripheral nervous system (as in hereditary sensorimotor neuropathies), or by progressive muscle weakness and atrophy (as in ALS), or by progressive ataxia (as in spinocerebellar ataxias), or by alterations in postures and movements (as in PD ), or progressive dementia, whether associated with neurological signs (such as muscle weakness and wasting in DFT associated with ALS or parkinsonism in most cases of dementia with Lewy bodies, DCL) or unassociated, at least at presentation and for most of the clinical course, with neurological signs (as in most cases of AD).

NERVOUS PLASTICITY

    Characteristics of critical periods. Intermodal plasticity from early sensory deprivation. Environmental enrichment. Cortical plasticity in the adult. Reorganization of cortical maps after injury and rehabilitation therapy. Increase cortical plasticity in adults to promote brain repair. Bibliography

The cerebral cortex is made up of modules formed by columns of neurons that are repeated throughout the cortical surface. Each cortical area comprises many of these modules, each of them responsible for analyzing a particular aspect of the function of this cortical area. Both in the cortical areas dedicated to the analysis of sensory stimuli and in the motor areas or areas dedicated to cognitive aspects, such as short-term memory (working memory), it is noted that the cortical representation forms an ordered map. For example, in the visual cortex the cortical columns close to each other analyze the visual information relating to points of the visual field close to each other, thus forming a real map. Cortical maps are created during development, thanks to mechanisms mediated by molecular factors encoded by specific gene expression programs. However, it has been observed that changes in experience can induce profound alterations in the organization of cortical maps during postnatal development. Cortical maps are therefore not rigid and immutable, but malleable structures that can be modified by experience. The mechanism underlying the action of experience appears to be synaptic plasticity, i.e. the property that the connections between neurons have to modify their strength as a function of their previous activation (Levelt, Hübener 2012). The action of experience on the formation of cortical circuits is greatest during denominated developmental time windows it has been observed that changes in experience can induce profound alterations in the organization of cortical maps during postnatal development. Cortical maps are therefore not rigid and immutable, but malleable structures that can be modified by experience. 

The mechanism underlying the action of experience seems to be synaptic plasticity, i.e. the property that the connections between neurons have to modify their strength as a function of their previous activation (Levelt, Hübener 2012). The action of experience on the formation of cortical circuits is greatest during denominated developmental time windows it has been observed that changes in experience can induce profound alterations in the organization of cortical maps during postnatal development. Cortical maps are therefore not rigid and immutable, but malleable structures that can be modified by experience. The mechanism underlying the action of experience seems to be synaptic plasticity, i.e. the property that the connections between neurons have to modify their strength as a function of their previous activation (Levelt, Hübener 2012). The action of experience on the formation of cortical circuits is greatest during denominated developmental time windows but structures that are malleable and modifiable by experience. The mechanism underlying the action of experience seems to be synaptic plasticity, i.e. the property that the connections between neurons have to modify their strength as a function of their previous activation (Levelt, Hübener 2012). The action of experience on the formation of cortical circuits is greatest during denominated developmental time windows but structures that are malleable and modifiable by experience. The mechanism underlying the action of experience appears to be synaptic plasticity, i.e. the property that the connections between neurons have to modify their strength as a function of their previous activation (Levelt, Hübener 2012). The action of experience on the formation of cortical circuits is greatest during denominated developmental time windowscritical periods(Berardi, Pizzorusso, Maffei 2000). The existence of critical periods for experience-dependent plasticity has been clearly demonstrated for the auditory cortex, for the somatosensory cortex and in particular for the visual cortex, which constitutes the structure on which most of the studies on critical periods of plasticity focus cortical. Starting in the 1960s, the studies of David H. Hubel and Torsten N. Wiesel, who used the developing visual cortex as an experimental model, introduced the concept of critical period. This concept comes from Konrad Lorenz's ethological studies of innate and learned behaviors. There are critical periods for many functions, eg, a song in birds and speech in humans. In the latter case,

Characteristics of critical periods.

 – Classical studies have shown that sensory deprivations (such as monocular deprivation or occlusion of one ear) or sensory impairments (e.g., caused by strabismus or rearing in an auditory depleted environment) cause functional deficits in the sensory system only affected when manipulating the input sensory is carried out in the critical period. Similar deprivations and alterations in mature animals are much less or not at all effective. The length of the critical periods depends on the function being analyzed and on the type of manipulation of experience being carried out. For example, the pain periods measured in terms of recovery from the effects of sensory deprivation are longer than the pain periods measured in terms of the induction of sensory deprivation effects. It should be noted that critical periods have also been observed for forms of sensory plasticity induced by increased sensory experience: in humans, musical training initiated in infancy leads to the increased auditory cortical representation of musical stimuli only if practice begins before age of nine.

The modification of the circuits determined by the plasticity of the critical periods seems to leave a trace that would remain even after the functional effects of the alteration of the experience have regressed with the restoration of a normal experience. Studies in animals have shown that a short period of monocular deprivation causes a decrease in cortical responses to the deprived eye, which may return to normal if the deprivation is quickly removed. However these animals, despite the return to normal from a functional point of view, remain more sensitive to the effects of monocular deprivation. In fact, a second period of monocular deprivation, which in control animals would have no effect because it is too short, becomes effective (Hofer, Mrsic-Flogel, Bonhoeffer et al. 2009).

Intermodal plasticity from early sensory deprivation. 

– The high levels of plasticity observable during development can cause the deprivation of a specific sensory modality to determine changes in the other sensory modalities as well (intermodal plasticity). For example, subjects who became blind early show a better ability to locate sound sources than sighted subjects, especially for peripheral sources. This improved capability could result from the expansion of neural inputs that carry auditory information to visual areas lacking stimulation. Similar phenomena were also observed for tactile inputs in early blind and congenitally deaf subjects, indicating that it is a general phenomenon and not related to the specificity of a sensory area or modality.

Environmental enrichment. – A quality of lifestyle marked by physical activity, cognitive activity, and social interactions is beneficial not only for muscles and social life but also for brain functionality. In animals, exposure to an environment characterized by voluntary physical exercise and exploratory activity with new objects (enriched environment, AA) has both structural effects, with an increase in cortical thickness and synapse density, and functional effects, as it improves learning (Sale, Berardi, Maffei 2014). Exposure to AA from birth produces an acceleration of visual development and prevention of adverse effects due to lack of visual experience. AA acts by influencing the expression of factors crucial for visual cortical plasticities, such as neurotrophic factors and inhibitory circuitry. Even in adults, AA produces effects on visual cortical plasticity, increasing it to the point of favoring recovery from the effects of a previous monocular deprivation. Furthermore, in a mouse model of Alzheimer's, it has been demonstrated that early exposure to AA prevents the onset of cognitive and anatomical deficits, while late exposure slows down their progression.

NERVOUS SYSTEM

General neuron physiology. - The possibility of recording even the smallest bioelectrical manifestations without deformation has led to an unprecedented advance in our knowledge of neuron physiology.

It is appropriate to summarize what were the classical conceptions of neurophysiologists. By now accepted by all the theories of the neuron, it was believed that on the surface of its morphological constituents - the gynophore, the dendrites, the neurite (see nervous, tissue; XXIV, p. 659) - a membrane separated the inside from the outside of the nerve cell or its extensions. This membrane had never been directly observed, but its existence was postulated as the most likely explanation of the different concentrations of ions, and therefore of the potential difference, existing between the inside and the outside of the neuron. It was recognized that this membrane polarization characterized the cell or nerve fiber at rest. It was seen as the cause of the demarcation potential. It was then believed that the onset of the action potential, which accompanies the activity of the neuron and the propagation along the cylinder axis of the nerve impulse, was due to the disappearance of this polarization (depolarization of the membrane). These notions (cf.; App. II, 1, p. 837) have certainly been profoundly modified in the last quarter of a century, but it is a reason for admiration for the great electrophysiologists of the past to note that the central nucleus of their doctrine has stood the test of time well.

The surface membrane of the neuron.

 - Electron microscopy (see histology, nervous tissue, in this App.) revealed that a membrane of uniform thickness (about 50 Å) separates the inside from the outside of the neuron. It is probably formed by a bimolecular layer of phospholipids and cholesterol, supported by a protein skeleton. The whole general physiology of the neuron hinges on the chemical-physical properties of this membrane. Through the use of glass ultramicroelectrodes, which allow entering the interior of the neuron without killing it, JC Eccles and his collaborators have been able to demonstrate that the membrane potential of the intracellular electrode with respect to an indifferent external electrode is of the order of −60 to −80 mV. The membrane potential, whereby the inside of the neuron is electronegative with respect to the outside, it is a general property of all excitable tissues. The polarization of the surface membrane is the cause of the demarcation potential of classical physiology. The membrane represents a barrier to the free diffusion of ions, because it has a low ionic permeability. The specific resistance and the specific capacitance of the membrane are very high: of the order of 500-1000 Ω/cm respectively2 and 3μF/cm 2 , for mammalian motor neurons.

Origin of the membrane potential. - When the passage of ions through the membrane occurs by simple diffusion, it is regulated i) by the permeability of the membrane and ii) by the electrochemical potential gradient. This gradient is zero when the membrane potential is equal to the equilibrium potential for that particular ion. Knowing the concentration of the ion inside and outside the cell we can, as we shall see below, calculate the equilibrium potential; on the other hand, knowing the membrane potential, we can determine for each ion the concentration gradient between the outside and inside of the cell for which there is electrochemical equilibrium. Therefore, if we find that the equilibrium potential of a given ion is different from the membrane potential, we must conclude that the cell or the nerve fiber actively intervenes, producing work and consuming energy, to maintain a distribution that is different from that allowed by the electrochemical balance. It is then admitted that for that particular ion, there is a "pump" which ensures an "uphill" flow of ions, i.e. in the opposite direction to that which would be given by the electrochemical gradients. In summary, a different distribution of ions inside and outside the cell can be the expression of a chemical-physical equilibrium, such as one could have on the two sides of an inert membrane (provided it is electrically polarised), if the gradient electrochemical is zero, but if it is different from zero we must admit the intervention of an active process,

For there to be equilibrium, without any energy consumption by the cell, ECl must be equal to the membrane potential. But when

Thus a membrane potential of −70 mV equilibrium at a concentration gradient for Cl′ corresponding to the ratio

This is precisely the concentration ratio that normally occurs and therefore the electrochemical potential gradient is zero for Cl'. Therefore the difference in concentration in Cl′ is not linked to any active process but is only a consequence of the membrane potential.

Let us now apply the same considerations to the two cations (Na• and K • ), which also appear unequally distributed on the two sides of the membrane. The table shows the data calculated by JC Eccles (1957).

Let's summarize. Concentration imbalances are in part the simple consequence of the existence of the membrane potential, in part they are linked to active metabolic processes, designated under the name of "Na and K pumps". Naturally, the chemical-physical mechanisms which maintain the membrane potential and the chemical reactions which underlie the functioning of the "pumps" remain to be known. It is not possible to answer the second question at the moment. As regards the first, it has been calculated that an excess of anions equal to 2 × 10 -15 within the motor neuron is sufficient to determine a membrane potential of −70 mV. But what is the chemical nature of this anion is unknown to us. We have given the explanation of the membrane potential based on the ion theory of AL Hodgkin and AF Huxley (1952). We will see in the following section that this theory satisfactorily accounts for the action potential as well.

The action potential. 

- According to the theory of J. Bernstein (1902), the action potential was considered the consequence of the transient and localized annulment of the membrane potential, which would have occurred every time a cell or a fiber became active. It has actually been seen that the polarization of the membrane is not canceled, but reversed when the action potential appears (figs. 1, 2). This discovery, made almost simultaneously by AL Hodgkin (1939) and by HJ Curtis and KS Cole (1940), completely changed our conceptions. This is how the birth of the action potential, often referred to as a spike in the Anglo-American electrophysiological literature, is explained today.

The very large electrochemical potential gradient for Na• does not lead to a high flow of ions from the outside (where the concentration is much higher) into the cell or fiber, because the specific membrane conductance for this cation is far lower than that for K • and Cl′ (respectively of the order of 0.01 millimho/cm 2 and 0.5 millimho/cm 2 ). The poor inward flow of Na• is compensated for by the extrusion of ions by the Na• pump. The situation changes dramatically when the action potential arises. The conductivity for Na• increases dramatically, rising to about 15 millimho/cm 2. This leads to massive entry of Na• into the cell or fiber. The process takes about 1 msec. The entry of positive charges first cancels the polarization of the membrane, which reaches zero potential; but the flow continues and the polarization of the membrane is reversed, until almost reaching the equilibrium potential for Na• which is + 50, + 60 mV. This is the reason why the action potential is always greater than the membrane potential. The specific conductivity for K • also increases, but with a delay, and when this happens, there is an increased outflow of K • , also here according to the electrochemical gradient, i.e. from inside (where the concentration of K • is more elevated) outdoors. L'• initiates the descent of the action potential and that set of processes which will then lead to the re-establishment of the membrane potential and a return to the resting distribution of the ions. This refreshment process is related to the activity of the Na and K pump and, indirectly, to the metabolic activity of the cell or nerve.

The propagation of the nerve impulse. 

- When the action potential appears in a point of a nerve fiber, an electrode placed outside the membrane behaves as electronegative with respect to an electrode placed in the same position, but in a stretch of nerve at rest. On the active point it is as if a cathode connected to a direct current source had been placed. In both cases, positive charges leave the adjacent stretch of nerve at rest. In front of the action potential, the membrane of the fiber section still at rest is therefore depolarized, according to the classic mechanism of the electrotone. When the depolarization reaches a critical level, the specific Na• conductivity increases. Then sharply increases the flow of Na• towards the interior of the fiber, and this leads to a further depolarization of the membrane; this in turn increases the conductivity to Na•. This is how the self-regenerative process takes place which leads to the birth of the nerve impulse, i.e. to the inversion of the depolarization of the membrane in the stretch of fiber located immediately downstream, which was at rest a moment before. The reason for this irresistible march forward is that the action potential leaves behind a refractory-phase fiber so that under normal conditions conduction can only occur in one direction. Nerve conduction is therefore linked to the fact that the action potential represents an electrical stimulus for the downstream nerve section (fig. 3). that is, to the inversion of the depolarization of the membrane in the stretch of fiber located immediately downstream, which was at rest an instant before. The reason for this irresistible march forward is that the action potential leaves behind a refractory-phase fiber so that under normal conditions conduction can only occur in one direction. Nerve conduction is therefore linked to the fact that the action potential represents an electrical stimulus for the downstream nerve section (fig. 3). that is, to the inversion of the depolarization of the membrane in the stretch of fiber located immediately downstream, which was at rest an instant before. The reason for this irresistible march forward is that the action potential leaves behind a refractory-phase fiber, so that under normal conditions conduction can only occur in one direction. Nerve conduction is therefore linked to the fact that the action potential represents an electrical stimulus for the downstream nerve section (fig. 3). so under normal conditions conduction can occur in one direction only. Nerve conduction is therefore linked to the fact that the action potential represents an electrical stimulus for the downstream nerve section (fig. 3). so under normal conditions conduction can occur in one direction only. Nerve conduction is therefore linked to the fact that the action potential represents an electrical stimulus for the downstream nerve section (fig. 3).

The saltatory theory applies only to the myelinated peripheral fibers of Vertebrates, in which the myelin sheath is interrupted at regular intervals by the nodes of Ranvier (see nervous, tissue, XXIV, p. 663). The theory, which has the support of many experimental tests (R. Stämpfli, 1952; I. Tasaki, 1959), assumes that the internodal tract has a purely passive function in the propagation of the impulse. When the action potential appears in one node of Ranvier, the next node is depolarized, because positive charges carry themselves to the active node, flowing out of the myelin sheath that lines the internode. When the depolarization in the second node reaches a critical level, a new action potential arises, which will exert the same influence on a third node of Ranvier. In this way the nerve impulse propagates, almost "jumping" from node to node (fig. 4). This process has a considerable margin of safety: it has been shown that an impulse can "jump"

The synapse and the chemical mediation of the nerve impulse

. - The neuron theory assumes the discontinuity between individual neurons. These meet at well-circumscribed points, called synapses. The existence of synapses has been confirmed by electron microscopy. At the synapse, the membranes of two nerve cells - called the presynaptic membrane and the postsynaptic membrane, with reference to the direction of the transmission of the nerve impulse - come into contact, being separated by a granular space 50-500 Å thick. Within the presynaptic membrane are small vesicles of 300-500 Å in diameter, which are assumed to contain the chemical mediator.

The existence of a chemical mediator in central synapses had already been postulated by analogy with the behavior of peripheral synapses. JC Eccles deems this hypothesis necessary in view of the fact that the presynaptic impulse does not generate a flow of ions sufficient to produce a critical depolarization of the postsynaptic membrane. It, therefore, seems that the presynaptic impulse, having reached the last endings, frees the chemical mediator from an inactive precursor. This alters the ionic permeability of the postsynaptic membrane, giving rise to a new nerve impulse. The nature of the chemical mediator is unknown for most central synapses.