Somatosensory cortex: structural alterations following early injury to sense organs

In mouse somatosensory cortex there are discrete cytoarchitectonic units, called "barrels." Each barrel is related to one sensory vibrissa on the muzzle. Individual vibrissae were carefully injured at birth; 12 to 43 days later, the corresponding barrels proved to be absent. Evidently the sensory periphery has an important influence on the structure of the somatosensory cortex.

Mechanism of stimulation of type J pulmonary receptors

1. The responses of type J pulmonary receptors (identified according to existing criteria) were studied in anaesthetized cats by recording impulses in individual vagal afferent fibres whose conduction velocity ranged from 0.8 to 7 m/sec.2. Measurements of actual latencies between insufflation of halothane or ether into the lungs and the excitation of the endings, and the latencies before and after circulatory arrest have established that the endings are located in the interstitial tissues close to the pulmonary capillaries. Mainly for this reason, the term juxta-pulmonary capillary receptors (i.e. type J receptors) has been applied to these endings in preference to the term K deflation receptors used hitherto.3. The endings were stimulated by pulmonary congestion produced by occlusion of the aorta or left a-v junction for short periods. They were markedly stimulated during pulmonary congestion following injection of alloxan (150 mg/kg) or the addition of chlorine to the inspired air. This excitation was associated with a marked rise in pulmonary artery pressure and the occurrence of pulmonary oedema. However, the actual onset of excitation occurred some time after the rise in pressure and it was in fact more closely related to fall in pulmonary compliance. The frequency of discharge averaged over about 10-20 sec (in order to take the periods of relative inactivity into account) was 7.5 impulses/sec in 10 fibres (range 0.6-19 impulses/sec; S.D. 6.3). This is intense stimulation of the endings and the congestion so produced is therefore regarded as a severe stimulus for the endings.4. The pattern of excitation was variable. In some fibres the activity consisted of periodic bursts of impulses which seemed to be set off during the deflation phase of artificial respiration, sometimes during the inflation phase. This periodic activity was not due to contraction of smooth muscle as the endings are not stimulated following injection of histamine (into the right ventricle) which is known to stimulate smooth muscles in the alveolar ducts and respiratory bronchioles.5. It is postulated that the actual stimulus for the endings is a rise in interstitial pressure or volume produced by a rise in pulmonary capillary pressure. Evidence has been gathered to show that the latter rises during muscular exercise; this rise must stimulate the endings. It was therefore postulated that stimulation of the endings should cause reflex inhibition of limb muscles (for terminating exercise).

Processes of excitation in the dendrites and in the soma of single isolated sensory nerve cells of the lobster and crayfish

The stretch receptor organs of Alexandrowicz in lobster and crayfish possess sensory neurons which have their cell bodies in the periphery. The cell bodies send dendrites into a fine nearby muscle strand and at the opposite pole they give rise to an axon running to the central nervous system. Mechanisms of excitation between dendrites, cell soma, and axon have been studied in completely isolated receptor structures with the cell components under visual observation. Two sensory neuron types were investigated, those which adapt rapidly to stretch, the fast cells, and those which adapt slowly, the slow cells. 1. Potentials recorded from the cell body of the neurons with intracellular leads gave resting potentials of 70 to 80 mv. and action potentials which in fresh preparations exceeded the resting potentials by about 10 to 20 mv. In some experiments chymotrypsin or trypsin was used to make cell impalement easier. They did not appreciably alter resting or action potentials. 2. It has been shown that normally excitation starts in the distal portion of dendrites which are depolarized by stretch deformation. The changed potential within the dendritic terminals can persist for the duration of stretch and is called the generator potential. Secondarily, by electrotonic spread, the generator potential reduces the resting potential of the nearby cell soma. This excitation spread between dendrites and soma is seen best during subthreshold excitation by relatively small stretches of normal cells. It is also seen during the whole range of receptor stretch in neurons in which nerve conduction has been blocked by an anesthetic. The electrotonic changes in the cells are graded, reflecting the magnitude and rate of rise of stretch, and presumably the changing levels of the generator potential. Thus in the present neurons the resting potential and the excitability level of the cell soma can be set and controlled over a wide range by local events within the dendrites. 3. Whenever stretch reduces the resting membrane potential, measured in the relaxed state in the cell body, by 8 to 12 mv. in slow cells and by 17 to 22 mv. in fast cells, conducted impulses are initiated. It is thought that in slow cells conducted impulses are initiated in the dendrites while in fast cells they arise in the cell body or near to it. In fresh preparations the speed of stretch does not appreciably influence the membrane threshold for discharges, while during developing fatigue the firing level is higher when extension is gradual. 4. Some of the specific neuron characteristics are: Fast receptor cells have a relatively high threshold to stretch. During prolonged stretch the depolarization of the cell soma is not well maintained, presumably due to a decline in the generator potential, resulting in cessation of discharges in less than a minute. This appears to be the basis of the relatively rapid adaptation. A residual subthreshold depolarization can persist for many minutes of stretch. Slow cells which resemble the sensory fibers of vertebrate spindles are excited by weak stretch. Their discharge rate remains remarkably constant for long periods. It is concluded that, once threshold excitation is reached, the generator potential within slow cell dendrites is well maintained for the duration of stretch. Possible reasons for differences in discharge properties between fast and slow cells are discussed. 5. If stretch of receptor cells is gradually continued above threshold, the discharge frequency first increases over a considerable range without an appreciable change in the firing level for discharges. Beyond that range the membrane threshold for conducted responses of the cell soma rises, the impulses become smaller, and partial conduction in the soma-axon boundary region occurs. At a critical depolarization level which may be maintained for many minutes, all conduction ceases. These overstretch phenomena are reversible and resemble cathodal block. 6. The following general scheme of excitation is proposed: stretch deformation of dendritic terminals --> generator potential --> electrotonic spread toward the cell soma (prepotential) --> dendrite-soma impulse --> axon impulse. 7. Following release of stretch a transient hyperpolarization of slow receptor cells was seen. This off effect is influenced by the speed of relaxation. 8. Membrane potential changes recorded in the cell bodies serve as very sensitive detectors of activity within the receptor muscle bundles, indicating the extent and time course of contractile events.

Interoception: the inside story--a model for psychosomatic processes

OBJECTIVE

To comprehend psychosomatic processes, it will be necessary to understand the brain's influences on bodily functions and also the body's afferent sensory input to the central nervous system, including the effects of this input on behavior and cognitive functions, especially emotion. The objective of this Presidential Address is to review what is known circa the year 2000 of the processes and mechanisms of visceral sensory psychobiology, often called interoception.

METHODS

Over 1000 publications that have appeared since the 19th century were reviewed to prepare this review, including a group that are specifically cited here.

RESULTS

Factors and data were reviewed that were identified as germane to understanding interoception. These included definitional issues, historical roots, the neural basis, studies and results in the cardiovascular-respiratory and alimentary-gastrointestinal systems, studies of emotion, and studies in people with mental disorders. Drug and hormone effects, pain, proprioception, and phantom limb or organ factors, and the role of awareness were briefly described. Methodological issues, methods of study including functional imaging, and possible future directions for study were identified.

CONCLUSIONS

Understanding the physical basis of psychosomatic processes, including the so-called mind-body problem, will require a detailed understanding the psychobiology of interoception.

A THEORETICAL ANALYSIS OF NEURONAL VARIABILITY

A simple neuronal model is assumed in which, after a refractory period, excitatory and inhibitory exponentially decaying inputs of constant size occur at random intervals and sum until a threshold is reached. The distribution of time intervals between successive neuronal firings (interresponse time histogram), the firing rate as a function of input frequency, the variability in the time course of depolarization from trial to trial, and the strength-duration curve are derived for this model. The predictions are compared with data from the literature and good qualitative agreement is found. All parameters are experimentally measurable and a direct test of the theory is possible with present techniques. The assumptions of the model are relaxed and the effects of such experimentally found phenomena as relative refractory and supernormal periods, adaptation, potentiation, and rhythmic slow potentials are discussed. Implications for gross behavior studies are considered briefly.

ACTIVITY OF SINGLE NEURONS IN THE HYPOTHALAMIC FEEDING CENTERS: EFFECT OF GLUCOSE

Unit activity from neurons of hypothalamic feeding and satiety mechanisms, and from adjacent hypothalamic regions was recorded in anesthetized dogs with surgically exposed hypothalamus, and in Flaxedil-immobilized cats in which a stereotaxic approach was made. Intravenous glucose or insulin, or combinations of both, were given and the changes in spike activity observed. Glucose estimations were done on blood samples taken from femoral artery and vein. In starved animals the unit activity in satiety center neurons was slower than that obtained from feeding center neurons. Frequency of spikes recorded from satiety center neurons increased and that of feeding center neurons decreased significantly after glucose was given intravenously, while spike activity from these centers showed a reverse pattern of response after intravenous insulin. No significant changes were observed from other hypothalamic and cortical neurons. Activity of neurons of the satiety center did not show a significant correlation with blood glucose level per se, but a better correlation was found between unit activity and the A-V glucose difference. It is suggested that the satiety center is activated by increased glucose utilization in the body.

A theory of taste stimulation

The treatment in this paper of available quantitative data on the response of taste receptors to sodium salt stimulation clearly indicates that the ions of the chemical stimulus are loosely bound to some substance of the taste receptor. This can be thought of as an initial reaction which ultimately leads to stimulation of the receptor and an eventual depolarization of the associated sensory neuron. The speed of the total reaction suggests that the receptor substance is located on or near the surface of the receptor. The recently proposed (7) enzymatic reactions for chemoreceptors do not appear plausible for sodium salt stimulation of the taste receptors of the rat.

Distinct mechanisms for synchronization and temporal patterning of odor-encoding neural assemblies

Stimulus-evoked oscillatory synchronization of neural assemblies and temporal patterns of neuronal activity have been observed in many sensory systems, such as the visual and auditory cortices of mammals or the olfactory system of insects. In the locust olfactory system, single odor puffs cause the immediate formation of odor-specific neural assemblies, defined both by their transient synchronized firing and their progressive transformation over the course of a response. The application of an antagonist of ionotropic gamma-aminobutyric acid (GABA) receptors to the first olfactory relay neuropil selectively blocked the fast inhibitory synapse between local and projection neurons. This manipulation abolished the synchronization of the odor-coding neural ensembles but did not affect each neuron's temporal response patterns to odors, even when these patterns contained periods of inhibition. Fast GABA-mediated inhibition, therefore, appears to underlie neuronal synchronization but not response tuning in this olfactory system. The selective desynchronization of stimulus-evoked oscillating neural assemblies in vivo is now possible, enabling direct functional tests of their significance for sensation and perception.

Visual receptive fields of neurons in inferotemporal cortex of the monkey

Neurons in inferotemporal cortex (area TE) of the monkey had visual receptive fields which were very large (greater than 10 by 10 degrees) and almost always included the fovea. Some extended well into both halves of the visual field, while others were confined to the ipsilateral or contralateral side. These neurons were differentially sensitive to several of the following dimensions of the stimulus: size and shape, color, orientation, and direction of movement.

Structure of the Retinae of the Principal Eyes of Jumping Spiders (Salticidae: Dendryphantinae) in Relation to Visual Optics

1. The retinae of the principal (AM) eyes of jumping spiders contain four layers of receptors, one behind the other with respect to the incident light. The distribution of receptors in each layer has been determined.

2. The deepest layers (1 and 2) cover the whole area of the retina, and have the greatest density of receptors. The minimum receptor separation is 1.7 µm., or 11 min. visual angle. The more superficial layers (3 and 4) are confined to the central region of the retina.

3. In layers 1, 2 and 3 the rhabdomere-containing segments are rod-shaped, and are parallel to the incident light. In layer 4 they are ovoid, and are oriented approximately at right angles to the light.

4. At the first optic glomerulus the primary fibres from each receptor layer appear to terminate in separate regions of neuropile.

5. The focal lengths, radii of curvature and refractive indices of the lenses of the principal and side eyes have been measured. For the principal eyes, estimates have also been made of the diffraction limit, the depth of focus, and the magnitudes of chromatic and spherical aberration.

6. The normal position of the image in the eye was found by ophthalmoscopy. For blue-green light, the best image of distant objects is formed on the next-to-deepest layer (2).

7. The deepest layer (1) is conjugate with a plane about 2 cm. in front of the animal for blue-green light, or with infinity for red light, because of the eye's chromatic aberration.

8. Two theories are offered to account for the retinal layering. Either the spider uses different layers to examine maximally sharp images of objects at different dis tances; or each layer exploits the sharpest image of distant objects, but for light of different wavelengths.

9. Optical considerations indicate that either theory is possible, but the seconds (wavelength) theory is the more probable, because jumping spiders are known to possess colour vision. It predicts that layer 1 receptors contain red-sensitive, layer 2 blue-green sensitive and layer 3 violet-ultraviolet Sensitive pigments.

10. The structural peculiarities of the most superficial layer (4), and the fact that it is not conjugate with any plane in front of the animal for any visible wavelength, suggest that it is not resolving an image, but performing some other function. Reasons are given for thinking that this might be the analysis of the pattern of polarization of skylight.

The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages

Our objective was to investigate sympathetic and sensory nerve fibers in synovial tissue in rheumatoid arthritis (RA) and osteoarthritis (OA) in relation to histological inflammation and synovial cytokine and norepinephrine (NE) secretion. Immunohistochemistry was used to detect nerve fibers and inflammatory parameters. A superfusion technique of synovial tissue pieces was used to investigate cytokine and NE secretion. In RA, we detected 0.2 +/- 0.04 tyrosine hydroxylase-positive (TH-positive=sympathetic) nerve fibers/mm2 as compared to 4.4 +/- 0. 8 nerve fibers/mm2 in OA (P<0.001). In RA, there was a negative correlation between the number of TH-positive nerve fibers and inflammation index (RRank=-0.705, P=0.002) and synovial IL-6 secretion (RRank=-0.630, P=0.009), which was not found in OA. Substance P-positive (=sensory) nerve fibers were increased in RA as compared to OA (3.5+/-0.2 vs. 2.3+/-0.3/mm2, P=0.009). Despite lower numbers of sympathetic nerve fibers in RA than in OA, NE release was similar at baseline (RA vs. OA: 152+/-36 vs. 106+/-21 pg/ml, n.s.). Basal synovial NE secretions correlate with the number of TH-positive CD 163+ synovial macrophages (RA: RRank=0.622, P=0.031; OA: RRank=0.299, n.s.), and synovial macrophages have been shown to produce NE in vitro. Whereas sympathetic innervation is reduced, sensory innervation is increased in the synovium from patients with longstanding RA when compared to the synovium from OA patients. The differential patterns of innervation are dependent on the severity of the inflammation. However, NE secretion from the synovial tissue is maintained by synovial macrophages. This demonstrates a loss of the influence of the sympathetic nervous system on the inflammation, accompanied by an up-regulation of the sensory inputs into the joint, which may contribute to the maintenance of the disease.