Converging evidence now indicates that the amygdala plays a crucial role in the development and expression of conditioned fear. Conditioned fear is a hypothetical construct used to explain the cluster of behavioral effects produced when an initially neutral stimulus is consistently paired with an aversive stimulus. For example, when a light, which initially has no behavioral effect, is paired with an aversive stimulus such as a footshock, the light alone can elicit a constellation of behaviors that are typically used to define a state of fear in animals. To explain these findings, it is generally assumed (cf. McAllister & McAllister 1971 ) that during light-shock pair­ ings (training session ), the shock elicits a variety of behaviors that can be used to infer a central state of fear (unconditioned responses-Figure 1 ). After pairing , the light can produce the same central fear state and thus the same set of bchaviors formerly produced by thc shock. Moreover, the behavioral effects that are produced in animals by this formerly neutral stimulus (now called a conditioned stimulus-CS ) are similar in many respects to the constellation of behaviors that are used to diagnose gener­ alized anxiety in humans (Table 1 ). This chapter summarizes data sup­ porting the idea that the amygdala, and its many efferent projections, may represent a central fear system involved in both the expression and acquisition of conditioned fear.

The role of the amygdala in fear and anxiety

The location of neurons generating the rhythm of breathing in mammals is unknown. By microsection of the neonatal rat brainstem in vitro, a limited region of the ventral medulla (the pre-Botzinger Complex) that contains neurons essential for rhythmogenesis was identified. Rhythm generation was eliminated by removal of only this region. Medullary slices containing the pre-Botzinger Complex generated respiratory-related oscillations similar to those generated by the whole brainstem in vitro, and neurons with voltage-dependent pacemaker-like properties were identified in this region. Thus, the respiratory rhythm in the mammalian neonatal nervous system may result from a population of conditional bursting pacemaker neurons in the pre-Botzinger Complex.

https://science.sciencemag.org/content/sci/254/5032/726.full.pdf

Pre-Botzinger Complex: A Brainstem Region That May Generate Respiratory Rhythm in Mammals

The central sensory and motor connections of various respiratory, cardiovascular, and gastrointestinal viscera were analyzed using the transganglionic and retrograde transport of horseradish peroxidase (HRP).



In 42 adult cats, we examined the brain stem and peripheral ganglia following microinjections of HRP (10 μl) into individual visceral organs—larynx, extrathoracic trachea, intrathoracic trachea, right main bronchus, right lung (upper lobe), heart, and stomach. Comparison of individual cases led to the conclusion that distinct patterns of sensory and motor projections to the medulla exist for each visceral organ studied.



The nucleus of the tractus solitarius (nTS) receives the sensory projections from all the viscera listed above, with two exceptions: (1) a few sensory fibers from the larynx terminate in the ipsilateral spinal tract of the trigeminal nerve (spV), and (2) some sensory fibers from the bronchus, lung, and stomach terminate in the area postrema (ap). Within the nTS, the sensory fibers from each visceral organ terminate in a number of subnuclei. The dnTS, mnTS, and ncom receive sensory projections from all the viscera studied. The remaining five subnuclei (dlnTS, ni, nI, vlnTS, vnTS) of the nTS are not connected to all viscera, and the density of projections to these regions varies for different viscera. However, there does not seem to be any specific region of the medulla which is devoted entirely to receiving the sensory fibers from a particular visceral organ. Rather, the rostrocaudal extent of sensory fibers, from most of the viscera studied, spans the entire length of the medulla. Differences in the central representation of viscera were found to be subtle and to lie within the organization of the nuclear subgroups of the nTS.



The central representation of unpaired or midline viscera (e.g., trachea and heart) is bilateral for both sensory and motor innervation. However, for unilateral, paired viscera (e.g., bronchi and lungs), it was consistently found that over one third of the sensory and motor representation is contralateral. Control experiments involving vascular injections of HRP excluded the possibility that this contralateral labeling could have been due to vascular uptake of the enzyme. The localization of sensory perikarya of visceral afferents in the “principal visceral ganglion” of the vagus—the nodose ganglion—was overlapping, and no well-demarcated regions in the nodose ganglion could be identified that received projections primarily from a given visceral organ.



The motor nuclei providing parasympathetic (preganglionic) and somatic motor innervation to the viscera were primarily the dmnX, nA, and nRA. The entire dmnX (extending over 10–15 mm rostrocaudally), contributed fibers to each area injected with HRP, with the exception of the extrathoracic trachea. No region in the dmnX was found where preference was given to a specific viscus. The nA contributed efferents to all the viscera studied, and this contribution came from the entire 6 mm of nA contributing vagal efferents. Visceral containing smooth muscle as well as skeletal muscle were innervated by the nA. The caudal nRA provided motor fibers to the larynx, trachea, and stomach, and again no preferred rostrocaudal representation of motoneurons to a given viscus was found. Postganglionic sympathetic innervation to the viscera studied was found to be localized to the stellate and superior cervical ganglia. Within these sympathetic ganglia, some regional preference for different viscera was detected. In the case of motor innervation, unilateral paired viscera received motor fibers from both ipsilateral and contralateral sides of the medulla via both vagus nerves.

Brain stem projections of sensory and motor components of the vagus complex in the cat: II. Laryngeal, tracheobronchial, pulmonary, cardiac, and gastrointestinal branches.

https://escholarship.org/content/qt08d8g56h/qt08d8g56h.pdf?t=mi9rol

Spatial EEG Patterns, Non-linear Dynamics and Perception: the Neo-Sherringtonian View

An in vitro preparation was described for studying electrical activity of mammalian brain stem and spinal cord. The brain stem and the spinal cord were isolated from 0-4-day-old rats, placed in a bath and perfused with modified Krebs solution. Various reflex responses could be recorded from cranial nerves by stimulation of other cranial nerves. The preparation was viable for more than 7 h. Spontaneous periodic neural activity could be recorded from phrenic, hypoglossal and other spinal nerves. The periodic discharges of phrenic nerves are synchronized with those of ventral roots C4 and the upward movements of the thorax which was isolated together with the spinal cord. The rhythm of periodic activity seems to be generated in the brain stem. The periodic activity was enhanced by perfusion with low pH solution and depressed by high pH solution. It was markedly depressed by opioid compounds such as enkephalin. It is suggested that this periodic activity corresponds to the respiratory rhythm generated in the brain stem of intact animals. The present preparation may be valuable for elucidating cellular mechanisms of generation and control of respiratory rhythm in the mammalian central nervous system.

M. I. Cohen

Papers

Neurogenesis of respiratory rhythm in the mammal