Medical brain notes 24

  The Olfactory Region          A major difficulty in considering the olfactory pathways is that these involve numerous, rather obscure and small, structures and areas. For a proper understanding it is necessary that the names and positions of these areas be introduced at the outset.         The peripheral end organ for smell is the  olfactory mucosa  that lines the upper and posterior parts of the nasal cavity. Nerve fibres arising in this mucosa collect to form about twenty bundles that together constitute an  olfactory nerve.  The bundles pass through foramina in the cribriform plate of the ethmoid bone to enter the cranial cavity where they terminate in the  olfactory bulb  (Figs. 16.1, 16.3).         The olfactory bulb is an elongated oval structure that lies just above the cribriform plate. It is continuous posteriorly with the  olfa...

  The Olfactory Pathway          The fibres of the olfactory nerves are processes of  olfactory receptor cells  lying in the epithelium lining the olfactory mucosa (Fig. 16.3). These receptor cells are homologous to sensory neurons located in sensory ganglia. In other words, the first order sensory neurons of the olfactory pathway are located within the olfactory epithelium itself. (In the course of evolution  all  neurons have arisen by modification of epithelial cells and their migration, in most cases, into deeper tissues. The olfactory receptor cells retain their position in the epithelium and are, therefore, regarded as primitive).          Each receptor cell consists of a cell body and of two processes i.e., it is a bipolar cell. The peripheral process (dendrite) reaches the surface of the olfactory epithelium and ends in a small swelling. A number of cilia are attached to this...

  The Limbic Region         The term  limbic system  has been applied, in the past, to certain regions of the brain that are believed to play an important role in the control of visceral activity. Many of these areas have been considered as having a predominantly olfactory function; but it is now realised that this is not so. The areas of cerebral cortex included in the region are often referred to as the  limbic lobe .         Some of the functions attributed to the so called limbic system are as follows. a)                Integration of olfactory, visceral, and somatic impulses reaching the brain. b)               Control of activities necessary for survival of the animal, including the procuring of food and eating behaviour. c)      ...

  CONNECTIONS OF THE AMYGDALOID NUCLEI a.Connections with the brainstem 1.                The amygdala receive fibres from, and send fibres to, the reticular formation, particularly theparabrachial nucleus (Fig. 16.8). 2.                Noradrenergic fibres from the locus coeruleus, and serotoninergic fibres from the raphe nucleireach the amygdala through the medial forebrain bundle. Dopaminergic fibres ascend from the ventral tegmental area of the midbrain. 3.                Some fibres from the amygdala reach the nucleus of the solitary tract, and the dorsal nucleus ofthe vagus. The nucleus of the solitary tract projects back to the amygdala through the parabrachial nuclei. Through these connections the amygdala receive gustatory and visceral information can influence cardi...

  Septal Region        This term is used to designate certain masses of grey matter that lie immediately anterior to the lamina terminalis and the anterior commissure (Fig. 16.10). The cerebral cortex of this region shows two small vertical sulci called the anterior and posterior parolfactory sulci.          The region between the lamina terminalis and the posterior sulcus is the  paraterminal gyrus.  The anterior part of this region which adjoins the posterior parolfactory sulcus is called the  prehippocampalrudiment.  The region between the anterior and posterior parolfactory sulci is the  subcallosal area  (or  parolfactory gyrus ). Most workers agree that the paraterminal gyrus (along with the prehippocampal rudiment) forms part of the septal region. Some include the subcallosal area as well. The cortex of this region is referred to as the  septal area  in distinct...