THE LIMBIC SYSTEM The limbic (from limbus = ring) lobe refers to a group of structures that circlethe brain stem. The limbic system refers to the limbic lobe and other structhe brain stem. 1. The rhinencephalon or "smell brain" is concerned with olfaction a large part of the limbic system the anterior part of the limbic cortex concerned with gustatory sense Very good! No. The first and third foils are correct. The rhinencephalon is a small part of the human limbic system and is primarily involved in olfaction. 2. The rhinencephalon includes hippocampus olfactory tract fornix primary olfactory cortex Very good! No. The second and fourth foils are correct. Other structures in the rhinencephalon are the olfactory nerve rootlets, olfactory striae, olfactory bulb. 3. The olfactory nerves consist of myelinated fibers consist of unmyelinated processes of hair cells terminate on neurons in the primary olfactory cortex terminate on neurons in the olfactory bulb Very good! No. The second and fourth foils are correct. The unmyelinated processes from the olfactory hair cells in the nasal mucosa pierce the cribiform plate of the ethmoid bone and terminate on neurons in the olfactory bulb, which is in the frontal fossa of the cranial cavity. 4. The olfactory bulb appears laminated on histological sections is a relay station for olfactory pathways receives fibers from the anterior olfactory nucleus has a mitral cell layer of small granule neurons Very good! No. The first three foils are correct. The mitral layer of the olfactory bulb consists of large neurons (mitral neurons). The small granule neurons are in the granule layer. 5. The cell (s) that project (s) fibers from the olfactory bulb include granule cells mitral cells hair cells tufted cells Right! No. The second and fourth foils are correct. 6. The olfactory bulb fibers project to contralateral olfactory bulb anterior perforated substance subcallosal gyrus primary olfactory cortex Very good! No. All foils are correct. 7. The number of olfactory nerve fibers that converge on a single primary neuron in the olfactory bulb is believed to be about 10 100 1000 10,000 100,000 Very good! No, the answer is C. There are about 50 million olfactory hair cells in the nasal mucosa (each projecting an olfactory nerve fiber) and 50,000 primary neurons (mitral and tufted cells) giving a 1000 to 1 ratio. 7. The olfactory tract projects fibers from mitral and tufted cells in the olfactory bulb receives fibers from primary olfactory cortex divides into olfactory striae just rostral to the anterior perforated substance contains fibers from olfactory hair cells in the nasal mucosa Very good! No. The first three foils are correct. The olfactory tract divides into 3 striae as it enters the brain; lateral, medial and intermediate olfactory stria. 9. Fibers to the primary olfactory cortex in the temporal lobe pass through the medial stria intermediate stria lateral stria all of the above Very good! No, the answer is C. The medial stria projects to the subcallosal gyrus and the intermediate stria enters the anterior perforated substance. 10. The primary olfactory cortex is within the uncus of the temporal lobe is also called the lateral olfactory gyrus receives fibers directly without passing through a relay nucleus in the thalamus contains fewer principle type neurons (pyramidal cells) than the number of fibers in lateral stria Very good! No. The first three foils are correct. The primary olfactory cortex is the only primary sensory cortex (visual, taste, auditory, and somesthetic) that receives input without it passing througha thalamic relay nucleus. There are more principle neurons in primary olfactory cortex than incoming fibers. 11. Loss of smell (anosmia) is usually associated with lesions in the olfactory nerves and bulb primary olfactory cortex (uncus of the temporal lobe) both neither Very good! No, the answer is A. Lesions in the primary olfactory cortex usually produce hallucinations of smell (uncinate fits) and the hallucination is usually of an unpleasant smell. This symptom often indicates a tumor (glioma) in the uncus of the temporal lobe. 12. The limbic lobe includes subcallosal gyrus cingulate gyrus parahippocampal gyrus uncus Right! No. All foils are correct. These components of the limbic lobe are quite well accepted. As can be seen, the limbic lobe is made up of components of other lobes of the brain,i.e., the subcallosal gyrus is part of the frontal lobe. 13. The limbic lobe is concerned with olfactory function emotional behavior and memory both neither Right! No, the answer is C. 14. The limbic system includes the limbic lobe and septal nuclei hippocampal formation amygdala olfactory bulb Very good! No. The first three foils are correct. The limbic system includes all cortical and subcortical structures related to the limbic lobe. In addition to the septal nuclei, hippocampal formation and amygdala, thalamus, epithalamus (pineal,habenula) and brain stem reticular formation are also included. 15. The hippocampal formation is a major component of the limbic system. The hippocampal formation consists of the hippocampus dentate gyrus subiculum amygdala Very good! No. The first three foils are correct. The amygdala is part of the limbic system but it is not part of the hippocampal formation. 16. The layers of the hippocampus include the stratum oriens pyramidal layer molecular layer granular layer Very good! No. The first three foils are correct. The hippocampus also has been divided into fields, CA1, CA2, CA3 and CA4. 17. The principle neurons of the hippocampus are the pyramidal neurons in the pyramidal cell layer. Axons from the pyramidal cells are directed toward the ventricular surface form the alveus form the fimbria form the fornix Very good! No. All foils are correct. These tracts form the outflow of the hippocampus. Some axon collaterals terminate in stratum oriens and the molecular layer of the hippocampus. 18. The hippocampus receives direct fibers (afferent connections) from dentate gyrus amygdala septal area thalamus Very good! No. The first and third foils are correct. The afferent input to the hippocampus is from parahippocampal gyrus,dentate gyrus, cingulate gyrus, contralateral hippocampus and septal nuclei. The fibers from cingulate gyrus, septal nuclei and contralateral hippocampus reach the hippocampus via the fornix. Note: Some afferent fibers are present in the fornix and fimbria. These come from the anterior thalamic nuclei, the septal area, and the posterior part of the hypothalamus (Barr and Kiernan,1983, pge.266) However, most textbooks do not indicate this. 19. The efferent fibers are from pyramidal cells in the pyramidal cell layer. These fibers form the alveus fimbria and then fornix which makes connections with hypothalamus thalamus (anterior nucleus) septal nuclei cingulate gyrus Very good!. No. All foils are correct. Specifically, the fornix sends fibers to the following sites in the hypothalamus; preoptic region, lateral region and mammillary body. 20. Ablation (destruction) or stimulation of the hippocampus causes changes in olfaction (sense of smell) recent memory hearing behavior Very good! No. The second and fourth foils are correct. It is no longer believed the hippocampus is directly involved in olfaction. The hippocampus has a low seizure threshold (epilepsy). Seizures in the hippocampus are generally called temporal lobe epilepsy or psycho- motor seizures. They usually produce strange behavioral episodes but not the tonic, clonic movement changes seen with grand mal epilepsy. 21. The olfactory bulb sends fibers, via the lateral olfactory stria, to the corticomedial group of amygdaloid nuclei the basolateral group of amygdaloid nuclei both neither Right! No, the answer is A. The basolateral group receives indirect olfactory fibers from the pyriform cortex. Other afferent fibers to the amygdala arrive from prefrontal cortex, thalamus and hypothalamus. 22. The main outflow tract of the amygdala is the alveus fimbria fornix stria terminalis none of the above Right! No, the answer is D. The stria terminalis arises primarily from the corticomedial nuclei and its fibers project to septal nuclei, anterior olfactory nucleus, anterior hypothalamus, bed nucleus of stria terminalis and habenular nuclei via the stria medullaris. 23. The septal area lies posterior to the anterior commissure receives fibers from the hippocampus receives fibers from the cingulate gyrus receives fibers from the amygdala Very good! No. The second and fourth foils are correct. The septal nuclei lie in front of the anterior commissure. These nuclei also receive input from hypothalamus and midbrain. 24. The septal area receives fibers (afferent pathways) from hippocampus amygdala hypothalamus midbrain Right! No. All foils are correct. 25. Fibers from the amygdala reach the septal area via the fornix stria medullaris stria terminalis median forebrain bundle (MFB) Right! No, the answer is C. Fibers from the midbrain reach the septal area via the MFB. Efferent fibers from the septal area reach the habenula and hypothalamus via the stria medullaris. 26. The septal area stimulation causes bradycardia (slowing of heart rate) stimulation gives rise to pleasure or reward effects lesions in animals cause increased water intake lesions in animals decrease rate of learning tasks Right! No. The first three foils are correct. After septal lesions, tasks appear to be learned more quickly. 27. The Kluver-Bucy syndrome is due to bilateral lesions involving amygdala, hippocampal formation and adjacent structures. This syndrome includes decreased appetite hypersexuality aggressiveness docility Very good! No. The second and fourth foils are correct. Patients have increased appetite, and lack emotional response. 28. Temporal lobe epilepsy can cause a variety of symptoms which reflects the importance of the limbic system in cerebral function. These symptoms include amnesia docile behavior sensory hallucinations (olfactory, auditory and visual) simple motor acts Right! No. The first and third foils are correct. During a temporal lobe seizure behavior can be aggressive. The motor component is usually complex, involving twisting movements, running etc.. These combinations of symptoms result in the other name often given to these seizures, psychomotor epilepsy.