Question

Compare and contrast hippocampus, amygdala, and the prefrontal cortex in the following aspects:

a) Their neurocircuitries

b) Their associated behavior(s) and cognitive processes

c) How they adapt to environmental changes when the system is challenged (e.g., stress), and

d) What neurological / psychiatric disorders can exhibit defects in these systems?

Answer 1

	Hippocampus	Amygdala	Prefrontal cortex
Neurocircuitries	The major input to the hippocampus is through the entorhinal cortex (EC) whereas its major output is via CA1 to the subiculum. Information reaches CA1 via two main pathways, direct and indirect. These are primarily from the septum and hypothalamus via the fornix and from the adjacent entorhinal cortex. This cortical region receives input from diffuse areas of the neocortex, especially the limbic cortex, and from the amygdala. The entorhinal cortex projects to the dentate gyrus of the hippocampus via the perforant pathway, synapsing on granule cells.. This requires activation of glutamate receptors and results in long-term changes in neuronal excitability by way of calcium mediated physiologic effects. Outputs from the hippocampus pass primarily via two pathways. The first of these outputs is through the fornix. These fibers project to the mamillary bodies via the post-commissural fornix, to the septal nuclei, to the preoptic nucleus of the hypothalamus, to the ventral striatum and to portions of the frontal lobe through the precommisural fornix.	The amygdala is an important structure located in the anterior temporal lobe. The amygdala makes reciprocal connections with many brain regions in the thalamus, hypothalamus, septal nuclei, orbital frontal cortex, cingulate gyrus, hippocampus, parahippocampal gyrus, and brain stem. The olfactory bulb is the only area that makes input to the amygdala and does not receive reciprocal projections from the amygdala.	The prefrontal cortex is anterior to the premotor cortex. The orbital frontal cortex is the portion over the orbits. The prefrontal cortex receives input from the other areas of limbic cortex, from the amygdala and from septal nuclei and has reciprocal connections with each of these areas and with the dorsomedial nucleus of the thalamus.
Behaviors & Cognitive process		The amygdala is a critical center for coordinating behavioral, autonomic and endocrine responses to environmental stimuli, especially those with emotional content. It is important to the coordinated responses to stress and integrates many behavioral reactions involved in the survival of the individual or of the species, particularly to stress and anxiety. The amygdala reduce responses to stress, particularly conditioned emotional responses. Stimulation of the amygdala produces behavioral arousal and can produce directed rage reactions.
Adaptability		Various stimuli produce responses mediated by the amygdala. The convergence of inputs is important since it allows the generation of learned emotional responses to a variety of situations. The amygdala responds to a variety of emotional stimuli, but mostly those related to fear and anxiety.
Disorder	Alzheimer’s disease: - The hippocampus is one of the first areas to be affected by Alzheimer’s disease. An early sign of Alzheimer’s is when a person begins to lose their short-term memory. They may also find it difficult to follow directions. As the disease progresses, the hippocampus loses volume and it becomes harder to function in daily life. Epilepsy: - Autopsies have suggested that between 50 and 75 percent of people with epilepsy have damage to the hippocampus. Depression and stress: - In people with severe depression, the hippocampus loses volume. Scientists are unsure whether the small size is the result of depression or if it is a contributing factor. There is evidence that stress has a negative impact on the hippocampus.	Epilepsy Anxiety disorders   Neurodevelopmental disorders Mood disorders The amygdala is an integral part of a neural system that evolved early in order to detect environment dangers and modulate subsequent responses, a role which can profoundly influence human behavior.	Prefrontal cortex function is abnormal in mood disorders. Depression is most often associated with increased activity in portions of the frontal lobe, especially the medial regions including the subgenual portion of the anterior cingulate cortex, and decreased activity in the posterior cingulate gyrus.