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Flashcards in Limbic system and olfaction Deck (15)
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Overview of olfaction and limbic system

-Limbic system comprised of amygdala, hippocampus and septum (also cingulate gyrus, nucleus accumbens, hypothalamus)
-Olfactory system senses chemosensory stimuli disolved in nasal mucus and transmits the info to the amygdala and piriform (primary olfactory) cortex
-Main olfactory system (ours predominantly) detects volatile (airborne) stimuli, accessory olfactory system (not predominantly in humans) detects non-volatile stimuli related to social cues (pheromones)


Peripheral olfactory detection 1

-Airborne stimuli in external nare mucus contact olfactory receptors (specialized bipolar neurons w/ apical tuft of cilia and GPCR odorant receptors)
-We have >1000 different receptors, each for one odor. Each olfactory receptor expresses a single odor receptor
-Neurons w/ the same odor receptor all send projections to the same glomerulus in the olfactory bulb


Peripheral olfactory detection 2

-Olfactory receptor neurons only last 30-60 days (replenished by stem cells in nasal cavity), and are located in olfactory epithelium
-Projections of olfactory receptors form CN I to reach the glomeruli of common odor receptor input, in the olfactory bulb
-Glomeruli project to mitral cells in the olfactory bulb (olfactory tract)


Central olfactory processing

-Mitral cells in olfactory bulb project to central olfactory targets thru the lateral olfactory tract (ipsilateral, unlike other sensory systems)
-Projections terminate on the piriform cortex (primary olfactory), amygdala and entorhinal cortex
-These targets eventually send projections to the hippocampus, hypothalamus, thalamus, and cortex


Piriform cortex

-3 layered allocortex, responsible for olfactory discrimination thru connections w/ lateral orbitofrontal cortex
-This connection both directly and indirectly (via dorsomedial thalamus)


Corticomedial amygdala and entorhinal cortex

-Corticomedial amygdala receives and integrates socially-relevant inputs from different sensory systems
-Connections are important for social behavior, sexual behavior, aggressive behavior, and parental behavior
-Has receptors for gonadal steroids and connections w/ relevant hypothalamic nuclei
-Connections w/ entorhinal cortex allows olfaction to influence memory, and contributes to the emotional response to odors


Anosmia and hyperosmia

-Anosmia: loss of smell, can be developmental or the result of tumors, trauma, or metabolic diseases. Also compromises the sense of taste
-Hyperosmia: develops during pregnancy and w/ some psychiatric d/o


Regions and function of amygdala 1

-Corticomedial amygdala: integrates olfactory info w/ memory/emotions (endocrine signals and ascending somatosensory info) to modulate social behavior
-Has projections to hypothalamic nuclei via stria terminalis
-Basolateral amygdala: larger and is involved in learned emotional responses to sensory stimuli (mostly negative emotions like fear and anxiety), connects to brainstem nuclei involved w/ autonomic responses in emotion


Regions and function of amygdala 2

-Basolateral amygdala receives info from sensory (visual and auditory), hippocampus, cingulate, prefrontal, and parietal cortices
-These inputs shape emotional responses to threat, hunger, and social interaction
-Is related to associative memory since most of these responses are learned
-Projections of the basolateral amygdala to orbitofrontal cortex are important for perception of memory
-Projections to hypothalamus contribute to expression of emotion


Clinical implications of amygdala lesions

-Basolateral amygdala damage results in kluver-bucy syndrome: lack of emotional response, hyperorality (eating shit) due to loss of visual recognition, and hypersexuality. Very rare
-Anxiety and PTSD: Very common. Due to learned hyper vigilance to environmental stimuli. Rx only unlearn the fear memories
-Autism in part due to dysfunction of amygdala


Septum and nucleus accumbens 1

-Septum extends medially btwn the LVs, dorsal to hypothalamus and anterior to thalamus
-Septal nuclei have connections w/ the hippocampus and amygdala (also other limbic regions)
-Lateral septum is important for social behaviors, mostly aggression (testosterone receptors) and pair-bonding (vasopressin neurons)


Septum and nucleus accumbens 2

-Medial septum contains large and abundant cholinergic neurons, which have wide-spread connections including to forebrain to regulate attention and cortical arousal
-Nucleus accumbens: key element of ventral striatum. The NA and midbrain ventral tegmental area (VTA) together form the mesolimbic DA system
-DA neurons in VTA project to NA to produce reward. The NA then projects to the basal ganglia to then facilitate reinforcement of habit to get the reward
-This pathway is largely implicated in drug abuse


Hippocampus 1

-Critical for learning and memory, comprised of a 3-layered allocortex: dentate gyrus, corpu ammon's (CA1-4), and subiculum
-Dentate gyrus and CA folded onto one another
-Hippocampus has bidirectional connections w/ other brain regions thru the fornix and perforant pathway
-Movement of info: afferents enter the dentate gyrus and move to CA4->CA1, then to subiculum, which is major source of hippocampal efferents, which feeds back to entorrhinal cortex


Hippocampus 2

-Hippocampus connects (via fornix) to septum, thalamus, hypothalamus, and mammillary bodies
-Also connects (via perforant pathways) to temporal association areas
-One of the most active and dynamic brain regions, has ongoing neurogenesis and high metabolic activity
-Thus has low seizure threshold (temporal lobe epilepsy) and sensitive to ischemic change


Function of hippocampus

-Important for declarative memory: memory of facts, experiences, and info
-Damage interferes w/ formation of new memories (anterograde amnesia), but does not affect existing long-term memory (no retrograde amnesia)
-Thus it is required for memory formation, but not for storage of long-term memory
-AD pts largely have loss of hippocampal neurons
-Korsakoff's syndrome: memory loss w/ confabulation. Occurs in chronic alcoholism
-Lesions in medial dorsal thalamus can give korsakoff's syndrome

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