Week 6: Limbic System and Olfaction

Question 1

What are the components of the limbic system?

Answer 1

Amygdala, hippocampus, septum
Olfactory system
Sometimes included:
cingulate gyrus, nucleus accumbens, hypothalamus

Question 2

Describe peripheral olfactory detection.

Answer 2

-volatile chemical signals travel through external nares, air is humidified, laminar air flow disrupted by nasal turbinates, bringing chemical signals in contact with olfactory epithelium at top of nasal cavity

Question 3

Describe the olfactory receptors.

Answer 3

• olfactory receptors=bipolar neurons, apical tuft of cilia w/ G protein coupled odorant receptors
• > 1000 different odorant receptors
• each olfactory receptor neuron expresses only 1 odorant receptor
• located in the olfactory epithelium underneath the cribriform plate
• neurons with the same odorant receptors send projections to same glomerulus in olfactory bulb

Question 4

How is olfaction different from other sensory systems?

Answer 4

• spatial relationship among different odorants unclear
• don’t have abstract terms to describe odorants, described in relation to characteristic smells, e.g. banana
• olfactory system projects directly to limbic regions, bypasses thalamic-cortical connections
• developed from nasal cavity not the brain

Question 5

What is the pathway from olfactory receptors to the brain?

Answer 5

• Olfactory receptor neurons project to glomeruli, which communicate with mitral cells
• Mitral cells projections terminate in piriform cortex, corticomedial amygdala, and entorhinal cortex via lateral olfactory tract
• predominantly IPSILATERAL

Question 6

Describe the piriform cortex.

Answer 6

3-layered allocortex

Important for olfactory discrimination, via connections of piriform cortex with lateral orbitofrontal cortex

Question 7

Describe the corticomedial amygdala and entorhinal cortex.

Answer 7

-Corticomedial amygdala: receives and integrates socially relevant inputs from different sensory systems. Olfactory inputs to here are important for social behavior: parental, sexual, aggression
-Entorhinal cortex: olfactory inputs influence meemory
Together they influence emotional responses to olfactory info.

Question 8

What is anosmia? hyposmia? hyperosmia?

Answer 8

anosmia: Loss of smell
-developmental problem or due to trauma, tumors, metabolic diseases
hyposmia: reduced sense of smell
-aging and neurologic disorders
Hyperosmia: increased sense of smell
-pregnancy, psych disorders

Question 9

What are the two major functional divisions of the amygdala?

Answer 9

Corticomedial and basolateral amygdala

Question 10

What is the function of the corticomedial amygdala?

Answer 10

• components: medial nucleus and 3 cortical nuclei
• function: regulates olfaction.
• integrates olfactory input with endocrine signals and with ascending SS info to modulate social behavior
• projects to hypothalamic nuclei

Question 11

What is the function of the basolateral amygdala?

Answer 11

• more important part of amygdala for humans
• components: basolateral, basomedial, alteral, and central nuclei
• function: regulates emotions
• learned emotional responses to sensory stimuli, mainly negative emotions such as fear and anxiety
• central nucleus connects with brainstem involved in ANS responses in emotion, i.e. racing heart and sweaty palms
• receives info from sensory cortex, many other cortical regions
• projection to hypothalamus: contribute to expression of emotion
• stria terminalis-connects with midline structures

Question 12

What are some clinical problems that are associated with the amygdala?

Answer 12

• Kluver-Busy Syndrome-damage to basolateral amygdala, results in lack of emotional response, hyperorality due to loss of visual recognition (eating and putting in mouth inappropriate items), hypersexuality
• anxiety and PTSD
• Autism

Question 13

What is the function of the septum?

Answer 13

• arousal, attention, and reward
• lateral septum: social behaviors, agression, pair bonding
• medial septum has cholingeric neurons: regulates attention and cortical arousal to incoming sensory inputs
• septal nuclei have widespread connections

Question 14

What is the function of the nucleus accumbens?

Answer 14

• reward and reinforcement, including natural rewards and drugs
• NA and ventral tegmental area (VTA) form mesolimbic dopamine system

Question 15

Describe the anatomy of the hippocampus.

Answer 15

• 3 parts: dentate gyrus, hippocampus proper (Ammon’s horn), and subiculum
• input: association areas–> parahippocampal/perirhinal cortex–> entorhinal cortex–>dentate gyrus–>CA3–>CA2–>CA1–> subiculum (feeds back to entorhinal cortex)
• hippocampus is connected to septum, thalamus, hypothalamus, and mammilary bodies
• bidirectional connections
• high metabolic activity, high neural plasticity, possible neurogenesis, sensitive to ischemic damage

Question 16

What is the function of the hippocampus?

Answer 16

• memory formation
• important for declarative memory (facts, experiences, information)
• damage interferes with formation of new memories
• long term memory largely intact

Question 17

What are clinical syndromes associated with the hippocampus?

Answer 17

• anterograde amnesia
• Alzhemier’s disease: memory loss, lose hippocampal neurons
• Korsakoff’s syndrome: memory loss and confabulation (fabrication of recent events). Assoc. with chronic alcoholism.

Question 18

Give a brief overview of the limbic system.

Answer 18

Neocortex (providing cognitive and sensory input)–> septal nuclei, amygdala, and hippocampus (links cognitive info w/ emotion and memory)

• ->hypothalamus (integrates input w/ endocrine and ANS) and back to neocortex (integrates input can causes motor activity)
• ->gives rise to survival response