Limbic System – Hippocampus and Amygdala

Prof. K. Sivapalan

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Hippocampus

• Hippocampus forms the floor of the lateral ventricles.

• It abuts the amygdaloid nucleus anteriorly

• It fuses with parahippocampus laterally which is the ventromedial portion of the temporal lobe.

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Function of the Hippocampus.

• Stimulation of parts of hippocampus can elicit pleasure, rage, passivity, or excessive sex drive.

• Short term memory is lost on removal of bilateral hippocampus for treatment of epilepsy.

• It appears to be involved in deciding the relevance of the input for survival, and provides the drive that causes translation of short term memory into long term memory.

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Amygdala

• It is located below the medial anterior pole of each temporal lobes.

• Connected to hypothalamus and the rest of the limbic system.

• One division of olfactory tract ends in amygdala.

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Autonomic Effects of Stimulation of Amygdala

• Changes in heart rate and blood pressure.• Changes in gastrointestinal motility and

secretion.• Defecation and micturition.• Pupilary dilation and sometimes constriction.• Piloerection.• Secretion of anterior pituitary hormones.-

gonadotrophins and ACTH

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Other Effects of Stimulation of Amygdala

• Tonic movements such as raising head or bending body.• Circling movements.• Clonic rhythmic movements.• Movements associated with olfaction and eating.- licking,

chewing, and swallowing.• Rage, escape, punishment, severe pain and fear.• Reward and pleasure.• Sexual activities such as erection, copulatory movements,

ejaculation, ovulation, uterine activity, and premature labor

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Bilateral Ablation of Amygdala

• Not afraid of anything.• Extreme curiosity about everything.• Forgets rapidly.• Place everything in mouth and sometimes

eating solid objects.• Strong sex drive –attempts to copulate with

immature animals, animals of wrong sex, different species.

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Correlation Between Neocortex and Limbic System.

• “The neocortex sits astride the limbic system like a rider on a horse without reins”

• Actually there are some connections and neocortical activity does modify emotional behavior and vice versa.

• But emotions cannot be turned on and off at will.

• ?Discipline. ?Value of habits and culture