Anatomy And Physiology

Anatomy Physiology

Dr. Bryan Katz's image for:
"Anatomy Physiology"
Image by: 

The term amygdala refers to a bilateral cluster of nuclei located deep in the brain's temporal lobes. The word amygdala itself is derived from the Greek word for almond. Apparently, the shape of these neuronal clusters vaguely resembles an almond, at least in the eyes of neuroanatomists.

The amygdala is usually considered to be part of the limbic system, a region of the brain involved with emotion, autonomic reactions, and memory. In addition to the amygdala, the limbic system includes the olfactory tracts, cingulate gyrus, hippocampus, anterior thalamus, and hypothalamus. The amygdala is thought to regulate the emotional content of memory in humans as well as other mammals.

The Papez circuit

Over 70 years ago, a scientist named George Papez proposed that the structures of the limbic system act as a circuit to integrate human emotional and physiological reactions to various stimuli. The output of the limbic system manifests as stereotyped, reflexive behaviors that becomes hardwired into long term memory. For example, many people have a strong fear of snakes and spiders. Regardless of the reason for this phenomenon, the sight of a snake or spider (or even viewing a photograph of one) triggers a predictable set of responses. The person's heart rate rises, as does blood pressure; the pupils and airways dilate; digestion stops; and the person may start to sweat or even hyperventilate. Clearly, visual stimuli can evoke a fight/flight response - the domain of the sympathetic nervous system - as well as the emotions that accompany this response, such as fear, anger, and a heightened sense of alertness.

Animal models

Few if any people have ever suffered brain damage exclusively affecting the amygdala. As such, scientists have turned to animal models to better understand the amygdala's role in behavior. Predictably, scientists were curious to find out what would happen in animals following ablation (surgical destruction) of the amygdala. In mice, the result was a complete loss of their normal fear response. Whereas the sight of a cat would cause normal mice to flee, mice lacking a functional amygdala approached cats and other predators nonchalantly.

Chimpanzees subjected to bilateral destruction of the amygdala developed a complex pattern of abnormal behaviors called Kluver-Bucy syndrome, after the researchers who first reported these findings. Similar to the mice, the chimpanzees reacted in bizarre ways to their surroundings. For example, when a hungry chimpanzee is offered food along with inedible items, it immediately eats the food and ignores the other objects. Instead, the Kluver-Bucy chimpanzees leisurely examined each object, visually and orally, as though they had lost all sense of object recognition, a condition called psychic blindness. The chimpanzees also exhibited bouts of hypersexual behavior and unprovoked aggression.

In humans, the closest correlate to Kluver-Bucy syndrome is damage to the temporal lobes secondary to trauma, uncontrolled seizures, viral encephalitis, or a brain tumor. Patients with these conditions may exhibit blunted emotions, or alternatively, display a labile affect, manifesting as paranoia and a hair trigger temper. People with these aberrant emotional responses sometimes benefit from treatment with mood stabilizers or neuroleptic medications.

More about this author: Dr. Bryan Katz

From Around the Web

  • InfoBoxCallToAction ActionArrow
  • InfoBoxCallToAction ActionArrow
  • InfoBoxCallToAction ActionArrow
  • InfoBoxCallToAction ActionArrow