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The Amygdala: Central Component in the Neurological Pathway of Fear

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Introduction

The Amygdala: Central Component in the Neurological Pathway of Fear Fear produces physiological symptoms in a wide variety of animals. Symptoms that are displayed when an animal is afraid include changes in autonomic activity for example changes in heart rate, blood pressure, or respiration. Fear can also be inferred when an animal freezes or suddenly stops some ongoing behavior such as eating or interacting with other animals. Also a change in simple reflexes or a change in facial expressions can also be a measure fear. When an organism is afraid and at the onset of fear these symptoms seem to kick in almost automatically as well as simultaneously. These highly correlated sets of responses seen during fear along with existing knowledge of the visual and auditory neural pathways suggest that stimuli elicited fear results from activation of a single area of the brain, the amygdala. Research in primates show that projections to the amygdala from sensory regions of the cortex are important in the processing of emotional significant complex stimuli. The results of which are then projected to a variety of target areas, which are critical for reacting accordingly to the stimulus. In order for the brain to recognize a fear causing stimulus information from the physical world must reach the brain along a specific pathway. ...read more.

Middle

Primate experiments by Amoral et. al. demonstrate that the lateral nucleus projects to all subdivisions of the basal nucleus and the accessory basal nucleus. The basal nucleus projects to the accessory basal nucleus as well as the central nucleus. The accessory basal nucleus sends a dense projection to the central nucleus. The central nucleus is the pivotal component of fear conditioning circuitry it provides various connections to brain stem areas that control a range of responses. Kapp's conclusion of the central nucleus explains that it is a crucial part of a system where autonomic conditioned responses are expressed. The central nucleus of the amygdala has direct projections to hypothalamic and brain stem areas that are involved in many of the symptoms of fear. Projections from the central nucleus of the amygdala to the hypothalamus appear to be involved in the activation of the sympathetic autonomic nervous system responsible for Paleness, pupil dilation and blood pressure elevation during fear. Projections from the central nucleus to the dorsal motor nucleus of the vagus nerve can cause parasympathetic activation resulting in ulcers, urination and defecation. Direct projections to the parabrachial nucleus cause increased respiration, for example panting and respiratory distress. The activation of dopamine, norepinephrine and acetylcholine during fear causes behavioral and EEG arousal as well as increased vigilance and may be attributed to projections from the amygdala to the ventral tegemental area. ...read more.

Conclusion

Electrical stimulation of the amygdala has also been shown to produce the pattern of behaviors resembling a state of fear. Stimulation of the amygdala can alter heart rate, blood pressure and also respiration, prominent symptoms of fear. Electrical stimulation can also simulate other symptoms of fear including cessation of behavior (freezing) startle reflex. In humans electrical stimulation of the amygdala elicits feelings of fear and anxiety as well as autonomic reactions associated with fear. A great deal of evidence from many laboratories using a variety of experimental techniques indicates that the amygdala plays a crucial role in conditioned fear. Many of the amygdaloid areas are involved in specific signs that are used to measure fear. Electrical stimulation of the amygdala produces a pattern of behaviors that mimic natural or conditioned states of fear. Lesions of the amygdala block innate or conditioned fear. Finally the amygdala appears to be a critical node in the pathway of fear where plasticity that mediates both the acquisition and extinction of conditioned fear. "A better understanding of the chemical composition and transmission properties of the amygdala may eventually lead to more effective pharmacological treatments for fear and anxiety disorders" (Davis 1992). Aggelton, J The Amygdala: Neurobiological Aspects of Emotion, Memory and Mental Dysfunction, Wiley-Liss, 1992 Davis, M The role of the Amygdala in Fear and Anxiety Annual Review of Neuroscience, Vol. 15, 1992 LeDoux, J Emotion, Memory and the Brain Scientific American. June1994 ...read more.

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