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Pathways of emotion - from cortex to peripheral organs

October 07, 2003

Walking down a dark alley late at night is enough to give anyone the heebie-jeebies. Your heart starts racing, your palms get clammy and you get ready to run. Now researchers from Boston University have unravelled the neural pathways that transmit information about your surroundings to your organs, enabling them to respond appropriately.

The research, to be published on Friday in BMC Neuroscience, has shown that neurons originating in high-order brain structures transmit signals about the environment relatively directly to low-order structures in the spinal cord. There is just one structure in the middle - the hypothalamus. The pathway then connects to autonomic nerves, which originate in the spinal cord, to regulate organ function.

Helen Barbas, the research team leader, says: "The existence of these pathways has implications for several psychological conditions. For example, these pathways may be excessively active in anxiety, post-traumatic stress, and obsessive-compulsive disorder - conditions in which the emotional experience is extreme relative to the situation. Similarly, these pathways may be abnormally inactive in psychopathic individuals, who lack appropriate emotional responses."

Other research in humans has implicated the prefrontal cortex in these conditions. When this area is damaged, patients lack emotional propriety and do not show the changes in heart rate and the skin responses that normally accompany emotional arousal.

To map the neural pathways the researchers injected two different tracer molecules into the prefrontal cortex and the spinal cord. These molecules travel along the nerve axons, and the neurons can be seen as coloured structures through the microscope - the axons of the neurons originating in the cortex were red and the neurons that terminated in the spinal cord were blue. Both blue and red structures could be seen close together in the hypothalamus, specifically in those areas that are involved in the control of peripheral organs. This finding suggested that prefrontal cortex neurons could interact with neurons in the hypothalamus that send messages to the spinal cord.

The team then used electron microscopy to look at nerve endings in the hypothalamus. They saw that neurons from one area of the prefrontal cortex terminated in large 'boutons' in the hypothalamus. The appearance of these boutons suggested that the prefrontal neurons have an excitatory effect on the neurons in the hypothalamus.

Barbas said, "By combining two independent approaches to label multiple pathways simultaneously, we have demonstrated that the prefrontal cortex has relatively direct access to central autonomic structures. This evidence suggests that high-level prefrontal areas can rapidly influence autonomic areas in complex emotional situations".

BioMed Central Limited