What is the primary effect of SNS activation during a threat?

The primary effect of sympathetic nervous system (SNS) activation during a threat is increased heart rate and alertness. When a person perceives a threat, the body enters a "fight or flight" response, which is orchestrated by the SNS. This response leads to physiological changes that prepare the body for immediate action, including an increase in heart rate to pump more blood to vital organs and muscles.

Along with the increased heart rate, alertness is also heightened as the body prepares to assess the situation and respond appropriately. This can include an increase in adrenaline levels, which further enhances the body's readiness to react. This preparation is crucial for survival in threatening situations, allowing for quicker responses and heightened awareness of the environment.

While options like cognitive enhancement and enhanced motor skills can occur to some degree during SNS activation, they are not the primary effects. Relaxation, on the other hand, is contrary to the physiological responses initiated by the SNS and would not be expected during a threat. Therefore, the increased heart rate and alertness are the most significant and immediate effects of SNS activation in response to perceived dangers.