This study investigates the neural circuitry underlying the acoustic startle reflex in rats, focusing on the short latency (8 msec) response. The reflex is mediated by a primary circuit involving the auditory nerve, ventral cochlear nucleus, lateral lemniscus nuclei, nucleus reticularis pontis caudalis (RPC), spinal interneuron, lower motor neuron, and muscles. Lesions of these structures abolish the reflex, while stimulation of these regions elicits startle-like responses with latencies of 7-5 msec. The RPC is critical for the reflex, as lesions of this region significantly reduce startle, whereas lesions of more rostral or caudal areas do not. Horseradish peroxidase (HRP) studies show that the RPC projects to the lateral lemniscus, suggesting a direct auditory pathway. The study also highlights the role of the reticulospinal tract in mediating the motor component of the reflex. Electrical stimulation of the RPC elicits startle-like responses with a latency of about 5 msec, and stimulation of the lateral lemniscus nuclei produces responses with a latency of about 6 msec. The study confirms that the RPC is essential for the reflex, as lesions of this region abolish startle, while stimulation of this region elicits startle-like responses. The study also shows that the reflex is mediated by a simple circuit involving the auditory pathway and spinal motor neurons, making it a useful model for studying behavioral plasticity, such as habituation, sensitization, and classical conditioning. The study provides a detailed anatomical and functional analysis of the acoustic startle circuit, identifying key structures and pathways involved in the reflex. The findings suggest that the reflex is mediated by a relatively simple neural circuit, making it an ideal model for studying the neural mechanisms underlying behavioral changes in complex organisms.This study investigates the neural circuitry underlying the acoustic startle reflex in rats, focusing on the short latency (8 msec) response. The reflex is mediated by a primary circuit involving the auditory nerve, ventral cochlear nucleus, lateral lemniscus nuclei, nucleus reticularis pontis caudalis (RPC), spinal interneuron, lower motor neuron, and muscles. Lesions of these structures abolish the reflex, while stimulation of these regions elicits startle-like responses with latencies of 7-5 msec. The RPC is critical for the reflex, as lesions of this region significantly reduce startle, whereas lesions of more rostral or caudal areas do not. Horseradish peroxidase (HRP) studies show that the RPC projects to the lateral lemniscus, suggesting a direct auditory pathway. The study also highlights the role of the reticulospinal tract in mediating the motor component of the reflex. Electrical stimulation of the RPC elicits startle-like responses with a latency of about 5 msec, and stimulation of the lateral lemniscus nuclei produces responses with a latency of about 6 msec. The study confirms that the RPC is essential for the reflex, as lesions of this region abolish startle, while stimulation of this region elicits startle-like responses. The study also shows that the reflex is mediated by a simple circuit involving the auditory pathway and spinal motor neurons, making it a useful model for studying behavioral plasticity, such as habituation, sensitization, and classical conditioning. The study provides a detailed anatomical and functional analysis of the acoustic startle circuit, identifying key structures and pathways involved in the reflex. The findings suggest that the reflex is mediated by a relatively simple neural circuit, making it an ideal model for studying the neural mechanisms underlying behavioral changes in complex organisms.