This study aimed to identify the neural circuit responsible for the short latency acoustic startle reflex in rats, which has a latency of 8 msec from tone onset to the beginning of the electromyographic response in the hindleg. The authors used a combination of electrical stimulation, lesion, and anatomical tracing techniques to trace the neural pathways involved. Key findings include: 1. **Lesions and Stimulation of Auditory Structures:** - Bilateral lesions of the ventral cochlear nucleus (VCN) abolished acoustic startle, while lesions of the dorsal cochlear nucleus, vestibular nuclei, and other auditory structures did not. - Electrical stimulation of the VCN elicited startle-like responses with a latency of about 7 msec. 2. **Lesions and Stimulation of Lateral Lemniscus:** - Bilateral lesions of the dorsal and ventral nuclei of the lateral lemniscus abolished acoustic startle, while lesions of adjacent structures did not. - Electrical stimulation of these nuclei elicited startle-like responses with a latency of about 6 msec. 3. **Lesions and Stimulation of Reticular Formation:** - Lesions of the ventral regions of the nucleus reticularis pontis caudalis (RPC) abolished acoustic startle, while lesions of more rostral or caudal areas did not. - Electrical stimulation of the RPC elicited startle-like responses with a latency of about 5 msec. 4. **Lesions and Stimulation of Spinal Cord:** - Lesions of the spinal cord eliminated acoustic startle. - Electrical stimulation of the spinal cord elicited startle-like responses with a latency of about 2.5 msec. 5. **Conclusion:** The primary acoustic startle circuit in rats consists of the auditory nerve, ventral cochlear nucleus, nuclei of the lateral lemniscus, nucleus reticularis pontis caudalis, spinal interneuron, lower motor neuron, and muscles. This circuit involves five synapses plus the neuromuscular junction. The study provides a foundation for further research on habituation, sensitization, prepulse inhibition, classical conditioning, and drug effects on behavior using the acoustic startle reflex as a model system.This study aimed to identify the neural circuit responsible for the short latency acoustic startle reflex in rats, which has a latency of 8 msec from tone onset to the beginning of the electromyographic response in the hindleg. The authors used a combination of electrical stimulation, lesion, and anatomical tracing techniques to trace the neural pathways involved. Key findings include: 1. **Lesions and Stimulation of Auditory Structures:** - Bilateral lesions of the ventral cochlear nucleus (VCN) abolished acoustic startle, while lesions of the dorsal cochlear nucleus, vestibular nuclei, and other auditory structures did not. - Electrical stimulation of the VCN elicited startle-like responses with a latency of about 7 msec. 2. **Lesions and Stimulation of Lateral Lemniscus:** - Bilateral lesions of the dorsal and ventral nuclei of the lateral lemniscus abolished acoustic startle, while lesions of adjacent structures did not. - Electrical stimulation of these nuclei elicited startle-like responses with a latency of about 6 msec. 3. **Lesions and Stimulation of Reticular Formation:** - Lesions of the ventral regions of the nucleus reticularis pontis caudalis (RPC) abolished acoustic startle, while lesions of more rostral or caudal areas did not. - Electrical stimulation of the RPC elicited startle-like responses with a latency of about 5 msec. 4. **Lesions and Stimulation of Spinal Cord:** - Lesions of the spinal cord eliminated acoustic startle. - Electrical stimulation of the spinal cord elicited startle-like responses with a latency of about 2.5 msec. 5. **Conclusion:** The primary acoustic startle circuit in rats consists of the auditory nerve, ventral cochlear nucleus, nuclei of the lateral lemniscus, nucleus reticularis pontis caudalis, spinal interneuron, lower motor neuron, and muscles. This circuit involves five synapses plus the neuromuscular junction. The study provides a foundation for further research on habituation, sensitization, prepulse inhibition, classical conditioning, and drug effects on behavior using the acoustic startle reflex as a model system.