The precise origin of the tectospinal pathway in three common laboratory animals: a study using the horseradish peroxidase method.

The horseradish peroxidase tracing method has been used to study the cells of origin of the tectospinal projections in the opossum, the tree shrew, and the cat. The present data show that only those collicular neurons which occupy the deep (ventral to the stratum opticum) tectal laminae send axons to the cervical spinal cord. In particular, layer IV contains the greatest number of spinal projecting neurons. Our results also reveal that while only the large sized collicular neurons project upon the cervical spinal cord in the opossum and the tree shrew, neurons comprising several different size categories do so in the cat. We thus suggest that several different descending channels exist over which the superior colliculus can influence the neck musculature in the cat.