Fig. 1. The circuitry of the cutaneus trunci muscle (CTM) reflex and its interruption by injury. Afferent sensory axons project from nociceptive and mechanosensory receptors in the skin into the spinal cord at the dorsal root of each vertebral segment on both the left and right sides via the dorsal cutaneous nerves, as shown in red. These synapse on second- and third-order neurons whose long-tract projections ascend the spinal cord in the ventrolateral funiculus to synapse on CTM motor neurons clustered as bilateral nuclei at the cervical–thoracic junction. There is no anatomical connection between the left- and right-side motor neuron pools. Efferent axons (blue) project out of the spinal cord as a component of the brachial plexus terminating on the cutaneus trunci muscle beneath the skin of the back via the lateral thoracic branch of the plexus. In the intact animal, the green and yellow/green area represents the entire normal bilateral receptive fields of the CTM sensory neurons. Tactile stimulation of this region produces skin contractions, but contractions are not elicited outside this region. Note that a lesion to the spinal cord blocks ascending conduction of the afferent sensory tracts of the CTM. This produces a region of areflexia below the level of the lesion in which tactile stimulation no longer produces CTM contractions (highlighted in yellow/green); the rostral fields are unaffected by the injury. The orange-shaded area represents a region of partial recovery within the area of areflexia below the level of the lesion. This drawing is not to scale and, for clarity, does not include every component in the CTM circuit (such as commissural or contralateral projections) (see Blight et al., 1990; Borgens et al., 1990).