Nociception

Clinical Examination

Pinch the lateral and then the medial digit (down to the depth of the underlying bone) with fingers (or if there an equivocal response use hemostats), and watch for a conscious response (i.e., the animal’s head turns, it growls / vocalizes, or bears its teeth).  The patient in the video is normal and so hemostats were not used.  Note that the dog displays a conscious response (sits up, and turns toward the examiner) to her digit being pinched.

The presence/absence of nocioception does NOT need to be assessed if the patient has voluntary motor function.

Anatomical Pathway

The digit is pinched with a hemostat (across the bone) and the patient displays a conscious response to the noxious stimulus.

Afferent Pathway

The pinch of the hemostat stimulates nociceptive receptors.  The sensory input travels up the associated peripheral nerve.  The cell bodies associated with the peripheral nerve axons are located in the spinal ganglion and the axons travel into the spinal cord via the dorsal root and into the dorsal grey column (DGC) where they synapse on neurons. Axons from the DGC travel in the lateral spinothalamic tract (both contra- and ipsi-lateral) up the cord to the brain.  In the brainstem, fibers travel in the lateral aspect of the medial lemniscus (ML).  As the impulse passes through the ML, collaterals are sent to the ascending reticular activating system (ARAS).  The fibers from the ML synapse in the thalamus (ventrocaudal lateral nucleus).  Axons from the thalamic neurons enter the internal capsule and are distributed to the somatosensory cortex of the forebrain.