Phrenic nerve inputs to upper cervical (C1-C3) spinothalamic tract neurons in monkeys

Area of Science:

• Neuroscience
• Spinal Cord Physiology
• Pain Pathways

Background:

• Spinothalamic tract (STT) neurons in the upper cervical spinal cord process sensory information.
• Previous studies in monkeys indicated mid-cervical STT neurons respond to phrenic afferent input from the diaphragm.
• Rodent studies show differential phrenic nerve input organization in dorsal horn neurons.

Purpose of the Study:

• To investigate if high cervical STT neurons in primates are preferentially activated by phrenic inputs from thoracic structures.
• To compare responses to ipsilateral phrenic nerve stimulation above and below the heart.
• To assess the influence of contralateral phrenic input on these neurons.

Main Methods:

• Extracellular recordings of C1-C3 STT neuron discharge rates in monkeys.
• Electrical stimulation of ipsilateral and contralateral phrenic nerve fibers.
• Comparison of responses to stimulation from phrenic input originating above and below the heart.

Main Results:

• Upper cervical STT neurons showed strongest excitation from ipsilateral, small-diameter phrenic fibers originating in thoracic structures.
• Group III and IV input from diaphragmatic phrenic fibers had a lesser excitatory effect.
• Contralateral phrenic input also demonstrated a reduced effect on C1-C3 STT neurons.

Conclusions:

• High cervical STT neurons in monkeys are preferentially responsive to phrenic input from thoracic structures.
• Differential organization of phrenic afferent input suggests a role in sensory processing, potentially including nociception.
• Lesser activation by diaphragmatic and contralateral inputs may indicate their involvement in pain signaling pathways.