Phrenic afferent input excites C1-C2 spinal neurons in rats

Area of Science:

• Neuroscience
• Spinal Cord Physiology
• Pain Pathways

Background:

• The phrenic nerve transmits motor information to the diaphragm and sensory information from thoracic structures.
• Understanding the sensory pathways of the phrenic nerve is crucial for diagnosing and treating referred pain.

Purpose of the Study:

• To investigate the effects of electrical stimulation of the phrenic nerve on neurons in the C1-C2 spinal cord segments.
• To determine if afferent fibers exist in the phrenic nerve above the heart and their role in activating spinal cord neurons.

Main Methods:

• Electrical stimulation of the ipsilateral phrenic nerve (above and below the heart), contralateral phrenic nerve, and ipsilateral vagus nerve in anesthetized rats.
• Recording cellular responses in C1-C2 spinal cord segments.
• Mapping excitatory receptive somatic fields and testing responses to noxious and non-noxious stimuli.

Main Results:

• Electrical stimulation of the phrenic nerve above the heart excited 45 C1-C2 spinal cord cells.
• Stimulation below the heart had no effect.
• Contralateral phrenic nerve stimulation excited 7 of 16 cells.
• Vagus nerve stimulation excited 37 of 45 cells, with greater activity increase and longer latency compared to phrenic nerve stimulation.
• Somatic receptive fields often included the ipsilateral neck and jaw, and cells responded to noxious stimuli.

Conclusions:

• Afferent fibers in the phrenic nerve above the heart, but not below, excite C1-C2 spinal cord cells.
• The phrenic nerve may serve as a pathway for referred pain from thoracic structures to the neck and jaw.
• These findings contribute to understanding the neuroanatomy of referred pain.