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Breathing and the spinal cord

How the brainstem's breathing rhythm reaches the diaphragm and rib muscles through the cord. Why the phrenic nerve's origin in the mid-neck matters after a spinal cord injury.

Updated 2026-10-024 sources

Depth:

Why it matters

Every breath is driven by muscles that are controlled through the spinal cord. The phrenic nerve to the diaphragm leaves the cord in the mid-neck, so spinal cord damage below the mid-cervical level does not make ventilation impossible [1].

What it is

Two muscle groups drive normal inspiration: the diaphragm and the external intercostal muscles [2].

  • The phrenic nerve, which drives the diaphragm, is connected to the cord at cervical levels 3 to 5 [1]. It is a branch of the cervical plexus [3].
  • The intercostal nerves, from thoracic spinal nerves T2 to T11, run between the ribs [3].

How it works

The rhythm comes from the brain. The respiratory centre in the medulla oblongata controls the respiratory rate and responds mainly to carbon dioxide, oxygen and pH in the blood [2].

  1. The ventral respiratory group in the medulla keeps a constant rhythm by stimulating the diaphragm and external intercostals to contract, causing inspiration [2].
  2. When its activity stops, the muscles relax and expiration follows [2].
  3. The dorsal respiratory group integrates input from lung stretch receptors and peripheral chemoreceptors and modifies the rhythm [2].

When the diaphragm contracts it moves down, enlarging the thoracic cavity; contraction of the external intercostals moves the ribs up and out [2]. Quiet expiration is passive: the lungs recoil as the muscles relax [2].

The phrenic nerve's roots are classically C3, C4 and C5, but dissection studies also find two-root patterns (C3–C4 or C4–C5) and single-root patterns (C4 or C5) [4].

The pontine respiratory group adds control: its apneustic centre stimulates the dorsal respiratory group to deepen inspiration, and its pneumotaxic centre inhibits it, allowing relaxation after inspiration [2]. The major stimulus for breathing is the concentration of carbon dioxide in the blood rather than oxygen [2].

MedullaCervical cordThoracic cordMusclesVentral respiratory grouprhythmPhrenic motor neuronsC3 to C5Intercostal motor neuronsthoracicDiaphragmmoves downExternal intercostalsribs up and out
Breathing: from the medulla to the diaphragmThe ventral respiratory group in the medulla drives the diaphragm through the phrenic nerve, which leaves the cord at C3 to C5, and the external intercostals through thoracic nerves.
  • Excitatory synapse (filled arrowhead)
  • Inhibitory synapse (bar)
  • Modulatory (open circle)
  • Signal or data flow, not a synapse (dashed)
  • Midline crossing (decussation)
Figure 1. Breathing: from the medulla to the diaphragm. The ventral respiratory group in the medulla drives the diaphragm through the phrenic nerve, which leaves the cord at C3 to C5, and the external intercostals through thoracic nerves.[2, 1] Schematic
  • Because the phrenic fibres leave the cord at C3 to C5, cord damage below the mid-cervical level does not make ventilation impossible.[1]

Simplified: The dorsal and pontine respiratory groups, which adjust the rhythm, are not drawn; the descending route from medulla to cord is drawn as one link.

Text description of this diagram
  1. The ventral respiratory group in the medulla drives the diaphragm through the phrenic nerve, which leaves the cord at C3 to C5, and the external intercostals through thoracic nerves.
  2. Ventral respiratory group (midline, Medulla) to Phrenic motor neurons (midline, Cervical cord); signal or data flow (not a synapse); stimulates contraction.
  3. Ventral respiratory group (midline, Medulla) to Intercostal motor neurons (midline, Thoracic cord); signal or data flow (not a synapse).
  4. Phrenic motor neurons (midline, Cervical cord) to Diaphragm (midline, Muscles); excitatory synapse; phrenic nerve.
  5. Intercostal motor neurons (midline, Thoracic cord) to External intercostals (midline, Muscles); excitatory synapse; intercostal nerves.

When it does not behave

Because the motor fibres of the phrenic nerve leave the cord at C3 to C5, cord damage below the mid-cervical level spares the diaphragm [1]. In central sleep apnoea, the respiratory centres do not respond properly to rising carbon dioxide and do not regularly stimulate the diaphragm and intercostals [2].

Common misconceptions

Misconception: The lungs pull air in by themselves.

The lungs are passive; the diaphragm and intercostal muscles change the pressure that moves air in and out [2].

Check yourself

Which cord levels does the phrenic nerve come from?

Cervical levels 3 to 5 [1].

Which group of neurons sets the basic breathing rhythm?

The ventral respiratory group in the medulla [2].

Read next

References

  1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 15.2 Autonomic Reflexes and Homeostasis. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/15-2-autonomic-reflexes-and-homeostasis
  2. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 22.3 The Process of Breathing. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/22-3-the-process-of-breathing
  3. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 13.4 The Peripheral Nervous System. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/13-4-the-peripheral-nervous-system
  4. Sharma A, Rani A, Pankaj AK, Chopra J, Manik P, Dewan RK. Variational anatomy of cervical part of phrenic nerve: An observational human cadaveric study. National Journal of Maxillofacial Surgery. 2025;16(2):285-291. doi:10.4103/njms.njms_172_23

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