Phrenic Nerve: Anatomy and Function | GetWellGo

Explore the phrenic nerve's anatomy and its crucial function in breathing and diaphragm movement. Learn more with GetWellGo, your guide to health!

Phrenic Nerve: Anatomy and Function | GetWellGo

  • Category

    General

  • Published By

    GetWellGo Team

  • Updated on

    12-May-2025

Phrenic Nerve Anatomy

The phrenic nerve is an important nerve of the human body that mainly governs the diaphragm, which is needed for breathing. Here's an overview of its anatomy:

Origin

The phrenic nerve arises from the cervical spinal cord, i.e., from:

  • C3, C4, and C5 nerve roots (with C4 being the chief contributor).
  • Common mnemonic: "C3, 4, 5 keep the diaphragm alive."

Course

Neck (Cervical Region):

  • Forms at the lateral margin of the anterior scalene muscle.
  • Descends vertically over the anterior surface of the anterior scalene, beneath the prevertebral fascia.
  • Passes behind the subclavian vein and in front of the subclavian artery.

Thorax:

  • Enters thoracic cavity between subclavian artery and vein.
  • Passes in front of root of lung (critical differentiation from vagus nerve, which goes posteriorly).
  • Courses upon the fibrous pericardium, along with the pericardiacophrenic vessels.

Diaphragm:

  • Pierces the diaphragm and supplies motor innervation.
  • Also supplies sensory fibers to the central portion of the diaphragm, pericardium, mediastinal pleura, and diaphragmatic peritoneum.

Branches

  • Motor: Diaphragm (exclusive motor supply).

Sensory:

  • Central diaphragm (through diaphragmatic pleura and peritoneum).
  • Pericardium (fibrous and parietal serous layers).
  • Mediastinal pleura.

Clinical Significance

  • Phrenic nerve palsy can result in paralysis of the diaphragm and respiratory complications.
  • Iatrogenic injury in surgeries (e.g., thoracic, neck) can compromise respiration.
  • Referred pain: Irritation of the diaphragm causes shoulder pain (C3–C5 dermatomes).

Phrenic Nerve Function

The phrenic nerve is motor and sensory, with functions largely concerning the diaphragm and surrounding structures.

Principal Functions of the Phrenic Nerve

Motor Function

  • Chief role: Innervates the diaphragm, the primary breathing muscle.
  • Function: Initiates contraction of the diaphragm, resulting in inhalation.
  • When the phrenic nerve is activated, the diaphragm contracts and flattens, expanding thoracic volume and bringing air into the lungs.

Sensory Function

Supplies sensory innervation to:

  • Central portion of diaphragm (parietal pleura and peritoneum above it).
  • Fibrous pericardium and parietal layer of serous pericardium.
  • Mediastinal pleura.
  • Diaphragmatic pleura and peritoneum (particularly centrally).

Phrenic Nerve Injury

Injury to the phrenic nerve can result in severe respiratory issues since it plays a critical role in diaphragmatic movement. Following is an in-depth description:

Causes of Phrenic Nerve Injury

Surgical trauma (most frequent):

  • Neck operations (e.g., thyroidectomy, cervical spine operation)
  • Cardiac surgery (e.g., coronary artery bypass procedure)
  • Thoracic surgery (e.g., lung resection surgery)

Trauma:

  • Neck or chest injury
  • Clavicle fractures

Tumors or masses:

  • Compressing the nerve (e.g., apical lung tumors such as Pancoast tumors)

Neurological disorders:

  • Motor neuron disease
  • Guillain–Barré syndrome

Infectious/inflammatory:

  • Viral neuritis (e.g., herpes zoster)
  • Idiopathic or iatrogenic causes
  • Such as central line placement or nerve blocks

Symptoms of Phrenic Nerve Injury

Unilateral injury:

  • Usually asymptomatic at rest
  • Shortness of breath on exertion or lying down (orthopnea)
  • Paradoxical diaphragmatic movement during inspiration (elevates rather than flattens)

Bilateral injury:

  • Severe dyspnea (shortness of breath) even at rest
  • Hypoventilation, particularly when lying down
  • Potential respiratory failure

Phrenic Nerve Palsy

Phrenic nerve palsy is partial or complete paralysis of the diaphragm caused by phrenic nerve impairment. It interferes with normal breathing by restricting diaphragmatic motion.

Causes of Phrenic Nerve Palsy

Similar to phrenic nerve injury, and may be:

  • Surgical trauma (thoracic or neck surgery)
  • Spinal cord injuries (C3–C5)
  • Compression by tumor or lymphadenopathy (e.g., lung cancer, Pancoast tumor)
  • Infection or inflammation (e.g., viral neuritis)
  • Neurological disorders (e.g., ALS, multiple sclerosis)
  • Iatrogenic (e.g., central line, regional nerve blocks)

Symptoms of Phrenic Nerve Palsy

Unilateral Palsy

  • Usually asymptomatic or minimal symptoms
  • Shortness of breath with exertion
  • Orthopnea (worse when recumbent)
  • Hiccups or chest pain (rarely)

Bilateral Palsy

  • Severe dyspnea at rest
  • Sleep-disordered breathing or nocturnal hypoventilation
  • Accessory muscle use to breathe
  • Respiratory failure in advanced cases

Phrenic Nerve Origin

The phrenic nerve arises from the cervical spinal cord, i.e., from the anterior rami of the following spinal nerves:

Phrenic Nerve Origin

  • C3, C4, and C5 spinal nerves
  • C4 is the largest contributor
  • Mnemonic: "C3, 4, 5 keep the diaphragm alive."

Formation

  • Rises in the neck, below the prevertebral fascia, and emerges at the lateral margin of the anterior scalene muscle.
  • Developed from motor and sensory fibers of the cervical plexus (and not the brachial plexus).

Functional Significance of Origin

  • Because it originates high in the cervical spine, trauma at or above C3–C5 may compromise the phrenic nerve and lead to paralysis of the diaphragm, which can result in respiratory failure.

Phrenic Nerve Innervation

The phrenic nerve supplies motor and sensory innervation to important structures within the thoracic and upper abdominal cavities.

Phrenic Nerve Innervation

Motor Innervation

  • Diaphragm (right half and left half)
  • The sole motor supply to the diaphragm
  • Critical for inspiration and respiration

Sensory Innervation

The phrenic nerve conveys sensory fibers to:

  • Diaphragmatic pleura: Central portion
  • Mediastinal pleura: Completely
  • Pericardium: Fibrous and parietal serous layers
  • Diaphragmatic peritoneum: Central region (below diaphragm)
  • Diaphragm (sensory component): Central part (peripheral through lower intercostal nerves)

Phrenic Nerve Sensory Function

The sensory role of the phrenic nerve consists of conveying sensations—mainly pain, touch, and pressure—from a number of important thoracic and upper abdominal structures to the central nervous system.

Sensory Roles of the Phrenic Nerve

Diaphragmatic Pleura

  • Middle part of parietal pleura covering the diaphragm.
  • Feels pleuritic pain from inflammation or irritation.

Mediastinal Pleura

  • Innervates the whole mediastinal pleura, assisting in feeling pain from mediastinal pathology.

Pericardium

  • Fibrous pericardium and parietal layer of serous pericardium.
  • Pain due to pericarditis can be referred through the phrenic nerve.

Diaphragmatic Peritoneum

  • Central portion (belly of the diaphragm), which rests in relation to the abdominal organs above.
  • Irritation (e.g., due to liver abscess or subphrenic abscess) may refer pain to the shoulder.

Phrenic Nerve Motor Function

The motor action of the phrenic nerve is to innervate the diaphragm, the main muscle of breathing.

Phrenic Nerve – Motor Function Summary

Major Motor Target:

  • Diaphragm (both the right and left halves)
  • Only motor supply to the diaphragm.
  • Rises from C3–C5 spinal roots (predominantly C4).

Role in Breathing:

  • Triggers contraction of the diaphragm.

During inspiration:

  • The diaphragm contracts and flattens.
  • This increases volume of the thoracic cavity and decreases intrathoracic pressure, pulling in air into the lungs.

Effect of Motor Loss (Palsy or Injury):

  • Unilateral loss → Asymptomatic or mild dyspnea on exertion.
  • Bilateral loss → Severe breathing difficulty, particularly when lying flat (orthopnea), potentially needing ventilatory support.

Phrenic Nerve Diaphragm Control

The phrenic nerve is the only motor nerve that controls the diaphragm, the major breathing muscle.

Phrenic Nerve and Control of Diaphragm

Motor Supply

  • Phrenic nerve innervates the right and left hemidiaphragms.
  • Arises from C3, C4, and C5 spinal cord segments (primarily C4).

Diaphragm Function

  • When the phrenic nerve activates, the diaphragm contracts and descends.
  • This flattens the diaphragm, increases the thoracic cavity, decreases intrathoracic pressure, and pulls air into the lungs (inspiration).

Breathing Control

  • Voluntary control: through the cerebral cortex for actions such as breath-holding or speech.
  • Involuntary control: through the respiratory centers of the brainstem (medulla and pons) that transmit rhythmic impulses through the phrenic nerve.

Unilateral vs. Bilateral Control

One phrenic nerve controls one hemidiaphragm:

  • Right phrenic → right diaphragm
  • Left phrenic → left diaphragm
  • Injury of one side leads to elevation and decreased mobility of that hemidiaphragm.
  • Bilateral injury leads to severe impairment of breathing.

Phrenic Nerve and Breathing

The phrenic nerve is important for breathing as it controls the diaphragm, the primary muscle of respiration.

Phrenic Nerve and Breathing: How It Works

Motor Control of the Diaphragm

  • The phrenic nerve arises from C3, C4, and C5 spinal roots.
  • It provides motor impulses to the diaphragm, which contracts.
  • When the diaphragm contracts, it flattens downward, expanding the volume of the thoracic cavity and pulling air into the lungs (inspiration).

Role in Inhalation

Inhalation is an active process:

  • Phrenic nerve excites diaphragm → diaphragm contracts → lungs expand → air enters.

Role in Exhalation

Exhalation is generally passive:

  • The diaphragm relaxes, returns to dome shape, decreasing thoracic volume → air is forced out.

Brainstem Regulation

  • The medulla oblongata (in the brainstem) sends rhythmic impulses to the phrenic nerve through the descending spinal tracts.
  • This automatic adjustment varies breathing on the basis of COâ‚‚ concentrations, oxygen requirement, and exercise.

Phrenic Nerve Disorders

Disorders of the phrenic nerve can result in several respiratory complications because of the nerve's central role in regulating the diaphragm. Disorders of the phrenic nerve can lead to paralysis or dysfunction of the diaphragm, which can have a profound impact on breathing. This is an overview:

Common Phrenic Nerve Disorders

Phrenic Nerve Palsy (Paralysis)

  • Definition: Dysfunction of one or both phrenic nerves, resulting in paralysis of the diaphragm

Bilateral Phrenic Nerve Dysfunction

  • Definition: Bilateral dysfunction of both phrenic nerves, resulting in marked diaphragmatic paralysis or weakness.

Phrenic Nerve Compression

  • Definition: Extrapericardial compression of the phrenic nerve by adjacent structures (e.g., tumors, lymphadenopathy).

Congenital Phrenic Nerve Dysfunction

  • Definition: Uncommon, hereditary disorder where the phrenic nerve is absent or congenitally malformed at birth, resulting in diaphragmatic dysfunction.

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