What is the pathway of the stapedial reflex?

Stapedial Reflex Pathway

The stapedial reflex is a bilateral brainstem reflex arc with afferent input from the cochlear nerve (CN VIII) and efferent output through the facial nerve (CN VII) to the stapedius muscle, with central processing occurring in the superior olivary complex and facial motor nucleus. 1

Afferent Pathway (Sensory Limb)

• Acoustic stimulation activates hair cells in the cochlea, which transmit signals via the cochlear nerve (CN VIII) 1
• The afferent fibers project to the ventral cochlear nucleus in the brainstem 2, 1
• From the cochlear nucleus, signals travel to the superior olivary complex bilaterally, which explains why the reflex is bilateral regardless of which ear is stimulated 2

Central Processing

• The brainstem centers in the superior olivary complex serve as the integration point for the reflex 2, 1
• This bilateral central processing allows the stapedius muscle in both ears to respond to ipsilateral, contralateral, and binaural stimulation 1
• The binaural interaction at this level plays an important role in maintaining sound direction localization 2

Efferent Pathway (Motor Limb)

• From the superior olivary complex, signals project to the facial motor nucleus 1
• The facial nerve (CN VII) carries efferent signals through its stapedial branch 3, 4
• The stapedial nerve innervates the stapedius muscle in the middle ear 4

Reflex Response

• The stapedius muscle contracts reflexively, pulling the stapes neck in the direction of the stapedius tendon 2
• This movement stiffens the ossicular chain (incus and malleus) and changes perilymph pressure in the inner ear 2
• Normal reflex threshold: 90-95 dB SPL for pure tones and 70-75 dB SPL for wideband noise 1

Clinical Significance

• The reflex provides protective action against noise-induced hearing damage by attenuating sound transmission 2
• Testing requires intact middle ear mechanics and both afferent (CN VIII) and efferent (CN VII) pathways 3
• Facial nerve lesions distal to the stapedial nerve branch will abolish the reflex on the affected side, making this test useful for localizing facial nerve injury 4
• The stapedial nerve branches from the facial nerve within the temporal bone, so lesions proximal to this branch (closer to the brainstem) will affect the reflex, while more distal lesions may spare it 4