Anatomy of the Reflex Arc
The acoustic reflex is carried by a compact circuit: a muscle, the ossicles it acts on, a sensory limb into the brainstem, and a motor limb back out to the muscle.
The stapedius muscle
The stapedius is the smallest skeletal muscle in the human body. It arises within a bony canal in the posterior wall of the middle ear and its tendon emerges to insert on the neck of the stapes. When it contracts it pulls the stapes, stiffening the ossicular chain and damping its transmission of sound — particularly at low frequencies.
A second middle-ear muscle, the tensor tympani, attaches to the malleus and is supplied by the trigeminal nerve. In humans the tensor tympani responds mainly to non-acoustic stimuli; the reflex measured clinically in response to sound is essentially a stapedius reflex.
The afferent limb
The afferent — sensory — limb begins at the cochlea. Sound transduced by the inner hair cells generates activity in the auditory nerve fibres, and this signal travels along the eighth (vestibulocochlear) nerve to the cochlear nucleus in the brainstem[1]. From the cochlear nucleus, fibres project to the superior olivary complex on both sides.
Because the afferent limb runs through the cochlea and eighth nerve, a lesion at either site affects the reflex whenever that ear is stimulated — regardless of which ear holds the probe. This is the mechanistic root of the stimulus-ear pattern.
The efferent limb
The efferent — motor — limb begins at the facial nerve nucleus. Motor fibres leave the nucleus, travel within the facial (seventh) nerve, and the stapedial branch peels off in the descending mastoid segment of the nerve to supply the stapedius muscle[1].
Because the efferent limb ends at the muscle of one particular ear, a lesion here affects the reflex recorded from that ear no matter which ear is stimulated — the mechanistic root of the probe-ear pattern. The fact that the stapedial branch arises partway along the facial nerve also makes the reflex a topodiagnostic marker: its presence or absence helps localise a facial nerve lesion relative to that branch point.
Crossed and uncrossed pathways
Within the brainstem the circuit divides. Some fibres drive the facial nucleus on the same side as the stimulated ear — the uncrossed pathway, which produces the ipsilateral reflex. Others cross the midline to drive the facial nucleus on the opposite side — the crossed pathway, which produces the contralateral reflex[1].
This division matters clinically. A lesion confined to the midline crossing fibres can abolish the crossed reflexes while leaving the uncrossed reflexes intact — the diagonal pattern of an intra-axial brainstem lesion. It is only visible because the reflex is routinely recorded in both ipsilateral and contralateral conditions.