Jaw Jerk Reflex

Definition: The jaw jerk reflex, also known as the masseter reflex, is a reflex contraction of the jaw muscles induced by tapping or striking the chin region. It is an involuntary response involving the sensory and motor components of the trigeminal nerve and is primarily used to assess the integrity of cranial nerve V (trigeminal nerve).

Components of the Jaw Jerk Reflex:

1. Sensory Component:

The sensory component of the jaw jerk reflex involves the activation of the sensory receptors, primarily the muscle spindles, present in the masseter muscle. These muscle spindles detect changes in muscle length and rate of change, sending sensory signals to the brain via the trigeminal nerve.

2. Motor Component:

The motor component of the jaw jerk reflex involves the contraction of the masseter muscle in response to the sensory signals received. The motor neurons responsible for this contraction are located in the brainstem and are activated by the sensory input from the trigeminal nerve.

Procedure to Elicit the Jaw Jerk Reflex:

The jaw jerk reflex can be elicited using the following procedure:

1. The person being examined should be in a relaxed sitting or supine position.

2. The examiner places their index and middle fingers on the patient’s chin to provide a stable target.

3. Using a reflex hammer, the examiner taps or strikes the patient’s chin below the lower lip. The tap should be quick and firm.

4. The response of the patient’s jaw muscles is observed and assessed.

Interpretation of the Jaw Jerk Reflex:

The interpretation of the jaw jerk reflex depends on the observed response. The reflex can be:

1. Normal: A normal response includes a brief contraction of the masseter muscle on the same side as the tap, with minimal or no deviation of the jaw.

2. Hyperactive: A hyperactive response is characterized by an exaggerated contraction of the masseter muscle, causing the jaw to jerk forcefully.

3. Absent: An absent or diminished response may indicate damage or dysfunction of the trigeminal nerve or its associated motor pathway.

It is important to note that the interpretation of the jaw jerk reflex should be considered alongside other clinical findings and assessments to make an accurate diagnosis.