From the Guidelines
In an upper motor neuron lesion, the jaw jerk reflex would be increased. This is because upper motor neurons normally provide inhibitory control over lower motor neurons in the brainstem that mediate the jaw jerk reflex. When an upper motor neuron lesion occurs, this inhibitory control is lost, resulting in disinhibition of the reflex arc. The jaw jerk reflex is mediated by the trigeminal nerve (cranial nerve V), with afferent signals traveling through the sensory component and efferent signals through the motor component. The reflex center is located in the pons. This increased reflex response is part of a broader pattern of upper motor neuron syndrome, which typically includes hyperreflexia, spasticity, and positive Babinski sign, as seen in conditions such as stroke, where spasticity is a common component of the upper motor neuron syndrome 1. Clinically, an exaggerated jaw jerk is tested by placing a finger on the patient's chin and tapping it with a reflex hammer while the jaw is slightly open and relaxed, and observing for an abnormally brisk closure of the jaw. Some key points to consider in the clinical assessment of upper motor neuron lesions include:
- Hyperreflexia: an increase in reflex response, such as the jaw jerk reflex
- Spasticity: a velocity-dependent resistance to stretch of a muscle, which can be correlated with activity limitations and reduced quality of life 1
- Positive Babinski sign: a neurologic sign indicating upper motor neuron damage These clinical findings are essential in diagnosing and managing upper motor neuron lesions, and guiding treatment to improve patient outcomes and quality of life.
From the Research
Upper Motor Neuron Lesion and Jaw Jerk Reflex
- The jaw jerk reflex is a monosynaptic stretch reflex that is used to assess the integrity of the trigeminal nerve and its motor nucleus, as well as the corticobulbar tract 2.
- In an upper motor neuron lesion, the jaw jerk reflex can be affected due to the disruption of the corticobulbar tract, which is responsible for the regulation of the reflex 3.
- Studies have shown that upper motor neuron lesions can lead to an increase in muscle tone, including the jaw muscles, resulting in a brisk jaw jerk reflex [(2,3)].
- The increase in jaw jerk reflex is due to the loss of inhibition from the upper motor neurons, leading to an increase in the excitability of the spinal reflexes 3.
- Therefore, in an upper motor neuron lesion, the jaw jerk reflex would likely be increased.