The tendon reflex, sometimes also referred to as the stretch reflex or myotatic reflex, is an involuntary muscle contraction that occurs in response to the stretching of a tendon.
Understanding the Tendon Reflex
The tendon reflex is a fundamental neurological response. Here's a breakdown:
- Stimulus: The reflex is initiated by a tap on a tendon, usually with a reflex hammer. This tap causes a brief stretch of the muscle associated with that tendon.
- Mechanism: The stretch activates specialized sensory receptors within the muscle called muscle spindles. These receptors transmit a signal through nerve fibers to the spinal cord.
- Response: Within the spinal cord, the signal triggers a motor neuron that travels back to the muscle, causing it to contract. This contraction is the tendon reflex itself.
- Purpose: The main purpose of the tendon reflex is to maintain muscle tone and posture. It helps in adjusting muscle contraction to changes in position or loading.
Tendon Reflex vs. Stretch Reflex
The terms "tendon reflex," "stretch reflex," and "myotatic reflex" are often used interchangeably. However, as the provided reference suggests, there's some debate:
The deep tendon reflex is sometimes called the stretch reflex or myotatic reflex because of the stretch action and the muscle response involved. Some authors argue that they are not the same reflex.[5] They believe the tendon reflex occurs after the tendon's active stretching when it is tapped with the hammer.
While they are highly related, some distinguish them because the tendon reflex is thought to be the result of active tendon stretching caused by the tap of a hammer, whereas a stretch reflex may be a more general term that refers to the overall stretch of a muscle which does not need to be initiated by an outside stimulus. However, for practical purposes, they are often considered as similar concepts.
Practical Examples
Tendon reflexes are often tested during a neurological exam to assess nerve and muscle function.
- Knee-jerk Reflex: Tapping the patellar tendon below the knee cap causes the quadriceps muscle in the thigh to contract, resulting in the extension of the lower leg.
- Biceps Reflex: Tapping the biceps tendon in the inner side of the elbow causes the biceps muscle to contract, leading to flexion of the elbow.
- Ankle Jerk Reflex: Tapping the Achilles tendon at the back of the ankle causes the calf muscle to contract, leading to plantarflexion of the foot.
These reflexes give clinicians insight into the health of the spinal cord, nerves, and muscle function. Diminished or absent reflexes can indicate a neurological issue, while exaggerated reflexes may also suggest a problem.
Summary
In essence, the tendon reflex is a rapid, automatic contraction of a muscle in response to a mechanical stretch of the tendon. It serves a critical role in maintaining posture, tone, and overall muscle function, though some consider it a distinct subset of the more general stretch reflex.
