The Hoffmann reflex, often confused with Hoffmann's sign, is a spinal reflex elicited by electrically stimulating a sensory nerve and observing the corresponding muscle contraction. It's a measure of spinal cord excitability, not to be confused with the clinical test for an upper motor neuron lesion.
Understanding the Hoffmann Reflex
The Hoffmann reflex (H-reflex) is an electrical analog of the muscle stretch reflex. Unlike the muscle stretch reflex which involves a mechanical stimulus (e.g., tapping a tendon), the H-reflex is induced by electrical stimulation. Here’s a breakdown of key aspects:
Key Characteristics:
- Electrical Stimulation: The H-reflex is activated by electrically stimulating a peripheral nerve, typically a mixed nerve (containing both sensory and motor fibers), like the tibial nerve in the leg, although other nerves may be used.
- Sensory Afferents: The electrical stimulation activates the sensory fibers (Ia afferents) within the nerve.
- Spinal Pathway: These Ia afferents send a signal to the spinal cord, where they synapse directly onto the alpha motor neurons.
- Motor Efferents: The alpha motor neurons then send a signal back to the muscle, causing a muscle contraction (typically the soleus muscle when stimulating the tibial nerve.)
- Latency: The time between the electrical stimulus and the muscle contraction is known as the H-reflex latency. This is longer than the latency for the direct motor response (M-wave), as the neural signal has to travel both to and from the spinal cord.
- Reflects Spinal Excitability: The size and characteristics of the H-reflex reveal information about the excitability of the spinal motor neuron pool.
Difference from the Hoffmann's Sign
It is crucial to distinguish the Hoffmann reflex (H-reflex) from the Hoffmann's sign. The latter is a clinical neurological test for upper motor neuron lesions, usually in the upper extremity. The video referenced, titled, "Hoffmann's Sign or Reflex | Upper Motor Neuron Lesion - YouTube" is specifically covering the clinical test - Hoffmann's sign, not the Hoffmann reflex. They are entirely separate concepts.
| Feature | Hoffmann Reflex (H-reflex) | Hoffmann's Sign |
|---|---|---|
| Nature | Electrical spinal reflex | Clinical exam |
| Stimulus | Electrical stimulation of a nerve | Flicking or tapping the fingernail or nailbed of the middle finger. |
| Response | Muscle contraction (e.g. soleus). | Involuntary flexion or adduction of the thumb and/or flexion of the index finger or other fingers. |
| Purpose | To assess spinal cord excitability. | To test for upper motor neuron dysfunction. |
Clinical and Research Applications:
- Spinal Cord Studies: The H-reflex is used to study the physiology of spinal circuits and assess the effects of various interventions (e.g., training, medication) on spinal cord excitability.
- Neuromuscular Disease Diagnosis: Changes in the H-reflex can indicate alterations in spinal cord processing, which may contribute to certain neurological diseases.
- Gait Analysis: H-reflexes can provide insight into how spinal reflexes contribute to gait, balance, and muscle tone.
- Research Tool: It provides a way to examine how different neural pathways and inputs affect the sensitivity of the spinal motor neurons.
Summary
The Hoffmann reflex is a valuable physiological measure of spinal cord activity, obtained via electrical nerve stimulation, while Hoffmann's sign is a clinical exam to detect upper motor neuron lesions.
