Assessment of reflexes is based on a clear understanding of the following principles and relationships:
- Tendons connect muscles to bones, usually crossing a joint. When the muscle contracts, the tendon pulls on the bone, causing the attached structure to move.
- When the tendon is struck by the reflex hammer, stretch receptors contained within it generate an impulse that is carried via sensory nerves to the spinal cord. At this juncture, the message is transmitted across a synapse to an appropriate lower motor neuron. An upper motor neuron, whose cell body resides in the brain, also provides input to this synapse.
- The signal then travels down the lower motor neuron to the target muscle.
- The sensory and motor signals that comprise a reflex arc travel over anatomically well characterized pathways. Pathologic processes affecting discrete roots or named peripheral nerves will cause the reflex to be diminished or absent. This can obviously be of great clinical significance. The Achilles Reflex is dependent on the S1 and S2 nerve roots. Herniated disc material (a relatively common process) can put pressure on the S1 nerve root, causing pain along its entire distribution (i.e. the lateral aspect of the lower leg). If enough pressure if placed on the nerve, it may no longer function, causing a loss of the Achilles reflex. In extreme cases, the patient may develop weakness or even complete loss of function of the muscles innervated by the nerve root, a medical emergency mandating surgical decompression. The specific nerve roots that comprise the arcs are listed for each of the major reflexes described below.
- A normal response generates an easily observed shortening of the muscle. This, in turn, causes the attached structure to move.
- The vigor of contraction is graded on the following scale:
No evidence of contraction
Decreased, but still present (hypo-reflexic)
Clonus: Repetitive shortening of the muscle after a single stimulation
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Assessment of Reflexes
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