Activity in three segmental pathways was compared in normal subjects, patients with spinal shock, and patients with established spinal spasticity. The Achilles tendon reflex (ATR) was used to estimate transmission in the Ia monosynaptic pathway. Evidence is produced implying that vibration activates motoneurones principally through a polysynaptic pathway. The tonic vibration reflex (TVR) was used to estimate transmission in this Ia polysynaptic pathway. The percentage of the motoneurone pool (M-response) that could be activated by these pathways was used as a measure of transmission. The H reflex (vibration)/H reflex (control) ratio was used as an estimate of the degree of presynaptic inhibition of the Ia monosynaptic pathway. The findings led to the following conclusions. (1) In spinal shock presynaptic inhibition is greater than normal, transmission in the Ia monosynaptic pathway is reduced, and in the Ia polysynaptic pathway virtually abolished. (2) In established spasticity presynaptic inhibition is impaired, transmission in the Ia monosynaptic pathway is increased, but transmission in the Ia polysynaptic pathway never recovers. (3) The failure of presynaptic inhibition associated with spasticity is a gradual process. A hypothesis to explain these findings is proposed.
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