1. The reflex responses to stretch were studied in single motor units
and the surface electromyogram in human masseter. 2. Controlled stretches
of the isometrically contracting jaw-closing muscles evoked short-latency
(10-15 ms) and long-latency (35-70 ms) excitatory reflex responses in the
masseter surface electromyogram. 3. The majority (65%) of tonically active
masseter motor units were excited in both short- and long-latency phases
of the reflex. The timing of the stimulus determined whether the unit discharged
in the short- or long-latency phase. If a non-tonically active motor unit
was recruited by the stimulus, it invariably discharged in the long-latency
phase. 4. Although short-latency responses were strongly time-locked to
the stimulus, there was very little shortening of interspike intervals
(ISIs) in this phase of the reflex. The shortening of ISIs was more prominent
and prolonged during the long-latency phase, which explains why this phase
produces most of the reflex force changes following the stretch. 5. Within
pairs of concurrently active motor units there was a tenfold range in the
size of the short-latency response to the same stretch. 6. A substantial
proportion (35%) of the twenty-two masseter motor units tested had no statistically
significant short-latency reflex response. 7. In contrast to other human
muscles, there was no functional connection between a population of Ia
afferents and some masseter motoneurons. There are two possible explanations
for this result. The short-latency, presumably monosynaptic, Ia afferent
inputs may not be uniformly distributed to human masseter motoneurons.
Alternatively, these inputs may be subject to tonic presynaptic inhibition
that is not uniformly distributed throughout the masseter motoneuron pool.