THE PLANTAR REFLEX - RePub

eflex arc must have its impact at or near these motoneurones. In other
words, an excitatory action (direct and selective innervation of distal motoneurones)
is closely connected with an inhibitory action (counteracting the
recruitment of the extensor hallucis longus via the flexion reflex pathways).
The inhibitory neurone might be activated by collaterals of corticomotoneuronal
fibres or consist of separate, but intimately related, pyramidal fibres.
The proposed interaction between descending tracts and flexion reflex
pathways is represented schematically in figure 14.
Nevertheless, there are exceptions. Some patients show a Babinski sign
together with other pyramidal features but without demonstrable weakness
or loss of skill (cases 1-4, table XVIII), others lack it whilst having
some motor deficit in the foot (case 5, table XX). There are two ways of
explaining these inconsistencies. The first is that direct excitation of distal
motoneurones and inhibition of impulses via flexion reflex afferent nerve
fibres can be dissociated because they are mediated by different neurones,
however closely they are linked. The alternative is to invoke individual
factors in exceptional patients: motor abnormalities that are too subtle for
testing on the one hand, and an idiosyncratically poor influx of segmental
impulses to motoneurones of the extensor hallucis longus on the other.
Bilateral Babinski signs without other pyramidal features
In normal subjects, even in those with rather brisk flexion reflexes, a
normal pyramidal system keeps the extensor hallucis longus muscle out of
the flexion reflex synergy, at least after infancy. This is of course the basis
for the great practical value of the plantar reflex. However, one meets
exceptional cases where extremely brisk flexion reflexes are found on both
sides, in the absence of other abnormalities: mere touch activates all flexor
muscles of the leg, including the extensor hallucis longus, and this can be
reproduced indefinitely.
Two such patients were described in Chapter IV. One had sustained a
(probably) epileptic seizure two years before and the other showed a raised
CSF protein, but in neither were there abnormalities on neurological
examination. Rather than to invoke an occult disturbance of distal
pyramidal projections, it may be more reasonable to assume that in these
patients the flexion reflex - either released from supraspinal control or
being only at one extreme of normal variation - breaks through the
normal descending control of extensor hallucis longus motoneurones. That
such a view is not too heretical is illustrated by the fact that up going toes
can be produced in many normal subjects by stimulating the base or pads of
the toes (Verger and Abadie, 1900; Dosuzkov, 1932). It is unnecessary to
honour Babinski by postulating absolute impermeability of extensor
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