1. Several lines of evidence suggest that the I7-I10 muscle group cont
ributes to the radula opening phase of behavior in Aplysia; 1) extrace
llular stimulation of these muscles in reduced preparations causes the
halves of the radula to separate, 2) synaptic activity can be recorde
d from muscles I7-I10 in intact animals when the radula is opening, an
d 3) motor neurons innervating I7-I10 are activated out of phase with
retractor/closer motor neurons during cycles of buccal activity driven
by the cerebral-to-buccal interneuron 2 (CBI-2). 2. All of the opener
muscles are innervated by the B48 neurons, a bilaterally symmetrical
pair of cholinergic motor neurons. B48 neurons produce excitatory junc
tion potentials (EJPs) in opener muscle fibers that summate to produce
muscle contractions. Contraction size is determined by the size of de
polarization in muscle fibers and/or by action potentials that are tri
ggered by summation of B48-evoked EJPs. 3. In addition to input from B
48 neurons, opener muscles also receive excitatory input from the chol
inergic multiaction neurons B4/B5. EJPs evoked by stimulation of neuro
ns B4/B5 are 1/10 the size of B48-evoked EJPs. Consequently, changes i
n muscle tension produced by B4/B5 activity are relatively small. In c
ontrast to B48 neurons, neurons B4/B5 are likely to be active during t
he closing/retraction phase of behavior. During cycles of buccal activ
ity driven by neuron CBI-2, neurons B4/B5 fire in phase with closer/re
tractor motor neurons. Thus opener muscles may develop a modest amount
of tension during the closing/retraction phase of behavior as a resul
t of synaptic input from neurons B4/B5. 4. Opener muscles may also dev
elop tension during closing/retraction simply by virtue of the fact th
at they have been stretched. When isolated opener muscles are lengthen
ed, depolarizations are recorded from individual muscle fibers, and mu
scle tension increases. With sufficient changes in fiber length, actio
n potentials are elicited. These action potentials produce twitchlike
muscle contractions that become rhythmic with maintained stretch. Stre
tch-activated depolarizations are generally first apparent when muscle
length is increased by 1 mm. Length changes of 4-5 mm are generally n
ecessary to elicit twitchlike muscle contractions. Changes of 1-2 mm i
n muscle length are observed when the opener muscle's antagonist, the
accessory radula closer, is activated in reduced preparations. 5. Stre
tch may also modulate B48-induced contractions of the opener muscles.
When muscle length is increased, B48-elicited contractions of the I7 m
uscle are larger. These increases in contraction amplitude are accompa
nied by decreases in contraction latency. 6. We conclude that muscles
I7-I10 contract vigorously in response to strong excitatory input from
neuron B48 and contribute to radula opening. Stretch may potentiate t
his activity. Thus, if radula closer muscles contract vigorously and p
ull on the opener muscles, the opener muscles will respond by contract
ing more vigorously themselves. This may be a mechanism for maintainin
g amplitude relationships between antagonistic muscles. Additionally,
it is likely that the opener muscles will develop at least a modest am
ount of tension during closure/retraction of the radula. Part of this
activation may derive from the weak excitatory input that the muscles
receive from neurons B4/B5. Another part may derive from the stretch.
One function of this co-contraction may be to act as a brake on closur
e, bringing this phase of feeding behavior to a smooth halt.