102
LVAN OOODBODY
functional significance is clew. Ascidians must frequently be subjected
to minor mechanical or othw stimuli which are of little concern to the
animal but prsistentJ ~timrili, regardless of strength, evoke a defensive
response. According t o Hoyle the first small response is a slight closing
of the siphons, the second and large contraction is the contraction of the
mantle musculature cmd squirting of water from the branchial sac.
Subsequent contrtlctions represent siphon closure and withdrawal. One
of the featurcs of the syAtem is that it is easily exhausted and immediately after one burst of stimulation it responds feebly to a fresh burst.
Persistent Rtirnrrlation results in relaxation of the animal and a period of
relative inoxcitability. Hoyle considered the possibility that such
exhaustion might irivolve a junetion transmitter, either by progressive
depletion of the trmsmitter concerned or by its accumulation at the
junction and failure of the system to remove it. He concluded that both
possibilities are unlikely and suggested that fatigue might be in the
muscle system itself. There is still much controversy over the nature of
synaptic transmitters in ascidians but recent work points towards a
cholinergic system in which cholinesterase activity is low (Florey, 1963,
1!)67). Such a systeiii might permit the accumulation of transmitter
substance tLt the junction and give rise to fatigue (vide in&).
Hoyle’s cxpcrimeiits throw considerable light on the nature of the
ascidiari response merhanisrn and its ability to elicit fast withdrawal
responses by the devclopmcint of facilitation in the nervous system.
However, hi8 work throws no light on the most important of all
nervous responses--the crosHed reflex in the siphons, and its nervous
control.
There has been no detailed investigation of the nervous control of
the crossed reflex and our understanding of it is limited to the result^
obtained when the cerebral ganglion is removed as has been done by
Hecht (1918b), Day (l!jl9), Yamaguchi (1931), Bacq (1934b, 1935a),
Das (1 948a) and Flomy ( 1951). All of these workers are agreed that
ablation of the ganglion destroys or impedes the cross reflex but does not
interfere with the direct response. In view of the nature of the innervation of the siphons from the ganglion (p. 89) this result is not surprising,
but removal of the ganglion also results in a general loss of tonus in the
siphonal muscles, increased tonus in the mantle musculature and more
frequent squirting (Hccht, lrtl8b ; Day, 1919; Bacq, 1934b). Fiorey
(Zoc. cit.) considered that the outer layers of circular muscle only are
innervated by central nerve connections and that inner muscle layers
have only peripheral connections through a nerve net. In the absence of
the ganglion stimulation of orie siphon will still result in closure of the
other through such connections.
LVAN OOODBODY
functional significance is clew. Ascidians must frequently be subjected
to minor mechanical or othw stimuli which are of little concern to the
animal but prsistentJ ~timrili, regardless of strength, evoke a defensive
response. According t o Hoyle the first small response is a slight closing
of the siphons, the second and large contraction is the contraction of the
mantle musculature cmd squirting of water from the branchial sac.
Subsequent contrtlctions represent siphon closure and withdrawal. One
of the featurcs of the syAtem is that it is easily exhausted and immediately after one burst of stimulation it responds feebly to a fresh burst.
Persistent Rtirnrrlation results in relaxation of the animal and a period of
relative inoxcitability. Hoyle considered the possibility that such
exhaustion might irivolve a junetion transmitter, either by progressive
depletion of the trmsmitter concerned or by its accumulation at the
junction and failure of the system to remove it. He concluded that both
possibilities are unlikely and suggested that fatigue might be in the
muscle system itself. There is still much controversy over the nature of
synaptic transmitters in ascidians but recent work points towards a
cholinergic system in which cholinesterase activity is low (Florey, 1963,
1!)67). Such a systeiii might permit the accumulation of transmitter
substance tLt the junction and give rise to fatigue (vide in&).
Hoyle’s cxpcrimeiits throw considerable light on the nature of the
ascidiari response merhanisrn and its ability to elicit fast withdrawal
responses by the devclopmcint of facilitation in the nervous system.
However, hi8 work throws no light on the most important of all
nervous responses--the crosHed reflex in the siphons, and its nervous
control.
There has been no detailed investigation of the nervous control of
the crossed reflex and our understanding of it is limited to the result^
obtained when the cerebral ganglion is removed as has been done by
Hecht (1918b), Day (l!jl9), Yamaguchi (1931), Bacq (1934b, 1935a),
Das (1 948a) and Flomy ( 1951). All of these workers are agreed that
ablation of the ganglion destroys or impedes the cross reflex but does not
interfere with the direct response. In view of the nature of the innervation of the siphons from the ganglion (p. 89) this result is not surprising,
but removal of the ganglion also results in a general loss of tonus in the
siphonal muscles, increased tonus in the mantle musculature and more
frequent squirting (Hccht, lrtl8b ; Day, 1919; Bacq, 1934b). Fiorey
(Zoc. cit.) considered that the outer layers of circular muscle only are
innervated by central nerve connections and that inner muscle layers
have only peripheral connections through a nerve net. In the absence of
the ganglion stimulation of orie siphon will still result in closure of the
other through such connections.