TH R PIFYSIOLOQY OF ASCIDIANS
91
tlw innervution for u nrrve network in both the branchial sac and the
visrertt (cf. p. 124). The systc.ni of fibrrs around the dorsal strand is
piirticrilurly wcll tlevt4opi~d in thc vicinity of the branrhial sac and
Pedele belicved that this is iLssociated with the nervous control of
ciliary activity. 8imilurly Fcdele helieved that the visceral elements
around the dorsal strand supply i~ fibrillar network surrounding the
alimenttwy cibnul and which controls ciliary activity in the gut. Fedele
noted that a frsture of all thew fibrillar networks is their lack of polarity
and this is further substuntiiLted by the work of Mackie et al. (1974)
on the branchinl SAC of’ Corelln.
Mackie anti his collcaguex hibve studied branchial innervation and
ciliary control in Corelln! willmeriano. A network of fine nerve fibres
runs through the brunchial sac sending branches to some of the ciliated
cells lining tlw stigmata; neuro-ciliary junctions have been identified
in this epithclium by electron microscopy. The nerve network is
connected to the ganglion by way of the visceral nerve but electrophysiological stiicties provide evidence of alternative conduction routes
into the branchid SAC along the endostyle and around the anterior
margin ; these alternative routes have not been identified histologically.
The branchid nerve network is through conducting and unpolarized and
it is presumed to be responsible for spreading excitation to the ciliated
(%ells of thc stigtnata. The spreiid of response can be identified by ciliary
arrests anti arcompunying electrical potentials. Ciliary arrests usually
occur ill a rliythnircil series along with muscular contractions in the
inuntle and siphons. Isolihted parts show inherent rhythmicity but in
the intart animal a single rhythm prevails. There are evidently
numerous, scattered peripherd pacemaker sites which are normally
coupled and whose output controlfi ciliary and muscular effectors alike.
Ciliary arrests are accompanied by large electrical signals recordable
externally from the brunchial surface with suction electrodes ; intracellular recordings with mirro-electrodes show 50 millivolt depolarizations lasting over one second. I~epolarizations probably spread from
cell to cell in the riliated epithelium since most of the cells are not
directly innervuted. 60 Angstriim close junctions, associated with interrellular electrical communication in a number of other tissues are shown
to be present between the stigmata1 ciliated cells. Observations on
ciliary arrest btshaviour in artificial sea waters containing differing ionic
balances suggest that the response depends on the sudden influx of
calcium ions as in the comparable casr of ciliary reversal in Parumecium.
Resumption of normal beating would occur when the resting potential
is restored by outward pumping of calcium ions.
Co-ordination between muscular rontrurtion and ciliary arrest is
91
tlw innervution for u nrrve network in both the branchial sac and the
visrertt (cf. p. 124). The systc.ni of fibrrs around the dorsal strand is
piirticrilurly wcll tlevt4opi~d in thc vicinity of the branrhial sac and
Pedele belicved that this is iLssociated with the nervous control of
ciliary activity. 8imilurly Fcdele helieved that the visceral elements
around the dorsal strand supply i~ fibrillar network surrounding the
alimenttwy cibnul and which controls ciliary activity in the gut. Fedele
noted that a frsture of all thew fibrillar networks is their lack of polarity
and this is further substuntiiLted by the work of Mackie et al. (1974)
on the branchinl SAC of’ Corelln.
Mackie anti his collcaguex hibve studied branchial innervation and
ciliary control in Corelln! willmeriano. A network of fine nerve fibres
runs through the brunchial sac sending branches to some of the ciliated
cells lining tlw stigmata; neuro-ciliary junctions have been identified
in this epithclium by electron microscopy. The nerve network is
connected to the ganglion by way of the visceral nerve but electrophysiological stiicties provide evidence of alternative conduction routes
into the branchid SAC along the endostyle and around the anterior
margin ; these alternative routes have not been identified histologically.
The branchid nerve network is through conducting and unpolarized and
it is presumed to be responsible for spreading excitation to the ciliated
(%ells of thc stigtnata. The spreiid of response can be identified by ciliary
arrests anti arcompunying electrical potentials. Ciliary arrests usually
occur ill a rliythnircil series along with muscular contractions in the
inuntle and siphons. Isolihted parts show inherent rhythmicity but in
the intart animal a single rhythm prevails. There are evidently
numerous, scattered peripherd pacemaker sites which are normally
coupled and whose output controlfi ciliary and muscular effectors alike.
Ciliary arrests are accompanied by large electrical signals recordable
externally from the brunchial surface with suction electrodes ; intracellular recordings with mirro-electrodes show 50 millivolt depolarizations lasting over one second. I~epolarizations probably spread from
cell to cell in the riliated epithelium since most of the cells are not
directly innervuted. 60 Angstriim close junctions, associated with interrellular electrical communication in a number of other tissues are shown
to be present between the stigmata1 ciliated cells. Observations on
ciliary arrest btshaviour in artificial sea waters containing differing ionic
balances suggest that the response depends on the sudden influx of
calcium ions as in the comparable casr of ciliary reversal in Parumecium.
Resumption of normal beating would occur when the resting potential
is restored by outward pumping of calcium ions.
Co-ordination between muscular rontrurtion and ciliary arrest is