herirt does not htive to imply that that drug is part of the normal
physiologictd cquipincnt of thc hetirt.
Iri Humnttwy thori, the existing c.vidcnce is totally against the
presctncce of IL neurogeniu corit rol mechimisni for the ascidian heart but
the evidence in favour of hormond control is inadequate and contradictory. It is possible that some form of intrinsic regulation may be
exercised by sccretions from the cgtoplasmic lobes of the myoendothelium (Kalk, 1970).
E. Conduction
It1 tho rLbsrricct of ncurogeiiic. or iicurohuniorttl control it Reems
probublra t tin t hciirt t)wt is riiitlvr niyogcnic control. Myogctnic control
requiren thtit ttrcrr Hhoirld tw i ~ i i tLtl(yuut,c* mesnx of c~leotrical conductanw between cells untl t hiit thcy n i u x t be capable of co-ordination.
The resting poten tiul of ascidian myocndothelial cells has been
given ~ L R from -48 to -75 mV (Miller and McCann, 1963; Kriebel,
1967; Anderson, 1968) and there is very little overshoot during the
action potential. The electricd ciharactcristics of these cells remain the
same irrespctive of the direction in which the wave of mechanical
contraction is moving (Ebara, 1967 ; Kriebel, 1967) and this suggests
that there i,q only a singlo physiological cell type which can drive the
heurt mrchiiiiically iri either direction. Nevertheless, Anderson (1968)
has shown that all cells in the heart are capable of a certain amount of
independent activity ; the acbtion potential in a cell may for instance
show sniall pre- and post-pot entials, sniall potential changes may occur
independent of an*v mthchanical or electrical stimulation or a cell may
show no potwitid changc at all during a contraction of the heart.
Mechanical coridriction in heurt tissue will take place through
strips which arc conipletdy isolated from the raphe and the undiffertwtiated line und Kric.hel ( 1967) has fihown that elcc.trica1 conduction (.an
pass through isolated strips of hcart tixxue arid will cross the undifferentiated linc but not the raphe. Furthermore (Kriebcl, lH6Hh) the heart
(tun continue to function and pump blood when the raphe han hecn
mushed or sectioned. The raphe appears to hth no more than a suspcnsory striict lire with little physiological signifiwnce.
Kriebcl (1967) 118s shown that under uormal cirwmstatxcs the
wave front of acltivity pauses along the heart perpendicular to the long
axis of the heart cells. The average speed of conduction along the heart
is 13 mm/scc and, since each cell is only 4 p across, the wave front
must cross the cell in 0.3 msec. Kriebel brings forward evidence to show
that this is too fast to be mediated by non-electrical means and he
concludrs t~hrtt> conduction in the heart must be by local current flow
physiologictd cquipincnt of thc hetirt.
Iri Humnttwy thori, the existing c.vidcnce is totally against the
presctncce of IL neurogeniu corit rol mechimisni for the ascidian heart but
the evidence in favour of hormond control is inadequate and contradictory. It is possible that some form of intrinsic regulation may be
exercised by sccretions from the cgtoplasmic lobes of the myoendothelium (Kalk, 1970).
E. Conduction
It1 tho rLbsrricct of ncurogeiiic. or iicurohuniorttl control it Reems
probublra t tin t hciirt t)wt is riiitlvr niyogcnic control. Myogctnic control
requiren thtit ttrcrr Hhoirld tw i ~ i i tLtl(yuut,c* mesnx of c~leotrical conductanw between cells untl t hiit thcy n i u x t be capable of co-ordination.
The resting poten tiul of ascidian myocndothelial cells has been
given ~ L R from -48 to -75 mV (Miller and McCann, 1963; Kriebel,
1967; Anderson, 1968) and there is very little overshoot during the
action potential. The electricd ciharactcristics of these cells remain the
same irrespctive of the direction in which the wave of mechanical
contraction is moving (Ebara, 1967 ; Kriebel, 1967) and this suggests
that there i,q only a singlo physiological cell type which can drive the
heurt mrchiiiiically iri either direction. Nevertheless, Anderson (1968)
has shown that all cells in the heart are capable of a certain amount of
independent activity ; the acbtion potential in a cell may for instance
show sniall pre- and post-pot entials, sniall potential changes may occur
independent of an*v mthchanical or electrical stimulation or a cell may
show no potwitid changc at all during a contraction of the heart.
Mechanical coridriction in heurt tissue will take place through
strips which arc conipletdy isolated from the raphe and the undiffertwtiated line und Kric.hel ( 1967) has fihown that elcc.trica1 conduction (.an
pass through isolated strips of hcart tixxue arid will cross the undifferentiated linc but not the raphe. Furthermore (Kriebcl, lH6Hh) the heart
(tun continue to function and pump blood when the raphe han hecn
mushed or sectioned. The raphe appears to hth no more than a suspcnsory striict lire with little physiological signifiwnce.
Kriebcl (1967) 118s shown that under uormal cirwmstatxcs the
wave front of acltivity pauses along the heart perpendicular to the long
axis of the heart cells. The average speed of conduction along the heart
is 13 mm/scc and, since each cell is only 4 p across, the wave front
must cross the cell in 0.3 msec. Kriebel brings forward evidence to show
that this is too fast to be mediated by non-electrical means and he
concludrs t~hrtt> conduction in the heart must be by local current flow