TIIE PIIYSIOLOQY OF ASCIDIANS
63
There is no innervatioii of t lie hwrt and conduction and co-ordination are rntircly myogeniv. All cells of the heart have potential for
acting M paccmakers and small dristws of cells call behave as independent unite with a single rhythm. A similar typ" of activity is found in
isolated heart cells and groups of wlls from the early chick embryo
(Hecht, 1966) and it may bc a general property of unspecialized heart
cells. In the active ascidian heart ro-ordination is achieved through
two pacemaker centres, oiie at either end (terminal pacemakers), and
conduction through the heitrt is by meaiis of apical junctions between
the cells. The terminal pacemakers probably maintain their dominance
by having slightly different thresholds for activity than other heart
cells. Myogenio hearts are known from ot!her animals and in the chick
embryo the heart is totally myogenic in the early stages of development
before neurd connections are made (Hwht, 1965). In the hagfish
(Myzine) the systemic heart is myogenic and never has a nervous innervation, and hcitrt cells having the characteristics of pacemakers are
found througliout d l chambers of the heart (Jensen, 1964). In molluscs
the heart, is essentially rnyogenic but is influenced extrinsically by
arcelerator and inhibitor nerves (Hill and Welsh, 1966). Krijgsman and
Divaris (1955) state that, the molluscan heart is of a diffuse myogenic
t8ype and, like the ascidian, isolated hearts and fragments of heart will
continue to contract endogenously.
The physiological mechanisms which vontrol reversal of heart beat
are Htill not fully understood but it secnis likely that the visceral terminal pacemaker is subject, to fiequency variations by secretions from
the myoendothelial cells thcmwlvcs. The direction of beat may then
depend on whether the viscwal pucerneker is in a phase of low frequency
or high frequcncy. Thc precise nioment of reversal may, however, be
modified by mechttniral st9imuli resulting from back pressure in the
arterial system. A similar mechanism for the maintenance of contraction, but not reversal, in the molluscan heart has been suggested by
Krijgsman and Ilivaris (1!)65).
Recent advances in our knowledge of the ascidian heart enable UB to
have a much rlrarer understanding than before of the physiological
mechanisms controlling the heart's activity. However, we still do not
know what is the functional necessity for reversing the circulation of
blood and this problem doe8 not appear to have been examined at all.
The phenomenon of reversal is found without exception in all members
of the Tunicatrb-the pelagic forms as well as the sessile ascidians-and
it therefore seeins likely that i t is of fiinctional importance and not just
4 1 physiological accident. Its gcncral occurrence in the group suggests
t hat' it is amocittted with some special featiire of t h e tunicatc circulation.
63
There is no innervatioii of t lie hwrt and conduction and co-ordination are rntircly myogeniv. All cells of the heart have potential for
acting M paccmakers and small dristws of cells call behave as independent unite with a single rhythm. A similar typ" of activity is found in
isolated heart cells and groups of wlls from the early chick embryo
(Hecht, 1966) and it may bc a general property of unspecialized heart
cells. In the active ascidian heart ro-ordination is achieved through
two pacemaker centres, oiie at either end (terminal pacemakers), and
conduction through the heitrt is by meaiis of apical junctions between
the cells. The terminal pacemakers probably maintain their dominance
by having slightly different thresholds for activity than other heart
cells. Myogenio hearts are known from ot!her animals and in the chick
embryo the heart is totally myogenic in the early stages of development
before neurd connections are made (Hwht, 1965). In the hagfish
(Myzine) the systemic heart is myogenic and never has a nervous innervation, and hcitrt cells having the characteristics of pacemakers are
found througliout d l chambers of the heart (Jensen, 1964). In molluscs
the heart, is essentially rnyogenic but is influenced extrinsically by
arcelerator and inhibitor nerves (Hill and Welsh, 1966). Krijgsman and
Divaris (1955) state that, the molluscan heart is of a diffuse myogenic
t8ype and, like the ascidian, isolated hearts and fragments of heart will
continue to contract endogenously.
The physiological mechanisms which vontrol reversal of heart beat
are Htill not fully understood but it secnis likely that the visceral terminal pacemaker is subject, to fiequency variations by secretions from
the myoendothelial cells thcmwlvcs. The direction of beat may then
depend on whether the viscwal pucerneker is in a phase of low frequency
or high frequcncy. Thc precise nioment of reversal may, however, be
modified by mechttniral st9imuli resulting from back pressure in the
arterial system. A similar mechanism for the maintenance of contraction, but not reversal, in the molluscan heart has been suggested by
Krijgsman and Ilivaris (1!)65).
Recent advances in our knowledge of the ascidian heart enable UB to
have a much rlrarer understanding than before of the physiological
mechanisms controlling the heart's activity. However, we still do not
know what is the functional necessity for reversing the circulation of
blood and this problem doe8 not appear to have been examined at all.
The phenomenon of reversal is found without exception in all members
of the Tunicatrb-the pelagic forms as well as the sessile ascidians-and
it therefore seeins likely that i t is of fiinctional importance and not just
4 1 physiological accident. Its gcncral occurrence in the group suggests
t hat' it is amocittted with some special featiire of t h e tunicatc circulation.