1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
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classes: One group of cells is small whereas the other is extremely large.
The neurosecretory neurons send small unmyelinated axons into the extensive plexus of blood vessels ventral to the spinal cord.
In actinopterygian fish, caudal neurosecretory neurons are ubiquitous
and are located in a definite neural center in the distal portion of the
spinal cord. Although neurosecretory cells have been found in all fish
that have been investigated, the morphology of the caudal neurosecretory
system is extremely variable (Fridberg and Bern, 1968). The morphological variation is usually confined to the size and location of the units
in the spinal cord. In some small species such as the guppy Lebistes
reticulutus ( Fridberg, 1962a; Sano and Kawamoto, 1959), the neurosecretory cells are extremely small. However, neurosecretory cells of
extremely small size appear to be the exception and not the rule (Fridberg and Bern, 1968).
Caudal neurosecretory neurons have unmyelinated axons which in
some species gather into an infundibulum or stalk. The infundibulum is
reminiscent of the infundibulum of the hypothalamus which leads into
the hypophysis. Variation in the location of the nerve fiber tract of the
caudal neurosecretory system is a result of the variability in location of
the urophysis ventral to the spinal cord. To reach the spinal cord the
infundibulum must pierce the leptomenix (primitive meninges) of the
spinal cord. Thus, the length and location of the infundibulum is also
determined by the shape and location of the urophysis (Fridberg and
Bern, 1968). The caudal neurosecretory tract of teleosts usually is free
of glial cell processes; however, at the zone of termination within the
urophysis one finds ependymal and glial elements interspersed among
the neurosecretory fibers (Bern et aZ., 1965).
The Dahlgren or neurosecretory cells are truly neurosecretory as
demonstrated by their ultrastructure. These modified neurons contain
elementary neurosecretory granules in the size range of 800-2500 A
within the perikaryon ( Afzelius and Fridberg, 1963; Bern and Takasugi,
1962; Enami and Imai, 1958; Fridberg, 1962a; Holmgren and Chapman,
1960; Palay, 1957, 1960; Sano and Knoop, 1959). These elementary
neurosecretory granules are found not only in the perikaryon but also
have been located along the axons of neurosecretory cells, indicating a
migration of neurosecretory material from the perikaryon to the urophysis. The elementary secretory granules are associated with and produced by the Golgi apparatus in the perikaryon of neurosecretory cells
in elasmobranchs and teleosts (Bern et al., 1961; Fridberg, 1963; Sano
and Knoop, 1959).
The physiology of this extremely interesting system is in doubt.
Maetz et nl. (1963, 1964) using the goldfish, Carassius auratus, found
71
classes: One group of cells is small whereas the other is extremely large.
The neurosecretory neurons send small unmyelinated axons into the extensive plexus of blood vessels ventral to the spinal cord.
In actinopterygian fish, caudal neurosecretory neurons are ubiquitous
and are located in a definite neural center in the distal portion of the
spinal cord. Although neurosecretory cells have been found in all fish
that have been investigated, the morphology of the caudal neurosecretory
system is extremely variable (Fridberg and Bern, 1968). The morphological variation is usually confined to the size and location of the units
in the spinal cord. In some small species such as the guppy Lebistes
reticulutus ( Fridberg, 1962a; Sano and Kawamoto, 1959), the neurosecretory cells are extremely small. However, neurosecretory cells of
extremely small size appear to be the exception and not the rule (Fridberg and Bern, 1968).
Caudal neurosecretory neurons have unmyelinated axons which in
some species gather into an infundibulum or stalk. The infundibulum is
reminiscent of the infundibulum of the hypothalamus which leads into
the hypophysis. Variation in the location of the nerve fiber tract of the
caudal neurosecretory system is a result of the variability in location of
the urophysis ventral to the spinal cord. To reach the spinal cord the
infundibulum must pierce the leptomenix (primitive meninges) of the
spinal cord. Thus, the length and location of the infundibulum is also
determined by the shape and location of the urophysis (Fridberg and
Bern, 1968). The caudal neurosecretory tract of teleosts usually is free
of glial cell processes; however, at the zone of termination within the
urophysis one finds ependymal and glial elements interspersed among
the neurosecretory fibers (Bern et aZ., 1965).
The Dahlgren or neurosecretory cells are truly neurosecretory as
demonstrated by their ultrastructure. These modified neurons contain
elementary neurosecretory granules in the size range of 800-2500 A
within the perikaryon ( Afzelius and Fridberg, 1963; Bern and Takasugi,
1962; Enami and Imai, 1958; Fridberg, 1962a; Holmgren and Chapman,
1960; Palay, 1957, 1960; Sano and Knoop, 1959). These elementary
neurosecretory granules are found not only in the perikaryon but also
have been located along the axons of neurosecretory cells, indicating a
migration of neurosecretory material from the perikaryon to the urophysis. The elementary secretory granules are associated with and produced by the Golgi apparatus in the perikaryon of neurosecretory cells
in elasmobranchs and teleosts (Bern et al., 1961; Fridberg, 1963; Sano
and Knoop, 1959).
The physiology of this extremely interesting system is in doubt.
Maetz et nl. (1963, 1964) using the goldfish, Carassius auratus, found