1. ANATOhlY AND PHYSIOLOGY O F THE CENTRAL NERVOUS SYSTEM
7
Neurotubules (2OOA in diameter) may also be present but are more
frequently observed in unmyelinated axons. The axoplasm contains oval
or elongated mitochondria with double membranes and often tubular
patterns of cristae and occasional membrane bound vacuoles. Neurofilaments are frequent, especially in niyelinated axons. Myelin sheaths of
central ncrvous system axons have laminations showing a periodicity of
major densc lines lying 120-140 A apart, separated by intraperiod lines.
Schniidt-Lanterman clefts are occasionally observed in the myelin sheath
of central nervous system axons. Unlike mammalian central nervous
system, nodes of Ranvier are frequent along axons of the fish central
nervous system. Internodal segments are often of great length in the
central nervous system of fish and the area of the node devoid of the
myelin sheath may be considerable.
Bouton terminaux are characterized by numerous synaptic vesicles,
mitochondria, neurofilaments, and occasional vesicles ( Long et al., 1968;
Pappas, 1966; Robertson, 1963; Robertson et al., 1963). The synaptic
specialization has pre- and postsynaptic membrane thickenings ( synaptic
bars) and a 250-A synaptic cleft (Fig. 2). Electrotonic synaptic junctions
have been described in the medulla oblongata and spinal cord of puffers
and electric fish (Bennett et al., 1967a-d; Nakajima et al., 1965; Pappas,
1966) and has been postulated for the club endings on Mauthner neurons ( Robertson, 1963; Robertson et al., 1963).
In the primitive myxinids, Myxine glutinosa, nerve cells have a clear,
rounded, relatively large nucleus occupying a central position in the cell
body ( Mugnaini, 1967). The cytoplasm contains ergastoplasm, free ribosomes, Nissl bodies, and large, heterogeneous dense bodies. Dendrites
and axoiis contain neurotubules which pass from the perikaryon into the
cell processes. Axosomatic synapses are usually absent, but axodendritic
synapses occur frequently ( Mugnaini, 1967).
The neurons of the tiger, Galeocerdo cuvieri, hammerhead, Sphyrnu
zygaena, and nurse shark, Ginglymostoma ciriratum, frequently have
round nuclei containing a prominent nucleolus and abundant cytoplasm
containing neuronal organelles typical of vertebrates ( Bakay and Lee,
1966; Long et al., 1968). Spccial neurosecretory neurons are located in
the preoptic nucleus and have been described in the goldfish (Palay,
1957, 1960) and Zoarces viviparus (Oztan, 1967), in the adenohypophysis of Zoarces vivipariis (Oztan, 1966), and in the caudal
neurosecrctory systcin of teleosts ( Fridberg and Bern, 1968). The neurohypophyscal neurosecretory neurons in the preoptic nucleus contain
marginated Nissl substance, an eccentric folded nucleus, a central zone
of Golgi apparatus, mitochondria, multivesicular bodies, and two size
7
Neurotubules (2OOA in diameter) may also be present but are more
frequently observed in unmyelinated axons. The axoplasm contains oval
or elongated mitochondria with double membranes and often tubular
patterns of cristae and occasional membrane bound vacuoles. Neurofilaments are frequent, especially in niyelinated axons. Myelin sheaths of
central ncrvous system axons have laminations showing a periodicity of
major densc lines lying 120-140 A apart, separated by intraperiod lines.
Schniidt-Lanterman clefts are occasionally observed in the myelin sheath
of central nervous system axons. Unlike mammalian central nervous
system, nodes of Ranvier are frequent along axons of the fish central
nervous system. Internodal segments are often of great length in the
central nervous system of fish and the area of the node devoid of the
myelin sheath may be considerable.
Bouton terminaux are characterized by numerous synaptic vesicles,
mitochondria, neurofilaments, and occasional vesicles ( Long et al., 1968;
Pappas, 1966; Robertson, 1963; Robertson et al., 1963). The synaptic
specialization has pre- and postsynaptic membrane thickenings ( synaptic
bars) and a 250-A synaptic cleft (Fig. 2). Electrotonic synaptic junctions
have been described in the medulla oblongata and spinal cord of puffers
and electric fish (Bennett et al., 1967a-d; Nakajima et al., 1965; Pappas,
1966) and has been postulated for the club endings on Mauthner neurons ( Robertson, 1963; Robertson et al., 1963).
In the primitive myxinids, Myxine glutinosa, nerve cells have a clear,
rounded, relatively large nucleus occupying a central position in the cell
body ( Mugnaini, 1967). The cytoplasm contains ergastoplasm, free ribosomes, Nissl bodies, and large, heterogeneous dense bodies. Dendrites
and axoiis contain neurotubules which pass from the perikaryon into the
cell processes. Axosomatic synapses are usually absent, but axodendritic
synapses occur frequently ( Mugnaini, 1967).
The neurons of the tiger, Galeocerdo cuvieri, hammerhead, Sphyrnu
zygaena, and nurse shark, Ginglymostoma ciriratum, frequently have
round nuclei containing a prominent nucleolus and abundant cytoplasm
containing neuronal organelles typical of vertebrates ( Bakay and Lee,
1966; Long et al., 1968). Spccial neurosecretory neurons are located in
the preoptic nucleus and have been described in the goldfish (Palay,
1957, 1960) and Zoarces viviparus (Oztan, 1967), in the adenohypophysis of Zoarces vivipariis (Oztan, 1966), and in the caudal
neurosecrctory systcin of teleosts ( Fridberg and Bern, 1968). The neurohypophyscal neurosecretory neurons in the preoptic nucleus contain
marginated Nissl substance, an eccentric folded nucleus, a central zone
of Golgi apparatus, mitochondria, multivesicular bodies, and two size