The External, Musculoskeletal and Neuro-Anatomy of Sea Turtles
69
The spinal nerves leave the spinal cord as paired dorsal and ventral nerve roots
and exit the vertebrae via intervertebral foramina. The dorsal nerve roots are
composed of somatic and visceral sensory nerve fibers and may contain motor
fibers as well; the ventral roots are generally composed of both somatic and visceral
motor nerve fibers. These nerves function as the autonomic nervous system. The
autonomic nervous system of turtles has both sympathetic and parasympathetic
components. However, these are not anatomically segregated as thoracolumbar
sympathetic and craniosacral parasympathetic regions, as in mammals (Kardong,
2002). Hence, nerves arising along the length of the spinal cord may have both
sympathetic and parasympathetic components (Kardong, 2002). Two networks of
interconnected spinal nerves, the brachial and sacral (lumbosacral) plexes, are
associated with control of the limbs. They are poorly described in the literature
on sea turtles, but are covered generally in freshwater species (Ashley, 1962) and
other reptiles (Grasse, 1948).
Two tissue layers, the meninges (“menix” is singular), cover the brain and spinal
cord. The outer menix is the tough dura mater. The pia mater or leptomenix is more
delicate and lies directly on the brain’s surface (Kardong, 2002). There are spaces
within the braincase that are subdural (beneath the dura mater) and epidural (above
the dura mater). Epimeningeal veins occupy some of the epidural space. The brain
is bathed in clear cerebrospinal fluid within the subdural space. It is produced by
the tela choroidea (a vascular region of the brain) and the pia mater dorsal to the
medulla and fourth ventricle.
2.5.1 PERIPHERAL NERVOUS SYSTEM
In cheloniids, the plexes are formed by ventral nerve roots and their branches. The
brachial plexus (Figure 2.24) arises at the level of cervical vertebrae VI–VIII in sea
turtles (Grasse, 1948; Wyneken, 2001). These cervical nerves form a complex network innervating the shoulder, brachial (humerus), and flipper muscles (Table 2.2);
they also send separate branches to the respiratory muscles. Most muscles receive
innervation from more than one branch of the plexus, so multiple innervations are
common. The cervical plexus (designated as simply the brachial plexus by some
authors) develops as dorsal and ventral parts. The dorsal cervical plexus branches
form the deltoid, superior brachial, superficial radial, and deep radial nerves to the
forelimb adductors and extensors (Walker, 1973). The ventral sets of branches form
the supracoracoid and the inferior brachial nerves (the latter subdivides into the
ulnar and median nerves). A ventral branch of cervical spinal nerve VI makes a
large contribution to the median nerve. Nerves VII and VIII give rise to the superior
brachial nerve, which immediately divides to form the superficial radial nerve and
the deep radial nerve to the anterior shoulder and dorsal flipper (Figure 2.24). The
supracoracoid, subscapular, inferior brachial, and ulnar nerves travel to the supracoracoideus and subscapular pectoral muscles, the brachialis muscle, and the ventral
side of the flipper. The deltoideus nerve, to the anterior shoulder muscles, arises
primarily from cervical spinal nerves VI and VII. Not all cervical spinal nerves
contribute to the plexes.
1123_C02.fm Page 69 Friday, November 15, 2002 11:16 AM
69
The spinal nerves leave the spinal cord as paired dorsal and ventral nerve roots
and exit the vertebrae via intervertebral foramina. The dorsal nerve roots are
composed of somatic and visceral sensory nerve fibers and may contain motor
fibers as well; the ventral roots are generally composed of both somatic and visceral
motor nerve fibers. These nerves function as the autonomic nervous system. The
autonomic nervous system of turtles has both sympathetic and parasympathetic
components. However, these are not anatomically segregated as thoracolumbar
sympathetic and craniosacral parasympathetic regions, as in mammals (Kardong,
2002). Hence, nerves arising along the length of the spinal cord may have both
sympathetic and parasympathetic components (Kardong, 2002). Two networks of
interconnected spinal nerves, the brachial and sacral (lumbosacral) plexes, are
associated with control of the limbs. They are poorly described in the literature
on sea turtles, but are covered generally in freshwater species (Ashley, 1962) and
other reptiles (Grasse, 1948).
Two tissue layers, the meninges (“menix” is singular), cover the brain and spinal
cord. The outer menix is the tough dura mater. The pia mater or leptomenix is more
delicate and lies directly on the brain’s surface (Kardong, 2002). There are spaces
within the braincase that are subdural (beneath the dura mater) and epidural (above
the dura mater). Epimeningeal veins occupy some of the epidural space. The brain
is bathed in clear cerebrospinal fluid within the subdural space. It is produced by
the tela choroidea (a vascular region of the brain) and the pia mater dorsal to the
medulla and fourth ventricle.
2.5.1 PERIPHERAL NERVOUS SYSTEM
In cheloniids, the plexes are formed by ventral nerve roots and their branches. The
brachial plexus (Figure 2.24) arises at the level of cervical vertebrae VI–VIII in sea
turtles (Grasse, 1948; Wyneken, 2001). These cervical nerves form a complex network innervating the shoulder, brachial (humerus), and flipper muscles (Table 2.2);
they also send separate branches to the respiratory muscles. Most muscles receive
innervation from more than one branch of the plexus, so multiple innervations are
common. The cervical plexus (designated as simply the brachial plexus by some
authors) develops as dorsal and ventral parts. The dorsal cervical plexus branches
form the deltoid, superior brachial, superficial radial, and deep radial nerves to the
forelimb adductors and extensors (Walker, 1973). The ventral sets of branches form
the supracoracoid and the inferior brachial nerves (the latter subdivides into the
ulnar and median nerves). A ventral branch of cervical spinal nerve VI makes a
large contribution to the median nerve. Nerves VII and VIII give rise to the superior
brachial nerve, which immediately divides to form the superficial radial nerve and
the deep radial nerve to the anterior shoulder and dorsal flipper (Figure 2.24). The
supracoracoid, subscapular, inferior brachial, and ulnar nerves travel to the supracoracoideus and subscapular pectoral muscles, the brachialis muscle, and the ventral
side of the flipper. The deltoideus nerve, to the anterior shoulder muscles, arises
primarily from cervical spinal nerves VI and VII. Not all cervical spinal nerves
contribute to the plexes.
1123_C02.fm Page 69 Friday, November 15, 2002 11:16 AM