Transcript Axial Skeleton

Axial Skeleton
Chapter 8
Introduction
• The notochord and vertebral column define the
long axis of the body, offer sites for muscle
attachment, prevent telescoping, and support
body weight.
• The notochord is a long, continuous rod of
fibrous connective tissue wrapping a fluid filled
core.
• The vertebral column consists of a repeating
series of cartilaginous or bony elements.
– The vertebral column debuts early with evidence of
segmental blocks along a notochord.
Vertebrae
• The first parts of the vertebrae to appear were
the dorsal (neural) and ventral (hemal) arches
that rested upon the notochord.
– The neural arches protect the neural tube
– The hemal arches protect the blood vessels.
• The next part to evolve was the two centra
(the intercentrum and pleurocentrum)
– The centra serve to anchor and support the arches
• At one time, the vertebral type was used to
define tetrapod taxa.
• However, problems became evident with this
theory:
– Many early tetrapods evolved from aquatic ancestors,
and their vertebrae became modified for life on land
– Many of these secondarily returned to the aquatic
environment and their vertebrae resumed their more
primitive shape
• Thus, the morphologically similar types of
vertebrae represent multiple evolutionary trends,
evidence of functional convergence, not close
phylogeny.
Centra
• The centra represent the body of the vertebra.
• Among vertebrates there is great diversity in
the structure of the centra.
– They may be absent: Aspondyly
– Singular: Monospondyly
– Or Doubled: Dispondyly
• Centra are linked
successively into a chain
of vertebrae, the axial
column.
• The shapes of the
surfaces at the
articulations between
vertebrae affect the
properties of the
column and the way
forces are distributed.
• In both procoelous and opisthocoelous centra,
the convex articular surface of one fits into the
concave surface of the next to for a ball and
socket joint.
– This permits extensive motion in most directions
without stretching the nerve cord.
• Flexion of acoelous or amphicoelous vertebrae
causes adjacent centra to hinge on their edges.
– Flexion causes a hinge like motion stretching the
nerve cord.
• Heterocoelous centra allow great lateral and
vertical flexion, but prevent rotation of the
vertebral column
• The notochord, or its adult derivatives, often
run throughout and fill the concavities on the
centra.
• The term intervertebral disk has been used to
describe any pad between centra.
– However, in specifically applies to a pad of fiberocartilage with a core, the nucleus pulposes,
derived from the notochord.
• These are found only in mammals
– In other groups, the pad is referred to as the
intervertebral body
Ribs
• Ribs are struts that can fuse with vertebrae or
articulate with them.
• Ribs provide sites for muscle attachment, help
suspend the body form a protective case
around the organs, and can serve as accessory
breathing devices.
• In many fishes, there are two sets of ribs with
each vertebral segment, a dorsal and ventral
set.
• In tetrapods, one of these sets of ribs is lost,
and the other (dorsal ribs) become the trunk
ribs.
– Ribs of primitive tetrapods are bicipital, having
two heads that articulate with the vertebrae.
– The ventral head, or capitulum, articulates with
the parapophysis, a ventral process.
– The dorsal head, or tuberculum, articulates with
the diapophysis, on the neural arch.
• Although ribs function in locomotion, in
tetrapods they also become important in
respiration.
• Classification of tetrapod ribs is based on the type
of association with the sternum.
– Ribs that meet ventrally with the sternum are true
ribs.
• Consist of two jointed segments, the vertebral (costal) rib
and the proximal (sternal) rib.
– The joint between these two rib segments allows for changes in
chest shape during respiration
– Those that articulate with each other but not the
sternum are false ribs.
– And those that do not associate with anything are
floating ribs.
• In birds, cervical ribs are reduced and fuse with
the vertebrae.
– In the thoracic region, the first several, followed by
true ribs that articulate with the sternum.
– Some floating, and most true ribs bear a unicate
process, which offer sites for the attachment of
respiratory and shoulder muscles.
• In Mammals, ribs are present on all thoracic
vertebrae and define this region.
– Most are true ribs and they meet with the sternum
through a cartilaginous rib segment.
– Within the cervical and lumbar regions, remnants
exist only as remnants fused with the transverse
process.
Sternum
• The sternum is a midventral skeletal structure
that is endochondral in origin
• The sternum is the site of origin for chest
muscles.
• It also secures the ventral tips of the true ribs.
– The rib cage consists of the ribs and sternal
elements that enclose the viscera.
• The sternum may consist of a single bony
plate or several elements in seriese.
• Fishes lack a sternum.
– When it first appears in tetrapods it appears to not be
a phylogenetic derivative of either the ribs or pectoral
girdle.
• In anurans, a single sternal element lies posterior
to the pectoral girdle.
• The sternum is absent in turtles, snakes, and
other limbless lizards, but common in other
reptiles.
• In flying birds, the massive flight muscles arise
from a large sternum that bears the keel.
• In most mammals the sternum consists of a chain
of ossified elements in series.
Gastralia
• Posterior to the sternum in some vertebrates
is a separately derived set of skeletal
elements, called the gastralia, or abdominal
ribs.
• Unlike the true ribs, gastralia are dermal in
origin.
• And are restricted to the sides of the ventral
body wall and do not articulate with the
vertebrae.
• Gastralia are common in lizards and
crocodiles, serving as accessory attachment
sites for abdominal muscles.
• Within turtles the plastron is a composite
bony plate forming the floor of the shell.
– It consists of a fused group of dermal elements,
including the clavicles and gastralia.
Phylogeny
Fishes
• Agnathans:
– Among ostracoderms the notochord is large and
prominent.
• Vertebral elements are harder to document, due to lack of
preservation of the internal skeleton.
– Among living hagfishes and lamprey, the situation is
similar.
• Hagfishes possess a prominent notochord but lack any
vertebral elements
• Lamprey possess small, cartilaginous vertebral elements
resting dorsally upon the prominent notochord.
• Primitive Gnathostomes:
– In most primitive fishes, the axial column
consisted of a prominent notochord.
– There is no evidence of vertebral centra, although
dorsal and ventral arches were usually present.
– Primitive chondrichthyans exhibit a similar
pattern, with a prominent notochord with only
cartilaginous neural and hemal arches
representing the vertebral column.
• In advanced sharks these elements enlarge to become
the prominent element of the body axis, although the
notochord persists within the vertebrae.
• Bony fishes:
– Sturgeons and paddle fishes have unossified
vertebral columns, a secondarily derived
condition.
– Teleosts have a completely ossified vertebral
column, and the centra become more prominent,
replacing the notochord
• Neural spines and ribs become more developed.
• Sarcopterygians:
– The notochord continues to serve as the primary
axial support in this phyla.
– In many species, such as the rhipidistians,
vertebral elements are usually ossified and
composed of three separate vertebral elements.
• Mechanically, the axial column of fishes
represents an elastic beam.
– Lateral bending movements produced by the body
musculature place the column in compression.
• Even during peak bursts of speed, the fish’s notochord,
or vertebrae, experience stresses well within their
capacity to resist breaking or collapse.
• Centra seem to function as compression
members, and the stiffness that resists
buckling is controlled by the degree of lateral
flexure permitted by ligaments
Caudal skeleton and Fins
• In most fishes the axial skeleton continues into
the tail.
• The tail is usually asymmetrical, with a long
dorsal and small ventral lobe separated by a
notch.
• The different types of tail are based on the
deflection of the vertebral column.
a) Diphycercal: vertebral column extends straight
into the base of the tail.
b) Heterocercal: Vertebral column extends upwards
into a large dorsal lobe of the fin.
c) Homocercal: Small upward swing of vertebral
column, but tail halves are mostly equal.
d) Rounded homocercal: Same as before, fin rays
form rounded shape.
• Tetrapods:
– As animals went from water to air their bodies went
from a buoyant support design to one in which the
body was supported between the limbs.
– Changes in the axial skeleton are especially indicative
of the new mechanical demands.
• Amphibians have a vertebral column composed of single,
solid vertebrae at each segment, like their ancestors.
– The characteristic mode of fish locomotion that depends on
undulations of the body has been retained in modern
salamanders and most reptiles.
– Synchronized with these lateral body swings are limb movements
that lift and plant the foot to establish points of rotation.
• What was mechanically new was the tendency to twist the
vertebral column, placing it in torque.
• Several features of the designs of early tetrapod vertebrae
can be seen as functional modifications that address these
new demands.
• One of the vertebral innovations of tetrapods
was the zygapophyses.
• These bony processes reach across the
vertebral joints to interlock gliding
articulations.
• They are oriented to allow bending in the
vertical and horizontal plane, but resist
twisting.
• The other new feature of the vertebrate
skeleton is a delineation of the sacral region,
the site of attachment of the pelvic girdle.
– evidence that direct transfer of propulsive forces
in the hind limb to the axial skeleton became an
important part of terrestrial locomotion.
• Other changes that relate to extended
exploration of land involve a loss of the
connection between the head and pectoral
girdle.
– This was accompanied by a redesign of the 1st
vertebrae into a cervical vertebrae.
• For early tetrapods, life on land meant that
the lower jaw rested on the ground.
– Opening the jaw required lifting the head, since
the lower jaw could not move.
– With removal from the pectoral girdle the head
was now able to be lifted without interference
from the shoulder.
– This also meant that the head experienced less
jarring during locomotion, a benefit to visual
predators.
• Amniotes:
– Phylogenetically carry over their traits from the
archosaour line, so the major centrum is the
pleurocentrum.
– In many reptiles and birds, and all mammals the
intercentrum is lost.
– In amniotes the head rotates primarily on two
anterior cervical vertebrae .
• The first is the atlas, the second the axis
• Vertical and horizontal movements are controlled by
the atlas, twisting by the axis.
• In turtles, the shell into which the limbs and
head retreat is a composite unit made of
expanded ribs, vertebrae, and dermal bones
of the integument that hold the soft tissues.
• Turtles are unique in that the appendicular
skeleton lies within the rib cage.
• The vertebral column of amniotes is often
specialized.
• In snakes, additional zygapophyses provide
additional support but do not restrict normal
lateral bending.
• In birds, numerous cervical vertebrae have
highly mobile, heterocoelous articulations
giving bird skulls great range of motion.
• At the posterior end, the thoracic, lumbar,
sacral, and some caudal vertebrae fuse to
form a synsacrum.
• In mammals, the vertebral column is
differentiated into distinct regions.
– 7 cervical vertebrae
– 15-20 thoracic and lumbar vertebrae
– 2-3 sacral vertebrae (humans have 5)
• Caudal vertebrae vary in number with
differences in the size of the tail.
• Arches and zygapophyses diminish toward the
posterior tip of the tail so that only centra
remain at the tip.
Form and Function
Fluid Environment
• In an aquatic medium n organism does not
depend on the endoskeletal framework for
support.
• The body takes advantage of the surrounding
water for support.
• For active, aquatic organisms 2 problems exist.
1. Drag on the body
•
Fixed by streamlining
2. Orientation in three dimensional space
•
Stabilized by fins
Terrestrial Environment
• Land generally presents a 2-D surface across
which to maneuver.
• The major problem living on land is resistance
to gravity.
– When at rest, the body either sits on the ground
or between the legs.
– The legs act as an abutment that supports the
body.
– The vertebral column serves as a bridge between
the support posts, the legs
• The mammalian vertebral column, if viewed in
engineering terms, might be represented by
two suspension bridges, with the body
between.
– The spines and centra represent the compression
members; the ligaments and muscles are tension
members, and the legs act as the peirs.
• To carry weight any arch must maintain a
bowed shape.
– Between the pairs of limbs, the abdominal
muscles and sternum keep the arched vetebral
column from sagging
Design of Vertebrae
Direction of the Neural Spine
• Local mechanical forces on a spine arise from
contraction of axial muscles.
– Bones are weakest to tension and torsion and
strongest when resisting compression.
• The neural spine is oriented in a way that its
long axis is parallel with the resultant forces.
– This means that the spine experiences these
forces as compressive forces only
Height of the Neural Spine
• The height of the spine is proportional to the
mechanical leverage that the muscles exert.
• In effect, the spines are levers and transfer
forces to the centra.
• Increasing spine length increases the lever
arm and, therefore, increases the mechanical
advantage of the lever.
Regionalization
• In fishes
• The vertebral column is differentiated into two
regions, the caudal and trunk regions.
– Zygapophyses are generally absent.
– Centra are unspecialized.
• The vertebral column basically offers
attachment to swimming muscles.
• In tetrapods, the vertebral column supports the
body against gravity and receives and transmits
the propulsive forces the limbs generate.
– Trough the pelvic girdle, the hind limbs are directly
attached to the adjacent region of the vertebral
column, defining the sacral region.
– The cervical region is also differentiated, allowing the
skull freedom of movement independent of the body.
• Rapid locomotion causes the vertebral column to
flex laterally on itself.
– This can cause the ribs to crown.
– Loss of ribs in the area of greatest flexion is the
solution to this problem.
• Five distinct regions are differentiated in
mammals.
– The musculature is attached to the vertebral column
in complex ways, corresponding to the demands of
their active lifestyle.
• Birds have major modifications that match the
demands of flight.
– Cervical vertebrae are flexibly articulated to give the
head great freedom of movement and reach.
– Conversely, most of the vertebrae in the middle and
posterior part of the column are fused to each other
and the pelvic girdle
• This brings rigidity to the vertebral Column and establishes a
firm and stable axis for control while birds are in flight.
• The form and function of the vertebral column
are related directly to the static and dynamic
forces placed upon it.
• Overall, the axial skeleton and associated
musculature contribute to:
– bending the body,
– storing elastic energy,
– and transmitting useful forces for locomotion
generated by the limbs or caudle fin.
END