Download Muscular System

Muscular System
• All portions are derived from mesoderm
• Muscle tissue is capable of contraction, so
many bodily activities are carried out by the
muscles.
• All muscle cells are elongate, therefore they
are termed fibers.
• Muscle fibers lie in parallel arrays with the
longitudinal axis of the muscle.
• 3 Fiber Types exist, categorized based on their
histology and physiology.
Muscle Fiber Types
• Skeletal = striated, voluntary, multinucleate,
peripherally located nuclei.
– Forms bulk of the body musculature.
• Cardiac = striated, involuntary, uninucleate,
centrally located nucleus, branching (bifurcated)
fibers, intercalated discs.
– Found in the heart and dorsal aorta as it exits the
heart.
• Smooth = non-striated, involuntary, single
centrally located nucleus.
– Found in the walls of the gut, blood vessels, urinary
and genital ducts.
Figure 10.2 – Skeletal Muscle
Figure 10.3 – Cardiac Muscle
N = central nucleus; arrowhead = branching of fibers; I = intercalated disc
Figure 10.4 – Smooth Muscle
Cross-section
Smooth
Muscle
Longitudinal
-section
Skeletal Muscle Fiber Subtypes
1. Red Fibers = High concentration of myoglobin
(involved in oxygen uptake from the blood), high
numbers of mitochondria, aerobic, slow-twitch,
fatigue-resistant
2. White Fibers = Lower myoglobin concentration and
lower numbers of mitochondria, glycolytic, fasttwitch, fatigue-rapidly
3. Intermediate Fibers = intermediate myoglobin
concentration and relatively high numbers of
mitochondria, fast-twitch, oxidative-glycolytic,
fatigue-resistant
• Different muscles have different ratios of these fiber
types.
Stained for Myosin ATPase
Stained for NADH dehydrogenase
Fig 10.7 – Skeletal muscle fiber subtypes
S = slow-twitch, red muscle fiber
FG = fast-twitch, white muscle fiber
FOG = fast-twitch, intermediate muscle fiber
Classification of Muscles
• Based on embryonic origin and innervation
Axial
Somatic
Epaxial
Hyaxial
Cranial
Cranial
(dorsal)
(ventral)
(eyes & branchiomeric)
(hypobranchial)
Appendicular
Cardiac (Secondarily striated)
Visceral
Smooth
Classification of Muscles
• Primitive Condition in vertebrates was 2 discrete sets
of musculature:
– Somatic = muscles of the “outer tube” of the body.
– Visceral = muscles connected mainly with the gut tube (=
“visceral animal”) or blood vessels (heart).
• Somatic Musculature derived from myotome region of
the somite (epimere)
– Innervated by somatic motor neurons.
• Visceral Musculature is derived from hypomere (lateral
plate mesoderm) or from mesenchyme
– Heart forms where lateral plate mesoderm meets at the
midline
– Innervated by visceral motor fibers of the Autonomic N.S.
• Somite = segmented; Hypomere = unsegmented, with
coelom in the middle.
Classification of Muscles
• Axial (somites)
• Cranial
– Branchiomeric and eye muscles (somitomeres =
modified somites anterior to ear region)
– Hypobranchial (somites – ventral myotomes)
• Appendicular (somites)
• Cardiac (splanchnopleure)
• Smooth (splanchnopleure)
Fig 10.22 – Embryonic
origin of cranial
muscles in a shark
embryo.
General Trends in Vert Musculature
• Muscles become more divided in Tetrapods as
movements become more complex.
– Increased division in musculature leads to finer,
more precise movements
• As legs become more important, appendicular
musculature increases in size and volume.
Muscle shapes also change.
• Axial musculature is less developed in volume,
but it is more subdivided.
Axial Musculature
• In Amphioxus and embryonic cyclostomes
(ammocoetes) somites form in complete,
unbroken series from front part of the head to
the trunk.
• In vertebrates, this series is interrupted by the
expansion of the braincase in the ear region.
• Anteriorly only 3-4 somites persist – these
develop into muscles that move the eyeball
(rectus muscles, oblique muscles, retractors and
levators) and muscles associated with pharynx
(branchiomeric musculature)
• Posteriorly somites form the trunk musculature
and muscles of the neck region. This musculature
consists of successive segments called myomeres.
24 h chick
Somites
(myomeres)
Amphioxus
Axial Musculature
• Most primitive (ancestral) condition = single myomere
(derived from a single somite) per body segment.
– No epaxial (dorsal) - hypaxial (ventral) division
– This is the apparent condition for Jaymoytius (fossil
ostracoderm) and for Amphioxus
• Cyclostomes  each myomere overlaps adjacent
myomere, but still no epaxial-hypaxial separation.
• Jawed Fish  myomeres show complicated folding,
overlapping several to many body segments.
– Results in easier lateral undulations as overlap increases
innervation and results in better coordination
– Separation of epaxial (dorsal) from hypaxial (ventral) divisions by
a horizontal septum
• Tetrapods  myomeres cover many body segments as
different muscles.
– Virtually complete loss of segmentation in advanced Tetrapods
(birds, mammals)
Evolutionary
trends in axial
musculature in
vertebrates.
Note the
increasing
subdivision and
reduction in
segmentation in
the advanced
Tetrapod
condition.
Cranial Musculature
• Extrinsic eye muscles = move eye within orbits
– Derived from 3-4 somitomeres anterior to ear region
• Jaw Musculature derived from two distinct
embryonic sources, each with separate
innervation
– Hypobranchial Musculature = derived from myotome
region of trunk somites
– Ventral tips of myotomes grow forward and into
pharyx region along sides of pharyngeal arches
– Innervated by nerves from cervical region of spinal
cord (somatomotor nerves)
Cranial Musculature
• Jaw Musculature, Part 2
– Branchiomeric Musculature = derived from
somitomeres in pharyngeal region and anterior to
ear region
– Formerly thought to be derived embryonically
from splanchnopleure (surrounding gut tube)
– Innervated by cranial nerves (somatomotor fibers)
Fig 10.22 – Embryonic
origin of cranial
muscles in a shark
embryo.
Pharyngeal Musculature
• Primitive Fish Condition = all pharyngeal (gill)
arches have the following components:
– Constrictors = above and below gill slit; act to
change size of gill slits
– Levators = attached to dorsal ends of gill arches;
act to raise dorsal part of arch
– Adductors = pull dorsal and ventral halves
together
– Interarcuals = bend upper end of arches backward
Pharyngeal Musculature
• In Tetrapods, much of branchiomeric and
hypobranchial musculature is no longer
associated with the digestive tract (pharynx), but:
– Assumes other orientations and functions
• Facial muscles
• Jaw muscles
• Part of shoulder musculature
– Or becomes lost
• Branchiomeric muscles are all supplied via cranial
nerves (somatomotor fibers).
• In contrast, hypobranchial musculature is
innervated via spinal nerves (also somatomotor
fibers).
Fig 10.29 – Branchiomeric and
shoulder musculature
Branchiomeric musculature is associated
with gill operation in sharks, becomes
associated with facial, jaw and shoulder
musculature in Tetrapods.
Appendicular Musculature
• Associated with limbs and limb girdles
• Derived from general myotomic musculature of the
trunk
• Limb musculature originates from myomeres in lower
vertebrates (e.g., sharks), as paired finger-like
processes extend from ventral ends of myomere to
become mesenchyme of embryonic fins.
• Tetrapod condition:
– Muscle mass develops as condensations from
mesenchyme of limb bud, rather than from myomere
processes.
– Limb bud mesenchyme originates from somites, so they
also have embryonic derivation from myotome.
– Contributions from axial and branchiomeric musculature
occur in pectoral musculature.
Shark Embryo: Note the finger-like processes extending from the
ventral portions of the myomere. These will enter the fin bud to
form the appendicular musculature.
In Tetrapods, the appendicular musculature develops from limb bud
mesenchyme (originates from myotome), but also receives
contributions from axial and branchiomeric musculature.
Appendicular Musculature
• Fish Condition = two opposing muscle masses
covering dorsal and ventral surfaces from
girdle to base of fin.
– Dorsal muscles elevate fin
– Ventral muscles depress fin
• Appendicular muscles used for steering
purposes; axial musculature powers
locomotion movements during swimming
• Forelimb and hindlimb musculature similar in
fishes
Appendicular Musculature
• Musculature of fore- and hindlimbs differs in
Tetrapods
• Pectoral Girdle and forelimb musculature from
4 sources that form muscular “sling” that acts
to support anterior region of body
– Dorsal limb
– Ventral limb
– Axial (levator scapulae, rhomboideus, serratus)
– Branchiomeric (trapezius and mastoid groups)
• Pelvic girdle solidly fused to vertebral column
so no muscular sling present there
Fig 10.28 – Muscular “sling” supporting
anterior portion of the body in
association with the pectoral girdle in
Tetrapods.
Appendicular Musculature
• General Evolutionary Trend …
• Appendicular musculature is of small
importance and volume in fish (operates fins),
but becomes dominant in the Tetrapods,
where limbs are responsible for locomotion.
• Not only do appendicular muscles increase in
size and volume, but complexity is vastly
greater (allows much finer movements).
Evolutionary trends in
appendicular musculature
in vertebrates.
Note the increasing size and
complexity of the appendicular
musculature in the advanced
Tetrapod condition.
Cardiac Muscle Development
• Heart tube forms where vitelline veins fuse into
subintestinal vein, near anterior end of subintestinal vein.
• Dorsal regions of lateral plate mesoderm grow to meet and
surround heart tube. From this the heart muscle develops.
– Cardiac muscle is secondarily striated and innervated by
Autononic NS (Vagus nerve).
• Heart muscle develops from this lateral plate mesoderm
surrounding heart tube.
– Primordial epicardium thickens  will become outer coat of
heart (epicardium) and muscular layer (myocardium).
– Endocardial cells form tubes continuous with vitelline veins and
the developing ventral aorta.
– Lateral plate mesoderm grows to surround heart tube.
– Paired endocardial tubes fuse to form a single tube  lining of
the heart.
– Subsequent folding of the heart tube and formation of medial
septa lead to formation of the four chambers of the heart.
Muscle Terminology and Function
• Muscles never attach to skeletal elements
directly
– Always via tendon, ligament or aponeurosis (= flat
sheet of connective tissue).
• Muscle generally attaches to skeletal elements
at either (both) end.
– Origin = most stable attachment
– Insertion = attachment at opposite end (generally
more mobile end)
Muscle Classification by Action
•
•
•
•
•
Extensor = opens a joint
Flexor = closes a joint
Adductor = draws segment toward midline of body
Abductor = draws segment away from midline of body
Pronator (Supinator) = rotates distal part of a limb to prone
(supine) position
– Prone = face downward, palms forward
– Supine = face upward, palms rotated rotated forward [upward]}
•
•
•
•
•
Rotator = twists a limb segment
Levator = raises a structure
Depressor = lowers a structure
Constrictor (Sphincter) = surrounds orifices to close them
Dilators = opens orifices
Muscle Homologies
• Several criteria exist for establishing muscle
homologies, although each criterion has its
uncertainties.
– Similar function between muscles suggests
homology despite differences in origins or
insertions
– Similar nervous innervation suggests homology,
even if functions differ
– Common embryonic pattern of development
Fig 10.19 – Criteria for muscle homologies.
a) Similar function despite differences in
origin or insertion
b) Jaw depressor muscles have different
innervations, suggesting absence of
homology
c) Limb musculature in shark and
Tetrapods both derived from
myotome during development,
suggesting homology
Muscle Homologies –
Remembering the general
evolutionary trends in the different
musculatures will help in
understanding homologies.
Axial = decreased size but increased
subdivision and reduction in
segmentation in advanced vertebrates
Appendicular = increase in size, volume
and complexity as they become
muscles of locomotion
Branchiomeric and Hypobranchial =
lose association with gills to become
facial, jaw or shoulder musculature; or
are lost.