Download Action and Support: The Muscles and Skeleton

Action and Support:
The Muscles and Skeleton
Chapter 39
Muscular and Skeletal Systems
• Muscles: tissues that produce movement
through contraction
– Composed of contractile cells
• The skeleton provides supportive
framework for animal
– Maintains animal shape
– Provides supportive framework so muscles
can generate movements
Types of Muscles
• Muscular work requires muscles to alternatively
contract and lengthen
• There are three types of muscles
– Skeletal
– Cardiac
– Smooth
• Skeletal muscle
– Attached to bones, moves the skeleton
– Striated appearance
– Under voluntary control
Types of Muscles
• Cardiac muscle
– Located in the heart, pumps blood
– Also striated, although less than skeletal
muscle
– Under involuntary control
• Smooth muscle
– Not striated
– Located in tubular organs
– Under involuntary control
Skeletal Muscle Structure
• Skeletal muscle attaches to the skeleton
by tendons
– Tough cords of collagen fibers
• Muscle tissue is made of
– Muscle fibers: long muscle cells, arranged in
parallel
– Myofibrils: intracellular contractile elements;
cylindrical bundles of proteins
Skeletal Muscle Structure
• Myofibril composition
– Thick filaments
– Thin filaments
• Thick filaments made of myosin proteins
– Myosin heads project out from each myosin protein
• Thin filaments made primarily of actin proteins
– Also include troponin and tropomyosin accessory
proteins
Skeletal Muscle Structure
• Thick and thin filaments are arranged in
repeating units called sarcomeres
Skeletal Muscle Structure
• Sarcomeres
– The fundamental contractile unit of muscle
– Thin actin filaments overlap thick myosin
filaments
– Actin filaments attach to Z-lines bordering
each sarcomere
Skeletal Muscle Structure
• Other muscle fiber structures
– Sarcoplasmic reticulum: calcium storage
site; surround myofibrils
– T tubules: “tunnels” leading from outside to
inside of muscle fiber; passes close to
myofibrils
Muscle Contraction
• Results from thick and thin filaments
sliding past one another, shortening
muscle length
– Described as the sliding filament
mechanism
The Sliding-Filament Mechanism
• Thin filament is composed of double chain of
actin
– Each actin has a myosin binding site
– Each binding site is blocked by accessory proteins
• During contraction
– Accessory proteins move aside and expose myosin
binding sites
– Myosin heads then bind to actin and bend
– Thin filaments are pulled toward sarcomere center,
sliding over thick filaments
The Sliding-Filament Mechanism
• Using ATP, myosin heads repeatedly
bend, release, and reattach farther along
the thin filament
– The sarcomere shortens
Control of Contraction
• Skeletal muscle contraction is controlled
by the nervous system
• The nervous system interacts with
skeletal muscle at neuromuscular
junctions
– “Joining” of motor neuron and muscle fibers;
specialized synapses
Neuromuscular Junctions
• Motor neurons release neurotransmitters
that diffuse across synaptic cleft
– Neurotransmitters bind to receptors on the
muscle fiber membrane
– Action potential forms on muscle fiber
membrane
• Neuromuscular junctions are always excitatory
Skeletal Muscle Contraction
• Motor neurons and muscle fibers are
arranged as motor units
– One branched motor neuron forming
synapses with many muscle fibers
Calcium Ions and ATP
• Muscle contraction depends on the availability
of calcium ions and ATP
• An action potential in the muscle cell travels into
the T tubules
– Opens calcium channels of the sarcoplasmic
reticulum, allowing calcium ions to flow into cytosol
surrounding the thick and thin filaments
– Calcium binds to troponin of the thin filament,
resulting in shape change that pulls the tropomyosin
proteins away from the myosin binding sites
Calcium Ions and ATP
• As long as binding sites are exposed and
ATP is available, myosin heads will
repeatedly bind, bend, release, and
reattach, contracting the muscle fiber
• ATP powers the movement of the head,
and is necessary for the myosin to release
from the actin
Calcium Ions and ATP
• When action potential is over
– Calcium is actively transported back into the
sarcoplasmic reticulum
• Calcium is removed from troponin
– Accessory proteins return to resting position
• Block myosin binding sites and myosin heads
– Muscle relaxes
Calcium Ions and ATP
• Muscle fibers require ATP as continuous
energy source
– Stored ATP is used up after a few seconds of
activity
• Creatine phosphate
– Molecules in muscle tissue that quickly resynthesize ATP from ADP
– Lasts only a few seconds
Steady Supply of Energy
• Cellular respiration
– Glucose and fatty acids used to form ATP
– Requires an oxygen source for efficient ATP
formation
– If not enough oxygen, glycolysis takes over
and lactic acid is produced
• Can lead to muscle fatigue
• Most lactic acid converted to glucose by liver
Exercise
• The number of muscle fibers does not
change as the result of exercise
– Muscle fibers can change size with training
– The number of myofibrils increases, causing
an increase in muscle fiber thickness
– Increased muscle fiber thickness causes
increased muscle strength
Athletic Ability
• There are two types of skeletal muscle fibers
– Slow-twitch fibers
– Fast-twitch fibers
• Slow-twitch fibers
–
–
–
–
Contract more slowly
Abundant mitochondria for cellular respiration
Highly vascularized for oxygen delivery
High levels of oxygen-storing protein myoglobin
• Fast-twitch fibers
– Fast, powerful contractions
– Fewer mitochondria, smaller blood supply
– Adapted to use glycolysis, which does not use oxygen and
supplies ATP much more rapidly than cellular respiration
Athletic Ability
• Most adults have even numbers of fiber
types
• Champion sprinters
– ~ 80% fast-twitch fibers
– Bursts of amazing speed for a very short time
• World-class marathoners
– ~ 80% slow-twitch fibers
– Contract for hours before exhausting the
energy supplied by cellular respiration
Gene Therapy
• The functional capacity of muscles is
diminished with aging and diseases such
as muscular dystrophy
• Experimental introduction of synthetic
genes encoding IGF-I (insulin-like growth
factor I) into mice leads to larger, more
robust muscles
Cardiac Muscle Powers the Heart
• The muscles fibers of the heart are cardiac
fibers
• Use the same sliding filament contraction
mechanism as skeletal muscle fibers
• In contrast to skeletal muscle, contractions can
be self-initiated
– Sinoatrial node (SA node) serves as the heart's
pacemaker
– Gap junctions allow for synchronized cell contraction
• Allowing action potentials to travel from muscle cell to
muscle cell
Smooth Muscle
• Surrounds blood vessels and tubular organs
(bladder, uterus, gastrointestinal tract)
• Smooth muscle cells lack regular arrangement
of sarcomeres
– No striations
• Contractions
– Slow, sustained
– Mostly involuntary
– Initiated by stretch, hormones, and/or the nervous
system
What Does the Skeleton Do?
• It is the support framework of body
• The skeleton comes in three forms
– Hydrostatic skeleton
– Exoskeleton
– Endoskeleton
Skeleton Types
• Hydrostatic skeleton
– Made of fluid-filled cavities surrounded by
muscle
– Provides fluid support: muscles around fluid
cause shape changes and locomotion
– Found in worms, mollusks, and cnidarians
Skeleton Types
• Exoskeleton
– Hard outside covering with flexible joints
– Provides support, armor protection, and
precise movement
– Found in arthropods like spiders,
crustaceans, and insects
– Must be molted periodically
Skeleton Types
• Endoskeleton
– Internal skeleton
– Provides support and some protection
– Grows more easily and can support more
weight than exoskeletons
– Found in echinoderms and chordates
Skeleton Types
• Animals move by contractions of
antagonistic muscles – muscles that
work in opposition to one another,
supported by their various skeletons
The Vertebrate Skeleton
• Supports the body and protects internal
organs
• Provides levers for locomotion
• Produces blood cells
• Storage site for calcium and phosphorus
• Assists sensory transduction
The Vertebrate Skeleton
• The bones of the vertebrate skeleton can
be placed in two categories
– Axial skeleton: forms the axis of the body
– Appendicular skeleton: form the
appendages and their attachments to the
axial skeleton
Vertebrate Skeleton Tissues
• Three types of connective tissues
– Ligaments
– Cartilage
– Bones
• Ligaments - Collagen bands that connect
bone to bone at joints
Cartilage
• Cartilage provides flexible support and
connections
• Functions
– Provides flexible support: ears, nose
– Connects bones: ribs to sternum
– Provides shock absorbency: knees,
intervertebral discs
– Precursor to developed vertebrate skeleton
Cartilage
• Structure
– Chondrocytes: mature cartilage cells;
produce collagen matrix
– Collagen matrix: flexible; elastic matrix of
protein fibers
– Lack blood vessels: slow healing
Bone
• Bone provides a strong, rigid framework
for the body
• Resembles cartilage, but the collagen
fibers of bone are hardened by deposits of
calcium phosphate
• Bone comes in two forms
– Compact bone
– Spongy bone
Bone
• Compact bone: hard, dense outer
covering; protects spongy bone
– Arranged in osteon units: concentric rings of
osteocytes around a central canal that
contains a capillary
• Spongy bone: lightweight, rich in blood
vessels, porous (contains bone marrow)
Bone
• Made of three cell types
– Osteoblasts
– Osteocytes
– Osteoclasts
Bone
• Osteoblasts: bone-forming cells
– Secrete hardened matrix of collagen and
calcium phosphate
• Osteocytes: mature bone cells
– Nourished by nearby capillaries
– Connected to other osteocytes by cell
extensions
• Osteoclasts: break down bone
Bone Remodeling
• Changes in bone composition due to
stresses (or lack thereof) placed on the
skeleton
– Bone gets stronger and weaker depending on
relative activity of osteoblasts and osteoclasts
– Influenced by hormones
How Does the Body Move?
• The skeleton allows movement by
providing a moveable framework
• Muscles move the skeleton by action of
antagonistic muscle pairs
– One muscle flexes, while the other is
passively extended
Flexible Joints
• In vertebrates, muscles move the skeleton
around flexible joints
• Joints are the points where two bones meet
• Joint anatomy
– Tendon: connective tissue that attaches muscle to
bone
– Ligament: connective tissue that attaches bone to
bone
– Origin: where muscle is attached to immovable bone
on one side of joint
– Insertion: where muscle is attached to movable
bone on other side of joint
Vertebrate Joint Examples
• Hinge joint
– Allows movement in one plane only
• The knee, an example of a hinge joint
– When the flexor muscle (biceps femoris)
contracts, the joint bends
– When the extensor muscle (quadriceps
femoris) contracts, it straightens the joint
– Alternate contractions of flexor and extensor
muscles cause the lower leg bones to swing
back and forth at the knee joint
Vertebrate Joint Examples
• Ball-and-socket joint
– Allows movement in many planes
– Example: the hip joint