Action and Support:
The Muscles and
Skeleton
Introduction to Life Processes - SCI 102 1
Lesson 15
Muscle Contraction
• Muscles: tissues that produce movement by
contracting
• Vertebrates have three types of muscles
(muscle tissue)
Skeletal
Cardiac
Smooth
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Muscle Contraction
• Vertebrate skeletal muscles have highly organized,
repeating structures
Skeletal muscles move the skeleton and appear striated
This type of muscle tissue consists of a series of nested,
repeating parts
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Muscle Contraction
• Parts of skeletal muscle
Tendons: muscles are encased in connective tissue sheaths
Muscle fibers are individual cells found within the sheath
Muscle fibers contain myofibrils encased in the sarcoplasmic
reticulum
Each myofibril consists of repeating elements called
sarcomeres, which contain the protein filaments that make up
muscles
• The filaments of myofibrils are made of myosin and actin, each of
which interacts with the other to contract the muscle fiber
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Muscle Contraction
• Muscle fibers contract through interactions between thin and
thick filaments
• The sliding-filament mechanism describes how thick and thin
filaments slide over each other, shortening muscle length
Each spherical actin molecule has a myosin binding site
• Myosin binding sites are blocked by tropomyosin
During contraction, tropomyosin moves aside and exposes the myosin
binding sites
• The myosin heads bind to actin
• The myosin heads pull the thin filaments toward the sarcomere center,
sliding over the thick filaments
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Muscle Contraction
• Muscle contraction uses the energy of ATP
Energy from ATP is used to extend the head on the myosin filament
so it can flex the thin filament toward the center of the sarcomere
A skeletal muscle’s reserves of ATP are used up quickly
Muscle cells carry creatine phosphate
• These molecules in muscle tissue quickly resynthesize ATP from ADP, but
only last a few seconds
Glycolysis can provide some ATP for short activities
Cellular respiration provides ATP for prolonged activity
• Glucose and fatty acids are used to form ATP
• An oxygen source is required for efficient ATP formation
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Muscle Contraction
• The nervous system controls the contraction of
skeletal muscles
• Motor neurons excite skeletal muscle fibers at
neuromuscular junctions
Motor neurons release neurotransmitters that diffuse
across the synaptic cleft to receptors on the muscle fiber,
triggering an action potential
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Muscle Contraction
The action potential travels along the muscle fiber cell membrane and
passes down T tubules, causing the sarcoplasmic reticulum to release
calcium ions into the myofibril cytoplasm
• Calcium ions bind to troponin accessory proteins
• Calcium-troponin binding causes tropomyosin to shift position, exposing
myosin binding sites on actin
• Myosin heads bind to actin
• The actin filaments slide past the myosin filaments, shortening the muscle fiber
When the action potential is over, calcium is actively transported back
into the sarcoplasmic reticulum
• Calcium is removed from the accessory proteins that block the actin binding
sites, and the muscle relaxes
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Muscle Contraction
• The nervous system controls the contraction of skeletal
muscles
The size of motor units and the frequency of action potentials
determine the force of muscle contraction
• A motor unit is one branched motor neuron synapsing with many
muscle fibers
The size of a motor unit regulates the type of movement it can
generate
• The nervous system controls the strength of muscle contractions by
varying the number of muscle fibers stimulated and the frequency
of action potentials to each fiber
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Muscle Contraction
• Fast-twitch and slow-twitch muscle fibers are
specialized for different types of activity
Muscle fibers come in two basic types, slow-twitch and
fast-twitch
• Slow-twitch fibers contract with less power than fast-twitch
• Fast-twitch fibers contract powerfully but fatigue more quickly
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Muscle Contraction: Leg Muscles
• Leg muscle contractions
The strength of leg muscle contractions when lifting a
heavy object is determined by:
• The frequency of action potentials and number of muscle fibers
stimulated
• The largest motor units in the human muscles are
found in the leg muscles
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Muscle Contraction: Leg Muscles
• Leg muscle fibers
Fast-twitch muscle fibers
• Contain fewer mitochondria than slow-twitch fibers
• Are found in the legs of sprinters
Slow-twitch fibers
• Have more mitochondria
• Contract with less power than fast-twitch fibers but can keep
contracting for a long period of time
• Are found in the legs of marathon runners
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Cardiac, Smooth, and Skeletal Muscle
Property Smooth Cardiac Skeletal
Muscle appearance Nonstriated Striated Striated
Cell shape Tapered at both ends Branched Tapered at both ends
Number of nuclei One per cell One per cell Many per cell
Speed of contraction Slow Intermediate Slow to rapid
Contraction stimuliSpontaneous, stretch,
nervous system, hormonesSpontaneous Nervous system
Function
Controls movement of
substances through hollow
organs and tubes
Pumps blood Moves the skeleton
Under voluntary control? No No Yes
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The three muscle types differ from each other structurally and functionally
Type of Muscle
Cardiac, Smooth, and Skeletal Muscle
• Cardiac muscle powers the heart
The structure and contraction mechanism is similar to that of
skeletal muscle
• One main difference is that cardiac muscle fibers are connected to
each other by intercalated discs
Unlike skeletal muscle, cardiac muscle can generate its own
contractions
• The heart’s pacemaker does this
• Gap junctions allow for the synchronization of contractions by
allowing action potentials to travel from muscle cell to muscle cell
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Cardiac, Smooth, and Skeletal Muscle
• Smooth muscle produces slow, involuntary
contractions
Smooth muscle surrounds blood vessels and most hollow
organs
Smooth muscle cells are not striated because the thin and
thick filaments are scattered throughout the cell
Contractions are initiated by stretch, hormones, or the
nervous system and are slow and sustained
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Cardiac, Smooth, and Skeletal Muscle
• When a person builds up muscle strength, the
muscle becomes resistant to fatigue
In a strong muscle, there are often elevated levels of ATP
to give the muscle the extra energy it needs
Example: marathon runner’s leg muscles are resistant to
fatigue
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Cardiac, Smooth, and Skeletal Muscle
• People who work out often (bikers, athletes, trainers for the
Olympics) often require rapid bursts of high-energy muscle
output
Many of these people will spend money on dietary supplements
such as:
• Amino acids, which are used by the body as building blocks to make more
high-protein muscle cells
• Energy increasers, which are used to increase energy or provide more
endurance during workouts
Many supplements contain creatine, which can donate phosphate
ions to regenerate ATP when there is a lack of oxygen
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Muscles and Skeletons Work Together to Provide Movement
• The actions of antagonistic muscles on skeletons
move animal bodies
Three different types of skeletons are seen in the animal
kingdom
Coordinated movement of an animal’s body is produced
by alternating contractions of antagonistic muscles
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Muscles and Skeletons Work Together to Provide Movement
• Worms, cnidarians, and many mollusks have a hydrostatic skeleton made of fluid and muscle Muscles around fluid cause shape changes and locomotion
• The bodies of arthropods are encased by rigid exoskeletons Movement occurs at joints and is based on antagonistic
muscles Arthropods must molt periodically in order to grow
• Endoskeletons are rigid structures found inside echinoderms and chordates Movement occurs at joints and is facilitated by antagonistic
muscles
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Muscles and Skeletons Work Together to Provide Movement
• Endoskeleton
Rigid internal skeleton found in vertebrae (animals with a
backbone and humans), as well as in seas stars and related
animals
• The vertebrate endoskeleton serves multiple functions
It supports the body and protects internal organs
It allows locomotion in many different forms
It participates in sensory function in the middle ear
Bones produce blood cells
Bones store calcium and phosphorus
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Muscles and Skeletons Work Together to Provide Movement
• The vertebrate skeleton consists of two parts The axial skeleton includes the head, vertebral column,
and rib cage
The appendicular skeleton includes the pectoral and
pelvic girdle and the appendages attached to them
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Muscles and Skeletons Work Together to Provide Movement
• The vertebrate skeleton is composed of:
Cartilage
Ligaments
Bone
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Muscles and Skeletons Work Together to Provide Movement
• Cartilage provides flexible support and connections
During embryonic development, most of the skeleton is
composed of cartilage, which is later replaced by bone
Cartilage covers the ends of bones and provides
flexible support to other parts of the body
The living cells of cartilage are called chondrocytes
• They secrete the collagen matrix
• Cartilage lacks blood vessels, and is slow to heal
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Muscles and Skeletons Work Together to Provide Movement
• Cartilage plays many roles in the vertebrae skeleton
Examples:
• The entire skeleton of sharks, and many other fish, is composed
of cartilage
• Cartilage covers the ends of bones at the joints
• Cartilage supports the flexible portions of the body (ear, nose)
• Provides the framework for the larynx, trachea, and bronchi of
the respiratory system
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Muscles and Skeletons Work Together to Provide Movement
• Composition of cartilage
Contains large amounts of glycoproteins
Often has elastic fibers
Lacks blood vessels
• Damage to cartilage tends to heal slowly because of the lack
of blood vessels
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Muscles and Skeletons Work Together to Provide Movement
• Ligaments connect bone to bone in joints
Are mainly made up of wavy collagen fibers
Do not have significant amounts of elastic fibers
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Muscles and Skeletons Work Together to Provide Movement
• Bone provides a strong, rigid framework for the body
Bone consists of a hard outer shell of compact bone that encloses spongy bone in its interior
• Compact bone is arranged in osteons, in which collagen and calcium phosphate surround a central canal containing blood vessels
• Spongy bone is lightweight, rich in blood vessels, and porous
There are three types of bone cells
• Osteoclasts break down bone
• Osteoblasts are bone-forming cells
• Osteocytes are mature bone cells
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Muscles and Skeletons Work Together to Provide Movement
• Bone remodeling allows skeletal repair
Bone remodeling occurs through the coordinated activity
of osteoblasts and osteoclasts
• Bones become thicker and larger as a person grows
Bone remodeling varies with age
The ultimate bone remodeling occurs after a fracture
• Healing begins when a cartilage callus forms at the break site
• The cartilage is replaced by bone
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Muscles and Skeletons Work Together to Provide Movement
• Bone strength is increased when it is given moderate stress
from weight-bearing exercises
• Many people hope that they will one day be able to travel
into space for extended periods of time
This comes with health risks to the bones and muscles
• Under weightless conditions, space travelers will lose bone and muscle
mass because there is no pull of gravity to strengthen the bones and
muscles.
• To maintain bone and muscle mass, they will have to do special, weight-
bearing exercises each day.
If they do not do exercise, their bones and muscles will atrophy (deteriorate),
which could have additional negative side effects on their body physiology.
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Muscles and Skeletons Work Together to Provide Movement
• Antagonistic muscles move joints in the vertebrate
skeleton
Joints are held together by ligaments, which are strong
and flexible
• Tendons hold muscles to bones
The origin of any joint is where muscle is attached to the
immovable bone on one side of a joint
• Movement occurs in antagonistic muscle pairs when one muscle
flexes while the other is passively extended
• The insertion is where muscle is attached to the movable bone on
one side of a joint
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Muscles and Skeletons Work Together to Provide Movement
• Antagonistic muscles move joints in the vertebrate
skeleton
Hinge joints move in two dimensions
• Examples: elbows, knees, fingers
Ball-and-socket joints allow movement in many planes
• The round end of one bone fits into a hollow depression in another
• Allow movement in several directions
• Examples: hips, shoulder
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Muscles and Skeletons Work Together to Provide Movement
• The knee joint
If there would be a ball-and-socket joint in the knee, there
would be decreased joint stability and the person would be
unable to stand properly
The anterior cruciate ligament (ACL) is one of the major knee
joint ligaments in humans, and damage to this ligament is fairly
common in athletes.
• An ACL tear is most often a sports-related injury but can also result
from accidents, falls, and work-related injuries. Most sports-related
ACL tears occur when a person pivots or lands from a jump.
Instability or weakness of the knee joint happens when a person
has a torn ACL
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Muscles and Skeletons Work Together to Provide Movement
Introduction to Life Processes - SCI 102 33
The biceps femoris
bends the leg back
when crouching