HUMAN PHYSIOLOGY
MUSCLES
A muscle converts chemical energy into mechanical energy. The human body has 3
types of muscle:
- Skeletal muscle (attached to bones of the
skeleton, enabling these muscles to
control body movement)
- Cardiac muscle (found only in the heart
and moves blood through the circulatory
system)
- Smooth muscle (muscle of internal organs
and tubes, such as the stomach, urinary
bladder, and blood vessels. Its primary
function is to influence the movement of
material into, out of, and within the body)
Skeletal and cardiac muscles are classified as striated muscles.
Most skeletal muscles are attached to skeleton bones by tendons (made of collagen).
They control body movement by pulling bones, since they can never push a bone.
The origin of a muscle is the end of the muscle that is attached
closest to the trunk or to the more stationary bone. The insertion
of a muscle is the more distal or mobile attachment.
Most joints in the body have both flexor and extensor muscles -
antagonistic muscle groups:
- Flexor (the connected bones are brought closer together
when the muscle contracts)
- Extensor (the connected bones move away from each
other when the muscle contracts)
Skeletal muscle is a collection of muscle fibers. Each skeletal
muscle fiber is a long, cylindrical cell with up to several
hundred nuclei near the surface of the fiber. The
connective tissue contains satellite cells. These are
stem cells that become active and differentiate into
muscle when needed for muscle growth and repair.
Each skeletal muscle fiber is sheathed in connective
tissue, with groups of adjacent muscle fibers bundled
together into units called fascicles.
Sarcolemma: cell membrane of muscle fibers
Sarcoplasm: cytoplasm of muscle fibers
Sarcoplasmic reticulum: a form of modified endoplasmic reticulum that wraps around
each myofibril
1
,The sarcoplasmic reticulum concentrates and stores Ca2+ with the help of a Ca2+-
ATPase in the SR membrane. Calcium release from the SR creates calcium signals
that play a key role in contraction in all types of muscle.
The SR consists of longitudinal tubules with enlarged end
regions called the terminal cisternae. The terminal cisternae
are adjacent to a branching network of transverse tubules,
also known as t-tubules. The membranes of t-tubules are a
continuation of sarcolemma (muscle fiber membrane). T-
tubules are filled with extracellular fluid.
T-tubules allow action potentials to move rapidly from the
cell surface into the interior of the fiber so that they reach
the terminal cisternae nearly simultaneously. Without t-
tubules, the action potential would reach the center of the fiber only by conduction of
the action potential through the cytosol, a slower and less direct process that would
delay the response time of the muscle fiber.
Triad: one t-tubule and its two adjacent terminal cisternae
Muscle fibers contain myofibrils, organized bundles of contractile and elastic proteins
that carry out the work of contraction. These contractile units are called sarcomeres.
Myofibril proteins include:
- Myosin (thick filament)
- Actin (thin filament)
- Tropomyosin
- Troponin
- Titin
- Nebulin
Myosin is a motor protein with
the ability to create movement.
Myosin has 2 heads (motor
domain) that uses energy from ATP to create movement and bind to actin filaments.
Because myosin acts as an enzyme to hydrolyze ATP, the motor domain is
considered a myosin ATPase.
Crossbridges form when the myosin heads bind to actin. Each G-actin molecule has
a single myosin-binding site, and each
myosin head has one actin-binding site.
Crossbridges have two states: low-force
(relaxed muscles) and high-force (contracting
muscles).
Myosin and actin both have helix structures.
The arrangement of thick and thin filaments
in a myofibril creates a repeating pattern of
alternating light and dark bands. One repeat
of the pattern forms a sarcomere, the
contractile unit of the myofibril.
2
,Each sarcomere has the following elements:
1. Z disks. One sarcomere is composed of two Z
disks and the filaments found between them. Z disks
are zigzag protein structures that serve as the
attachment site for thin filaments.
2. I bands. These are the lightest color bands of the
sarcomere and represent a region occupied only by
thin filaments. A Z disk runs through the middle of
every I band, so each half of an I band belongs to a
different sarcomere.
3. A band. This is the darkest of the sarcomere’s
bands and encompasses the entire length of a thick
filament. At the outer edges of the A band, the thick
and thin filaments overlap. The center of the A band
is occupied by thick filaments only.
4. H zone. This central region of the A band is lighter than the outer
edges of the A band because the H zone is occupied by thick filaments
only.
5. M line. This band represents proteins that form the attachment site for
thick filaments, equivalent to the Z disk for the thin filaments. Each M line
divides an A band in half.
Titin and nebulin are giant accessory proteins. Titin spans the
distance from one Z disk to the neighboring M line. Nebulin,
lying along the thin filaments, attaches to a Z disk but does
not extend to the M line. Titin provides elasticity meaning that
it returns stretched muscles to their resting length.
Muscle contraction is the result of thick and thin filaments
sliding along each other. The Z disks of the sarcomere move
closer together as the sarcomere shortens. The I band and H
zone – regions where actin and myosin don’t
overlap in resting muscle – almost disappears.
However the length of the A band remains the
same.
3
, In resting skeletal muscle, tropomyosin wraps around actin filaments and partially
covers actin’s binding sites. Weak, low-
force actin-myosin binding can still take
place, but myosin is blocked from
completing its power stroke. Before
contraction can occur, tropomyosin must
be shifted so the binding site is active
again. Troponin (TN) is a calcium-
binding complex of three proteins.
Troponin controls the positioning of
tropomyosin.
1. An ATP molecule binds to the
myosin head. ATP-binding decreases the
actin-binding affinity of myosin, and myosin
releases from actin.
2. ATP hydrolysis provides energy for the
myosin head to rotate and reattach to actin.
Both ADP and Pi remain bound to myosin.
In this cocked position, myosin binds to a
new actin that is 1–3 molecules away from
where it started. The newly formed actin-
myosin crossbridge is weak and low-force
because tropomyosin is partially blocking
actin’s binding site. However, in this rotated
position myosin has stored potential energy.
Most resting muscle fibers are in this state,
cocked and prepared to contract, and just
waiting for a calcium signal.
3. The power stroke begins after Ca2+ binds to troponin to uncover the rest of the
myosin-binding site. The crossbridges transform into strong, high-force bonds
as myosin releases Pi. Release of Pi allows the myosin head to swivel. The
heads swing toward the M line, sliding the attached actin filament along with
them.
4. At the end of the power stroke, myosin releases ADP. With ADP gone, the
myosin head is again tightly bound to actin in the rigor state. The cycle is
ready to begin once more as a new ATP binds to myosin.
1 motor unit = 1 motor neuron (activate muscle
cells with action potentials)
Big movements: few motor units
Delicate movements: many motor units
4
MUSCLES
A muscle converts chemical energy into mechanical energy. The human body has 3
types of muscle:
- Skeletal muscle (attached to bones of the
skeleton, enabling these muscles to
control body movement)
- Cardiac muscle (found only in the heart
and moves blood through the circulatory
system)
- Smooth muscle (muscle of internal organs
and tubes, such as the stomach, urinary
bladder, and blood vessels. Its primary
function is to influence the movement of
material into, out of, and within the body)
Skeletal and cardiac muscles are classified as striated muscles.
Most skeletal muscles are attached to skeleton bones by tendons (made of collagen).
They control body movement by pulling bones, since they can never push a bone.
The origin of a muscle is the end of the muscle that is attached
closest to the trunk or to the more stationary bone. The insertion
of a muscle is the more distal or mobile attachment.
Most joints in the body have both flexor and extensor muscles -
antagonistic muscle groups:
- Flexor (the connected bones are brought closer together
when the muscle contracts)
- Extensor (the connected bones move away from each
other when the muscle contracts)
Skeletal muscle is a collection of muscle fibers. Each skeletal
muscle fiber is a long, cylindrical cell with up to several
hundred nuclei near the surface of the fiber. The
connective tissue contains satellite cells. These are
stem cells that become active and differentiate into
muscle when needed for muscle growth and repair.
Each skeletal muscle fiber is sheathed in connective
tissue, with groups of adjacent muscle fibers bundled
together into units called fascicles.
Sarcolemma: cell membrane of muscle fibers
Sarcoplasm: cytoplasm of muscle fibers
Sarcoplasmic reticulum: a form of modified endoplasmic reticulum that wraps around
each myofibril
1
,The sarcoplasmic reticulum concentrates and stores Ca2+ with the help of a Ca2+-
ATPase in the SR membrane. Calcium release from the SR creates calcium signals
that play a key role in contraction in all types of muscle.
The SR consists of longitudinal tubules with enlarged end
regions called the terminal cisternae. The terminal cisternae
are adjacent to a branching network of transverse tubules,
also known as t-tubules. The membranes of t-tubules are a
continuation of sarcolemma (muscle fiber membrane). T-
tubules are filled with extracellular fluid.
T-tubules allow action potentials to move rapidly from the
cell surface into the interior of the fiber so that they reach
the terminal cisternae nearly simultaneously. Without t-
tubules, the action potential would reach the center of the fiber only by conduction of
the action potential through the cytosol, a slower and less direct process that would
delay the response time of the muscle fiber.
Triad: one t-tubule and its two adjacent terminal cisternae
Muscle fibers contain myofibrils, organized bundles of contractile and elastic proteins
that carry out the work of contraction. These contractile units are called sarcomeres.
Myofibril proteins include:
- Myosin (thick filament)
- Actin (thin filament)
- Tropomyosin
- Troponin
- Titin
- Nebulin
Myosin is a motor protein with
the ability to create movement.
Myosin has 2 heads (motor
domain) that uses energy from ATP to create movement and bind to actin filaments.
Because myosin acts as an enzyme to hydrolyze ATP, the motor domain is
considered a myosin ATPase.
Crossbridges form when the myosin heads bind to actin. Each G-actin molecule has
a single myosin-binding site, and each
myosin head has one actin-binding site.
Crossbridges have two states: low-force
(relaxed muscles) and high-force (contracting
muscles).
Myosin and actin both have helix structures.
The arrangement of thick and thin filaments
in a myofibril creates a repeating pattern of
alternating light and dark bands. One repeat
of the pattern forms a sarcomere, the
contractile unit of the myofibril.
2
,Each sarcomere has the following elements:
1. Z disks. One sarcomere is composed of two Z
disks and the filaments found between them. Z disks
are zigzag protein structures that serve as the
attachment site for thin filaments.
2. I bands. These are the lightest color bands of the
sarcomere and represent a region occupied only by
thin filaments. A Z disk runs through the middle of
every I band, so each half of an I band belongs to a
different sarcomere.
3. A band. This is the darkest of the sarcomere’s
bands and encompasses the entire length of a thick
filament. At the outer edges of the A band, the thick
and thin filaments overlap. The center of the A band
is occupied by thick filaments only.
4. H zone. This central region of the A band is lighter than the outer
edges of the A band because the H zone is occupied by thick filaments
only.
5. M line. This band represents proteins that form the attachment site for
thick filaments, equivalent to the Z disk for the thin filaments. Each M line
divides an A band in half.
Titin and nebulin are giant accessory proteins. Titin spans the
distance from one Z disk to the neighboring M line. Nebulin,
lying along the thin filaments, attaches to a Z disk but does
not extend to the M line. Titin provides elasticity meaning that
it returns stretched muscles to their resting length.
Muscle contraction is the result of thick and thin filaments
sliding along each other. The Z disks of the sarcomere move
closer together as the sarcomere shortens. The I band and H
zone – regions where actin and myosin don’t
overlap in resting muscle – almost disappears.
However the length of the A band remains the
same.
3
, In resting skeletal muscle, tropomyosin wraps around actin filaments and partially
covers actin’s binding sites. Weak, low-
force actin-myosin binding can still take
place, but myosin is blocked from
completing its power stroke. Before
contraction can occur, tropomyosin must
be shifted so the binding site is active
again. Troponin (TN) is a calcium-
binding complex of three proteins.
Troponin controls the positioning of
tropomyosin.
1. An ATP molecule binds to the
myosin head. ATP-binding decreases the
actin-binding affinity of myosin, and myosin
releases from actin.
2. ATP hydrolysis provides energy for the
myosin head to rotate and reattach to actin.
Both ADP and Pi remain bound to myosin.
In this cocked position, myosin binds to a
new actin that is 1–3 molecules away from
where it started. The newly formed actin-
myosin crossbridge is weak and low-force
because tropomyosin is partially blocking
actin’s binding site. However, in this rotated
position myosin has stored potential energy.
Most resting muscle fibers are in this state,
cocked and prepared to contract, and just
waiting for a calcium signal.
3. The power stroke begins after Ca2+ binds to troponin to uncover the rest of the
myosin-binding site. The crossbridges transform into strong, high-force bonds
as myosin releases Pi. Release of Pi allows the myosin head to swivel. The
heads swing toward the M line, sliding the attached actin filament along with
them.
4. At the end of the power stroke, myosin releases ADP. With ADP gone, the
myosin head is again tightly bound to actin in the rigor state. The cycle is
ready to begin once more as a new ATP binds to myosin.
1 motor unit = 1 motor neuron (activate muscle
cells with action potentials)
Big movements: few motor units
Delicate movements: many motor units
4
