ch 11.5- muscular system Flashcards
smooth muscle traits
present in aitways, organs, blood vessels
involuntary
1 nuc per cell
not striated
cardiac muscle
in heart
involuntary
1 nuc per cell
striated
Cardiac muscle contains intercalated discs, made of
desmosomes (hold cells together) and gap junctions
(channels for rapid ion exchange) that enable
coordinated contractions of the heart.
skeletal muscle
present around bone
voluntary
many nuc per cell
striated
striations in muscle meaning
Striated means the muscle contains sarcomeres.
Smooth muscle therefore lacks sarcomeres, whereas
cardiac and skeletal muscle contain them.
skeletal muscle bundles
Muscle → muscle fascicles → muscle fibers (muscle
cells) → myofibrils (composed of contractile protein
filaments)
sarcolemma
The sarcolemma is the muscle fiber’s cellular
membrane, and it protects each muscle fiber.
contains T-tubules, invaginations
that propagate action potentials throughout the
muscle, ensuring coordinated contraction.
how do muscles work
All muscles always contract (pull) across a joint to
move body parts, they never push.
myofibrils
Myofibrils are made of repeating units of sarcomeres,
the functional unit of skeletal muscle that shortens to
cause muscle contraction.
sarcomeres
Sarcomeres are made of
thin actin filaments and thick myosin filaments.
These filaments slide past each other to shorten
sarcomeres through the sliding filament model of
muscle contraction.
stimulation of muscle contraction steps
- Action potential propagation reaches the motor
neuron axon terminal, causing voltage-gated
calcium channels to open, which allows Ca2+ ions
to enter the neuron. - The influx of Ca2+ ions causes synaptic vesicles
containing acetylcholine to fuse with the
presynaptic membrane, releasing acetylcholine
into the neuromuscular junction. - Acetylcholine binds to and opens ligand-gated
sodium channels on the postsynaptic muscle
fiber, causing sodium to enter the postsynaptic
cell. This creates graded potentials on the
muscle fibers. - The graded potentials open voltage-gated
sodium channels, further depolarizing the
postsynaptic cell membrane. - If the membrane reaches threshold potential, an
action potential is triggered that spreads along the
muscle fiber membrane, initiating a process that
leads to muscle contraction.
sarcoplasmic reticulum
is the endoplasmic
reticulum of muscle fibers that releases stored
calcium ions into the muscle cell through
voltage-gated calcium channels when triggered
by the depolarization of the muscle cell
The calcium ions then bind to troponin, which
removes tropomyosin from the
myosin-binding-sites on actin, allowing myosin to
interact with actin and cause sarcomere shortening,
via sliding filaments.
cross bridge cycling
- Initiation: Calcium ions expose the
myosin-binding-sites on actin. - A cocked back, high energy myosin head
(containing ADP) binds to actin, forming a cross
bridge. - The myosin head contracts (known as a power
stroke) and releases ADP, bringing the myosin
head back to a low energy state. The power
stroke causes the sarcomere to shorten. - A new ATP molecule binds to myosin, causing
detachment of the myosin head from the actin
filament. - The myosin head hydrolyzes the ATP into ADP
and re-enters a cocked back, high energy state.
If calcium is still present, the cross-bridge cycle
continues. - Termination: Motor neuron signaling ends. The
sarcoplasmic reticulum pumps calcium back into
itself, and troponin brings tropomyosin back to
cover myosin-binding-sites on actin.
z lines
the ends of the sarcomeres. Thin
actin filaments branch from the Z lines towards the
middle of the sarcomere.
M lines
the midpoints of the sarcomeres.
Thick myosin filaments branch from the M lines
towards the ends of the sarcomere.
I band
the area in the sarcomere where only
actin filaments are present.
