Chapter 9: The Muscular System Part 1

What are the 3 types of muscle tissue?

Skeletal, cardiac, and smooth

How do the three types of muscle tissue differ?

Structure, location, function, and activation

What are skeletal and smooth muscle cells called?

Muscle fibers (elongated cells)

What two myofilaments are needed for muscle contraction?

Actin and myosin

What are the sarcolemma and sarcoplasm?

• Sarcolemma: muscle cell plasma membrane

• Sarcoplasm: muscle cell cytoplasm

What do the prefixes myo, mys, and sarco mean?

Muscle

Where is skeletal muscle located, and what is its main function?

Attached to and covering the bony skeleton; responsible for overall body movement (motility).

What are the structural and control characteristics of skeletal muscle?

Striated and voluntary.

How does skeletal muscle contract, fatigue, and function?

Contracts rapidly, tires easily, and produces overall body movement

How much force can skeletal muscle exert?

From a fraction of an ounce to over 70 pounds

Where is cardiac muscle found, and what are its characteristics?

Found only in the heart; striated and involuntary.

What controls cardiac muscle contraction and allows it to respond to bodily needs?

The heart's pacemaker sets a fairly steady contraction rate. Neural controls allow it to respond to changes in bodily needs.

Where is smooth muscle found, and what are its characteristics and function?

Found in walls of hollow visceral organs. Nonstriated and involuntary

What is excitability

The ability to receive and respond to stimuli

What is contractility?

The ability to shorten forcibly

What is extensibility?

The ability to be stretched or extended

What is elasticity?

The ability to recoil and return to its original resting length

What are the 4 important functions of muscle tissue?

• Produce movement

• Maintain posture and body position

• Stabilize joints

• Generate heat

How does muscle tissue produce movement?

Responsible for locomotion and manipulation, including walking, digesting, and pumping blood

How does muscle tissue generate heat?

Muscle contractions generate heat

What are the 3 connective tissue sheaths of muscle?

Endomysium, perimysium, and epimysium

What is the endomysium?

A fine layer of reticular connective tissue surrounding each muscle fiber

What is the perimysium?

Fibrous connective tissue surrounding groups of muscle fibers called fascicles

What is the epimysium?

Dense regular connective tissue surrounding the entire muscle

What supplies each muscle?

One nerve, one artery, and one or more veins

What are the functions of nerves, arteries, and veins in skeletal muscle?

• Nerves: Supply each muscle fiber with a nerve ending that controls contraction

• Arteries: Deliver oxygen and nutrients to contracting fibers

• Veins: Remove waste

Where are most skeletal muscles attached?

They span joints and attach to bone in at least two places

What are the origin and insertion of a muscle?

Origin = immovable bone attachment

Insertion = movable bone attachment

What happens to the insertion when a muscle contracts?

The insertion moves toward the origin

What are the two ways muscles attach to bones?

Directly and indirectly

What is a direct muscle attachment?

The muscle's epimysium fuses to the periosteum of a bone

What is an indirect muscle attachment?

Connective tissue extends beyond the muscle as a tendon or aponeurosis

What are the characteristics of a skeletal muscle fiber?

A long, cylindrical cell with multiple nuclei located just beneath the sarcolemma

What is the size of a skeletal muscle fiber?

10–100 μm in diameter and up to hundreds of centimeters long

What does the sarcoplasm contain?

Numerous glycosomes and myoglobin, an oxygen-binding protein.

What structures are found inside muscle fibers?

Usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules.

What are myofibrils?

Densely packed, rod-like contractile elements that make up most of the muscle fiber's volume

What gives skeletal muscle its striped appearance?

The repeating, aligned arrangement of dark A bands and light I bands within the muscle fiber.

What are A bands and I bands?

A bands are DARK; I bands are LIGHT. They form a repeating pattern within muscle fibers

What is a sarcomere?

The smallest contractile unit of a muscle

Where is a sarcomere located?

The region of a myofibril between two successive Z discs

What is a sarcomere composed of?

Myofilaments made of contractile proteins. They are either thick or thin.

How far do thick filaments extend?

The entire length of the A band

How far do thin filaments extend?

Across the I band and partway into the A band

What is the Z-disc, and what does it do?

A coin-shaped sheet of proteins (connectins) that anchors thin filaments and connects myofibrils to one another.

What is the H zone?

The lighter region where thin filaments do not overlap thick filaments

Why do M lines appear darker?

Because of the presence of the protein desmin

What are thick filaments made of?

Myosin

What are the two main parts of a myosin molecule?

A rod-like tail and two globular heads

What are myosin tails and heads made of?

• Tails: Two interwoven heavy polypeptide chains

• Heads: Two smaller, light polypeptide chains called cross bridges

What protein are thin filaments made of?

Actin.

What are actin molecules made of?

A helical polymer of globular subunits called G actin.

What is the function of G-actin, and which regulatory proteins are bound to actin?

• G actin contains active sites where myosin heads attach during contraction.

• Tropomyosin and troponin are regulatory proteins bound to actin.

What is the sarcoplasmic reticulum (SR)?

Smooth endoplasmic reticulum that surrounds each myofibril and runs mostly longitudinally.

What are terminal cisternae?

Paired structures that form perpendicular cross channels.

What is the function of the SR?

Regulates calcium levels inside the muscle cell

What are T tubules, and where are they located?

Elongated tubes that extend into the muscle cell at each A band–I band junction.

What is a triad?

One T tubule associated with two terminal cisternae.

How do T tubules and the SR help muscle contraction?

They provide tightly linked signals for muscle contraction.

What are T tubules continuous with?

The sarcolemma.

What is the function of T tubules?

Conduct impulses deep into the muscle, signaling Ca²⁺ release from adjacent terminal cisternae.

How do T tubules and the SR work together?

• T tubule proteins = voltage sensors.

• SR foot proteins = receptors that regulate Ca²⁺ release from SR cisternae.

What connects the T tubule and SR membranes?

A double zipper of integral membrane proteins that protrudes into the intermembrane space.

What happens to thin and thick filaments during muscle contraction?

Thin filaments slide past thick filaments, increasing their overlap.

How much do thin and thick filaments overlap in a relaxed muscle?

They overlap only slightly.

What happens when a muscle is stimulated?

Myosin heads bind to actin, and filament sliding begins.

How do myosin heads generate tension during contraction?

They repeatedly bind and detach from actin like a ratchet, pulling thin filaments toward the center of the sarcomere.

What happens as sliding occurs throughout the sarcomeres?

Sarcomeres shorten, causing the muscle to shorten

What 3 things must happen for a skeletal muscle to contract?

1. A nerve ending stimulates the muscle.

2. An action potential travels along the sarcolemma.

3. Intracellular Ca²⁺ levels rise (final trigger).

What is excitation-contraction coupling?

The process that links the electrical signal to muscle contraction.

Which nervous system stimulates skeletal muscles?

The somatic nervous system through motor neurons

What happens to motor neuron axons as they enter muscles?

They branch extensively and travel to muscle fibers

What does each motor neuron axonal branch form?

A neuromuscular junction (NMJ) with a single muscle fiber

What are found in motor neuron axon endings?

Synaptic vesicles containing the neurotransmitter acetylcholine (ACh).

What is the motor end plate?

A specialized region of the sarcolemma containing ACh receptors.

What are the two main parts of the neuromuscular junction?

• Axonal endings: contain synaptic vesicles with acetylcholine (ACh).

• Motor end plate: part of the sarcolemma containing ACh receptors.

What is the synaptic cleft?

The space separating the axonal endings from the muscle fiber.

What happens when a nerve impulse reaches the axon terminal at the NMJ?

Voltage-regulated Ca²⁺ channels open, allowing Ca²⁺ to enter the axon.

What causes axonal vesicles to release acetylcholine (ACh)?

Ca²⁺ causes axonal vesicles to fuse with the axonal membrane, releasing ACh into the synaptic cleft by exocytosis.

What happens after ACh is released into the synaptic cleft?

ACh diffuses across the cleft, binds to receptors on the sarcolemma, and initiates an action potential in the muscle.

What are synaptic vesicles, and what do they contain?

Small membranous sacs in axonal endings that contain the neurotransmitter acetylcholine (ACh)

What is the motor end plate?

A specific part of the sarcolemma containing ACh receptors that helps form the neuromuscular junction

What enzyme breaks down acetylcholine (ACh)?

Acetylcholinesterase

Why is ACh quickly destroyed after binding to its receptors?

To stop continued muscle contraction without additional stimulation

What is an action potential?

A brief electrical event involving membrane depolarization, polarity reversal, and propagation along the membrane

What happens when ACh binds to receptors at the motor end plate?

1. Chemically (ligand)-gated channels open

2. Na⁺ and K⁺ diffuse out

3. The inside of the sarcolemma becomes less negative. This is called depolarization

What is an end plate potential?

A local electrical event at the motor end plate that triggers an action potential.

What happens after an action potential is generated?

• Travels along the sarcolemma

• Travels down the T tubules

• Triggers Ca²⁺ release from the terminal cisternae

What does Ca²⁺ do during muscle contraction?

Ca²⁺ binds to troponin, causing tropomyosin to move away and expose actin's active binding sites for myosin heads.

What happens during myosin cross-bridge cycling?

Myosin cross-bridges repeatedly attach to and detach from actin.

What powers myosin cross-bridge cycling?

ATP hydrolysis.

How does a muscle fiber relax?

Ca²⁺ is actively transported back into the SR, tropomyosin blocks actin's binding sites again, and the muscle fiber relaxes.

What are the steps of the cross-bridge cycle?

1. Myosin attachment

2. Power stroke

3. Detachment

4. Cocking of the myosin head

What happens during cross-bridge formation?

When a myosin head attaches to an actin filament.

What happens during the power stroke?

The myosin head pivots and pulls the actin filament toward the M line.

What causes cross-bridge detachment?

ATP attaches to the myosin head, causing it to detach from actin.

What happens during the cocking of the myosin head?

ATP is hydrolyzed, providing energy to return the myosin head to its high-energy position.

What does muscle contraction refer to?

Activation of myosin's cross-bridges (force-generating sites).

When does muscle shortening occur?

When the tension generated by cross bridges exceeds the forces opposing shortening.

When does muscle contraction end?

When cross bridges become inactive and the tension they generate declines, allowing relaxation.