Lec 3 Excitation and Contraction of Skeletal and Smooth Muscle

Comprehensive flashcards covering the excitation and contraction of skeletal and smooth muscles based on the lecture notes.

What are the five primary functions of muscle tissue identified in the lecture?

Produce body movements, stabilize body position, regulate organ volume, move fluids and solid food and wastes, and produce heat.

What property allows muscle tissue to respond to certain stimuli by producing electrical signals?

Electrical excitability

What is the muscle property known as contractility?

The ability of muscle tissue to contract forcefully when stimulated by an action potential.

Which muscle property refers to the ability to stretch without being damaged?

Extensibility

What does the property of elasticity describe in muscle tissue?

The ability of muscle tissue to return to its original length and shape after contraction or extension.

What is a tendon?

A cord of dense regular connective tissue that attaches muscle to bone.

What term describes a broad, flat sheet of connective tissue that attaches muscle to muscle or muscle to bone?

Aponeurosis

How does the epimysium relate to muscle structure?

It is the outermost layer of connective tissue (fascia) that covers the entire muscle.

What is the perimysium?

A layer of connective tissue that surrounds a fasciculus (bundle of muscle fibers).

What is the endomysium?

A thin layer of connective tissue that surrounds each individual muscle fiber (cell).

What is a fasciculus (fascicle)?

A discrete bundle of muscle cells segregated from the rest of the muscle by a connective tissue sheath.

What term is used for an elongated multinucleate cell with a striated appearance in muscle tissue?

Muscle fiber

What is the sarcolemma?

The cell membrane of a skeletal muscle fiber.

What is the sarcoplasm?

The cytoplasm of a muscle fiber.

What are myofibrils?

Threads of contractile proteins that pack the muscle fiber and have a banded appearance.

Define a sarcomere.

The basic functional unit of a myofibril, extending from one Z disk to the next.

What are the boundaries of a sarcomere called?

Z disks (or Z lines).

What characterizes the A band of a sarcomere?

The dark, middle part of the sarcomere that extends the entire length of the thick filaments.

What is the I band?

The lighter area of the sarcomere that contains only thin filaments.

What is the H zone?

The narrow center of each A band that contains only thick filaments.

Define the M line.

A region in the center of the H zone that contains proteins that hold thick filaments together at the center of the sarcomere.

Which proteins are classified as contractile proteins?

Actin and Myosin

Which proteins are classified as regulatory proteins in muscle fibers?

Troponin and Tropomyosin

What are the four structural proteins mentioned that help align and provide elasticity to myofibrils?

• Titin

• Dystrophin

• Myomesin

• Nebulin

What is the role of Titin?

It is a structural protein that connects the Z disk to the M line, helping stabilize the position of thick filaments.

What is the sarcoplasmic reticulum (SR)?

A network of fluid-filled membrane-enclosed tubules that stores $Ca^{2+}$ ions for muscle contraction.

What is the function of Transverse (T) tubules?

They are tiny invaginations of the sarcolemma that allow muscle action potentials to travel deep into the muscle fiber.

What constitutes a triad in a skeletal muscle fiber?

• A transverse tubule

• two terminal cisternae of the SR on either side

What is the Neuromuscular Junction (NMJ)?

The synapse between a somatic motor neuron and a skeletal muscle fiber.

Which neurotransmitter is released at the NMJ to trigger muscle contraction?

Acetylcholine (ACh)

Where are acetylcholine molecules stored within the motor neuron axon?

Synaptic vesicles

What is the synaptic cleft?

The space between:

• the axon terminal of a motor neuron

• the motor end plate of a muscle fiber.

What is the motor end plate?

The specialized region of the sarcolemma opposite the synaptic end bulbs that contains ACh receptors.

What enzyme decomposes acetylcholine in the synaptic cleft to end a muscle impulse?

Acetylcholinesterase

What initial change in the sarcolemma occurs when ACh binds to its receptors?

The permeability of the sarcolemma changes and $Na^+$ enters the cell.

According to the sliding filament theory, what happens to the filaments during contraction?

Thick and thin filaments slide past one another, moving the actin filaments toward the center of the sarcomere.

What ion must bind to troponin to initiate the movement of tropomyosin?

$$Ca^{2+}$$

What is the role of tropomyosin in a relaxed muscle?

It covers the active binding sites on actin, preventing myosin heads from attaching.

What event causes the 'power stroke' during the cross-bridge cycle?

• The release of $ADP$

• Phosphate ($P_i$) from the myosin head

What is required to cause the myosin head to uncouple from the actin filament?

The binding of a new $ATP$ molecule to the myosin head.

What process provides the power to 'cock' the myosin cross-bridges?

The splitting (hydrolysis) of $ATP$ into $ADP$ and $P_i$.

Where is the primary source of calcium for skeletal muscle contraction located?

Sarcoplasmic reticulum

What is the characteristic shape of smooth muscle fibers?

Small, spindle-shaped cells.

How many nuclei are typically found in a single smooth muscle fiber?

One centrally located nucleus.

How do smooth muscle fibers compare in size to skeletal muscle fibers?

Skeletal muscle fibers are about 20 times as wide and thousands of times as long as smooth muscle fibers.

What structural feature common in skeletal muscle is notably absent in smooth muscle?

Transverse (T) tubules

Why is smooth muscle called 'smooth'?

Because it lacks visible striations.

What is the ratio of thick to thin filaments in smooth muscle compared to skeletal muscle?

$1:16$ in smooth muscle vs. $1:2$ in skeletal muscle.

What regulatory protein is absent in smooth muscle that is present in skeletal muscle?

Troponin

What are dense bodies in smooth muscle?

Dark-staining structures that serve as attachment points for thin and intermediate filaments, acting as counterparts to Z lines.

What is the function of intermediate filaments in smooth muscle?

They form a cablelike intracellular cytoskeleton that harnesses the pull generated by myofilament sliding.

What are the two layers of smooth muscle sheets typically found in visceral organs?

The longitudinal layer and the circular layer.

Define peristalsis.

The cyclic contraction and relaxation of opposing muscle layers that constricts and dilates an organ's lumen to move substances.

What are varicosities in the context of smooth muscle innervation?

Bulbous endings of autonomic nerve fibers that release neurotransmitters into diffuse junctions.

What structures allow smooth muscle cells to exhibit synchronized contractions by providing electrical coupling?

Gap junctions

What are pacemaker cells in smooth muscle?

Self-excitatory cells that can depolarize spontaneously and set the contractile pace for the muscle sheet.

What are the two sources of calcium for smooth muscle contraction?

The sarcoplasmic reticulum and extracellular calcium entering through channels.

What is the primary calcium-binding protein in smooth muscle according to the comparison table?

Calmodulin

How does the contraction duration of smooth muscle compare to skeletal muscle?

Smooth muscle remains contracted for $1-3\,\text{sec}$, which is approximately 30 times longer than a single skeletal muscle contraction.

How does the energy requirement of smooth muscle compare to skeletal muscle for maintaining tension?

Smooth muscle can maintain the same tension at less than $1\%$ of the energy cost.

What is the ATPase activity level of myosin in smooth muscle relative to skeletal muscle?

It is one-tenth that in skeletal muscle.

What is smooth muscle tone?

A state of moderate, sustained contraction maintained by smooth muscle without fatiguing.

What metabolic pathway produces most $ATP$ in smooth muscle due to its low energy requirements?

Anaerobic pathways

How does the effect of acetylcholine differ between skeletal and smooth muscle?

It always excites skeletal muscle, but can excite or inhibit smooth muscle depending on the receptors present.

What is the effect of acetylcholine on the smooth muscle of the bronchioles?

It causes the muscle to contract strongly, narrowing the bronchioles.

What is the effect of norepinephrine (NE) on the smooth muscle of the bronchioles?

It causes the smooth muscle to relax, dilating the air passageways.

What is the effect of norepinephrine (NE) on the smooth muscle in most blood vessel walls?

It causes the smooth muscle cells to contract and constrict the vessel.

List three chemical factors besides hormones that can promote smooth muscle contraction or relaxation.

• Lack of oxygen

• Excess carbon dioxide

• Low pH

What hormone is noted for stimulating increased contractile activity in stomach smooth muscle?

Gastrin

What is the stress-relaxation response?

A response where smooth muscle tension returns to its original level a few minutes after being stretched, allowing organs to expand.

Why is the stress-relaxation response important for the stomach and intestines?

It allows them to store contents temporarily without rushing foodstuffs through too quickly for digestion and absorption.

What system regulates skeletal muscle contraction?

The somatic nervous system

What system regulates smooth muscle contraction?

The autonomic nervous system

Which muscle type is classified as voluntary?

Skeletal muscle

Which muscle type is classified as involuntary?

Smooth muscle

What happens to the sarcoplasmic reticulum during the relaxation of a skeletal muscle fiber?

The SR actively pumps $Ca^{2+}$ back into its storage cisternae.

What structural protein is specifically associated with thin filaments in both skeletal and smooth muscle?

Tropomyosin

What is the result of the longitudinal layer of smooth muscle contracting?

The organ shortens.

What happens to the organ's lumen when the circular layer of smooth muscle contracts?

The lumen constricts (narrows).

In skeletal muscle, where does the action potential travel after leaving the surface sarcolemma?

Into the T tubules.

True or False: Smooth muscle has the same highly structured neuromuscular junctions as skeletal muscle.

False (It has diffuse junctions with varicosities).