Chapter 10 A&P

What are the three types of muscle tissue?

Skeletal, cardiac, and smooth muscle.

What is the basic function of all muscle tissue?

Generating force, also known as muscle tension.

List four functions of muscle tissue besides generating movement.

Maintain posture, stabilize joints, generate heat, and regulate flow through hollow organs.

What is skeletal muscle tissue composed of?

Long multinucleated cells arranged parallel to one another called muscle fibers.

What controls skeletal muscle contraction?

Voluntary (conscious) control.

What are skeletal muscles typically attached to?

Skeleton by connective tissue.

Where is cardiac muscle found?

Only in the heart.

Describe the structure of cardiac muscle cells.

Short, branched, with one to two nuclei, joined by intercalated discs containing gap junctions and desmosomes.

Is cardiac muscle contraction voluntary or involuntary?

Involuntary.

What type of junctions are found in cardiac muscle intercalated discs?

Gap junctions and desmosomes (modified tight junctions).

What are smooth muscle cells shaped like?

Long and flat with pointed ends (spindle-shaped) and a single centrally located nucleus.

Do smooth muscle cells have striations?

No, they are non-striated.

Where are smooth muscle cells found?

In walls of hollow organs, eyes, skin, and some glandular ducts.

Are smooth muscle contractions voluntary or involuntary?

Involuntary.

What connects many smooth muscle cells to one another?

Gap junctions, allowing synchronized contraction.

What are the five key properties of muscle cells?

Contractility, excitability, conductivity, extensibility, and elasticity.

What is contractility?

The ability of muscle proteins to draw closer together and generate force.

What is excitability?

The ability to respond to a stimulus such as chemicals or electrical signals.

What is conductivity?

The ability to conduct electrical changes across the plasma membrane.

Define extensibility.

The ability of a muscle cell to stretch up to three times its resting length without rupturing.

Define elasticity.

The ability of muscle tissue to return to its original length after being stretched.

What is the sarcoplasm?

The cytoplasm of a muscle cell.

What is the sarcolemma?

The plasma membrane of a muscle cell.

What is the sarcoplasmic reticulum (SR)?

A modified endoplasmic reticulum forming a web-like network around myofibrils that stores calcium ions.

What are myofibrils?

Bundles of specialized proteins within muscle cells that allow for contraction.

How do smooth muscle myofibrils differ from those in skeletal or cardiac muscle?

They are arranged differently and less organized.

What surrounds individual muscle fibers?

Endomysium (a type of extracellular matrix).

What is the typical length and thickness of a skeletal muscle fiber?

Up to $30$ cm long and about $100$ µm thick.

How are skeletal muscle fibers formed?

By the fusion of many embryonic myoblasts, giving each fiber multiple nuclei.

What is the most abundant organelle in skeletal muscle fibers?

Myofibrils.

What does the sarcoplasmic reticulum do in skeletal muscle fibers?

Surrounds myofibrils and stores/releases calcium ions for contraction.

What are transverse tubules (T-tubules)?

Deep inward extensions of the sarcolemma that surround each myofibril and conduct electrical impulses.

What fluid fills T-tubules?

Extracellular fluid.

What are terminal cisternae?

Enlarged sections of the SR that flank each T-tubule.

What structure is formed by two terminal cisternae and one T-tubule?

A triad.

What are myofilaments?

Protein filaments that make up myofibrils; responsible for muscle contraction.

Name the three types of myofilaments.

Thick filaments, thin filaments, and elastic filaments.

What are thick filaments made of?

Myosin, a contractile protein.

Describe the structure of a myosin molecule.

Two intertwining tails with globular heads connected by a hinge-like neck.

What do myosin heads bind to during contraction?

Active sites on actin.

What proteins make up thin filaments?

Actin, tropomyosin, and troponin.

What is the active site on actin used for?

Binding to myosin heads during contraction.

What is tropomyosin?

A long, rope-like regulatory protein that covers actin’s active sites.

What is troponin?

A small globular protein that binds to tropomyosin, actin, and calcium ions to regulate contraction.

What are elastic filaments composed of?

The protein titin.

What is the function of titin?

Stabilizes myofibril structure and resists excessive stretching.

What is a sarcomere?

The functional unit of muscle contraction, extending from one Z-disc to the next.

What gives skeletal muscle its striated appearance?

Alternating light (I bands) and dark (A bands) regions.

What does the I band contain?

Only thin filaments.

What is the Z disc?

A structural protein that anchors thin and elastic filaments and links myofibrils together.

What is the A band?

The dark region containing both thick and thin filaments (zone of overlap).

What is the H zone?

The middle of the A band where only thick filaments are present.

What is the M line?

The center of the A band that holds thick filaments in place.

What combines to form a fascicle?

Multiple muscle fibers surrounded by endomysium.

What surrounds each fascicle?

Perimysium.

What surrounds the entire skeletal muscle?

Epimysium.

What happens to the sarcomere during contraction?

It shortens as thick and thin filaments slide past each other.

What happens to the I band and H zone during contraction?

They both narrow.

What happens to the A band during contraction?

It remains unchanged.

What causes the filaments to slide?

Myosin heads pull actin filaments toward the M line.

What happens when all sarcomeres shorten simultaneously?

The entire muscle fiber shortens.

What creates a membrane potential?

Unequal distribution of ions across the plasma membrane.

What is the resting membrane potential of a muscle fiber?

Approximately $–85$ mV (inside more negative).

What is voltage?

The difference in electrical charge between two points.

What are ion channels?

Proteins that allow ions to pass through the plasma membrane.

What are leak channels?

Ion channels that are always open, allowing continuous ion flow.

What are gated ion channels?

Channels that open or close in response to a specific stimulus.

Name the three types of gated ion channels.

Ligand-gated, voltage-gated, and mechanically-gated channels.

What opens a ligand-gated channel?

Binding of a specific chemical (ligand) such as a neurotransmitter.

What opens a voltage-gated channel?

A change in the voltage across the membrane.

What opens a mechanically-gated channel?

Physical stimuli like pressure, stretch, or vibration.

What pump maintains sodium and potassium gradients?

The sodium-potassium pump ($Na^+$/$K^+$ ATPase).

How many ions are moved by each cycle of the $Na^+$/$K^+$ pump?

Three $Na^+$ out and two $K^+$ in.

What type of transport does the $Na^+$/$K^+$ pump use?

Active transport powered by ATP hydrolysis.

What is an electrochemical gradient?

The combined influence of an ion’s concentration gradient and electrical gradient.

Which direction does potassium’s concentration gradient push $K^+$?

Out of the cell.

Which direction does potassium’s electrical gradient pull $K^+$?

Into the cell (toward negative interior).

Which direction does sodium’s concentration gradient move $Na^+$?

Into the cell.

Which direction does sodium’s electrical gradient move $Na^+$?

Into the cell (toward negative interior).

Which ion has the stronger overall electrochemical gradient?

Sodium ($Na^+$).

What happens when potassium diffuses out of the cell through leak channels?

The inside becomes more negative, creating the resting membrane potential.

What is depolarization?

When the membrane potential becomes less negative due to $Na^+$ entering the cell.

What is repolarization?

The return of the membrane potential to a negative value due to $K^+$ leaving the cell.

What are action potentials?

Brief changes in membrane potential from negative to positive and back again.

What triggers the opening of voltage-gated $Na^+$ channels?

A stimulus causing depolarization.

During depolarization, which ions enter the cell?

Sodium ions ($Na^+$).

At what potential does depolarization typically peak?

Around $+30$ mV.

What happens after depolarization?

Voltage-gated $Na^+$ channels close and $K^+$ channels open, beginning repolarization.

What is conductivity in muscle fibers?

The ability of the action potential to propagate along the entire sarcolemma.

What structure carries the action potential deep into the fiber?

T-tubules.

What event does the arrival of an action potential at the T-tubules trigger?

The release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction.

What does it mean that all skeletal muscles are innervated?

Each muscle fiber is connected to a neuron that stimulates contraction.

What is a motor neuron?

A nerve cell that transmits signals from the brain or spinal cord to muscle fibers.

What is a neuromuscular junction (NMJ)?

The synapse where a motor neuron communicates with one or more muscle fibers.

What is the purpose of the NMJ?

To transmit a nerve impulse (action potential) from the neuron to the muscle fiber’s sarcolemma.

What are the three components of the NMJ?

Axon terminal, synaptic cleft, and motor end plate.

What is contained in the axon terminal?

Synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

What is the synaptic cleft?

The small space between the axon terminal and the muscle fiber filled with collagen fibers and gel.

What is the motor end plate?

The specialized region of the muscle fiber’s plasma membrane containing ligand-gated $Na^+$ channels.

What chemical binds to the motor end plate receptors?

Acetylcholine (ACh).