Chapter 12 muscles - integrated physiology

What are the two parts of the efferent division of the peripheral nervous system (PNS)?

Somatic motor division and autonomic division

What do autonomic neurons control?

Smooth muscle, cardiac muscle, many glands, and some adipose tissue

What do somatic motor neurons control?

Skeletal muscles

Describe the pathway of a somatic motor neuron

A single neuron originates in the CNS and extends its axon directly to skeletal muscle

Where are the cell bodies of somatic motor neurons located?

In the brain or the ventral horn of the spinal cord

Are somatic motor neurons excitatory or inhibitory?

Always excitatory

What are axon collaterals?

Branches of an axon that allow a somatic motor neuron to communicate with multiple muscle fibers

What is the neuromuscular junction (NMJ)?

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

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh)

What triggers acetylcholine release from the presynaptic axon?

An action potential opens voltage-gated Ca2+ channels. Ca2+ enters the axon terminal and triggers the release of acetylcholine

What is the motor end plate?

The specialized region of the muscle fiber membrane containing nicotinic acetylcholine receptors

What happens when acetylcholine binds to nicotinic receptors?

Ligand-gated ion channels open, allowing Na+ to enter and K+ to leave. The net influx of Na+ depolarizes the muscle membrane

What is the function of acetylcholinesterase?

It breaks down acetylcholine in the synaptic cleft, terminating the signal

Explain how a somatic motor neuron communicates with a skeletal muscle fiber at the NMJ.

1. 1. An action potential reaches the presynaptic axon terminal

2. 2. Voltage-gated Ca2+ channels open

3. 3. Ca2+ enters the axon terminal

4. 4. Acetylcholine is released into the synaptic cleft

5. 5. ACh binds to nicotinic receptors on the motor end plate

6. 6. Ion channels open, causing net Na+ influx and depolarization

7. 7. Acetylcholinesterase breaks down ACh to terminate the signal

What three things do skeletal muscles generate?

Motion, force, and heat

What protein creates movement during muscle contraction?

Myosin, by changing its conformation (shape)

What are the three types of muscles?

1. 1. Skeletal

2. 2. Cardiac

3. 3. Smooth

What are three characteristics of skeletal muscle?

1. Contracts in response to somatic motor neurons

2. Cannot initiate its own contraction

3. Contraction is not directly influenced by hormones

What are three characteristics of cardiac and smooth muscle?

1. 1. Receive autonomic innervation

2. 2. Can initiate spontaneous contractions

3. 3. Can be modulated by the endocrine system

How are skeletal muscles usually attached to bones?

By tendons on opposite sides of a joint

What is the origin of a muscle?

The attachment point on the less movable bone

What is the insertion of a muscle?

The attachment point on the more movable bone

What is the difference between a flexor and an extensor?

Flexor: decreases the angle of a joint

Extensor: increases the angle of a joint

What are antagonistic muscle pairs?

Muscles that work in opposition to produce movement in opposite directions

Give an example of an antagonistic muscle pair

Quadriceps and hamstrings

What is a muscle fiber?

A muscle cell

Describe the shape and structure of a skeletal muscle fiber.

A long, cylindrical cell containing several hundred nuclei near its surface

Why do skeletal muscle fibers contain multiple nuclei?

Because many individual embryonic muscle cells fuse together during development

Where are the nuclei located in a skeletal muscle fiber?

Near the surface of the cell

What are satellite cells?

Adult stem cells located just outside the muscle fiber membrane

What is the function of satellite cells?

They become activated and differentiate into muscle cells when needed, helping with muscle repair

What are fascicles?

Bundles of muscle fibers (myofibers)

What surrounds individual muscle fibers?

A connective tissue sheath

What structures are found between fascicles?

Collagen, elastic fibers, nerves, blood vessels

What surrounds an entire skeletal muscle?

A connective tissue sheath that is continuous with the connective tissue around tendons

Describe the organization of skeletal muscle from largest to smallest.

Whole skeletal muscle → fascicle → muscle fiber (myofiber) → myofibril

What is the sarcolemma?

The plasma membrane of a muscle fiber

What is the sarcoplasm?

The cytoplasm of a muscle fiber

What is the sarcoplasmic reticulum?

The specialized endoplasmic reticulum of a muscle fiber

What are myofibrils?

Organized bundles of contractile and elastic proteins that carry out muscle contraction

What is the main function of the sacroplasmic reticulum?

To store high concentrations of Ca2+ needed for muscle contraction

What are the two structural components of the sarcoplasmic reticulum mentioned in the slides?

Longitudinal tubules and terminal cisternae

What are transverse tubules (T-tubules)

Invaginations of the sarcolemma that extend into the muscle fiber

What structures are T-tubules located next to?

The terminal cisternae of the sarcoplasmic reticulum

What is the function of T-tubules?

To conduct action potentials into the interior of the muscle fiber

Is the lumen of a T-tubule continuous with the intracellular or extracellular environment?

The extracellular environment

What two substances or structures are found in the spaces between myofibrils?

Mitochondria and glycogen

What is glycogen?

A polysaccharide that stores glucose for energy

Why do muscle fibers contain many mitochondria?

To produce ATP needed for muscle contraction

What structures should you be able to identify in the ultrastructure of muscle fiber diagram?

Sarcolemma, sarcoplasmic reticulum, T-tubules, myofibrils, mitochondria, thick filament, thin filaments

What structure surrounds the myofibrils and stores calcium?

Sarcoplasmic reticulum

Which structure carries action potentials into the muscle fiber?

At T-tubules

Which structure contains the proteins responsible for contractions?

Myofibrils

What structure apply ATP for muscle contraction?

Mitochondria

What is the relationship between the sarcolemma, T-tubules, and sarcoplasmic reticulum?

T-tubules are continuous with the sarcolemma and conduct action potentials into the muscle fiber. They are positioned next to the terminal cisternae of the sarcoplasmic reticulum, which stores Ca²⁺

What is the basic contractile unit of a myofibril?

The sarcomere

How are sarcomeres arranged within myofibrils?

Sarcomeres are repeating contractile units arranged end to end

What marks the boundaries of a sarcomere?

Two Z-discs (Z-lines). A sarcomere extends from one Z-disk to the next

What protein forms the thick filaments?

Myosin, a motor protein that uses ATP to produce movement

What protein forms the thin filaments?

Actin

Which proteins regulate muscle contraction?

Troponin and tropomyosin

What are the two giant accessory proteins in sarcomere?

Titin and Nebulin

What two functional domains are found on myosin heads?

Myosin ATPase and Actin-binding domain

What forms a crossbridge?

A crossbridge forms when a myosin head binds to actin

What does tropomyosin cover on actin?

The myosin-binding sites

What is the function of Z-disks?

They anchor actin filaments at each end of a sarcomere and are located in the middle of the I band

What filaments are found in the I band?

Thin (actin) filaments in the I band

What does the A band represent?

The entire length of the thick (myosin) filaments, including regions where thick and thin filaments overlap

What is found in the H zone?

Only thick (myosin) filaments. It is located in the center of the A band

What is the function of the M line?

It anchors thick filaments in the center of the sarcomere

What are the two major functions of titin?

Stabilizes the position of contractile filaments and helps stretched muscles return to their original length

What is the function of nebulin?

Nebulin attaches to the Z-disk and helps align actin filaments within the sarcomere. It is an inelastic accessory protein

What is muscle tension?

The force produced by a contracting muscle

What is a load?

A weight or force that opposes a contracting muscle

What is muscle relaxation?

The release of tension produced by muscle contraction

What is a muscle twitch?

One contraction-relaxation cycle

What are the three major stages of muscle contraction?

1. Neuromuscular junction signaling

2. Excitation-contraction (E-C) coupling

3. Contraction-relaxation cycle

What happens at the neuromuscular junction during muscle contraction?

An acetylcholine signal from a somatic motor neuron is converted into an electrical signal in the muscle fiber

What is excitation-contraction (E-C) coupling?

The process in which muscle action potentials initiate calcium signals that activate the contraction-relaxation cycle

What does sliding filament theory describe?

Thin filaments slide over thick filaments toward the center of the sarcomere, causing the sarcomere to shorten

Do thick and thin filaments shorten during muscle contraction?

No. Both remain the same length. They slide past each other, shortening the sarcomere

What enzyme on the myosin head hydrolyzes ATP?

Myosin ATPase

What products form when myosin hydrolyzes ATP?

ADP and inorganic phosphate (Pi)

What causes a myosin head to become cocked?

Energy released from ATP hydrolysis is stored in the myosin head, placing it in an energized or cocked position

What is the power stroke?

The myosin head releases Pi and bends, pulling actin toward the center of the sarcomere

What happens to ADP during the power stroke?

ADP is released as the power stroke finishes

Why do different myosin heads act at different times?

Different myosin heads form crossbridges and perform strokes at different times, allowing repeated pulling of actin

What intracellular signal initiates muscle contraction?

An increase in cytosolic Ca2+

What does Ca2+ bind to during skeletal muscle contraction?

Troponin

What is step 1 of the contraction cycle?

Crossbridge attachment

• myosin uses stored energy from ATP hydrolysis

• the energized myosin head binds to actin

• a crossbridge forms

What is step 2 of the contraction cycle?

Power stroke

• Pi is released

• The myosin head bends and pulls actin toward the center of the sarcomere

• ADP is released

What is step 3 of the contraction cycle?

Crossbridge detachment

• A new ATP molecule binds to the myosin head

• Myosin detaches from actin

What is step 4 of the contraction cycle?

Myosin reactivation

• Myosin hydrolyzes ATP into ADP and Pi

• Energy is stored in the myosin head

• The head returns to its cocked position

What happens if new ATP cannot bind to myosin?

Myosin cannot detach normally from actin, causing the crossbridges to remain attached

What are the first two steps of excitation-contraction coupling?

1. Acetylcholine is released from the somatic motor neuron

2. An action potential is initiated in the muscle fiber

What happens after the muscle action potential begins?

Ca2+ is released from the sarcoplasmic reticulum into the cytocol

How does released Ca2+ initiate contraction?

1. Ca2+ binds to troponin

2. Tropomyosin shifts away from the myosin-binding sites

3. Myosin can bind to actin

4. Crossbridge cycling begins

What must happen for muscle contraction to end?

Ca2+ must be removed from the cytosol

How is Ca2+ returned to the sarcoplasmic reticulum?

Ca2+-ATPase pumps Ca2+ back into the sarcoplasmic reticulum lumen using ATP

What happens to tropomyosin when cytosolic Ca2+ decreases?

Tropomyosin returns to its original position, covering the myosin-binding sites on actin and preventing further crossbridge formation