A&P 1 Lab (Chapter 8-9)

The Muscular system

What is the function of Muscle Tissue?

generate force and produce movement

What are skeletal muscles?

composed of organized connective tissue wrappings and contractile units with each layer supporting and transmitting force

• electrically excitable cells

• maintains a resting membrane potential (approx. -70 to -90 mV)

What are the layers of skeletal muscle (4) (in order - from superficial to deep)?

1. Muscle (the entire organ)

2. Fascicle

3. Muscle fiber

4. Myofibril

What is the first layer of the skeletal muscle? What is it surrounded by and attached to?

muscle

• Surrounded by the epimysium

• Attached to bone via tendons and supported by the broader sheets of fascia

What is epimysium?

dense connective tissue sheath

What is the second layer of the skeletal muscle? What is it?

fascicle

• A bundle of muscle fibers wrapped by the perimysium

What is the third layer of the skeletal muscle? What is it? Function?

muscle fiber

• surrounded by the endomysium (the innermost connective tissue layer)

• also carries capillaries and nerves

What is the fourth layer of the skeletal muscle? What is it? Function?

myofibril

• Cylinder-shaped, contractile organelles composed of bundled specialized proteins that allow the fiber to shorten and generate force

What are the 5 structures of the muscle cells?

• myocyte

• sarcoplasm

• sarcolemma

• sarcoplasmic reticulum

• myofibrils

What is a myocyte?

• The muscle cell itself

• Large, multinucleated

• Function: force production

What is sarcoplasm?

• the cytoplasm of the myocyte

• rich in glycogen, myoglobin, enzymes, and the contractile proteins needed for contraction

What is Sarcolemma?

• the myocytes plasma membrane

• function: electrically capable of transmitting action potentials across the entire fiber surface

What is a sarcoplasmic reticulum?

• modified endoplasmic reticulum

• Function: stores and releases calcium ions (Ca2+), which is the key trigger for muscle contraction

What are myofibrils?

• Cylinder-shaped, contractile organelles composed of bundled specialized proteins that allow the fiber to shorten and generate force

What are T-Tubules? What do they do?

• they are the deep invaginations in the sarcolemma that form a tunnel like network that runs throughout the interior of the muscle fiber.

• function: carry action potentials inward, making sure all fibers receive the electrical signal and enabling contraction.

What is the Terminal Cisternae? Function?

• enlarged bulging sections of the sarcoplasmic reticulum that has one T-tubules between two terminal cisternae, forming the structure triad

• Function: when the action potential arrives via T-tubule, the terminal cisternae releases stored Ca2+ in the sarcoplasm to initiate contraction

Sarcomere (description)

• the myofibril is made up of sarcomere

• at the ends of the sarcomere is the Z disk or Z line

• The lines contains thick (myosin) and thin (actin) filaments

• the I bands only contain actin (thin filaments)

• the A band only contains myosin (thick filaments + overlap zone)

• the H zone/band only contains myosin and is in the center

• the M line is in the center of the A band; anchors thick filaments

• During the contraction, the thin filaments go toward the center, the I band and H zone shorten, BUT the A band STAYS the same length - the sliding filament theory

What is the Sliding Filament Theory?

During the contraction, the thin filaments go toward the center (to the M line), the I band and H zone shorten, BUT the A band STAYS the same length

What are the protein strands in the myofibril called?

myofilaments

What is the function of myofilaments?

• produce the striated (striped) appearance of skeletal muscle

• produce the sliding motion of contraction

What are the 3 filaments of the myofibril?

1. thick

2. thin

3. elastic

thick filaments are… + function

myosin

• function: pull thin filaments during contraction using globular heads

What do the globular heads contain?

• actin-binding site

  • Basically theres active sites on each myosin head that physically attaches to a active site on an actin during contraction, forming a cross bridge

• ATPase activity

  • Basically each myosin head can bind and hydrolyze ATP to release energy, which allows it to ‘cock’ (re-energize) into position like a spring then snap forward to pull the actin filament, creating muscle contraction.

thin filaments are… and composed of… + function

actin

composed of actin, tropomyosin and troponin

• function: contain binding sites for myosin head to attach to during contraction

What are the 3 proteins thin filaments composed of?

actin, troponin, and tropomyosin

Actin is…

Bead-shaped subunits arranged in two intertwining strands. Each actin subunit has an active site that can bind to a myosin head.

Tropomyosin is…

A long, rope-like protein that coils around the actin strands and physically covers the active sites at rest

• function: prevents myosin from binding and keeping the muscle relaxed.

Troponin is…

a regulatory protein complex that holds tropomyosin in the blocking position; when Ca²⁺ binds to the troponin, it changes shape, pulling tropomyosin away from actin active sites.

elastic filaments are composed of… + function

composed of titin (‘giant’ protein - pronounced like titan)

• molecular springs that help return the sarcomere to resting length and prevent overstretching

What is the resting membrane potential of skeletal muscle fibers?

Skeletal muscle fibers maintain a resting membrane potential of approximately –70 to –90 mV.

What is an action potential in a skeletal muscle fiber?

An action potential is a rapid, self-propagating change in membrane voltage triggered by a stimulus (which the sarcolemma genereates), which initiates muscle contraction.

what are the 2 phases of action potentials?

1. phase 1: depolarization

2. phase 2: repolarization

Depolarization occurs when…

• Na⁺ channels open in response to stimulation.

• Sodium ions rush into the cell down their concentration gradient.

• The membrane potential rapidly shifts from negative to more positive (approximately +30 mV), reversing the resting charge.

Repolarization occurs when…

• Na⁺ channels close and K⁺ channels open.

• Potassium ions flow out of the cell, restoring the negative charge inside.

• Once resting membrane potential is reached (~–70 mV), K⁺ channels close and the cell is ready for the next stimulus.