PSIO Lecture 3.1-3.8

What is the main function of muscle tissue?

To generate tension within the body for movement.

What causes muscles to move the body?

When they shorten, they pull on bones, producing movement.

Name the three types of muscle tissue.

Skeletal, cardiac, and smooth muscle.

List the key functions of skeletal muscle.

Locomotion, facial expression, posture and body position, and regulation of body temperature.

What does baseline muscle contraction maintain?

Basic body activities like sitting upright or maintaining posture.

How does ATP contribute to muscle activity?

ATP breakdown releases energy for muscle contraction.

What is the mechanical efficiency of muscle contraction?

Less than 30% of ATP energy becomes mechanical work; ~70% is lost as heat.

Why does exercise warm the body?

The majority of ATP energy (~70%) becomes heat during contraction.

What system is tightly linked with skeletal muscle metabolism?

The blood glucose (metabolic) system.

Is skeletal muscle voluntary or involuntary?

Voluntary (requires nervous input), though some actions can be automatic (e.g., reflexes).

🦴 Movement Mechanics

How do muscles move bones?

Muscles pull on bones; they never push.

Define origin of a muscle.

The fixed attachment site that stays stationary during movement.

Define insertion of a muscle.

The movable attachment that moves toward the origin when the muscle contracts.

During flexion of the forearm, what happens?

The angle at the elbow decreases as the insertion moves toward the origin.

In standard anatomical position, how are most joints positioned?

Extended, except for the feet.

What are the naming conventions for body movements?

Include both the action and the segment that moves (e.g., “flexion of the forearm,” “adduction of the thigh”).

What is flexion?

Movement that decreases the angle between articulating bones, typically in the sagittal plane.

What is extension?

Movement that increases the angle between articulating bones.

What is abduction?

Movement away from the body’s midline, typically in the frontal plane.

What is adduction?

Movement toward the body’s midline, typically in the frontal plane.

What is Reverse Muscle Action (RMA)?

When the insertion is fixed and the origin moves toward it (e.g., during pull-ups).

What is an agonist (prime mover)?

The muscle primarily responsible for producing a movement.

What is an antagonist?

A muscle that opposes or reverses the action of an agonist.

What is a synergist?

A muscle that assists an agonist to make the movement more efficient.

What is a fixator?

A stabilizing muscle that prevents unwanted movement at the origin.

In the musculoskeletal system, what acts as a lever?

the bone

What determines the movement efficiency of a lever?

The relative positions of the load, fulcrum, and effort.

What are the three classes of levers?

First-class, second-class, and third-class levers.

Describe a first-class lever (LFE).

Fulcrum between load and effort (e.g., neck nodding); few in body, efficient for heavy loads.

Describe a second-class lever (FLE).

Load between fulcrum and effort (e.g., standing on tiptoes); rare but provides mechanical advantage.

Describe a third-class lever (FEL).

Effort between fulcrum and load (e.g., biceps flexion); most common, allows wide range of motion but less efficient for force.

Why are third-class levers predominant in the body?

They favor speed and range of motion over force efficiency.

What covers groups of muscles?

Deep fascia.

What connective layer covers an entire muscle?

Epimysium (outermost layer).

What covers fascicles (bundles of muscle fibers)?

Perimysium.

What covers individual muscle fibers?

Endomysium.

Is the endomysium the same as the muscle cell membrane?

No — the cell membrane is the sarcolemma.

What are skeletal muscle cells called?

Muscle fibers (myofibers).

Describe typical muscle fiber size.

Up to 100 µm in diameter and several cm in length.

Why are muscle fibers multinucleate?

They form from fusion of myoblasts during development.

What are satellite cells?

Muscle stem cells attached to fibers that can divide for repair and growth.

Can muscle fibers undergo mitosis after birth?

No — growth occurs by hypertrophy, not hyperplasia.

Define hypertrophy.

Increase in cell size (existing fibers enlarge).

Define hyperplasia.

Increase in cell number (rare in adult muscle).

How do satellite cells contribute to regeneration?

They can undergo mitosis and fuse with existing fibers to repair or enlarge them.

What is the sarcolemma?

The plasma membrane of a muscle fiber; conducts electrical signals.

What are T-tubules (transverse tubules)?

Invaginations of the sarcolemma that transmit electrical signals deep into the fiber.

What is the sarcoplasmic reticulum (SR)?

Specialized smooth ER that stores and releases calcium ions (Ca²⁺) to regulate contraction.

What organelles surround the myofibrils closely?

The sarcoplasmic reticulum and T-tubules (forming the triad).

How is calcium involved in contraction control?

Calcium released from the SR triggers interaction between actin and myosin.

What are myofibrils?

Long, cylindrical bundles of contractile proteins inside a muscle fiber.

What proteins make up myofibrils?

Actin (thin filaments) and myosin (thick filaments).

What are sarcomeres?

Repeating structural and functional units within myofibrils responsible for contraction.

What happens when sarcomeres shorten?

The muscle fiber shortens, producing contraction.

Outline the main steps of the sliding filament model.

Myosin heads bind actin → cross-bridge forms.

ATP hydrolysis energizes myosin → pulls actin (“power stroke”).

Myosin releases, rebinds → cycle repeats.

Z-lines move closer; sarcomere shortens.

During contraction, which bands or zones change length?

I-band → decreases

H-zone → decreases

A-band → remains constant

Zone of overlap → increases

Z-lines → move closer together

⚛️ Muscle Proteins

What are the three categories of muscle proteins?

Contractile, regulatory, and structural proteins.

Name the contractile proteins.

Actin and myosin.

What is the role of actin?

Forms thin filaments; has myosin-binding sites for cross-bridge formation. (role)

What is the role of myosin?

Motor protein of thick filaments; uses ATP to generate movement and pull actin filaments.

Name the two main regulatory proteins.

Troponin and tropomyosin.

What is tropomyosin?

A thin-filament protein that covers myosin-binding sites on actin when the muscle is relaxed.

What is troponin?

A complex that anchors tropomyosin; when Ca²⁺ binds to troponin, it shifts tropomyosin to uncover actin’s binding sites. (what it is, not what it does)

How does calcium enable contraction?

Ca²⁺ binds troponin → tropomyosin moves → myosin can bind actin → cross-bridges form.

(enable)

What is titin?

A giant elastic protein spanning from Z-disc to M-line; stabilizes thick filaments and provides elasticity and recoil.

What is dystrophin?

Cytoskeletal protein linking thin filaments to the sarcolemma and extracellular matrix; transmits tension to tendons. (what is it?)

What is α-actinin?

Z-disc protein that binds actin and titin, anchoring thin filaments.

What is myomesin?

M-line protein that links thick filaments and titin together at the sarcomere center. (what is it?)

What is nebulin?

Protein that runs along the length of thin filaments, anchoring them to the Z-disc and maintaining alignment.

🧱 Sarcomere Structure Summary

What are the main components of a sarcomere?

Thin filaments, thick filaments, Z-discs, M-line, and the various protein connections (titin, α-actinin, myomesin, nebulin). (components)

What structure stores and releases calcium to trigger contraction?

The sarcoplasmic reticulum. (what)

What structure conducts electrical impulses into the muscle fiber interior?

The sarcolemma and T-tubules.

Which structure anchors thin filaments and defines the sarcomere boundaries?

The Z-disc

What are the contractile proteins?

Actin and myosin.

What is actin and where is it found?

Found in thin filaments; contains myosin-binding sites for crossbridge formation.

What is myosin and where is it found?

A motor protein in thick filaments; has heads that bind to actin to form crossbridges during contraction.

What are the regulatory proteins?

Troponin and tropomyosin.

What is tropomyosin’s role in muscle contraction?

It covers myosin-binding sites on actin when the muscle is relaxed. (role)

What does troponin do?

Holds tropomyosin in place when relaxed; when Ca²⁺ binds, troponin shifts tropomyosin to expose binding sites on actin.(what it do)

What triggers exposure of myosin-binding sites?

Ca²⁺ binding to troponin.

Name the structural proteins of the sarcomere.

Titin, dystrophin, α-actinin, myomesin, and nebulin.

What does titin do?

Spans half the sarcomere (Z disc → M line), stabilizes thick filaments, provides elasticity and extensibility, and restores resting length. (what does it do?)

What is dystrophin’s function?

Links thin filaments to the sarcolemma and extracellular matrix, transmitting tension from sarcomeres to tendons.

What is nebulin’s role?

Spans thin filaments and anchors them to Z discs.(NOT what it is,)

What does α-actinin do?

Found in Z discs; binds actin and titin to stabilize filament alignment.

What does myomesin do?

Found in the M line; links thick filaments and titin at the sarcomere center.

What is the functional unit of muscle contraction?

The sarcomere.

What are the main elements of a sarcomere?

Thin filaments, thick filaments, and Z discs. (elements)

What is not part of the sarcomere structure?

The sarcolemma.

Which structure stores and releases Ca²⁺?

The sarcoplasmic reticulum (SR).

What structure conducts electrical signals along the muscle fiber?

The sarcolemma.

What happens during the sliding filament model?

Myosin heads bind actin → pull thin filaments → sarcomere shortens.

Outline the steps of the sliding filament model.

Myosin binds actin forming crossbridges.

ATP hydrolysis energizes myosin for the power stroke.

Thin filaments slide along thick filaments.

Myosin releases and rebinds to continue the cycle.

What are the results of contraction?

Z-lines move closer.

Sarcomere and myofibril shorten.

Muscle fiber shortens.

How do band lengths change during contraction?

I-band → decreases

H-zone → decreases

A-band → stays constant

Zone of overlap → increases
(how)

When does crossbridge cycling begin?

When Ca²⁺ binds to troponin and myosin-binding sites on actin become exposed.

What happens before contraction begins (in resting state)?

ATP binds to the myosin head.

ATP hydrolysis “cocks” the myosin head (ADP + Pi remain attached).

List the steps of the contraction cycle.

Ca²⁺ binds to troponin → exposes actin sites.

Myosin heads bind to actin → crossbridge forms.

Myosin heads pivot (power stroke).

ATP binds → crossbridge detaches.

ATP hydrolyzes → re-cocks myosin head.

Cycle repeats as long as Ca²⁺ and ATP are present.

What is the rigor complex?

The state where the myosin head remains attached to actin after the power stroke.

What causes rigor mortis?

Absence of ATP prevents detachment of myosin heads from actin, locking muscles in contraction.