Bio K103 Chapter 40: Protection, Support, & Movement

What is the function of the muscular system?

Generates mechanical forces necessary for locomotion, manipulation of objects, and physiological actions needed for homeostasis.

What type of energy conversion occurs in muscles?

Chemical energy (ATP) is converted into mechanical energy.

Which animals have muscle tissue?

All metazoans except cnidarians.

Which animals have contractile proteins?

All metazoans.

What are the two major contractile proteins in muscle?

Actin and myosin.

What is actin?

The thin myofilament involved in muscle contraction.

What is myosin?

The thick myofilament involved in muscle contraction.

What is contraction?

The development of tension or force by a muscle fiber, potentially shortening the muscle.

What is relaxation?

The return of a muscle fiber toward its resting state as contraction ends.

What is an agonist?

A muscle that works with another muscle to produce a particular movement.

What is an antagonist?

A muscle that opposes the movement produced by another muscle.

Why do muscles often work in pairs?

One muscle produces a movement while its antagonist opposes that movement.

Skeletal Muscle Control

Voluntary; regulated by the somatic nervous system and initiated by motor neurons.

Skeletal Muscle Structure

Striated, multinucleated, and organized into long cylindrical fibers.

Skeletal Muscle Location & Function

Attached to bones; produces voluntary movements and helps maintain posture.

Cardiac Muscle Control

Involuntary.

Cardiac Muscle Structure

Striated, usually one nucleus per cell, branched fibers with intercalated discs.

Smooth Muscle Control

Involuntary; regulated by the autonomic nervous system, hormones, and stretch.

Smooth Muscle Structure

Non-striated, spindle-shaped cells with a single nucleus per cell.

Smooth Muscle Location

Walls of hollow organs, including the digestive tract and blood vessels.

Smooth Muscle Function

Produces involuntary movements, such as moving food through the digestive tract and constricting blood vessels.

What is the endomysium?

Connective tissue surrounding an individual muscle fiber.

What is the perimysium?

Connective tissue surrounding a bundle of muscle fibers.

What is the epimysium?

Connective tissue surrounding an entire muscle.

What is the sarcolemma?

The plasma membrane of a muscle cell.

What is the sarcoplasm?

The cytoplasm of a muscle cell.

What is the sarcoplasmic reticulum (SR)?

The specialized endoplasmic reticulum of a muscle cell that stores calcium ions.

What is the function of the sarcoplasmic reticulum?

Stores and releases Ca2+ needed for muscle contraction.

What are T-tubules?

Inward extensions of the sarcolemma that conduct electrical signals into the muscle cell.

What is a triad?

A region of skeletal muscle containing a T-tubule between two sections of sarcoplasmic reticulum (SR-T-SR).

What is a myofiber?

An individual muscle cell.

What is a myofibril?

A cylindrical structure within a muscle cell made of repeating sarcomeres and composed of myofilaments.

What are myofilaments?

Individual protein filaments (actin and myosin) that make up myofibrils.

What are contractile proteins?

Actin and myosin, which directly interact to produce muscle contraction.

What are regulatory proteins?

Proteins such as troponin and tropomyosin that control actin-myosin interactions.

What are structural proteins?

Proteins that help maintain the organization and stability of muscle structures.

What is a sarcomere?

The individual contractile unit of skeletal muscle, extending between two Z discs.

What is the Z disc?

The boundary of a sarcomere where actin filaments are anchored.

What is the I band?

The region containing the Z disc and actin filaments without overlapping myosin.

What is the A band?

The region extending the entire length of the myosin filaments, including areas where actin overlaps myosin.

What is the H band (zone)?

The region in the center of the A band containing only myosin filaments.

What is the M line?

The center of the H zone containing proteins that hold neighboring myosin filaments together.

Which sarcomere regions contain only actin?

The portions of the I band outside the A band.

Which sarcomere region contains only myosin?

The H zone.

Which sarcomere region contains overlapping actin and myosin?

The portions of the A band outside the H zone.

What happens to the Z discs during contraction?

They move closer together as the sarcomere shortens.

What happens to the I band during contraction?

It becomes narrower.

What happens to the H zone during contraction?

It becomes narrower or disappears as actin slides toward the center.

What happens to the A band during contraction?

It remains the same length because the myosin filaments do not shorten.

What is the sliding filament theory?

Actin filaments slide past myosin filaments, increasing their overlap and shortening the sarcomere.

Do actin and myosin filaments shorten during contraction?

No. They slide past one another while maintaining their original lengths.

What is tropomyosin?

A regulatory protein that covers myosin-binding sites on actin when calcium is absent.

What is troponin?

A regulatory protein complex containing a calcium-binding site that controls tropomyosin.

What happens when calcium binds to troponin?

Troponin changes shape, causing tropomyosin to move and expose myosin-binding sites on actin.

What are the main components of a myosin filament?

Two interwoven protein chains with projecting myosin heads.

What is a crossbridge?

The connection formed when a myosin head binds to an exposed binding site on actin.

Muscle Contraction Step 1

A nerve impulse (action potential) travels along the sarcolemma and moves inward through the T-tubules.

Muscle Contraction Step 2

The electrical signal in the T-tubules stimulates the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm.

Muscle Contraction Step 3

Ca2+ binds to specific sites on troponin subunits located along the actin filaments.

Muscle Contraction Step 4

Troponin changes shape, pulling tropomyosin fibers aside to expose the myosin-binding sites on actin.

Muscle Contraction Step 5

Myosin heads in the cocked position (due to previous ATP hydrolysis) bind to exposed sites on actin, forming crossbridges.

Muscle Contraction Step 6

The power stroke is triggered when phosphate detaches from the myosin head. The head pivots, pulling actin toward the center of the sarcomere. ADP is released at the end.

Muscle Contraction Step 7

A new ATP molecule binds to the myosin head, breaking the crossbridge attachment between myosin and actin.

Muscle Contraction Step 8

The myosin head hydrolyzes ATP into ADP and Pi, using the energy to return to its cocked position.

Muscle Contraction Step 9

The cycle repeats, increasing actin-myosin overlap and shortening the sarcomere without changing the lengths of the individual filaments.

Muscle Contraction Step 10

If neural stimulation continues, Ca2+ remains available and contraction repeats. Once stimulation stops, Ca2+ is pumped back into the SR and the muscle relaxes.

How does ATP contribute to muscle relaxation?

ATP provides energy for pumping Ca2+ back into the sarcoplasmic reticulum.

What is the immediate energy source for muscle contraction?

ATP.

What is the role of creatine phosphate in muscle activity?

Provides a rapid energy reserve for regenerating ATP.

What is the role of glycogen in muscle activity?

Serves as stored carbohydrate that can be broken down to help produce ATP.

What is oxygen debt?

The additional oxygen needed after exercise to help restore the body to its resting condition.

Do muscles run out of ATP when tired?

No. ATP is continually regenerated, but fatigue can involve depletion of energy reserves and other factors.

What factors influence muscle fatigue?

1. Depletion of energy sources (glycogen/ATP). 2. Accumulation of metabolic byproducts (lactic acid). 3. Central nervous system fatigue. 4. Failure of motor unit recruitment.

What is rigor mortis?

Muscle stiffening after death because ATP is no longer produced, preventing myosin heads from detaching from actin and keeping muscles contracted.

What factors influence the strength of muscle contraction?

1. Type of muscle fiber. 2. Aerobic training. 3. Strength training. 4. Number of contracting muscle fibers. 5. Frequency of stimulation. 6. Amount of stretch. 7. Number of motor units recruited.

How does the number of contracting muscle fibers affect strength?

Recruiting more muscle fibers increases the overall force produced.

How does sarcomere length affect contraction strength?

It affects actin-myosin overlap and the number of crossbridges that can form.

How does calcium availability affect muscle contraction?

Calcium allows troponin and tropomyosin to expose actin-binding sites, permitting crossbridge formation.

How does stimulation frequency affect muscle contraction strength?

More frequent stimulation can produce summation and stronger, sustained contractions.

What is a motor unit?

A single motor neuron (nerve axon) and all the muscle fibers it innervates.

How does motor unit recruitment affect muscle strength?

Recruiting more motor units activates more muscle fibers and increases force.

What is a single muscle twitch?

The response of a muscle to a single stimulus.

How long does a muscle action potential last?

Approximately 1-2 milliseconds.

Why does muscle contraction last longer than its action potential?

Excitation-contraction coupling and the contraction process continue after the action potential ends.

What is the latent period of a muscle twitch?

The time between the action potential and the beginning of muscle contraction.

What happens during the latent period?

Excitation-contraction coupling occurs, including calcium release from the sarcoplasmic reticulum.

What is contraction time?

The period during which muscle tension develops following stimulation.

What is summation?

The increased force produced when multiple stimuli arrive before a muscle completely relaxes.

Why does summation occur?

The muscle has not completely relaxed before another stimulus arrives, allowing tension to increase.

What is tetanus?

A sustained muscle contraction caused by repeated stimulation at a high frequency.

What is unfused tetanus?

A sustained contraction with partial relaxation between successive stimuli.

What is fused (complete) tetanus?

A sustained contraction with no noticeable relaxation between successive stimuli.

What is the difference between unfused and fused tetanus?

Unfused tetanus includes partial relaxation; fused tetanus has no relaxation between stimuli.

What is muscle tone?

A continuous, low-level state of muscle contraction present even when a muscle is at rest.

What is the difference between a twitch and tetanus?

A twitch follows one stimulus; tetanus results from repeated, high-frequency stimulation.

What are dense bodies in smooth muscle?

Structures that anchor actin filaments in smooth muscle cells.

What is the difference between smooth and skeletal muscle regarding sarcomeres?

Smooth muscle lacks sarcomeres; skeletal muscle contains highly organized sarcomeres.

What is the difference between smooth and skeletal muscle regarding gap junctions?

Smooth muscle has gap junctions; skeletal muscle does not.

What is the difference between smooth and skeletal muscle regarding calcium sources?

Smooth muscle relies heavily on extracellular Ca2+ and the SR; skeletal muscle relies primarily on the SR.

What is the difference between smooth and skeletal muscle regarding myofilaments?

Smooth muscle has loosely arranged actin and myosin; skeletal muscle has highly organized actin and myosin in sarcomeres.