Chapter 8Muscular System

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Recap:

-The three types of muscle in the body are skeletal, smooth, and cardiac muscle.

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Structure of a Skeletal Muscle

Each muscle is an organ comprised of -skeletal muscle tissue -connective tissues

-nervous tissue-blood

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Connective Tissue Coverings

Fascia: Layers of dense connective tissue that surround and separate each muscle.

Fascia extends beyond the ends of the muscle and gives rise to

tendons which are fused to bones

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Epimysium: The layer of connective tissue around each whole muscle

Perimysium : The layer of connective tissue surrounding individual

bundles (fascicles) within each muscle

Endomysium: The layer of connective tissue around each muscle cell (fiber)

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Skeletal Muscle Fibers

-Each muscle fiber is a single cell with…

-Sarcolemma: cell membrane

-Sarcoplasm : cytoplasm containing many mitochondria and nuclei

-Myofibrils: organelles that are separated into compartments called sarcomeres

-Sarcomeres: structures that have thick and thin protein filaments

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The Sarcomere:

-Thick filaments of myofibrils are made up of the protein myosin.

-Thin filaments of myofibrils are made up of the protein actin.

-The organization of these filaments produces striations.

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A sarcomere extends from Z line to Z line.

-I bands (light bands) made up of actin filaments are anchored to Z lines.

-A bands (dark bands) are made up of overlapping thick and thin filaments.

-In the center of A bands is an H zone, consisting of myosin filaments only.

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Skeletal Muscle Contraction

Muscle contraction involves several components that result in

-the shortening of sarcomeres

-the pulling of the muscle against its attachments.

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-Sliding filament theory: the myosin attaches to the binding site on the actin filament, pulls and releases it

*After release, it will move on to the next actin binding site

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Stimulus for Contraction

1. The motor neuron releases acetylcholine (signalled

from brain)*Signals muscle to contract

2. Calcium is released

3. Myosin is able to bind to actin and the fiber contracts (sarcomeres shorten)

How does this stop?8 - 16

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Energy Sources for Contraction

1. Energy for contraction comes from ATP.

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Muscle Fatigue

-Muscle Fatigue: A muscle loses its ability to contract during strenuous exercise

-Usually arises from the accumulation of lactic acid (lowered pH) in the muscle.

-As a result, no muscle response to stimulation

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Muscle Cramp

Muscle cramp: sustained involuntary contraction*Changes in fluid around cell*Uncontrolled stimulation of muscle

(dehydration…)

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THRESHOLD STIMULUSMuscles will only respond

(contract) if the stimulus strength reaches a certain point

Minimum strength required to make a muscle fiber contract is called the

THRESHOLD STIMULUS8 - 22

Muscular Response

ALL – OR – NONE RESPONSE

Muscles can’t “partially” contract

They will either contractfully or not at all

Increasing the stimulusintensity does not increase

the force of a single muscle’s response 8 - 23

Muscular Response

Twitch: period of contraction and relaxation of a single muscle fiber

Movements result from multiple muscle fibers contracting together

Fast Twitch: weight lifting, sprints,

Slow Twitch: low intensity, long distance run, swimming

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Twitches

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Skeletal muscle contractsinvoluntarily Can be caused by: *Muscle fatigue/strain *Stress/anxiety *Medication *Caffeine

MUSCLE TONE –Even at “rest” a muscle is contracting

Important in maintaining posture

When a person collapses, muscle tone has been lost

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Sustained Contractions

ISOTONIC - Muscle changes length and produces force; causes motion

○ Ex. Lifting weights

ISOMETRIC - Muscle doesn’t move but creates force

Ex. Yoga

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Types of Contractions

Smooth Muscles Smooth muscle cells:

-elongated with tapered ends -lack striations

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tw0 relaxed smooth muscle cells

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MultiunitBlood vesselsIris of the eye

Individual fibers

Walls of hollow organs

SheetsSelf-stimulating and

rhythmic

Peristalsis in tubes and hollow organs

Types of Smooth Muscle

Visceral

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Smooth Muscle Contraction

-Slower to contract and relax than skeletal muscle

-Can contract longer using the same amount of ATP

(sustainable contractions)

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Cardiac Muscle

- has structures that supply extra calcium, and can thus contract for longer periods

intercalated disks: join cells and transmit impulses throughout the heart

self-exciting and rhythmic

the whole heart contracts as a unit

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