Anatomy & Physiology

Chapter 6: The

Muscular System

Muscles are responsible for causing movement in the body.

Three types:SkeletalCardiacSmooth

What are their differentiating characteristics?

Some Prefixes that relate to muscles:-Myo & -mys refer to muscle directly-Sacro refers to “flesh”

Muscular System-General Information

Comparison of Muscle Types

Comparison of Muscle Types

Anatomy of Skeletal MuscleCells are surrounded

and bundled by connective tissueEndomysium—wraps

around A SINGLE muscle fiber

Perimysium—wraps around a FASCICLE (bundle) of muscle fibers

Epimysium—covers the entire skeletal muscle

Fascia—on the outside of the epimysium*chicken breast example

Epimysium blends into one of the following connective tissue attachmentsTendons—cord-like structures

Mostly made of collagen fibersConnect _________ to ______________.

Aponeuroses—sheet-like structuresAttach muscles indirectly to bones,

cartilages, or connective tissue coverings*Think of it as “roots” of a muscle.

Skeletal Muscle Attachments

• Name 4 characteristics of Smooth Muscle:

1. Appearance:2. Nuclei:3.Common Location:4.Action:

Smooth Muscle Characteristics

Cardiac Muscle CharacteristicsName 5

characteristics:1. Appearance:2. Location:3. Nuclei:4. Action:5. Other:

In this chapter we will be focusing mainly on skeletal muscle

What are the 4 main functions:1.2.3.4.

Functions of Skeletal Muscles

Microscopic Anatomy of Skeletal MuscleSarcolemma—

specialized plasma membrane of muscle cells

Myofibrils—long organelles inside muscle cellLight and dark band

alternate giving muscles a “striated/striped” appearance

Light “I” bandsContain thin filaments

Another name for “thin filaments”?

Dark “A” bandsContain thick filaments

Myofibrils-Closer Look

Sacromeres are found within myofibrils and are composed of actin and myosin filaments.

It is known as the part of the muscle fiber that creates contractions.

Structure:Thick filament: Myosin filament

Made of protein myosinHave projections called myosin “heads”Has enzymes that break ATP, known as ATPase

Thin filament: Actin filamentComposed of protein actin

Myosin and Actin overlap to create cross bridging

Sacromeres

CrossBridge Cycling Occurs when thick and

thin filaments connect during a muscle contraction.

The myosin heads help with the binding process.

When they’re aren’t in a cross bridge cycle there is a “bare-zone”This occurs when

muscle is relaxed

Microscopic Anatomy of Skeletal MuscleSarcoplasmic

reticulum—specialized smooth ERThe SR surrounds

each myofibril and stores calcium.

Calcium is used to aid in muscle contraction.

How would a deficiency in electrolytes inhibit the sacroplasmic’s optimum action?

Excitability (also called responsiveness or irritability)—ability to receive and respond to a stimulusWhat are some examples of a stimulus?

Contractility—ability to shorten a muscle when needed

Extensibility—ability of muscle cells to be stretched/elongated.

Elasticity—ability to recoil and resume resting length after stretching.Muscle returns to normal state after a stretch

phase

Terms Related To Contraction of Skeletal Muscle

A motor nerve which innervates muscle must be stimulated in order to create a contraction.

Motor nerves have a neuron/axon They come in contact with a muscle via axon terminalsHence, the term “neuromuscular junction”The nerve endings and muscle cells never directly touch

The gap is known as “synaptic cleft” which absorbs the necessary neurotransmitter

Commonly used neurotransmitter: acetylcholine/AchCauses somewhat of a sodium/potassium pump mechanism

which creates an unbalanced electrical currentThe imbalance leads to an “action potential”The action potential travels along the cell and creates a

MUSCLE CONTRACTION

Nerve Stimulus & Action Potential

Synaptic gap contains ACh which attaches to the ACh receptors

GENERAL OVERVIEW OF HOW A NERVE STIMULUS OCCURS

Action Potential causing a muscle contraction

Transmission of Nerve Impulse to MuscleNeurotransmitter—

chemical released by nerve upon arrival of nerve impulseThe neurotransmitter

for skeletal muscle is acetylcholine (ACh)

ACh attaches to receptors on the sarcolemma

Sarcolemma becomes permeable to sodium (Na+)

Transmission of Nerve Impulse to MuscleSodium rushes into

the cell generating an action potential

Once started, muscle contraction cannot be stopped

How does it stop?Allowing entry of

Potassium ions

Neuromuscular Junction

Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filamentThin filament is???

Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere

The attachment/detachment of myosin heads causes crossbridgingResult?

the muscle is shortened (contracted)

The Sliding Filament Theory of Muscle Contraction

Crossbridge cycling

Muscle fiber contraction is “all or none”Within a skeletal muscle, not all fibers may be

stimulated during the same intervalDifferent combinations of muscle fiber

contractions may give different responses

Graded responses—different degrees of skeletal muscle shortening

Contraction of Skeletal Muscle

Contraction of Skeletal Muscle

Graded responses can be produced by changingThe frequency of

muscle stimulationThe number of

muscle cells being stimulated at one time

Muscle fiber contraction is “all or none”Once a muscle is stimulated all of it is

stimulated but not necessarily the WHOLE muscle

Within a skeletal muscle, not all fibers may be stimulated during the same interval

Different combinations of muscle fiber contractions may give differing responses

Graded responses—different degrees of skeletal muscle shortening

Contraction of Skeletal Muscle-General

Tetanus (summing of contractions)One contraction is immediately followed by anotherThe muscle does not completely

return to a resting stateThe effects are added

Leads to a stronger and smoother muscle contraction and

Fused (complete) tetanusNo evidence of relaxation before the following

contractionsResults in a sustained muscle contraction

Graded Responses-Tetanus

Muscle force depends upon the number of fibers stimulated

More fibers contracting results in greater muscle tensionDepends on action being performed

Muscles can continue to contract unless they run out of energy

Muscle Response to Stimuli

Initially, muscles use stored ATP for energy

ATP bonds are broken to release energy

Only 4–6 seconds worth of ATP is stored by muscles

After this initial time, other pathways must be

utilized to produce ATP

Muscles ONLY use ATP as their energy source

Energy for Muscle Contraction

Direct phosphorylation of ADP by creatine phosphate (CP)Muscle cells store CP

CP is a high-energy moleculeAfter ATP is depleted, ADP is

leftCP transfers energy to ADP by

adding a phosphate group, to regenerate ATP

Supplies are exhausted in less than 15 seconds

What exercises/activities would use this energy pathway???

Energy for Muscle Contraction

Aerobic respirationGlucose is broken down to carbon dioxide and

water, releasing energy (ATP)This is a slower reaction that requires

continuous oxygenCANNOT occur w/o oxygen

A series of metabolic pathways occur in the mitochondria

This pathway yields the MOST ATPAppx: 36 ATP/1 glucose molecule

Energy for Muscle Contraction

What exercises/activities would use this energy pathway???

Energy for Muscle ContractionAnaerobic glycolysis and lactic acid formation

Reaction that breaks down glucose without oxygenOccurs in cytosol

Glucose is broken down to pyruvic acid to produce some ATP

Pyruvic acid is converted to lactic acidThis reaction is not as efficient, but is fast

Huge amounts of glucose are neededLactic acid produces muscle fatigueOnly produces about 5% as much ATP as aerobic

respiration

What exercises/activities would use this energy pathway???

When a muscle is fatigued, it is unable to contract even with a stimulus

Common cause for muscle fatigue is oxygen debtOxygen must be “repaid” to tissue to remove

oxygen deficitOxygen is required to get rid of accumulated

lactic acidIncreasing acidity (from lactic acid) and lack

of ATP causes the muscle to contract lessIn extreme cases a person can collapse (i.e.

marathon runners)

Muscle Fatigue and Oxygen Deficit

Isotonic contractionsMyofilaments are able to slide past each other

during contractionsThe muscle shortens and movement occurs

Isometric contractionsTension in the muscles increasesThe muscle is unable to shorten or produce

movementThe static resistance causes myofibrils to spin

but NOT slide across each other

Types of Muscle Contractions

Some fibers are contracted even in a relaxed muscle

Different fibers contract at different times to provide muscle toneMuscle is firm and ready for activity

The process of stimulating various fibers is under involuntary control

Muscle Tone

Exercise increases muscle size, strength, and enduranceAerobic (endurance) exercise results in

stronger, more flexible muscles with greater resistance to fatigueHow is this resistance created at the cellular

level?Makes body metabolism more efficient

Resistance (isometric) exercise (weight lifting) increases muscle size and strengthMuscle fibers don’t increase rather the number

of myofibrils/myofilaments within the muscle fiber

Effect of Exercise on Muscles