Exam unit #4 BI121 Lecture GRCC Jones

How many types of muscle tissue are in the muscular system?

3; Skeletal, cardiac, and smooth

Skeletal muscle

• Attached to bones of skeleton

• Voluntary (consciously controlled)

• Organ of the muscular system

Cardiac Muscle

• Makes up most of the wall of the heart

• Involuntary (non-consciously-controlled)

• Responsible for pumping action of the heart

Smooth muscle

• Found in walls of internal organs, such as those of digestive tract

• Involuntary (non-consciously-controlled)

Skeletal muscles are composed of:

Skeletal muscle tissue

Nervous tissue

Blood

connective tissues

Connective tissue coverings over skeletal muscles

Fascia

Tendons

Aponeuroses

Epimysium

Surrounds whole muscle

Perimysium

Surrounds fascicles within a muscle

Endomysium

Surrounds muscle fibers (cells) within a fascicle

What do myofibrils consist of?

Sarcomeres connected end-to-end

What makes skeletal muscle cells striated?

Arrangement of myofilaments in myofibrils

Sarcomeres contain these structures

I Band (thin filaments)

A band (thick and thin filaments)

H zone (thick filaments)

Z line (or z disc)

M line

Striation pattern has 2 main parts

I band and A band

I Band

Light band, composed of thin actin filaments

A Band

dark band, composed of thick myosin filaments with portions overlapped with thin actin filaments

H zone

Center of A band; composed of thick myosin filaments

Z line

Anchors filaments in place; sarcomere boundary; center of I band

M line

Anchors thick filaments; center of A band

Thick filaments

• Composed of myosin protein; heads from cross-bridges

Thin Filaments

• Composed of actin protein; associated with troponin and tropomyosin, which prevent cross-bridge formation when muscle is not contracting

Requirement for contraction of a muscle fiber

Interaction from several chemical and cellular components

Causes of contraction of a muscle fiber

A movement within the myofibrils, in which the actin and myosin filaments slide past one another, shortening the sacromeres

Contraction of a muscle fiber

Muscle fiber shortens and pulls on attachment points

The Sliding Filament Model

When sarcomeres shorten, thick and thin filaments slide past one another

Sliding Filament Model Cont.

• H zones and I bands narrow

• Z lines move closer together

• Thin and thick filaments do not change length

• Overlap between filaments increases

The action of each muscle mostly depends upon:

• The type of joint it is associated with

• The way the muscle is attached on either side of the joint

When bones or body parts move, bones and muscles act as what?

Levers

4 basic components of levers

• Rigid bar or rod (bones)

• Fulcrum or pivot on which bar moves (joint)

• Object moved against resistance (weight)

• Force that supplies energy for movement (muscles)

Origin

less moveable end

Insertion

more movable end

When a muscle contracts:

Insertion is pulled toward origin

Most skeletal muscle function in

groups

Agonist

muscle that causes an action

Prime mover

agonist primarily responsible for movement

Prime mover and agonist note:

(in some cases, the terms “agonist” and “prime mover” are used interchangeably)

Synergists

muscles that assist agonist/prime mover

Antagonist

Muscles whose contraction causes movement in the opposite direction of the prime mover

Lifespan Changes with Muscles

• Myoglobin, ATP, and creatine phosphate decline, starting in the 40

• CT and adipose cells replace some muscle tissue

• By age 80, almost half of muscle mass has atrophied

• Muscle strength decreases, and reflexes become slower

• Exercise helps to maintain muscle mass and function

Neuromuscular Junction (myoneural junction)

• a type of synapse

• site where an axon of motor neuron and skeletal muscle fiber interact

• skeletal muscle fibers contract only when stimulated by a motor neuron

Parts of a NMJ

• Motor neuron

• Motor end plate

• Synaptic cleft

• Synaptic vesicles

• Neurotransmitters

Stimulus for contraction neurotransmitter

Acetylcholine (ACh)

Nerve impulse causes

Release of ACh from synaptic vesicles

ACh binds to ACh receptors on

motor end plate

ACh causes changed in membrane permeability to

NA+ and K+ (which generates a muscle impulse [action potential])

Impulse causes relase of CA+2 from SR, which leads to

muscle contraction

Excitation-contraction coupling

Connection between muscle fiber stimulation and muscle contraction

During Muscle Relaxation

• CA+2 ions are stored in SR

• Troponin-tropomyosin complexes cover binding sites on actin filaments

Upon Muscle Stimulation

• Muscle impulses cause SR to release CA+2 ions into cytosol

• CA+2 on binds to troponin to change its shape

• Each tropomyosin is held in place by a troponin molecule. the change in shape of troponin alters the position of tropomyosin.

• Binding sites on actin are now exposed

• Myosin heads bind to actin, forming cross-bridges.

Cross-Bridge Cycling

• Myosin head attaches to actin binding site, forming cross-bridge

• Myosin cross-bridge pulls thin filament toward center of sarcomere

• ADP and phosphate are released from myosin

• New ATP binds to myosin

• Linkage between actin and myosin cross-bridge break

• ATP splits

• Myosin cross-bridge goes back to original position

Threshold stimulus

Minimum strength of stimulation of a muscle fiber required to cause contraction

When strength of stimulus reaches threshold,

an action potential is generated

Impulse spreads through muscle fiber,

releasing CA+2 from SR and activating cross bridge formation

One action potential from a motor neuron releases enough ACh to produce threshold stimulus in muscle fiber, causing:

a muscle impulse

Motor Unit

• A motor neuron plus all of the muscle fibers it controls

• A whole muscle consists of many motor units

• Coarse movements are produced with large numbers of fibers in a motor unit

• Precise movements are produced with fewer muscle fibers in a motor unit

Sustained Contractions

• Smaller motor units (smaller diameter axons) - recruited first

• Larger motor units (larger diameter axons) - recruited later

• Summation and recruitment can produce sustained contractions of increasing strength

• Whole muscle contractions are smooth movements

Muscle Tone (tonus)

Continuous state of partial contraction in resting muscles

Recruitment

• Increase in the number of motor units activated, to produce more force

• Certain motor units are activated first, and others are activated only when the intensity of stimulus increases

• As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated.

Muscular Responses

• Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of the muscle fiber.

• Electrical stimulator promotes the contractions

Twitch

Contractile response of a single muscle fiber to a single impulse

• Latent period

• Period of contraction

• Period of relaxation

Summation

• Process by which the force of individual muscle fiber twitches combine

• Produces sustained contractions

• Can lead to partial or complete tetanic contractions

Relaxation

When neural stimulation of muscle fiber stops:

• Acetylcholinesterase (enzyme) rapidly decomposes ACh remaining in the synapse

• Muscle impulse stops when ACh is decomposed

• Stimulus to sarcolemma and muscle fiber membrane ceases

• Calcium pump moves CA+2 back into sarcoplasmic reticulum (SR)

• Troponin-tropomyosin complex again covers binding sites on actin

• Myosin and actin binding are now prevented

• Muscle fiber relaxes

Energy sources for contraction

1. ATP reserves: small amount

2. Creatine-phosphate: initial source of energy to regenerate ATP from ADP and P

3. Cellular respiration

Cellular Respiration - Anaerobic Phase

• Glycolysis

• Occurs in cytoplasm

• Produces little ATP

Cellular Respiration - Aerobic Phase

• Citric acid cycle and electron transport system

• Occurs in the mitochondria

• Produces the most ATP

• Myoglobin stores extra oxygen in muscles

Oxygen Debt

• Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.

• During rest or moderate exercise, respiratory and cardiovascular systems supply enough O₂ to support aerobic respiration

Anaerobic (Lactic Acid) Threshold

• Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O₂. Lactic acid is produced.

Muscle Fatigue

Inability to contract muscle

Common causes of muscle fatigue:

• Decreased blood flow

• Ion imbalances across the sarcolemma

• Loss of desire to continue exercise

• Accumulation of lactic acid (controversial).

Muscle Cramp

• Sustained, involuntary muscle contraction

• May be caused by changes in electrolyte concentration in extracellular fluids in the area

Heat Production

• Heat is a by-product of cellular respiration in active cells

• Muscle cells are major source of body heat

  • If you need to warm up, move around (voluntary) or shiver (involuntary)

• More than half the energy released in cellular respiration becomes heat; less than half is transferred to ATP.

• Blood transports heat throughout body core.

Length-tension relationship

• Length of muscle fiber before stimulation determines amount of force it can develop

• Optimum starting length is resting length of the muscle fiber; this allows the greatest force to develop

• Stretched muscle fibers develop less force, since some myosin heads cannot reach binding sites on actin.

• Shortened muscle fibers also develop less force, since compressed

• Sarcomeres cannot shorten further

Isotonic

Muscle contracts and changes length; equal force

Concentric

Shortening contraction

Eccentric

Lengthening contraction

Isometric

Muscle contracts but does not change length; change in force

Slow-twitch fibers (Type I)

• Always oxidative

• Resistant to fatigue

• Red fibers

• Abundant myoglobin

• Good blood supply

• Many mitochondria

• Slow ATPase activity; slow to contract

Fast-twitch fatigue-resistant fibers (Type IIa)

• Intermediate twitch fibers

• Intermediate oxidative capacity.

• Intermediate amount ofmyoglobin.

• White fibers.

• Resistant to fatigue.

• Rapid ATPase activity.

Fast-twitch glycolytic fibers (Type IIb)

• Anaerobic respiration (glycolysis).

• White fibers (less myoglobin).

• Poorer blood supply.

• Fewer mitochondria than fast-twitch.

• More SR than fast-twitch.

• Susceptible to fatigue.

• Fast ATPase activity; contract rapidly.