MUSCLE TISSUEMuscle Contraction

TYPES OF MUSCLE TISSUE

A. Skeletal muscle-voluntary muscle, striated with light and dark bands, many nuclei per cell.

Found in limbs, trunk of body, jaws, face, eyes etc.

B. Smooth muscle—involuntary, visceral muscle forms muscular layer of organs. Has one nucleus per cell, contracts involuntarily, elongated. Rhythmic contractions.

Digestive tract, bladder, ducts, arteries and veins.

C. Cardiac muscle-involuntary, similar to skeletal-striated but controlled by the autonomic nervous system. Branched cells with central nuclei. Intercalated discs assure impulses travel between cells.

Heart muscle

SKELETAL MUSCLE COMPONENTS

MICROSOPIC ANATOMY OF SKELETAL MUSCLE

A. Muscles contain bundles of fascicles that contain muscle fibers (cells).

B. Muscle fibers contain Myofibrils which contain sarcomeres. Myofibrils play a key role in muscle contraction.

C. Sarcomeres contain: Actin-thin filaments Myosin-thick filaments The organization between actin

and myosin create the striations we see in muscle tissue.

THE SARCOMERE

The sarcomere is the basic unit of the muscle fiber.A. I band- contain thin actin filamentsB. Z lines-attach to the thin actin filamentsC. A bands-thick myosin and thin actin filaments overlapSarcomere Shortening Video

MUSCLE CONTRACTION The sarcoplasmic reticulum is a series of

membrane channels that surround the myofibrils.

The transverse tubules extend from the sarcoplasmic reticulum

Together these structures help activate muscle contraction

Neuromuscular junction

THE NEUROMUSCULAR JUNCTION

A. Connects nerve cells called motor neurons with muscle cells at a specialized region on the muscle called the motor end plate.

B. Neurotransmitters-chemicals released by the nerve cell which stimulate muscles to contract. For muscle contraction acetylcholine (ACh) is used.

C. Target of neurotransmitters is the sarcomere in the muscle.

THE NEUROMUSCULAR JUNCTION

STEPS OF MUSCLE CONTRACTION1. Action potential-electrical signal from a motor neuron (nerve fiber) causes a change in the electrical charge of the membrane in the nerve cell (Na+ rushes in).

2. The action potential causes a neurotransmitter (chemical that sends signals from the nervous system) called ACh ( acetylcholine) is released and bind to ACh receptors on muscle cell membrane,

3. This signals the muscle fibers in the sarcoplasmic reticulum which causes Ca++ rushes in from surrounding sarcoplasmic reticulum.

4. Ca++ binds to a molecule called troponin on actin which causes the shape to change.

5. Cross bridges attach and detach on actin and myosin causing contraction of muscle. The muscle fiber shortens.

Myofilament Contraction video

6. For the muscle to relax, ACh, and Ca++ are taken up and chemical energy ATP is needed to break actin apart from myosin.

Threshold stimulus-minimum stimulus needed for a muscle to contract.

All or None Response-partial contraction does not happen, the muscle contracts when the threshold stimulus occurs. A stronger stimulus does not affect the contraction.

WHAT IS ADENOSINE TRIPHOSPHATE (ATP) (SEE NEXT SLIDE FOR DIAGRAM)

ATP is the source of energy for muscle contraction/relaxation.

Muscles store a very limited amount of ATP (4-6 sec. worth at most).

ATP must be regenerated if contraction is to continue

ATP can be generated by one of three pathways….

1. AEROBIC METABOLISM/CELLULAR RESPIRATION

• Occurs in mitochondria, requires oxygen, and involves acomplex sequence of chemical reactions.

Glucose + oxygen carbon dioxide + water+ energy (36 ATP)

When this system is used:

• Used during prolonged, endurance activities which maintain a constant heart rate

• As long as there is enough oxygen available relative to ATPdemand, a muscle will utilize aerobic metabolism.

• Posture, most daily movements, marathon runs, bike riding

2.) ANAEROBIC METABOLISM/GLYCOLYSISo Description:

• When muscles contract vigorously, the bulging muscles compress blood vessels within, impairing oxygen delivery.

• This pathway can occur in the absence of oxygen. Chemical Formula:

Glucose lactic acid +energy (2 ATP)

When this system is used:

• When large amounts of ATP relative to available O2 are needed. Usually for exercise about 30-40 seconds long.

• Tennis, soccer, 100 meter swim

3. ANAEROBIC METABOLISM/CREATIN PATHWAY

1. Description:

• Utilizes the molecule creatine phosphate to regenerate ATP, after stored ATP is depleted.

2. Chemical Formula:

Creatine phosphate + ADP Creatine + ATP (1 ATP)

3. When this system is used:

• Moblized at the beginning of exercise, before other pathways ‘kick in’.

• Creates enough energy for about 15 seconds of contraction.

• 100 meter dash, weightlifting

MUSCLE FATIGUE

Definition: the state of physiological inability to contract

Causes:1. Occurs when ATP production fails to keep up with ATP use.• Deficiency of ATP, as opposed to total absence of ATP which leads to a continuous state of contraction (rigor mortis, writers cramp)

2. Accumulation of lactic acid and ionic imbalances alsocontribute.• decreases muscle pH• causes burning sensation• interferes with actin/myosin binding

SUMMARY OF MUSCLE METABOLISMCreatine pathway

Glycolysis Pathway

Aerobic pathway

Requires oxygen

no no yes

Amount of ATP produced

1 ATP 2 ATP 36 ATP

Duration 15 seconds 30-60 seconds hours

By product creatine lactic acid CO2 and H2O

Advantages • Rapid energy• Easily

mobilized

• Quick energy

• Does not require oxygen

• More ATP/glucose

• Waste products are easy to excrete

Disadvantages • Small amount of creatine available

• Small amount of ATP

• less ATP/glucose

• Lactic acid produces muscle fatigue

• Slower• Limited by

oxygen

FAST TWITCH VS. SLOW TWITCH MUSCLESSPRINTER VS. MARTHONER

SLOW TWITCH/FAST TWITCH

Analogy: birdsWhite meat-fast twitch, found in breast and wings,

used for flying-quick movements. Dark meat-slow twitch, found in legs, used for

walking.

Fast Twitch Slow Twitch

Lighter in color-less blood Darker in color-rich in blood

Few mitochondria Many Mitochondria

Mostly anaerobic Primarily aerobic

Easily fatigue Fatigue slowly

More forceful contractions Less forceful contractions

Increase in size with training Don’t increase in size with training

Sprinters Distance runners

MUSCLE CONDITIONS

Muscle cramps- (charley horse)sudden involuntary contraction of muscles due to injury, deficiency in calcium or phosphate, lack of blood flow, nerve tissue damage or dehydration.

Muscle twitches -often cause by a single nerve, due to fatigue, lack of water, stress and anxiety.

ALS-Lou Gehrig’s disease - damages motor neurons, leads to loss of motor movement.

Muscular dystrophy -skeletal muscle progressively weaken.