Fibers: composed of myofibrils

Myofibrils composed of protein filaments:

1. Myosin: thick filaments

2. Actin: thin filaments

Muscle structure:

Muscle Muscle fiber Myofibril

Actin

Myosin

Muscle Structure

Sarcomere: the muscle contraciton unit

Sarcomere Components

1. A Band: area of myosin filaments2. I Band: area between the “A” bands – both

actin and myosin3. H Zone: thickened area of myosin4. Z line: center of the I band5. Z to Z: equals one sarcomere

Muscle Contraction Components

1. Motor Neurons: cells of the nervous system – pass impulse from brain or spinal cord

Motor Unit: muscle fiber and motor neuron

2. Sarcolemma: the cell membrane of muscle fibers

3. Transverse Tubule: deep invaginations, enfoldings of the sarcolemma

4. Sarcoplasmic Reticulum: network of muscle contractions tubules that carry the muscle impulse

5. Mitochondria: cellular organelle which synthesizes adenosine triphosphate (ATP)

6. Tropomyosin: a protein attached to the actin filament which blocks myosin-binding sites

7. Troponin: a protein complex located on the tropomyosin

-Receives calcium from the sarcoplasmic reticulum

8. Myosin Cross Bridge: portion of the myosin filament which binds to the actin binding site

9. Calcium: ions released from sarcoplasm – bind to troponin – clear the myosin binding sites

10. Acetylcholine (ACh): the neurotransmitter released by the motor neurons

Neuromuscular Junction

Sliding Filament Contraction Theory

General Sequence of Events:-neural response is sent to the sarcoplasmic

reticulum-thin filament (actin) slides past the thick

filament (myosin)- “I” band becomes smaller- Actin/myosin slide back and forth

Sliding Filament Contraction Theory

Detailed Sequence of Events: 8 steps1. Neural impulse (polarization/depolarization)

from motor neuron moves to the synaptic knob and releases acetylcholine (ACh)

2. ACh carries the impulse across the synaptic cleft to the sarcolemma

Sliding Filament Contraction Theory

3. ACh carries the impulse along the sarcolemma to the transverse tubules. The ACh enters the transverse tubule to the sarcoplasmic reticulum.

4. ACh activates the sarcoplasmic reticulum to release Ca++ (calcium ions)

5. Ca++ binds to the troponin, moving the tropomyosin off of the binding sites on actin

Sliding Filament Contraction Theory

6. Cross bridge of myosin connects to actin7. The cross bridge performs a “power stroke” –

which shortens the sarcomere-ATP ADP is the energy source

8. Myosin grabs actin and pulls – the muscle contracts

Relaxation

1. Impulse stops – no ACh2. ACh in tubules breaks down by acetylcholine

esterase (AChE)3. Ca++ reabsorbed by sarcoplasmic reticulum4. Sarcomere lengthens and relaxes

Smooth Muscle:

FX:1. move food through intestine2. adjust the eye for light3. move blood through vessels4. moves babies from uterus through birth canal

Characteristics/Location- Muscle in hollow organs (except heart)- Digestive, urinary, circulatory and reproductive- One nucleus per myocyte- No striations/sarcomeres/ “T” tubules- Fusiform shape- lay in sheets at right angles- Alternating contraction/relaxation of sheets

causes elongation – peristalsis- Ca++ comes from extra-cellular fluid

Cardiac Muscle:

-makes up most of the heart

-has striations like skeletal muscle

-myocytes branched and interconnecting

-each myocyte stimulates its neighbor – chain reaction

Cardiac Muscle:

- Large T-tubules admit more Ca++ for contraction

- Sino-Atrial node: bundle of nervous tissue that stimulates contraction