Fibers: composed of myofibrils Myofibrils composed of protein filaments: Myosin: thick filaments Actin: thin filaments

Muscle structure: Actin Muscle Muscle fiber Myofibril Myosin

Muscle Structure

Sarcomere: the muscle contraciton unit

Sarcomere Components A Band: area of myosin filaments I Band: area between the “A” bands – both actin and myosin H Zone: thickened area of myosin Z line: center of the I band Z to Z: equals one sarcomere

Muscle Contraction Components

Motor Unit: muscle fiber and motor neuron 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 steps 1. 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 ACh carries the impulse along the sarcolemma to the transverse tubules. The ACh enters the transverse tubule to the sarcoplasmic reticulum. ACh activates the sarcoplasmic reticulum to release Ca++ (calcium ions) Ca++ binds to the troponin, moving the tropomyosin off of the binding sites on actin

Sliding Filament Contraction Theory Cross bridge of myosin connects to actin 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 Impulse stops – no ACh ACh in tubules breaks down by acetylcholine esterase (AChE) Ca++ reabsorbed by sarcoplasmic reticulum Sarcomere lengthens and relaxes

Smooth Muscle: FX: 1. move food through intestine 2. adjust the eye for light 3. move blood through vessels 4. 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