Anatomy Ch.10 Structure of a Skeletal Muscle

Connective Tissue coverings, skeletal muscle fibers and Skeletal muscle contraction

Fascia

Layers of dense connective tissue that hold a muscle in position, and separate it from adjacent muscles; between the skin and bones

Tendon

Cord-like, connects muscle bone

Aponeuroses

sheets of connective tissue that connect to bone or other muscles

epimysium

surrounds entire muscle

perimysium

separates/surrounds the Fascicles

Fascicles

bundles of muscle fibers (cells)

endomysium

surrounds each individual muscle fiber

deep fascia

surrounds and penetrates muscle

subcutaneous fascia

lies just beneath the skin (think of submarine)

Skeletal Muscle Fibers

long, thin cells

Sacrolemma

plasma membrane

sacroplasm

cytoplasm

sacroplasmic reticulum

like the ER

The sacromere

Functional unit of muscle cell; packet of microfilaments; many packets make up the entire length of the muscle fiber

Myofibrils

protein strangs running through the sacroplasm: produces striations and sacromeres

Myofibrils- Myosin

Thick filaments- made up of two twisted strands with globular cross-bridges (heads)

Myofibrils- Actin

thin filaments- have binding sites for myosin attachment

Actin filaments

Thins filaments, are lighter in color, and called I bands

Myosin filaments

thick filaments, are darker in color, called the A bands

Z discs/lines

I bands (actin) are attached to them; sacromeres extends from one Z line to the next

Titin

attaches myosin to Z lines

Troponin-tropomyosin complex

covers the binding sites

Transverse tubules

Membranous channels, are invaginations of the Sacrolemma; run between parts of the SR called the cisternae

NMJ

Neuromuscular Junction

Motor neurons

control effectors (skeletal muscle); each skeletal muscle fiber is connected functionally to an axon of a motor neuron - does not physically touch

Excitation-contraction coupling

How the neuron gives the skeletal muscle information; how the nervous system communicates with the skeletal muscle

Synapse

The space between the muscle fiber and the axon- the functional connection; indicated by a space called synaptic cleft

neurotransmitters

clinical release from the axon so that the nerve cell can communicate with muscle cell (think of powerline)

neuromuscular junction

site where an axon and a muscle fiber meet/synapse; there the sacrolemma specializes to form a motor end plate

Motor unit

motor neuron and the muscle fiber controls

Stimulus for contraction

acetylcholine (ACh), synaptic vesicles, muscle impulse

acetylcholine (ACh)

neurotransmitter that motor neurons use to control skeletal muscles

synaptic vesicles

Store ACh in distal end of axon → ACh is released into the synaptic cleft when a nervous impulse reaches the end of the axon → it diffuses the synaptic cleft and combines with ACh receptors on the motor endplate of the muscle fiber

Muscle impulse

response of the muscle; an electrical signal like a nerve impulse

The sliding filament model of muscle contractions

The sacromere shortens- the Z bands are pulled closer together all along the length of the muscle fiber

Myosin and actin slide past each other- due to cross bridge attachment- thin filaments move towards the center of the sacromere

Cross Bridge Cycling

Heads of Myosin are “cocked” when they attack → the pull the actin towards the center of the sacromere → releases them → re-clock → and then attach to the binding sites again

Use ATP to cock the myosin head → attach → move → release

Cross bridge attachment may happen over and over again AS LONG AS THERE IS ATP PRESENT, and a stimulus to contract (release of ACh at the neuromuscular junction)

Relaxation- acetylcholinesterase

Enzyme that breaks down ACh; prevents a single nerve impulse from continuously stimulation a muscle fiber

• No ACh, no stimulus to release calcium from cisternae - so the troonin goes back to its normal shape, and the tropomyosin blocks the binding sites on the actin once again.

Energy sources for contraction

ATP

ATP

cross bridge Cycling require ATP at every step = power stroke

Sources of ATP:

• Creatine phosphate (15 seconds of energy)

• Aerobic cellular respiration

• Anerobic cellular respiration

Anaerobic respiration

Glycolysis

Aerobic respiration

requires oxygen; used in the Electron Transport chain

• Blood carries oxygen to the muscles; muscles also have the pigment myoglobin to store oxygen

• blood flow may decrease during muscle contraction when muscle fibers compress blood vessels

Oxygen debt

the amount of oxygen necessary to metabolize lactic acid, amount of oxygen needed to compensate for ATP produced without oxygen; restores ATP and creatine phosphate levels

Muscle Fatigue

• loss of ability to contract

• may be due to decrease blood flow, ion imbalances, etc.

• Most likely due to accumulation of lactic acid

• cramps- sustained, painful, involuntary muscle contraction

Heat production

• Heat is a by-product off cellular respiration

• Muscle tissue is a major source of body heat

Muscle strength

• Genetically predetermined; depends on the number of myofibrils and sacromeres within each fiber

• when muscles become stronger → add more actin and myosin → causes the fibers to become larger

• when muscles atrophy - loss of myofibrils (but not fibers)