A&P Unit 4 - Muscles: Part 1

Skeletal muscle

• Location: attached to skeleton or other connective tissue

• Cell appearance: long and cylindrical

• Nucleus: multinucleated, located on periphery

• Function: move the body

• STRIATED

• VOLUNTARY

Cardiac muscle

• Location: heart

• Cell appearance: cylindrical, branch

• Nucleus: single nucleus, centrally located

• Function: contract the heart; force for moving blood through vessels

• Features: branching fibers; intercalated discs with gap junctions

• STRIATED

• INVOLUNTARY

Smooth muscle

• Location: walls of organs, blood vessels, eyes, glands, skin

• Cell appearance: spindle-shaped

• Nucleus: single, centrally located

• Function: muscular function of organs; multiple functions

• Features: gap junctions

• NON-STRIATED

• INVOLUNTARY

Functions of skeletal muscle tissue

• Produce skeletal movement

• Maintain posture and body position

• Support soft tissues

• Guard body entrances and exits

• Maintain body temperature

• Provide nutrient reserves

Properties of skeletal muscle tissue

• Excitability: react to stimuli

• Conductivity: spread electrical impulse through muscle cell

• Contractility: shorten when stimulated

• Extensibility: can stretch without harm

• Elasticity: can recoil from stretch

Skeletal muscles are organs made of

• Skeletal muscle tissue

• Connective tissue

• Nerves

• Blood vessels

Fascia

Wraps muscle group

Epimysium

Wraps muscle

Perimysium

Wraps fascicle

Endomysium

Wraps cell

Muscle cell (myocyte) =

Muscle fiber

Muscle words start with

Myo- or sarco-

Sarcoplasm

Muscle cytoplasm

Sarcolemma

Muscle cell plasma membrane

Myofibril

Inside the muscle cell

Myofilament

Make up the myofibrils

Transverse (T)-Tubules

Tubular infoldings of sarcolemma, go through cell

Sarcoplasmic Reticulum

Muscle cell ER

Terminal Cisternae

Enlarged portions of sarcoplasmic reticulum

Myofibrils features

• Repeating units of sarcomeres that run the length of the muscle fiber

• Has stripes called striations

• Made of thick and thin myofilaments

Sarcomere

• Muscle cell contractile unit

• During contraction, sarcomeres shorten

• Runs from one Z line to the next Z line

Z line

In the middle of I band, hold sarcomere together, actin connected here

A band

Runs the length of the thick filament

H band

Composed of thick filament only, in the middle of A band

M line

In the middle of H band

I band

Composed of thin filament only

Thick filaments

Composed of myosin

Thin filaments

• Composed of mostly actin

• Contains troponin and tropomyosin

Troponin

Switch - binds Ca2+, actin, tropomyosin

Tropomyosin

Blocks binding sites for myosin

Sliding Filament Theory of Contraction

When a skeletal muscle fiber contracts, the thin filaments slide past the thick filaments

When muscle contracts

• I band - decreases

• H band - decreases

• A band - no change

• Zone of overlap - increases

• Distance between Z-lines - decreases

Neuromuscular Junction (NMJ)

• Composed of the axon terminal, motor end plate, and the synaptic cleft

• Special type of synapse

Events at the NMJ

1. Electrical signal called an action potential travels along a nerve fiber

   1. An action potential is a sudden change in membrane potential

2. The action potential causes the opening of voltage-gated Ca²⁺ channels

3. Calcium (Ca²⁺) ions enter into the axon terminal

4. This causes the exocytosis of vesicles filled with Acetylcholine

   1. Acetylcholine is a neurotransmitter

5. Acetylcholine is released into the synaptic cleft and binds to Acetylcholine (Ach) Receptors

   1. Ach-Receptors are ligand-gated Na⁺ channels

6. Bind of Ach to Ach-Receptors causes sodium (Na⁺) to enter the cell and change the membrane permeability

7. This generates an action potential along the sarcolemma of the muscle fiber

Excitation-Contraction Coupling

1. Neural control - A skeletal muscle fiber contracts when stimulated by a motor neuron at a neuromuscular junction. The stimulus arrives in the form of an action potential at the axon terminal

2. Excitation - The action potential causes the release of Ach into the synaptic cleft, which leads to excitation - the production of an action potential in the sarcolemma

3. Release of calcium ions - This action potential travels along the sarcolemma and down T tubules to the triads. This triggers the release of Calcium ions (Ca2+) from the terminal cisternae of the sarcoplasmic reticulum

4. Contraction cycle begins - The contraction cycle begins when the calcium ions (Ca2+) bind to troponin, resulting in the exposure of the active sites on the thin filaments. This allows cross-bridge formation and will continue as long as ATP is available

5. Sarcomere shortening - As the thick and thin filaments interact, the sarcomeres shorten, pulling the ends of the muscle fiber closer together

6. Generation of muscle tension - During the contraction, the entire skeletal muscle shortens and produces a pull, or tension, on the tendons at either end

Cross-bridge formation

1. Contraction cycle begins - Arrival of calcium ions (Ca2+) within the zone of overlap in a sarcomere

2. Active-site exposure - Calcium ions bind to troponin, weakening the bond between actin and the troponin-tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads

3. Cross-bridge formation - once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges

4. Myosin head pivoting - After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This is called a power stroke. When it occurs, the bound ADP and phosphate groups are released

5. Cross-bridge detachment - When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge

6. Myosin reactivation - Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head

Steps that initiate a muscle contraction

1. Ach released - Ach is released at the NMJ and binds to Ach receptors on the sarcolemma

2. Action potential reaches T Tubule - An action potential is generated and spreads across the membrane surface of the muscle fiber and along the T tubules

3. Sarcoplasmic reticulum releases Ca2+ - The sarcoplasmic reticulum releases stored calcium ions

4. Active site exposure and cross bridge formation - Calcium ions bind to troponin, exposing the active sties on the thin filaments. Cross-bridges form when myosin heads bind to those active sites

5. Contraction cycle begins - The contraction cycle begins as repeated cycles of cross-bridge binding, pivoting, and detachment occur - all powered by ATP

Steps that end a muscle contraction

1. Ach is broken down - Ach is broken down by acetylcholinesterase (AchE), ending action potential generation

2. Sarcoplasmic reticulum reabsorbs Ca2+ - As the calcium ions are reabsorbed, their concentration in the cytosol decreases

3. Active sites covered, and cross-bridge formation ends - Without calcium ions, the tropomyosin returns to its normal position and the active sites are covered again

4. Contraction ends - Without cross-bridge formation, contraction ends

5. Muscle relaxation occurs - The muscle returns passively to its resting length

Important to know about contraction cycle

• Starts when calcium ions arrive from the sarcoplasmic reticulum

• Calcium ions bind to a protein called troponin, causing the exposure of the active (binding) site on actin

• Activated myosin heads form cross-bridge with the actin binding site

• When myosin heads pivot, they move the thin filament, this is the “power stroke”

• Myosin needs another ATP to repeat the cycle

• The cycle continues as long as calcium and ATP are present

• Can repeat several times each second