"Muscling" Through

Let's look at muscles on a molecular level:

Myosin- A protein that converts the chemical energy stored in the bonds of ATP into the kinetic energy of movement

- Pulls on actin filaments made of globular protein subunits that assemble linearly

ATPase- The binding site on myosin that hydrolyzes ATP to ADP, releasing energy

Tropomyosin & Troponin- Regulate contraction by controlling the interaction between actin and myosin filaments

Titin- A very large protein in muscle cells that forms an elastic filament to hold structures together and prevent overstretching

Dystrophin- A protein that helps bind actin to the muscle cell membrane

Microscopic Level

Sarcomere- The fundamental functional unit of muscle

Myofilaments- Organized structures in muscle cells that contain the actin (thin filaments) and myosin (thick filaments)

Microscopically visible feature Composition

A band Region of thick myosin proteins

H zone Central region of A band with no overlapping actin proteins when muscle is relaxed

I band Region of thin-filament actin proteins with no myosin

M line M line accessory proteins in the center of myosin thick filament perpendicular to the sarcomere

Z disc Zig-zag line of Z line proteins and actin binding proteins perpendicular to the sarcomere

Cell Level

- Muscle cells have extremely high numbers of mitochondria to produce ATP for force generation

- Each skeletal muscle fiber cell is multinucleate

- The plasma membrane is called the sarcolemma

- Cytoplasm of the fused muscle fiber is called sarcoplasm

Structures unique to muscle cells include...

- T tubules: Indentations of the sarcolemma into the interior of the cell

- Terminal cisternae: Dilated region of the sarcoplasmic reticulum that forms on either side of the T tubule

Skeletal Muscle Cells

- Individual myoblasts migrate to different regions in the body, then fuse to form a myotube

- Structural proteins are added to muscle fibers in a process called hypertrophy

- Satellite cells help to repair skeletal muscle fibers

Cardiac Muscle Cells (myocytes)

- Individual cells contact several adjacent cells at specialized junctions called intercalated discs

- Gap junctions are tunnels or protein channels in the cell membrane connecting adjacent cardiac muscle cells, allowing ions involved in electric currents to move quickly from one cell to the next (electric coupling), which drives the contraction of the heart

- Desosomes are especially strong cell junctions that maintain structural intergrity

- There are two specialized types of myocytes:

1. Contractile cells, which produce the force for the heartbeat

2. Pacemaker cells, which trigger contraction by spontaneously depolarizing at set intervals

Smooth Muscle Cells

- They drive peristalsis, the alternating dilation and contraction or lengthening and shortening of the organ to move substances through internal passages

- Can divide to produce more cells through hyperplasia

- Can maintain contractions through a latch state, during which actin and myosin remain locked together in the absence of Ca2+ ions without using energy in the form of ATP

- Not under voluntary control; can be stimulated by pacesetter cells (similar to those in the heart)

- Varicosities along nerves release neurotransmitters to smooth muscle cells

- Two types of smooth muscle cells:

1. Single-unit, which contain gap junctions and allow for electric coupling

2. Multi-unit, which rarely possess gap junctions: stimulation comes from nervous impulses or hormone release

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Acetycholine (Ach)- A neurotransmitter released by motor neurons that binds to receptors in muscle cells to allow for voluntary movement

ATP can be regenerated in muscles through creatine phosphate metabolism (fastest method, but produced less ATP), anaerobic glycolysis, and aerobic respiration (slowest method, but produces the most ATP)
Fun fact: After the first few seconds of exercise, available ATP is used up. After the first few minutes, cellular glucose and glycogen are depleted. After the next 30 minutes, the body's supply of glucose and glycogen is depleted, so after that time, fatty acids and other energy sources are used to make ATP. So, you have to exercise for more than 30 minutes to lose body fat!
Three main types of skeletal muscle fibers:

Slow oxidative (use aerobic respiration and are involved in endurance)
Fast oxidative (an intermediate fiber)
Fast glycolytic (use anaerobic glycolysis and are involved in speed)

Most muscles possess a mixture of each fiber type

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19) MUSCULAR HOMEOSTASIS

Skeletal muscle contribute to maintaining temperature homeostasis in the body by generating heat as a byproduct of metabolism.

20) INTEGRATION OF THE MUSCULAR SYSTEM

- Muscles require a constant supply of oxygen and nutrients from the cardiovascular system

- Skeletal muscles are under neural control; the epimysium contains fibers of the nervous system that connect to muscle fibers

- Smooth muscles play a role in many other organ systems, including the digestive, respiratory, and reproductive tracts. They are also in the walls of blood vessels.