Muscles Part 2

ATP Sources All muscle cells have some free ATP when relaxed but this is depleted almost immediately during contraction Once the ATP supply is depleted one of two things will happen: – More ATP must be produced rapidly – ATP will only be produced slowly by the mitochondria and the force produced will be less

Sources of ATP: Creatine Phosphate (anaerobic) – Rapidly converted to an ATP, runs out after a few more seconds Glycogen (anaerobic) – Stored glucose in the muscle fiber Blood Glucose (aerobic) – Only absorbed and metabolized slowly Fatty acids (aerobic) – Stored in muscle, metabolized slowly

ATP Depletion Once the muscle’s main ATP supply is depleted, the available force produced is much lower – Fatigue If all ATP is depleted, the muscle may lack the ATP supply to detach the myosin heads once the nerve signal ends – Cramp! Once the muscle is relaxed, blood glucose and O 2 are used to restore the ATP supply – This is known as repaying oxygen debt

Oxygen Debt If more energy is used than can be produced aerobically, anaerobic production of energy can sustain activity – Produces by products such as lactic acid Lactic acid can be reabsorbed and processed aerobically using more oxygen – This is why people pant and become winded if they have exerted themselves anaerobically

Muscle Fiber Types Muscle fibers come in three types: – Slow oxidative fibers – Fast oxidative fibers – Fast glycolytic fibers These are also described by their appearance – Oxidative = red muscle – Glycolytic = white muscle

Glycolytic Muscle Fibers Glycolytic muscle fibers are primarily anaerobic (use glycolysis and creatine phosphate) Produces lactic acid and creatinine as waste products The cytoplasm contains many glycosomes (vesicles that contain glycogen) Called white muscle because they lack myoglobin and mitochondria and so are pale in color

Oxidative Muscle Fibers Oxidative muscle contains mitochondria (for aerobic ATP production) and myoglobin (hemoglobin-like molecule) – Myoglobin increases oxygen concentration in muscles Can produce force constantly without becoming fatigued – Do not produce as much force as white muscle

Exercise Exercise may be sustained (aerobic) or unsustainable (anaerobic) – Aerobic exercise is also called endurance training – Anaerobic exercise is also called strength training Exercise can alter the composition of red and white fibers – Also influenced by genetics

Muscle Mass White muscle fibers are larger than red muscle fibers, and so strength training results in more “bulk” than endurance training Some hormones such as testosterone increase the number of muscle fibers – “Steroids” are usually testosterone analogs

Exercise Different people have different natural levels of red and white muscle – Many athletes are advised to train both unless they are in a very specialized sport (marathon running or weightlifting) – Athletes are also recommended to use stretching exercises to increase circulation to ligaments and tendons, preventing injury and increasing flexibility

Involuntary Muscles Skeletal muscles are voluntary muscles (part of a motor unit with a neuron under the control of the conscious mind) Cardiac and smooth muscle operate without conscious thought (involuntary)

Involuntary Muscle Contraction Both cardiac and smooth muscle have one feature in common that is absent from skeletal muscle: gap junctions Gap junctions allow ions to pass from one cell to another – This means when one muscle cell contracts it stimulates its neighbors to contract as well – Skeletal muscle does NOT have gap junctions

Pacemaker Cells In cardiac and smooth muscle, some cells are autorhythmic, which means they contract without any external stimulation Since they are also attached to other cells via gap junctions, they set the pace – Known as pacemaker cells – No external stimulation required

Involuntary Muscle Activity Note that when skeletal muscles cramp (cannot relax) or spasm (involuntary twitch) that is NOT induced by pacemaker cells or gap junctions – Can be caused by ion imbalance, energy depletion of the neurons or muscles, or unusual brain activity Only cardiac and smooth muscles are truly involuntary

Cardiac Muscle Cardiac muscle cells have the same sarcomeres and striped pattern as skeletal muscles Cardiac muscle cells are smaller and shorter and some are branched They attach to each other with intercalated discs that include gap junctions

Smooth Muscle Smooth muscle contracts differently than skeletal or cardiac muscle – Not striated Fibers running all through the cell are attached to dense bodies When the fibers contract the cell shortens, producing force in all directions rather than just in one direction

Muscle Injuries Muscle fibers may be depleted of energy and unable to relax (cramp) – Common cramp location is the arch of the foot Muscles may tear from excessive force placed upon the sarcomeres – Small tears produce soreness – Large tears may require surgery

Rhabdomyolysis Excessive overuse of a well- nourished muscle may cause the muscle to literally explode – This is very rare since usually a muscle will become fatigued instead – Typically only occurs in military training and CrossFit activity The key sign is myoglobin in the blood and urine (why?) – Typically leads to kidney failure and muscle scarring

Super cheerful weekend time! Please don’t forget to attempt the online quiz! See you Monday!