Muscles Part 2

By the end of this class you should understand: The behavior of muscle fibers and motor neurons as a motor unit The comparative behaviors and roles of slow- and fast-twitch muscles The types of exercise and the changes they produce in the body The functions of cardiac and smooth muscle in the body The structures that permit involuntary muscles to contract regularly and continuously

Muscle Activation Recall that every muscle fiber is stimulated by a single motor neuron Together they form a motor unit – One neuron signals multiple muscle fibers distributed throughout the whole muscle – Every muscle fiber in a muscle has one neuron it responds to

Motor Unit Note that the fibers are spread out so that the force is evenly distributed through the whole muscle – If they were all on one side it could cause the muscle to contract unevenly The motor unit follows the all- or-none principle – When the neuron fires, all fibers completely contract

All or None? This may seem a bit odd since we can clearly choose to exert more or less force The trick is that each muscle is made of many motor units Increasing the number of motor neurons firing results in increased recruitment of muscle fibers

Increased Recruitment The more motor neurons are firing, the more force the motor units exert Each motor unit firing is called a twitch As more twitches occur more rapidly, their forces add together until the muscle is at maximum tension (tetanus) – The disease tetanus is so named because it is a bacterial toxin that prevents relaxation of motor neurons

Muscle Fiber Types Muscle fibers come in two types: – Fast-twitch fibers – Slow-twitch fibers These are also described by their appearance – Fast-twitch = white muscle – Slow-twitch = red muscle

Fast-twitch Muscle Fast-twitch 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

Slow-twitch Muscle Slow-twitch muscle contains mitochondria (for aerobic ATP production) and myoglobin (hemoglobin-like molecule that stores extra oxygen) – 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

Have a good weekend! It’s fun to end on a cheerful topic!