 The muscular system produces movement and maintains posture.  There are three kinds of muscles: skeletal, cardiac, and smooth.  Muscles are excitable, contractile, extensible, and elastic.  Muscles are attached to bone by tendons and are arranged in opposing or antagonistic pairs.  Sarcomeres are the contractile units of muscle.  Muscle striations are caused by the arrangement of myofibrils within the muscle cell. Each myofibril contains groups of myofilaments composed of actin and myosin proteins.

 A muscle contracts when myosin binds to actin causing the filaments to slide past one another.  Contraction is powered by ATP and controlled by two regulatory proteins and calcium ions (sliding filament theory).  Nerves stimulate muscle contraction at the neuromuscular junction.  Acetylcholine, released from the motor neuron, causes a change in muscle cell permeability resulting in the release of calcium ions and contraction.  A motor neuron and all the muscle cells it stimulates are called a motor unit.

 A muscle twitch is the contraction caused by a stimulus to the muscle.  The ATP required for muscle contraction comes from creatine phosphate, aerobic respiration, and lactic acid fermentation.  Slow-twitch muscles, found in the abdomen and back, contract slowly, powerfully, and with endurance.  Fast-twitch muscles, found in the arms and legs, contract rapidly and powerfully, but with less endurance.  Aerobic exercise increases endurance while resistance exercise builds muscle mass.

1. Excitability – receive and respond to stimuli 2. Contractility – shorten and thicken 3. Extensibility – stretch and extend 4. Elasticity – return to original shape after contraction or extension

1. Motion 2. Maintenance of posture 3. Heat production 4. Protection of internal organs

 Forms from the mesoderm germ layer  Triploblasts  simplest: flatworms  Segmentation  annelids, arthropods, vertebrates

Vertebrate musclesInvertebrate muscles 1. Skeletal ◦ Attached to bones ◦ Striated and voluntary 2. Cardiac ◦ Heart wall ◦ Striated and involuntary 3. Smooth ◦ Walls of hollow internal structures ◦ Nonstriated and involuntary 1. Striated ◦ Voluntary movements ◦ Longitudinal or circular in shape 2. Smooth ◦ Involuntary movements

 Motion produced by action of muscles on bones  levers  Tendons  attach muscles to bones (e.g. Achilles tendon)  1 fixed point of attachment (origin) and 1 moving point of attachment (insertion)  Fascia  sheets that cover muscles  Muscles work in antagonistic or opposing pairs

 Adduction  Abduction  Flexion  Extension  Rotation  Sphincters  Levators  Depressors  Supination  Pronation

Skeletal muscle fiber (cell) Muscle Fascicle Surrounded by perimysium Surrounded by endomysium endomysium perimysium Skeletal muscle Surrounded by epimysium epimysium tendon

The muscle fiber is stimulated. Ca 2+ ions are released. Thin filaments move to middle of sarcomere. Muscle fiber contracts. Muscle tension increases.

 Cycle of contraction and relaxation generated from one stimulus  Force Summation ◦ Increasing # and size of fibers contracting ◦ Increasing frequency at which action potentials are sent to muscle fibers  Types of Twitches ◦ Slow Twitch  More ATP from aerobic respiration  Continuous extended contractions over time ◦ Fast Twitch  Anaerobic metabolism (creatine phosphate and lactic acid fermentation)  Short bursts of speed  Fatigue quickly

 Aerobic exercise ◦ Moderate intensity ◦ Extended periods of time  Anaerobic exercise ◦ High intensity ◦ Shorter periods of time

 Painful sensations caused by contraction/ overshortening of muscles  Causes ◦ Hyperflexion ◦ Inadequate oxygenation ◦ Changes in temperature ◦ Dehydration ◦ Low amount of electrolytes in blood  Treatment ◦ Correct breathing ◦ Increasing intake of water and electrolytes