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Muscles Human Biology Functions of the Muscular System • movement (~75 pairs [out of 434 muscles] are responsible for movement) • posture (as well as absorb shock) • respiration • heat • communication • constriction • heart beat Muscle Types • Skeletal (or voluntary) • Smooth (or involuntary organ) • Cardiac (or involuntary heart) • Muscles are considered organs of the muscular system Structure of the Muscle Muscle Structure • Muscles are surrounded by a thin connective tissue called fascia. Muscle Fibers • • • • Single muscle cells Can contract in response to a stimuli Thin, cylindrical, often multinucleated Contain fibers called myofibrils that run parallel along the length of the cell Myofibrils • Made of two types of protein – Actin (thin) – Myosin (thick) – Arranged in bands • Together, it is called a sarcomere • They cause muscle fibers to look striated (striped) Neuromuscular Junction • Skeletal muscle contracts as a result of a stimulus • Stimuli are electrochemical impulses from a motor neuron carried by a neurotransmitter • Motor neurons form between the spinal column, brain, and muscles. Muscle Structure • Humans have about 600 named muscles in the body • All muscles have two main parts – Origin: where the muscle attaches to a fixed bone – Insertion: where the muscle attaches to the bone across a joint Muscle Naming (handout) • Size: vastus (huge); maximus (large); longus (long); minimus (small); brevis (short). • Shape: deltoid (triangular); rhomboid (like a rhombus with equal and parallel sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a four-sided figure with two sides parallel). • Direction of fibers: rectus (straight); transverse (across); oblique (diagonally); orbicularis (circular). • Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm); supra- (above); infra- (below); sub- (under or beneath); lateralis (lateral). • Number of origins: biceps (two heads); triceps (three heads); quadriceps (four heads). • Origin and insertion: sternocleidomastoideus (origin on the sternum and clavicle, insertion on the mastoid process); brachioradialis (origin on the brachium or arm, insertion on the radius). • Action: abductor (to abduct a structure); adductor (to adduct a structure); flexor (to flex a structure); extensor (to extend a structure); levator (to lift or elevate a structure); masseter (a chewer). Muscular Responses • Threshold stimulus – The amount of stimuli needed to cause a muscle contraction • All or None Response – A muscle will contract fully with the threshold stimulus. Muscle fibers can not contract partially. – Fibers can, however, contract independently of each other causing a muscle to contract partially. Skeletal Muscles • Prime movers – The primary muscle that contracts to move a body part • Synergists – Smaller local muscles that contract to aid the prime mover • Antagonists – Larger muscles that oppose the prime movers and can act in the opposite direction Major Muscles Refer to p 195-205