The Muscular System and Integumentary System Ms. Hoffman September 13, 2004

How do Muscles and Bones Interact? Skeletal muscles generate force and produce movement by contracting, or pulling on body parts Skeletal muscles are joined together by tough connective tissues called tendons Tendons are attached so that they pull on bones and make them work like levers Most skeletal muscles work in opposing pairs When one muscle contracts, the other relaxes Ex. When the biceps muscle contracts, the triceps is relaxed and you can curl your arm Ex. When the biceps muscle relaxes, the triceps muscle contracts and you can extend your arm

What is Muscle Tone? Skeletal muscles remain in a state of partial contraction At all times, a few muscle cells are being stimulated while others are not Limited stimulation causes a tightening of some muscles called resting muscle tone Resting muscle tone is responsible for keeping the back and legs straight and the head upright, even when you are relaxed Regular exercise increases muscle tone

Muscles Provide the forces that put the body into motion More than 40% of the mass of the average human body is muscle Fxns: regulate blood pressure, move food through the digestive system, power every movement in the body

Types of Muscle Tissue Muscle tissue is found everywhere in the body…from beneath the skin to deep within the body There are three different types of muscle tissue: skeletal, smooth, and cardiac Each type of muscle tissue has a different structure and plays a different role in the body

Skeletal Muscles Usually are attached to bones Responsible for voluntary movements like dancing, running, jumping Have alternating light and dark bands or stripes called striations when viewed under the microscope Most skeletal muscles are controlled by the central nervous system Skeletal muscle cells are large cells, have many nuclei, and vary in length from 1 mm – 30 cm May be referred to as muscle fibers because these muscle cells are so long and slender Complete skeletal muscles consists of: muscle fibers, connective tissues, blood vessels, and nerves

Skeletal Muscle

Smooth Muscles Usually are not under voluntary control Spindle-shaped, one nucleus, no striations Found in hollow structures like the stomach, blood vessels, and small and large intestines Fxns: move food through your digestive tract, control the way blood flows through your circulatory system, decrease the size of the pupils of your eyes in bright light May function without nervous stimulation Connected to one another by gap junctions that allow electrical impulses to travel directly from one muscle cell to a neighboring muscle cell

Smooth Muscle

Cardiac Muscle Only found in the heart Striated like skeletal muscle Usually only have one nucleus like smooth muscle Usually not under the direct control of the central nervous system Cardiac cells are connected to their neighboring cells by gap junctions

Cardiac Muscle

Muscle Contraction Muscle fibers in skeletal muscles are made of smaller structures called myofibrils Each myofibril is made of even smaller structures called filaments Thick and thin filaments alternate to create the striated appearance of muscle cells Thick filaments are made of a protein called myosin Thin filaments are made of a protein called actin

Muscle Contraction Myosin and actin filaments are arranged along a muscle fiber in units called sarcomeres Sarcomeres are separated from each other by regions called Z discs During relaxation of a muscle there are no thin actin filaments in the center of a sarcomere

Muscle Contraction Myosin and actin filaments are what cause muscles to contract A muscle contracts when the thin actin filaments in a muscle fiber slide over the thick myosin filaments For contraction to occur, the thick myosin filaments must form a cross-bridge with the thin actin filament The cross-bridge changes shape and pulls on the actin filament which slides toward the center of the sarcomere The distance between the Z discs decreases The cross-bridge detaches from the actin filament and the cycle is repeated when the myosin binds to another site on the actin filament

What Supplies the Energy for Muscle Contraction? The energy of muscle contraction is supplied by ATP One molecule of ATP supplies the energy for one interaction between a myosin cross- bridge and an actin filament Lots of ATP is needed! ATP can be produced by cells via cellular respiration

Controlling Muscle Contractions Motor neurons connect the central nervous system (brain and spinal cord) to skeletal muscle cells Impulses from motor neurons control the contraction of skeletal muscle fibers Neuromuscular junction- the point of contact between a motor neuron and a skeletal muscle cell

Controlling Muscle Contractions At the neuromuscular junction there are pockets (vesicles) in the terminals of the motor neuron that release a neurotransmitter called acetylcholine Neurotransmitters are chemicals used by neurons to transmit an impulse across a synapse (space where a message can be relayed) to another cell Acetylcholine molecules cross the synapse and cause an impulse in the cell membranes of the muscle fiber This impulse causes calcium ions to be releases within the fiber The calcium ions affect proteins that regulate how the actin and myosin filaments interact The contraction of a muscle fiber is an all- or -none process