TIM: The heart is a kind of in-between case called cardiac muscle. It appears smooth because its cells are arranged in sheets.Īn image shows the arrangement of cells in smooth muscle tissue. Most involuntary tissue is made of smooth muscle. They're arranged in bundles of cells called muscle fibers. TIM:The voluntary tissue is striated, or striped. TIM:Under the microscope, voluntary and involuntary muscles look different.Īn image shows striated muscle fibers in voluntary muscle tissue. TIM:Involuntary muscles in your veins and arteries control the flow of blood.Īn animation shows blood cells moving through a blood vessel. The heart inside the human silhouette is replaced with an image of the digestive system, from the mouth to the stomach to the intestines. They contract slowly and rhythmically to move food through. These muscles line the walls of digestive organs, like the esophagus, stomach, and intestines. TIM:For one thing, you don’t have any control over them-they work all day long, whether you want them to or not. TIM:Involuntary muscles are totally different.Ī silhouette of a human appears with a beating heart in the chest. TIM:Some of your face muscles are a little different-they’re attached directly to the skin and other tissues, letting you make different expressions. Muscles surround the lips and eyes, and form bands over the cheeks, forehead, scalp, and down the neck. Only the muscles are visible, without the layer of skin. TIM:Skeletal muscles are attached to bones by thick bands of tough tissue called tendons.Īn image of a face appears. The arms bend in the opposite directions now, the upper muscle reads “relaxes” and the lower muscle read “contracts.” TIM:When the muscles switch jobs, your arm bends the other way. TIM:The contracting muscle pulls on the bone.Īn arrow points up toward the contracting muscle. The muscle on top reads “contracts.” The muscle on the bottom reads “relaxes.” An inset image of an arm with skin covering the muscles and joints mimics the same up and down motion. TIM:To move the elbow joint, one set of muscles shrinks, or contracts, while the other one relaxes and gets longer.Īn animation shows the elbow muscles moving up and down. TIM:Skeletal muscles work in teams at your joints, where two or more bones meet.Īn image appears of elbow muscles and bones joined together. Anyway, voluntary muscles are also called skeletal muscles because their job is to move bones. TIM:Of course, not all movement of voluntary muscle is consciously done-there are also reflexes, like when you automatically move your hand away from a hot object. An impulse flares in the nerve of the arm. TIM:When you decide to move a part of your body, a nerve signal travels from your brain to the right muscle, making it move.Ī fly buzzes and lands on the arm muscles of the figure. TIM:They’re called voluntary because they’re under your conscious control.Īn animation shows nerve signals travelling from the brain, down the spine, across the ribs, and down each arm and leg of the human figure. The images then reveal the network of muscles in each body part underneath the skin. TIM:Whenever you smile, lift weights, or walk around, your voluntary muscles are at work.Ī split screen shows images of a face smiling, an arm lifting a weight, and a leg walking. TIM:Voluntary muscles are the ones that let you move the different parts of your body. The other kind is voluntary.Īn animation shows the muscular system of a human figure. TIM:Well, that’s one of the two basic types of muscle. You can’t flex your heart because it’s an involuntary muscle.Īn animated heart pumps a steady heartbeat. TIM: Dear Tim and Moby, If the heart is a muscle, how come I can’t flex it? From Dorothy. Tim wipes sweat off his face and reads from a typed letter. Tim is lifting a weight, while Moby stands beside him. Text reads: The Mysteries of Life with Tim and Moby
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