Skeletal muscles are attached to the skeleton, and their contraction causes the movement of bones. Nerve impulses originating in the brain and spinal cord innervate skeletal muscles by way of nerves and, through a series of steps, bring about contraction. Blood vessels serve the muscles, bringing to muscle cell mitochondria the oxygen and nutrients needed to produce a supply of ATP for muscle contraction.
Skeletal muscles make up the bulk of the body, and their contraction accounts for the movement of bones. Nerves innervate skeletal muscles. and blood vessels bring skeletal muscles oxygen and nutrients.
Anatomy of a Skeletal Muscle
Muscles are covered by several layers of connective tissue called the fascia (fash'e-ah), which extends beyond the muscle to become its tendon. Fascia also surrounds bundles of muscle fibers called fascicles and it separates the muscle fibers within a fascicle.
Muscle Fiber
A muscle fiber can contract because it contains hundreds and sometimes even thousands of contractile elements, called myofibrils. Myofibrils run the length of a muscle fiber, and the striations seen in observations of muscular tissue are due to the placement of filaments within sarcomeres, the units of a myofibril. There are thin (actin) filaments and thick (myosin) filaments. When a myofibril contracts, the actin filaments slide past the myosin filaments, causing the myofibril, and therefore the muscle fiber, to shorten and thicken. This is the sliding filame~t theory of muscle contraction.
Myofibrils are the contractile elements of muscle fibers. The placement of the myofilaments actin and myosin accounts for the striations of skeletal muscle.
Innervation of Muscle
Muscles are innervated; that is, they are supplied with nerves, and nerve impulses cause muscles to contract. A motor neuron arising from the spinal cord branches to several muscle fibers; collectively, this is called a motor unit. A neuromuscular junction occurs where a motor neuron fiber meets a muscle fiber.
In muscle fibers, the plasma membrane, called the sarcolemma, forms tubules that penetrate and dip down into the cell. These tubules, which comprise the T system (T for transverse), come near to expanded portions of the endoplasmic reticulum (ER). The endoplasmic reticulum is called the sarcoplasmic reticulum and the expanded portions are calcium storage sacs, where calcium ions (Ca2+) are stored.
After nerve impulses arrive at a neuromuscular junction, they move down the T system, prompting the release of Ca2+ from the calcium storage sacs. If ATP is available as an energy source, the actin filaments now slide past myosin filaments, and the sarcomere contracts.
Motor neurons meet muscle fibers at neuromuscular junctions. Impulses travel dovvn the tubules of the T system and cause calcium to be released from the sarcoplasmic reticulum. Contraction occurs due to the presence of calcium and ATP in the muscle cells, which prompts actin filaments to slide past myosin filaments, shortening the length of the sarcomere.
