Muscles and Muscle Tissue Chapter 9 Anatomy & Physiology

Overview of Muscle Tissues Types of muscle tissue are: Skeletal muscle which is associated with the bony skeleton and consists of large striated cells that are controlled voluntarily. Cardiac muscle occurs only in the heart and consists of small cells that are striated and under involuntary control. Smooth muscle is found in the walls of hollow organs and consists of small elongated cells that are not striated and are under involuntary control.

Functional Characteristics of Muscle Tissue Excitability or irritability is the ability to receive and respond to a stimulus. Contractility is the ability to contract forcibly when stimulated. Elasticity is the ability to resume the cells’ original length once stretched.

Muscle Functions Muscles produce movement by acting on the bones of the skeleton. They pump blood and propel substances throughout the body. Aid in maintaining posture by adjusting the position of the body with respect to gravity. Muscles stabilize joints by exerting tension around the joint. Muscles generate heat as a function of their cellular metabolic processes.

Skeletal Muscle Gross Anatomy of Skeletal Muscle Each muscle has a nerve and blood supply that allows neural control and ensures adequate nutrient delivery and waste removal. Connective tissue sheaths are found at various structural levels of each muscle. Endomysium surrounds each muscle fiber Perimysium surrounds each group of muscle fibers. Epimysium surrounds the whole muscle.

Microscopic Anatomy of a Skeletal Muscle Fiber Skeletal muscle fibers are long and cylindrical with multiple nuclei. Myofibrils account for roughly 80% of cellular volume and contain the contractile elements of the muscle cell. Striations are due to the alternating of dark A bands and light I bands. Myofilaments make up the myofibrils and consist of thick and thin filaments

The sliding filament model of muscle contraction states that during contraction the thin filaments (actin) slide past the thick (myosin) filaments. Overlap between the myofilaments increases and the sacromere shortens.

Physiology of a Skeletal Muscle Fiber The neuromuscular junction is a connection between and axon terminal and a muscle fiber that is a the route of electrical stimulation of the muscle cell. A nerve impulse caused the release of acetylcholine to the synaptic cleft which binds to receptors on the motor end plate. This triggers a series of electrical events on the sarcolemma.

Generation of an action potential across the sarcolemma occurs in response to acetylcholine binding with receptors on the motor end plant. This involved the build up of sodium ions that makes the membrane slightly less negative in charge.

Contraction of a Skeletal Muscle A motor unit consists of a motor neuron and all the muscle fibers. The muscle twitch is the response of a muscle to a single action potential on its motor neuron.

Muscle tone is the phenomenon of muscles exhibiting slight contraction even when at rest which keeps muscles firm..healthy…and ready to respond. Isotonic contractions result in movement occurring at the joint and shortening of muscles. Isometric contractions result in increases in muscle tension but no lengthening or shortening of the muscle.

Muscle Metabolism Muscles contain very little stored ATP. Muscles will function aerobically as long as there is plenty of oxygen. If the oxygen runs out the muscles will function anaerobically through glycolysis. Muscle fatigue is the inability to contract due to the shortage of available ATP.

Force of Muscle Contraction As the number of muscle fibers stimulated increases the force of the contraction increases. Large muscle fibers generate more force than smaller muscle fibers.

Smooth Muscle Microscopic Structures of Smooth Muscle Fibers Smooth muscle cells are small and spindle-shaped with one central nucleus. Smooth muscle cells are usually arranged into sheets of opposing fibers forming longitudinal layers or a circular layer.

Contraction of the opposing layers of muscle leads to a rhythmic form of contractions called peristalsis which propels substances through the organs. Smooth muscle has no striations.

Contraction of Smooth Muscle Smooth muscle fibers exhibit slow and synchronized contractions. Smooth muscle contracts more slowly and consumes less ATP than skeletal muscle.

Developmental Aspects of Muscles Women have relatively less muscle mass than men. Muscular dystrophy is one of the few disorders that muscles experience and is characterized by atrophy and degeneration of muscle tissue.