Muscles are organized into three basic types:
- Striated: Skeletal muscle facilitates movement by applying forces to bones and joints through its contraction. They are generally under voluntary control. Muscles have an origin; a thick portion of the muscle between the insertion and origin is called the muscle belly or gaster and a tendon.
- Cardiac: Cardiac muscle is an involuntary striated muscle found exclusively in the heart. Cardiac muscle has unique properties; it stimulates its own contraction without the required electrical impulse from the central nervous system via special pacemaker cells in the sinoatrial node.
- Smooth: Smooth muscle is an involuntary non-striated muscle found in the walls of hollow organs such as the bladder, and in blood vessels. Smooth muscle can be directly stimulated by the CNS or can react to hormones secreted locally, such as vasodilators and vasoconstrictors.
Skeletal muscle fiber
- Anatomy: Skeletal Muscle Fibers are made up of many myofibrils surrounded by sarcoplasmic reticulum. The sarcolemma is on the outside of the muscle fiber and contains many nuclei.
- Sarcomere: The dark striations of skeletal muscle are made up of a lattice of thick and thin filaments, which are formed into a functional unit of contraction known as the sarcomere. Sarcomeres are the basic unit of muscle, made up of actin and myosin.
- Sliding filament model: Sliding filament model: after the signal to contract comes from the central nervous system, an action potential spreads over the muscle fiber.
Physiology of contraction
- Excitation-Contraction coupling: Excitation-Contraction coupling is the process by which an action potential causes calcium (Ca2+) release and cross bridge cycling. An incoming action potential, transmitted through the neuromuscular junction, causes the depolarization of the skeletal muscle cell.
- Length-Tension Relationship: The amount of tension (force of contraction) a skeletal muscle creates is dependent, in part, on the length of the muscle itself. The optimal length for a muscle is when the maximum number of myosin cross bridges make contact with thin filaments.
Muscle metabolism
- Breakdown of ATP: when a muscle is stimulated to contract, the initial ATP (stored as phosphocreatine) is the initial energy source, lasting seconds.
- Anaerobic Glycolysis: when the phosphocreatine source is depleted, muscle converts glucose into pyruvic acid and ATP.
- Aerobic Glycolysis: if sufficient oxygen is available muscle tissue will convert glucose into CO2, H2O and ATP to use as energy.
- Lipolysis: during long periods of exercise, muscle can use the byproducts of fat breakdown, which are free fatty acids (FFA). FFA are converted into ATP.
Types of Skeletal Muscle Fibers: There are 3 main types of skeletal muscle:
- Slow Oxidative Fibers
- Fast Oxidative-Glycolytic Fibers
- Fast Glycolytic Fibers.
Most skeletal muscles are a mixture of all 3 types.
Control of muscles
- Neuromuscular junction: Motor neurons axons connect with muscle fibers via a neuromuscular junction. The axon ends at the neuromuscular junction and is separated from the muscle fiber itself by a synaptic cleft. Neutortansmitters such as acetylcholine, cross the synaptic cleft and transmit the chemically converted electrical impulse to the muscle causing it to contract.
- Voluntary muscles: broadly only skeletal muscles are under voluntary control. Meaning we can contract the muscles at will.
- Involuntary muscles: Involuntary muscles are rhythmic, automatically controlled muscles. The muscles of breathing are under both voluntary and involuntary control. Cardiac (heart) muscle is under involuntary control. The pacemaker cells in the sinoatrial node set the rate and the autonomic nervous system can modulate that rate. Smooth muscles in the walls of organs and blood vessels are primarily under involuntary control.
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