The Muscular System

The muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture, and circulates blood throughout the body . The muscular system is responsible for the movement of the human body. Attached to the bones of the skeletal system are about 700 named muscles that make up roughly half of a person’s body weight. Each of these muscles is a discrete organ constructed of .skeletal muscle tissue, blood vessels, tendons, and nerves

3 Types of Muscles

Characteristics of Muscle -Skeletal and smooth muscle are elongated Muscle cell = muscle fiber. -Contraction of a muscle is due to movement of microfilaments (protein fibers) -All muscles share some terminology Prefixes myo and mys refer to muscle Prefix sarco refers to flesh

Characteristics of Muscle Tissue 1- Excitability -The ability to receive and respond to a stimulus -In skeletal muscle, the stimulus is a neurotransmitter chemical signal) release by a neuron (nerve cell). -In smooth muscle, the stimulus could be a neurotransmitter, a hormone or stretch. -In cardiac muscle, the stimulus could be a neurotransmitter, a hormone, or stretch. -The response is the generation of an electrical impulse that travels along the plasma membrane of the muscle cell.

Characteristics of Muscle Tissue 2- Contractility The ability to shorten forcibly when adequately stimulated. This is the defining property of muscle tissue. 3- Extensibility The ability to be stretched 4-Elasticity The ability to recoil and resume original length after being stretched.

Muscle Control Example Type of control Nervous control Type of muscle Lifting a glass Voluntary Controlled by CNS Skeletal Heart beating Involuntary Regulated by ANS Cardiac Peristalsis Controlled by ANS Smooth

Where Does the Energy Come From? -Energy is stored in the muscles in the form of .ATP -ATP comes from the breakdown of glucose .during Cellular Respiration -This all happens in the Mitochondria of the cell. -When a muscle is fatigued (tired) it is unable .to contract because of lack of Oxygen

STRUCTURE OF SKELETAL MUSCLE

-Skeletal muscle organs are dominated by muscle tissue but also contain nervous, vascular and assorted connective tissues. -The whole muscle is surrounded by a layer of dense irregular connective tissue known as the epimysium (mysium=muscle).

-Epimysium surrounds several bundles known as fascicles. -Each fascicle is a bundle of super-long skeletal muscle cells (muscle fibers), surrounded by a layer of dense irregular CT called the perimysium (peri=around). -Each muscle cell extends the length of the whole muscle organ and is surrounded by a fine layer of loose connective tissue, the endomysium. -The epi-, peri-, and endomysium are all continuous with one another.

Each T-tubule will be flanked by a terminal cisterna Each T-tubule will be flanked by a terminal cisterna. This forms a so-called triad consisting of 2 terminal cisternae and one T-tubule branch.

-The connection between the nerve and muscle is called the Neuromuscular junction -Each branch of a motoneuron forms a single junction with a muscle fiber. The myelin sheath surrounding the motor axon ends near the surface of the muscle fiber and the axon divides into a number of short processes that lie embedded in grooves on the muscle-fiber surface. This region of the sarcolemma (muscle membrane) is known as the motor end plate. Acetylcholine is the neurotransmitter in these synapses. End-plate potentials (EPPs) can be recorded at the motor end plate when the presynaptic membrane is activated to release vesicles containing the acetylcholine

Steps of neuromuscular activation 1- Action potentials arriving at the presynaptic terminal 2- cause voltage-gated Ca2+ channels to open allowing Ca+ entrance to the presynaptic terminal 3-Ca2+ uptake into the terminal causes the synaptic vesicles to come in contact with the presynaptic membrane and release of acetylcholine into synaptic cleft by exocytosis 4- Ach binds to the alpha subunits of nicotinic receptors 5- Binding of Ach to its receptors open ligand gated Na+ channels 6- Opening of Na+ channels cause Na+ influx 7- Na+ influx cause change of the internal of the membrane 8- Causing building of action potential in the muscle membrane causing muscle contraction

Some Clinical importance of the NMJ Botulinum toxin produced by Clostridium botulinum result from improperly canned food contaminated with clostridia bacteria botulinum toxin exerts its lethal effect by blocking the release of Ach Death is due to resparatory failure caused by inability to contract diaphragm Myasthenia gravis A disease involving N.M junction is characterized by the extreme muscular weakness It is an auto immune condition in which the body produces antibodies against its own motor end plate ach receptors. Thus not all Ach molecules can find functioning receptors site with which to bind.