MUSCULAR SYSTEM

FUNCTIONS OF MUSCLE TISSUE MOVEMENT SUPPORT HEAT PRODUCTION

PROPERTIES OF MUSCLES (p. 34 in Exercise Science) Irritability Contractibility Elasticity Extensibility Conductivity

Types of Muscle Skeletal muscle Smooth muscle Cardiac muscle The human body is comprised of 324 muscles Muscle makes up 30-35% (in women) and 42-47% (in men) of body mass. Three types of muscle: Skeletal muscle Smooth muscle Cardiac muscle

TYPES OF MUSCLE TISSUE (p. 20 Exercise Science) SKELETAL  attach to bones by tendons  striated appearance  voluntary SMOOTH  involuntary  non-striated; dense sheets CARDIAC  involuntary; controlled by the autonomic nervous system  striated

ANATOMY OF A SKELETAL MUSCLE 3 layers of connective tissue: EPIMYSIUM(FASCIA): envelopes the entire muscle; extends beyond the muscle and changes into a tendon PERIMYSIUM: sheath of connective tissue that binds groups of muscle fibers ENDOMYSIUM: surrounds each muscle fibers

Below the endomysium lies a plasma membrane called the SARCOLEMA which contains the cell’s cytoplasm (sarcoplasm) Running along the muscle fibers length are thread-like structures called MYOFIBRILS. Within the myofibrils are two proteins: ACTIN (thin) AND MYOSIN (thick) SACROMERE: repeating structural units of striated muscle containing actin and myosin

Sacromeres are separated by Z lines (discs) Within a sacromere, the darker area, called the A band extends from one end to the other of the thick filaments and includes portions of the thin filaments (actin) where they overlap the thick filaments (myosin) A lighter, less dense area called the I band contains only thin filaments (actin) Z discs run through the centre of each I band A narrow H Zone in the center of each A band contains only thick filaments (myosin) Cross bridges are the projecting heads of mysoin (will be discussed with the sliding filament theory)

Components of skeletal muscle d) myofibril c) muscle fibre b) muscle fibre bundle a) Muscle belly

THE NEUROMUSCULAR SYSTEM The neuromuscular system is the general term referring to the linkages between the muscular system and the nervous system Muscle contraction is under control of the CNS, which is made up of nerve cells called neurons MOTOR UNIT is the term referring to the motor neuron, the axon (pathway) and the muscle fibers it stimulates The point between the nervous and muscular system is called the NEUROMUSCULAR JUNCTION

Excitable cells (neurons and muscle fibers) make contact and communicate at specialized regions called SYNAPSES. At most synapses, a small gap, called the SYNAPTIC CLEFT, separates the two excitable cells. Because the cells do not physically touch, the neuron communicates with the muscle cell by releasing a chemical called a NEUROTRANSMITTER

Close to its target skeletal muscle fiber, the axon of a motor neuron branches into clusters of bulb-shaped AXON TERMINALS The distal end of an axon terminal contains many membrane-enclosed sacs called SYNAPTIC VESSICLES which contain neurotransmitter molecules  acetylcholine (ACh) When a nerve impulse (action potential) reaches the axon terminal, it triggers exocytosis of synaptic vessicles which liberate Ach into synaptic cleft

The region of the muscle fiber membrane (sarcolemma) that is adjacent to the axon terminals has what is called the MOTOR END PLATE There are ACh receptors on the motor end plate The binding of ACh to its receptor opens a channel that passes small cations, which trigger the action potential that leads to a muscle contraction In most skeletal muscle fibres, there is only one NMJ located near the midpoint of the fiber  the muscle action potential spreads from the centre toward the ends

All-or-none Principle The all-or-none principle stipulates that, when a motor unit is stimulated to contract, it will do so to its fullest potential muscle fibre ALWAYS contracts entirely the more fibres that are recruited determines the strength of the contraction Every motor unit has a specific threshold that must be reached for such activation to occur

Types of Muscle Contraction Forms and types of muscle contraction Static Dynamic

Types of Muscle Contraction Forms and types of muscle contraction Static Isometric Concentric Eccentric

Types of Muscle Contraction Concentric Eccentric Isometric Static Isotonic Auxotonic Isokinetic Plyocentric (overcoming, accommodating) (resistive) Forms and types of muscle contraction Dynamic

High microscope magnification of a single sarcomere within a single myofibril