Functional organization and properties of skeletal muscles Dr Tony Frugier Physiology Department – Room 704 tfrugier@medcol.mw

Muscle Have two common functions: To generate motion To generate force Skeletal muscles also generate heat

Types of muscle Skeletal Muscle Mostly attached to the bones

Cardiac muscle Found in the heart Create movement of blood thru the circulatory system Skeletal & cardiac muscles are striated muscles

Smooth muscle Muscle of the internal organs & tubes such as the stomach, urinary bladder & blood vessels Influence the movement of material into, out of, or within the body

Skeletal muscles are voluntary while smooth and cardiac muscles are involuntary Sometimes skeletal muscles can contract without conscious control and we can learn a certain degree of conscious control over some smooth and cardiac muscle Skeletal muscle can only contract in response to a signal from a somatic motor neuron They can not initiate their own contraction, nor is their contraction influenced directly by hormones

Cardiac and smooth muscles have multiple levels of control Their primary extrinsic control is from autonomic innervation The activity of cardiac and smooth muscles is subject to modulation by the endocrine system

Skeletal muscle Make up about 40% of body weight Responsible for positioning and movement of the skeleton Attached to bones by collagenous tendons If the centres of the connected bones are brought closer together when muscles contract, the muscle is called a flexor If the bones move away from each other when the muscle contracts, the muscle is called an extensor Flexor and extensor pairs are called antagonistic muscle groups

Skeletal muscle composition A skeletal muscle is a collection of muscle cells, or muscle fibers The muscle fibers that function together and are controlled by one motor neuron are called a motor unit Skeletal muscle fibers are the largest cells in the body created by fusion of many individual embryonic muscle cells

Muscle fiber (1) The cell membrane of a muscle fiber is called the sarcolemma The cytoplasm is called the sarcoplasm The main intracellular structures are the myofibrils, bundles of contractile and elastic proteins that carry out the work of contraction

Muscle fiber (2) Skeletal muscle fibers contain an extensive sarcoplasmic reticulum (SR) that wraps around each myofibril The SR consists of longitudinal tubules which release Calcium, and the terminal cisternae, which concentrate and sequester calcium A branching network of transverse tubules known as T-tubules is closely associated with the terminal cisternae One T-tubule with its flanking terminal cisternae is known as a triad

Muscle fiber (3) T-tubules allow action potentials that originate at the neuromuscular junction on the cell surface to move rapidly into the interior of the fiber Without t-tubules, the AP could reach the center of the fiber only by diffusion of positive charges through the cytosol thus a slower process The cytosol contains many glycogen granules and mitochondria

Myofibrils are the contractile structures of a muscle fiber Muscle fiber contains a thousand or more myofibrils that occupy most of the intracellular volume, leaving little space for cytosol and organelles Each myofibril contains several types of proteins: Contractile proteins myosin and actin Regulatory proteins tropomyosin and troponin Giant accessory proteins titin and nebulin

Myosin Myosin is the protein that makes up the thick filaments of the myofibril Various isoforms of myosin occur in different types of muscles in relation to their speed Myosin is composed by two heavy protein chains that intertwine to form a long coiled tail and a pair of tadpole-like heads Two light weight chains are associated with the heavy chain of each head In skeletal muscle, about 250 myosin molecules join to create a thick filament

Actin The protein that makes up the thin filaments of the muscle fiber One actin molecule is a globular protein (G-actin) Multiple globular actin molecules polymerize to form long chains or filaments (F-actin) In skeletal muscles two F-actin polymers twist together like a double strand of beads creating the thin filament of the myofibril The parallel thick & thin filaments are connected by crossbridges The crossbridges are myosin heads that loosely bind to the actin filaments Each G-actin molecule has a single binding site for a myosin head

Sarcomere Under light microscope, the arrangement of thick and thin filaments in a myofribril creates a repeating pattern of alternating light and dark bands One repeat of the pattern forms a sarcomere

Elements of a sarcomere (1) Z Disk Zigzag structure made of proteins that serve as the attachment site for the thin filaments One sarcomere has two Z disks and the filaments found between them I Band Lighest coloured bands of the sarcomere and represents the region occupied only by thin filament Z disk runs through the middle of an I Band, so each half of an I Band belong to a different sarcomere

Elements of a sarcomere (2) A Band The darkest of the band in the sarcomere and encompasses the entire length of the thick filament At the outer edge of the A Band, the thick and thin filaments overlap. The center of the A band constitutes of the thick filaments only

Elements of a sarcomere (3) H Zone This central region of the A band is ligher than the outer edges of the A band because the H zone is occupied by the thick filaments only M Line Represents the attachment site for the thick filaments. One A band is divided in half by an M line

Elements of a sarcomere (4) The proper alignment within a sarcomere is ensured by two types of proteins titin and nebulin Titin is a huge elastic molecule and largest protein known Titin has two functions: it stabilizes the position of contractile filaments, and its elasticity returns stretched muscle to their resting length Titin is help by nebulin an inelastic giant protein that lies alongside thin filaments and attaches to the Z disk Nebulin helps align the actin filaments of the sarcomere

Physiology Department (Room 704) Questions ??? Dr Tony Frugier Physiology Department (Room 704) tfrugier@medcol.mw