Skeletal Muscle Histology ANPS 19 Skeletal Muscle Histology Dr. Tompkins Department of Neurological Sciences Given D408 john.tompkins@uvm.edu Reading: Anatomy & Physiology: An Integrative approach, McKinley et al., Chapter 10, Muscle Tissue, pages 331-340 (Sections 10.1-10.2)

Muscle Tissue Muscle tissue distributed almost everywhere Some functions of muscular tissue Produces skeletal movement Propels food we eat along gastrointestinal tract Expels waste we produce Changes amount of air that enters the lung Pumps the blood to body tissues (all these functions accomplished by different types of muscle tissue) - smooth muscle in bladder - Cardiac muscle

Muscle Tissue Three types of muscle tissue: Skeletal muscle, cardiac muscle, smooth muscle In contrast to cardiac and smooth muscle, skeletal muscle is under voluntary control Composes 40-50% of weight of the adult 700 skeletal muscles in the muscular system Cardiac muscle- more specialized, cells communicate to each other, electrical signals Smooth muscle- digestive system

Skeletal Muscle Functions Produce skeletal movement Maintain posture and body position Support soft tissues Storage and movement of materials sphincters - circular bands of skeletal muscle ensure the voluntary expulsion of feces and urine Maintain body temperature sphincters: bladder Maintain body temperature metabolism

Characteristics of Skeletal Muscle Tissue Excitable - responds to electrical stimulation (like neurons) Conductive – action potential moves along membrane (like neurons) Contracts Elastic Extendable -Skeletal Muscle Tissue have resting membrane potential -Proteins of muscle cells allow contraction -Elasticity due to coiled protein

Levels of Functional Organization in Skeletal Muscle Fiber -muscle fiber= muscle cell

Structural Organization of Skeletal Muscle (Figure 10.1) A skeletal muscle is composed of thousands of muscle cells, connective tissue, blood vessels & nerves. Muscle cells (referred to as muscle fibers) are typically as long as the entire muscle. Bundles of muscle fibers termed fascicles. Tendon (connects muscle to bone) Deep fascia (ensheathes functionally distinct muscles) Epimysium Skeletal muscle Artery The epimysium, perimysium and endomysium are three concentric layers of connective tissue. Provide protection, sites for blood vessel and nerve distribution, and are a means of attachment to the skeleton or other structures. - Mysium-association with muscle Deep fascia connects muscle to tendon Vein Perimysium Nerve Fascicle Endomysium Muscle fiber (This should look familiar to you. It’s similar to the structure of a nerve.)

Organization of connective tissues Epimysium surrounds whole muscle Perimysium sheathes bundles of muscle fibers: surrounds fascicles Epimysium and perimysium contain blood vessels & nerves which branch into the muscle Endomysium which covers individual muscle fibers Tendons attach muscle to bone.

A cross sectional view of skeletal muscle. Where is the perimysium A cross sectional view of skeletal muscle. Where is the perimysium? Endomysium?

A high magnification view of skeletal muscle in cross section A high magnification view of skeletal muscle in cross section. C is for capillaries and P is for perimysium. Where is the endomysium? muscle fiber

Longitudinal section of skeletal muscle. Where is the endomysium? - Muscle fiber as long as muscle Purple nuclei- multinucleated Endomysium around each fiber

Skeletal muscle fibers formed by fusion of myoblasts myo- or  ( before a vowel ) my- from Greek mus muscle These are the embryonic cells which fuse to form skeletal muscle fibers during development. Each contributes a nucleus to total number of nuclei. Myoblasts Muscle fiber Myoblasts fuse to form a skeletal muscle fiber. Satellite cell Satellite cells myoblast cells that have not fused with muscle fiber- for regrowth of new fiber Muscle fiber Unfused myoblasts termed “satellite cells”. These cells may be stimulated to differentiate if muscle is injured. Nuclei Satellite cell muscle fiber is a multinucleated cell cytoplasm of muscle fiber termed sarcoplasm (Figure 10.2)

Box of spaghetti This is a diagram of a muscle fiber or a muscle cell showing the large number of myofibrils in the sarcoplasm

Inside a Skeletal Muscle Fiber Sarcoplasmic reticulum the endoplasmic reticulum of a muscle cell surrounds bundles of contractile proteins (myofibrils) contains the Ca2+ necessary for muscle contraction Muscle Triad Fascicle part of the sarcoplasmic reticulum a reservoir for Ca2+ ions - 2 terminal cisternae + 1 T-tubule = 1 Triad T-tubule Terminal cisternae Muscle fiber - plasma membrane of the muscle fiber - invaginations of this membrane termed T-tubules or transverse tubules Sarcolemma Nucleus - long cylindrical structure of fibers - extend length of muscle fiber - 80% of muscle fiber volume - each fiber has hundreds to thousands Myofibrils Sacro: muscle T tubules: pipes from outside into cell, connection with SR Terminal cisternae- storage tanks for Ca ions, where ca ions released from when action potential comes in Myofilaments protein filaments thick and thin Nucleus Openings into T-tubules Sarcoplasm Sarcomere Nucleus - functional unit of myofibril Mitochondrion (a) Skeletal muscle fiber (Figure 10.3 a)

Cross section of skeletal muscle fibers in which myofibrils can be seen.

Molecular Structure of Thick and Thin Filaments (Figure 10.4) Muscle fiber Myofilaments Myofibril - come in ‘thick’ and ‘thin’ Myosin molecule Heads Tail Actin binding site ATP and ATPase binding site Myosin heads Myosin molecule: intertwined Thin filament: composed of Actin molecules (g-actin=globular actin, fibrous strand actin=f-actin) Tropomyosin- twine for actin, covers myosin binding sites Troponin= calcium sensor (a) Thick filament Tropomyosin Troponin Ca2+ binding site G-actin F-actin Myosin binding site (b) Thin filament

Molecular Structure of Thick and Thin Filaments (Figure 10.4) Myosin molecule Heads Actin binding site ATP and ATPase binding site Myosin heads Tail Thick filaments assembled from bundles of protein molecules, myosin each myosin protein has two intertwined strands each strand has globular head and elongated tail tails pointing toward center of thick filaments heads pointing toward edges of thick filaments head with a binding site for actin (on thin filaments) head has site where ATP attaches and is split by ATPase

Molecular Structure of Thick and Thin Filaments (Figure 10.4) Tropomyosin - globular protein attached to tropomyosin - has binding site for Ca2+ - together form troponin-tropomyosin complex Troponin - twisted stringlike protein - covers small bands of F-actin - covers myosin binding sites in noncontracting muscle G-actin Tropomyosin F-actin Myosin binding site Troponin Ca2+ binding site Thin filaments composed primarily of two strands of actin strands twisted around one another actin strands composed of spherical molecules, globular actin (G-actin) connect to form a fibrous strand, filamentous actin (F-actin) G-actin has myosin binding site - where myosin head attaches during contraction We will learn more about how these proteins work together in the next lecture.

Organization of a Sarcomere Myofibrils consist of repeating units called sarcomeres. - number varies with length of myofibril. - composed of overlapping thick and thin filaments - delineated at both ends by Z discs A band (DARK regions) - central region of sarcomere - contains entire thick filament - contains partially overlapping thin filaments - appears dark under a microscope Muscle fiber Sarcomeres Myofilaments Myofibril I band A band Z disc H zone M line Sarcomere (Figure 10.5 a) Z discs - contains specialized proteins running perpendicular to myofilaments which anchor thin filaments I bands (LIGHT regions) - contains only thin filaments - extend from both directions of Z disc - appear light under a microscope - disappear at maximal muscle contraction H zone - central portion of A band - only thick filaments present; no thin filaments - disappears during maximal muscle contraction M line - protein meshwork structure at center of H zone - attachment site for thick filaments

A Sarcomere: The Functional Unit of Skeletal Muscle Contraction Connectin (Titin) Z disc Thin filament Thick filament Sarcomere I band A band H zone M line - Z disc- specialized proteins that anchor filaments A band can contain either thick or thin filaments Connectin- coiled protein gives springy nature to muscles H zone- only thick filaments, when muscle contracts, H zone fully disappears M line- proteins that contain thick filaments in place (LIGHT) (DARK) (Figure 10.5 b)

Specific Banding Patterns in a Skeletal Muscle Fiber I – I band Z – Z disk A – A band N – nucleus These units aligned in a single fiber which contains many stacked myofibrils.

Structure of a Sarcomere Transverse sectional plane M line Thick filaments and accessory proteins H zone Thick filaments A band Thick filaments Thin filaments I band Thin filaments Connectin Z disc Thin filaments Connectin and accessory proteins (Figure 10.5 c)

Sarcomere Shortening (Figure 10.12) Relaxed sarcomere Z disc Thick filament Z disc Connectin Thin filament Thin filament M line Z disc Z disc M line H zone H zone I band A band I band I band A band I band (a) Relaxed skeletal muscle Contraction Contraction M line Z disc Z disc Z disc Z disc M line A band A band Fully contracted sarcomere Fully contracted sarcomere (b) Fully contracted skeletal muscle (Figure 10.12)

Summary: Sacromere (many)  myofibril (thousands)  muscle fiber muscle fasicle  skeletal muscle  muscles of muscular system