C&T Structure Lecture 2 PDF

Cell and Tissue Structure Lect # 2 Muscle Excitable Tissue Prof Maeve Caldwell Department of Physiology  Four Tissue Types 1. Epithelial tissue – Lining/barrier of secretory – Skin and mucous membranes 2. Muscle (excitable) tissue –...

Cell and Tissue Structure Lect # 2 Muscle Excitable Tissue Prof Maeve Caldwell Department of Physiology  Four Tissue Types 1. Epithelial tissue – Lining/barrier of secretory – Skin and mucous membranes 2. Muscle (excitable) tissue – Skeletal (striated) muscle – Smooth muscle – Cardiac muscle 3. Nervous (excitable) tissue – Brain – Spinal cord 4. Connective tissue (cells, fibres, matrix) – Loose connective tissue – Dense fibrous tissue (Capsule, Ligament, Tendon) – Cartilage & Bone – Blood (originate from bone marrow)  MUSCLE (1) purposeful movement of the whole body or parts of the body (such as walking or waving your hand), (2) manipulation of external objects (such as driving a car or moving a piece of furniture), (3) propulsion of contents through various hollow internal organs (such as circulation of blood or movement of a meal through the digestive tract), and (4) emptying the contents of certain organs to the external environment (such as urination or giving birth).  Muscle Chemical energy ↓ Muscle Mechanical energy Muscle forms about 50% of the total body weight: q 40% skeletal muscle q 10% smooth & cardiac muscle Simply put, Muscles perform the following functions: q They contract… q They generate heat q They generate motion q They generate force q They provide support  Types of Muscle Striated Unstriated muscle muscle Skeletal Cardiac Smooth muscle muscle muscle Voluntary Involuntary muscle muscle  Types of Muscle 1. Skeletal muscle - strong, short contractions - fibres, striations, peripheral nuclei 2. Cardiac muscle - strong, continuous contractions - fibres (branching), striations, central nuclei 3. Smooth muscle - weaker, continuous contractions - cells, central nuclei  SKELETAL MUSCLE CELL STRUCTURE A single skeletal muscle cell is also called a MUSCLE FIBER b/c of its greater length than width. LENGTH: up to 75,000 µm or 2.5 feet. DIAMETER: from 10 to 100 micrometers. SHAPE: elongated & cylindrical. OUTER MEMBRANE: called sarcolemma. Nucleus & Organelles: present. Mitochondria & ER What is the chemical composition of the muscle? ü Proteins (20%) (either as enzymes or for muscle Control) üLactic Acid (in muscle that has undergone fatigue) üATP, ADP üMyoglobin (stores O2 & gives colour to the muscle)  Skeletal Muscle Cells – Motor Unit - all muscle fibers innervated by a single neuron Innervation ratio (motor neuron:muscle fibers, 1:100 to 1:2000) Motor neuron = spinal motor neuron Eye motor neurons 1:23, Gastrocnemius 1:1000 – Myoneural Junction Synapse between motor neuron & muscle fiber Neurotransmitter - acetylcholine Motor end plate - specialized area of sarcolemma under axon terminal  Basic Units of muscle organisation sarcolemma - plasmalemma of muscle cells. External to this cell membrane is a well developed basement membrane. sarcoplasm - cytoplasm of muscle cells excluding the myofibrils. sarcoplasmic reticulum - smooth endoplasmic reticulum of muscle cells. epimysium - thick layer of collagenous connective tissue that separates large bundles of muscle. perimysium - collagenous connective tissue that separates smaller bundles of muscle cells called fascicles. fascicle - bundle of muscle cells bounded by perimysium. endomysium - thinner layer of connective tissue that separates individual muscle cells.  Skeletal Muscle Skeletal muscle is generally connected to bone via a piece of connective tissue called a tendon. Individual muscle cells are syncytial (ie. each striated muscle cell contains multiple nuclei (multinucleate). Each individual muscle cell is called a muscle fiber. Within the sarcoplasm of these cells are myofibrils composed of repeating sarcomere units (see later). These sarcomeres are the actual contractile apparatus. The myofibrils are linear arrays of structures known as sarcomeres that are arranged in an end to end repeating pattern. The sarcomeres contain filaments of actin and myosin that interact to cause contraction of the muscle cells. Skeletal Muscle: organisation ─ collagen fibres blend to form tendon at end muscle ─ tendons attaches skeletal muscle to bone  Fascicle (bundle of fibers) ─ fascicle = bundle of muscle fibers ─ composed of several muscle cells (fibers)  Muscle Cell/Fiber ─ muscle cells make individual muscle fibers ─ muscle fiber cells are large and elongated ─ have multiple peripheral nuclei ─ produce voluntary contractions  Myofibril ─ myofibrils composed of myofilaments EM of skeletal muscle X 26 000  MYOFIBRIL A myofibril displays alternating dark & light bands. Myofibril Dark bands Light bands (A bands) (I bands) Thick & thin Thin filaments only filaments Sarcomere: The area between 2 consecutive Z discs/ lines is called a Sarcomere (Myofilament). It is the functional unit of a muscle. It has a length of 2.3 µm. Z disks I band A band H Zone M line Titin Nebulin The two- and three-dimensional organization of a sarcomere  Myofilament components Z-disc: are dense thin membranes made up of special lattice-like proteins present transversely. Dark or A-band: Thick filaments present overlapped by the thin filaments at the ends only. Light or I band: area present b/w the ends of the 2 thick filaments. It consists of thin filaments only. H-Zone: The lighter area in the middle of the A- band, where the thin filaments do not reach. It consists of thick filaments only. M-Line: A line that extends vertically down the middle of the A-band in the center of the H-zone.  Myofilaments (Myosin/Actin) ─ myofilaments/sarcomeres composed of ─ thick filament myosin ─ thin filaments actin ─ during contraction sarcomere shortens Sarcomere ─ myosin-binding protein Z line H zone I band A band Z line C binds myosin and Relaxed relaxation actin ─ striations due to I band A band same H zone shorter alignment of filaments shorter width of myofibrils Contracted contraction Thick filament Thin filament Sarcomere shorter ACTIN & THIN FILAMENTS G-actin is the monomer which will form the thin filament. It is a protein with a molecular weight of 43,000. It has a prominent site for cross-linkage with myosin. G-actin ↓ F-actin (6-7 nm long polymerized G-actin, double stranded in structure) ↓ Thin filaments  Regulatory Proteins of the Muscles TROPOMYOSIN TROPONIN Rod-like protein Globular protein complex Mol. Weight: 70,000 made of 3 polypeptides 2 chains: alpha & beta Forms part of thin filaments chains Under resting conditions, it Binds to Ca2+ covers the site for myosin attachment on F-actin Inhibitory in function molecule. Forms part of Thin filaments Attached to Tropomyosin THIN FILAMENTS  THIN FILAMENTS: Length: 1 µm Diameter: 5-8 nm No. of G-Actin mol: 300-400 Other Proteins: - Nebulin: provides elasticity to the sarcomere. - Titin: is the largest known protein in the body. It connects the Z-line to the M-line in the sarcomere & contributes to the contraction of skeletal muscle. Formation of Thin Filaments  MYOSIN & THICK FILAMENTS: Thick filaments consist of 2 symmetrical halves that are mirror images of each other. Chief constituent is MYOSIN, with a mol. weight of 480,000. Its molecule has 2 ends, a globular end having 2 heads & a rod-like tail. It has 6 peptide chains: - 2 identical heavy chains (200,000 each) - 4 light chains ( 20,000 each)  Binding sites on Myosin molecule: The myosin molecule has 2 binding sites: 1. Binding site for ACTIN 2. ATPase site Skeletal Muscle: summary  Skeletal Muscle: striated appearance LM of skeletal muscle LM of skeletal muscle EM of skeletal muscle X 320 (H&E stain) X 1200 X 2860 LM of skeletal muscle tongue skeletal muscle limbs X 300 (Masson’s trichrome) Skeletal Muscle Attachments Epimysium blends into a connective tissue attachment – Tendons—cord-like structures Mostly collagen fibers Often cross a joint due to toughness and small size – Aponeuroses—sheet-like structures Attach muscles indirectly to bones, cartilages, or connective tissue coverings  Types of Muscle 1. Skeletal muscle - strong, short contractions - fibres, striations, peripheral nuclei 2. Cardiac muscle - strong, continuous contractions - fibres (branching), striations, central nuclei 3. Smooth muscle - weaker, continuous contractions - cells, central nuclei  (1) Cardiac Muscle function and feature Cardiac muscle – all-or-none (‘twitch’) – continuous, rhythmic Purkinje fibres Pale (unstained activity glycogen). – resistant to fatigue innervation – involuntary Purkinje – pacemaker cells coordinate fibres contraction of tissue SA Node – electrical conduction AV Node (Purkinje fibres and gap junctions)  (2) Cardiac Muscle Nucleus cardiocytes – are cardiac muscle cells – contain myofibrils = striated – cells contact each other at Intercalated discs intercalated discs Nucleus – cells bound together by - gap junctions - desmosomes Intercalated discs Cardiac Muscle  Types of Muscle 1. Skeletal muscle - strong, short contractions - fibres, striations, peripheral nuclei 2. Cardiac muscle - strong, continuous contractions - fibres (branching), striations, central nuclei 3. Smooth muscle - weaker, continuous contractions - cells, central nuclei  Smooth Muscle innervation – involuntary (autonomic innervation) – dual (stimulatory/inhibitory) – graded, spreading & continuous contraction multi-unit – each cell innervated – variable force – e.g. airways, large arteries single-unit – few cells directly innervated – synchronous contraction (myogenic) – e.g. gut, uterus  Smooth Muscle function & feature – little/weak contractile apparatus – long, slender, spindle shaped cells – single central nucleus – nonstriated – composed of – thick filaments – thin filaments located in organs of – cardiovascular system (vessel walls) – respiratory system – digestive system – urinary system – reproductive system – Inside the eyes Smooth Muscle Summary Summary  Buzz Words Types Skeletal Cells Cardiac Myoblast mesodermal cells (Skeletal) Smooth Cardiocytes (Cardiac) Smooth Muscle Cell (Smooth) Skeletal Muscle Fascicle Epimysium Muscle Cell (Fibre) Endomysium Myofibril Perimysuim Myofilaments Thin actin filaments Thick myosin filaments