Lecture # 17: Muscular Tissue (Chapter 11) Objectives: 1- Compare the three types of muscle tissue with regard to microscopic appearance, location, function, and regulation of contraction. 2- Define epimysium, perimysium, endomysium, and tendon. 3- Describe muscle cell anatomy. 4- Describe the arrangement of thin and thick filaments in a sarcomere and its relationship to striations. 5- Define motor unit and describe the anatomy of a neuromuscular junction. 6- Explain the neural, chemical, and mechanical factors involved in the contraction of skeletal muscle. 7- Explain isotonic and isometric contractions.

Skeletal Muscle Tendon Muscle Fascicle Muscle fiber (cell) Epimysium (attachments between muscle and bone matrix) Muscle Fascicle Epimysium Muscle fiber (cell) Endomysium Perimysium (connective tissue surrounding entire muscle) (connective tissue around muscle cells) (connective tissue around muscle fascicles)

The Muscle Fiber T tubules Nucleus Muscle fiber They conduct the nerve impulse from the sarcolemma to the interior of the cell Muscle fiber Terminal cisterna Sarcoplasmic reticulum It stores and releases calcium for muscle contraction Mitochondria They produce the chemical energy (ATP) for muscle contraction Openings into transverse tubules Triad: Myofibrils 2 Terminal cisternae Transverse tubule Sarcolemma Sarcoplasm Myofilaments

SKELETAL MUSCLE MUSCLE FASCICLE MUSCLE FIBER (CELL) MYOFIBRIL Contains: Surrounded by: MUSCLE FASCICLE Muscle fascicles Epimysium Contains: Surrounded by: MUSCLE FIBER (CELL) Muscle fibers (cells) Perimysium Contains: Surrounded by: Myofibrils Endomysium MYOFIBRIL Contains: Myofilaments MYOFILAMENTS They are organized in sarcomeres Thick filaments: myosin Sarcomere Thin filaments: actin

Myofilaments Troponin Thick filament Thin filament Myosin molecule G actin Thin filament Tail Head Hinge region Tropomyosin Troponin complex Myosin molecule Contractile proteins Regulatory proteins Troponin Tropomyosin Myosin Actin They do the work of shortening the muscle fiber They act like a switch to determine when the fibers can contract

Striations M line Z line Titin Sarcomere Myosin (thick filaments) They attach the thin and elastic filaments Z line Titin (elastic filaments) Sarcomere They are the smallest functional units of the muscle fiber M line Myosin (thick filaments) Actin (thin filaments) I band (lighter) It contains thin filaments but not thick filaments Zone of overlap A band (dark) H band A band: Its length is equal to the length of the thick filaments. It contains both thin and thick filaments M line: It consists of proteins that connect each thick filament with its neighbors H band: It is a lighter region on either side of the M line, which contains only thick filaments I band Z line Zone of overlap Zone of overlap: It is the region where the thin filaments are situated between the thick filaments H band M line

When a skeletal muscle fiber contracts: A band H band I band Z line M line Z line Zone of overlap When a skeletal muscle fiber contracts: 1- The H bands and I bands get smaller 3- The Z lines move closer together 2- The zone of overlap get larger 4- The width of the A band remain constant

Contraction ADP Pi Active sites ADP ATP Pi Hydrolysis of ATP to ADP + Pi; activation and cocking of myosin head Formation of myosin–actin cross-bridge ADP Pi Sliding of thin filament over thick filament shorten the sarcomeres and muscle also shorten (contraction) Power stroke; sliding of thin filament over thick filament

Hydrolysis of ATP to ADP + Pi; activation and cocking of myosin head Binding site for myosin (active site) G-actin strand ADP + Pi ATP

Troponin Tropomyosin Ca +2 F-actin strand Cross-bridge ADP + Pi At low intracellular concentration of Ca the tropomyosin blocks the binding sites for myosin in the actin molecules and prevents the formation of cross-bridges +2

Troponin Ca +2 Ca +2 Tropomyosin Ca +2 Ca +2 Ca +2 F-actin strand Ca +2 Ca +2 Ca +2 Cross-bridge Ca +2 ADP + Pi Ca +2 Ca +2 Ca +2 Ca +2 Ca +2 At high intracellular concentration of Ca the troponin is activated and undergoes a conformational change that moves the tropomyosin away from actin’s binding sites for myosin heads +2

The Nerve-Muscle Relationship Motor unit: It is one nerve fiber and all the muscle fibers innervated by it The average motor unit contains 200 muscle fibers for each motor unit Neuromuscular junction (NMJ): It is the point where a nerve fiber meets a muscle fiber

Synaptic knob or axon terminal The Muscle Fiber Nucleus T tubules They conduct the nerve impulse from the sarcolemma to the interior of the cell. Muscle fiber Terminal cisterna Sarcoplasmic reticulum It stores and releases calcium for muscle contraction. Action potential Mitochondria Ca2+ They produce the chemical energy (ATP) for muscle contraction. . . .. . ... Neurotransmitter Myofibrils Synaptic cleft Synaptic knob or axon terminal Action potential Myofilaments

The Neuromuscular Junction Myelin The Neuromuscular Junction Motor nerve fiber Synaptic knob Sarcolemma Mitochondrion Synaptic knob Sarcolemma T tubule Synaptic vesicle Junctional folds They have ACh receptors which bind Ach Synaptic cleft Sarcoplasm They contain acetylcholine (Ach) Myofilaments

Two Types of Ion Channels Voltage gated ion channels: Ligands (Neurotransmitters, hormones) + + + + + _ _ _ _ + + + + + + + _ _ _ _ _ _ _ Chemically gated ion channels: Voltage gated ion channels: They open when the specific ligand binds to the receptor. They open in response to a voltage change in the plasma membrane.

_ _ _ + + + _ _ _ + + + Chemically Gated ion Channels Acetylcholine Na+ + + + _ _ _ + + + _ _ _ Resting Membrane Potential End plate Potential

Excitation 1- The Arrival of an Action Potential Axon of motor neuron Action potential 1- The Arrival of an Action Potential 2- The Release of Acetylcholine Synaptic terminal Sarcolemma Action potential Mitochondrion Ca 2+ Voltage gated ion channels open Sarcolemma T tubule Synaptic vesicle Junctional folds Fusing synaptic vesicle Synaptic cleft

Chemically gated ion channels Axon terminal 3- Binding of Ach to the receptors 4- Opening of ligand-gated ion channels and creation of end plate potential + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ Motor End Plate Acetylcholine End-plate potential Chemically gated ion channels + It is rapid fluctuation in the membrane potential that falls back to a level close to the resting membrane potential Na + K Acetic acid Choline

End-plate potential Action Potential 5- Opening of voltage-gated ion channels and creation of action potential Action Potential: It is a rapid voltage change in which a plasma membrane reverses its electrical polarity Action potential have self-propagating effect that produce a traveling wave of excitation in the nerves and muscles cells + + + + _ _ _ _ + + + + + + + + _ _ _ _ _ _ _ _ End-plate potential Voltage-gated ion channels Action Potential Ligand- gated ion channels

Acetylcholine Ca Na Active transport ATP - - + + + + + + + + + + 1 & 2- Nerve signal stimulates voltage-gated calcium channels that result in exocytosis of synaptic vesicles containing ACh = ACh release Acetylcholine Ca +2 Na + - - + + + + + + + + + + + + - - - - - - - - - - - - 5- Voltage change in end-plate region (EPP) opens nearby voltage-gated channels in plasma membrane producing an action potential 6 & 7- Action potential spreading over sarcolemma reaches and enters the T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SR 3 & 4- Binding of ACh to the surface of muscle cells opens Na+ and K+ channels resulting in an end-plate potential (EPP) 12 & 13- Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors so stimulation of the muscle cell ceases 8 & 9- Calcium released by SR binds to troponin. Troponin-tropomyosin complex changes shape and exposes active sites on actin 14- Active transport pumps calcium from sarcoplasm back into SR where it binds to calsequestrin. ATP is needed for muscle relaxation as well as muscle contraction 10 - Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking” it in an extended position. It binds to an active site on actin Active transport 11 – Myosin releases the ATP and P and flexes into a bent , tugging the thin filaments along with it (POWER STROKE). 15 & 16- Loss of calcium from sarcoplasm results in troponin-tropomyosin complex moving over the active sites which stops the production or maintenance of tension Muscle fiber returns to its resting length due to stretching of series-elastic components and contraction of antagonistic muscles ATP

Rigor Mortis Stiffening of the body beginning 3 to 4 hours after death -- peaks at 12 hours after death & diminishes over next 48 to 60 hours Deteriorating sarcoplasmic reticulum releases calcium Activates myosin-actin cross bridging & muscle contracts, but does not relax. Muscle relaxation requires ATP & ATP production is no longer produced after death Fibers remain contracted until myofilaments decay