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

Muscle fiber (cell)

Fascicle

Muscle

EndomysiumPerimysium

Epimysium

Tendon

(connective tissue around muscle cells)

(connective tissue around muscle fascicles)

(connective tissue surrounding entire muscle)

(attachments between muscle and bone matrix)

The Muscle Fiber

2 Terminal cisternaeTransverse tubule

Muscle fiber

Openings intotransverse tubules

Triad:

Sarcoplasmic reticulumIt stores and releases calcium for muscle contraction

Terminal cisterna

SarcolemmaSarcoplasm

MitochondriaThey produce the chemical energy (ATP) for muscle contraction

T tubulesThey conduct the nerve impulse from the sarcolemma to the interior of the cell

Myofibrils

Myofilaments

Nucleus

SKELETAL MUSCLEContains: Surrounded by:

MUSCLE FASCICLEContains: Surrounded by:

MUSCLE FIBER (CELL)Contains: Surrounded by:

Muscle fascicles Epimysium

Muscle fibers (cells) Perimysium

Myofibrils Endomysium

MYOFIBRILContains:

Myofilaments

They are organized in sarcomeres

Sarcomere

MYOFILAMENTSThick filaments: myosin

Thin filaments: actin

Thin filament

Myosin molecule

Thick filament

Tropomyosin

HeadHinge region

Tail

G actin

Troponin complex

Myofilaments

Myosin

Actin

Contractile proteinsThey do the work of shortening the muscle fiber

Regulatory proteinsTroponin

TropomyosinThey act like a switch to determine when the fibers can contract

They attach the thin and elastic filaments

They are the smallest functional units of the muscle fiber

Sarcomere

A band (dark)

H band

M line

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

Zone of overlap: It is the region where the thin filaments are situated between the thick filaments

Zone of overlap

Zone of overlap

A band: Its length is equal to the length of the thick filaments. It contains both thin and thick filaments

I band (lighter) It contains thin filaments but not

thick filaments

Z lineI band

Zone of overlap

H band M line

Actin (thin filaments)

Myosin (thick filaments)

Z line Z lineTitin(elastic

filaments)

Striations

I band I band

A band

H band

Zone of overlap

Zone of overlap

M lineZ line Z line

When a skeletal muscle fiber contracts:

1- The H bands and I bands get smaller

2- The zone of overlap get larger

3- The Z lines move closer together

4- The width of the A band remain constant

Active sites

Power stroke; sliding of thin filament over thick filament

Formation of myosin–actin cross-bridgeHydrolysis of ATP to ADP + Pi; activation and cocking of myosin head

Sliding of thin filament over thick filament shorten the sarcomeres and muscle also shorten (contraction)

ATP ADP

Pi

ADP Pi

ADP

Pi

Contraction

Binding site for myosin (active site)

G-actin strand

ATP

ADP + Pi

Hydrolysis of ATP to ADP + Pi; activation and cocking of myosin head

Ca +2

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

F-actin strand

Tropomyosin

ADP + Pi

Cross-bridge

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2

Ca +2Ca +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

Troponin

F-actin strand

Tropomyosin

ADP + Pi

Cross-bridge

The Nerve-Muscle Relationship

It is one nerve fiber and all the muscle fibers innervated by it

Motor unit:

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

The Muscle Fiber Muscle fiber

Sarcoplasmic reticulumIt stores and releases calcium for muscle contraction.

Terminal cisterna

MitochondriaThey produce the chemical energy (ATP) for muscle contraction.

T tubulesThey conduct the nerve impulse from the sarcolemma to the interior of the cell.

Myofibrils

Myofilaments

Nucleus

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+

. .

..

. . ..

. ...

Action potential

Neurotransmitter

Action potential

Synaptic knob or axon

terminal

Synaptic cleft

Motor nerve fiber

Synaptic knob

Sarcolemma

T tubule

Junctional folds

Synaptic vesicle

Synaptic cleft

Sarcolemma

Mitochondrion

Myelin

The Neuromuscular Junction

Myofilaments

Sarcoplasm

Synaptic knob

They have ACh receptors which bind Ach

They contain acetylcholine (Ach)

Chemically gated ion channels:

+ + + + + + + _ _ _ _ _ _ _+ + + + +

_ _ _ _

Voltage gated ion channels:

Ligands

(Neurotransmitters, hormones)

They open in response to a voltage change in the plasma membrane.

They open when the specific ligand binds to the receptor.

Two Types of Ion Channels

+ + +

_ _ _ + + +

_ _ _

Acetylcholine Na+

End plate Potential

Resting Membrane Potential

Chemically Gated ion Channels

Axon of motor neuron

Action potential

Synaptic terminal

Sarcolemma

T tubule

Junctional folds

Synaptic vesicle

Ca 2+

Fusing synaptic vesicle

Synaptic cleft

Action potential

Voltage gated ion channels

open

Excitation1- The Arrival of an Action Potential

Mitochondrion

2- The Release of Acetylcholine

Sarcolemma

+ + + + + + + +

_ _ _ _ _ _ _ _ _ _ _Acetylcholine

Na

K

Acetic acid

Choline

Axon terminal

Motor End Plate

Chemically gated ion channels+

+

End-plate potentialIt is rapid fluctuation in the membrane potential that falls back to a level close to the resting membrane potential

3- Binding of Ach to the receptors

4- Opening of ligand-gated ion channels and creation of end plate potential

+ + + + + + + +

_ _ _ _ __ _ _

Voltage-gated ion channels

+ + + +

_ _ _ _

End-plate potential

Action Potential

5- Opening of voltage-gated ion channels and creation of action potential

Ligand- gated ion channels

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

Action Potential:

- - + + + + + + + + + +

+ + - - - - - - - - - - - -

ATP

Acetylcholine

Ca+2

Na +

1 & 2- Nerve signal stimulates voltage-gated calcium channels that result in exocytosis of synaptic vesicles containing ACh = ACh release

3 & 4- Binding of ACh to the surface of muscle cells opens Na+ and K+ channels resulting in an end-plate potential (EPP)

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

8 & 9- Calcium released by SR binds to troponin. Troponin-tropomyosin complex changes shape and exposes active sites on actin

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

11 – Myosin releases the ATP and P and flexes into a bent , tugging the thin filaments along with it (POWER STROKE).

12 & 13- Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors so stimulation of the muscle cell ceases14- 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

15 & 16- Loss of calcium from sarcoplasm results in troponin-tropomyosin complex moving over the active sites which stops the production or maintenance of tensionMuscle fiber returns to its resting length due to stretching of series-elastic components and contraction of antagonistic muscles

Active transport

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 calciumActivates myosin-actin cross bridging & muscle contracts, but

does not relax.Muscle relaxation requires ATP & ATP production is no longer

produced after deathFibers remain contracted until myofilaments decay

Rigor Mortis