Chapter 10 - Muscle Tissue

Unit 2 PHYL 141

Skeletal Muscle

long fused cells

multiple nuclei

visible striations

voluntary

connected to skeleton

Smooth muscle

spindle shaped

single nuclei

no visible striations

no voluntary movement

located in organs and vessels

Cardiac Muscle

branced and interconnected

single nuclei

visible striations

not voluntary

only in the heart

Organization of muscle fibers from smallest to largest

myofilaments, myofibril, muscle fiber, fascicle, muscle

Muscle fibers

made of myofibrils wrapped in sarcolemma

have dark (A bands) and light (I bands) portions

Endomysium

muscle fibers are wrapped in this

Perimysium

muscle fibers (with endomysium) are bundled and wrapped in this. 

now called a fascicle

Epimysium

Outermost layer, covering the entire muscle

fascicles are wrapped in this

Motor Unit

functional unit of a muscle

composed of a single motor neuron and all the muscle cells it stimulates

sends a signal that causes muscle fibers to contract simultaneously

Sarcomere

Part that contracts when moving muscle

individual unit of contraction

makeup myofibril

Components of sarcomere

A band

I band

H zone

Z line/disc

M line

A band

contains both thick (myosin) and thin (actin) filaments

more static area

Zone of overlap: regions where thin and thick filaments overlap (this can change with contraction)

I Band

contains only thin filaments (actin) 

good indicator of how much more space you have for contraction

Z line/Z disc

anchors for actin

forms the actual function part for muscle 

sarcomere is between 2 of this

M-line

region that holds myosin together

very little zone of overlap

H Zone

region that contains only myosin, no actin

m line is the center of this

H zone

I Band

A band

M line

Z line/Z disc

Neuromuscular Junction Excitation

1. neuronal action potential

2. acetylcholine release

3. Na+ enters the cell

4. Sarcoplasmic reticulum Caa2+ release

Sarcoplasmic Reticulum

releases Ca+ to initiate muscle contractions

energy for cells are produced here (smooth ER for muscles)

Acetylcholine

neurotransmitter/stimulus for muscles (action potential)

A neuron will secrete ths across a synaptic cleft

Action Potential 

change in electrical charge that occurs in a neuron when a nerve impulse is transmitted 

depolarization and repolarization 

Resting potential

initial phase

naturally allows for the movmeent of more K+ ions across membrane

generally netaive

muscles return to after their electrical potential was charged

Depolarization

Na+ into the cell

Neurotrasmitter released —> cell becomes permeable to sodium

Repolarization 

K+ ions out 

cell becomes permeable to K+ again

T-tubules

incaginations of membranes that allow the electric flow to get deep into the cell

ACH causes Na+ to enter the cell through voltage gated chanels

Na+ entering near the Sarcoplasmic reticulum (via t-tubules) causes Ca2+ release

Myofibrils

composed of repeating units called sarcomeres

thick (myosin) filaments and thin (actin) filaments

Tropomyosin and troponin

Thick and Thin filaments contribution to contraction

Myosin grabs onto actin

Myosin pulls actin inward causing contraction

Troponin

Locks tropomyosin in place; when Ca2+ binds to it, it releases tropomyosin

Tropomyosin

prevents myosin binding to actin

Steps of Contraction

1. ATP hydrolyzes into ADP + P powering the myosin head

2. Myosin binds to actin (cross bridge formation) 

3. Myosin puylls actin inward towards the center (power stroke) 

4. ATP binds to myosin cuasing it to detach from actin

Contraction

tightening and shortening of a muscle fiber to generate force

Twitch

singular muscle contraction

Tension

force generated by muscle contractions

strength

Ideal Sarcomere length

80-120% resting length

tension = strength

produces maximum tension

Decreased sarcomere length

below 80% 

zone of overlap increases until thin filaments have nowhere else to go 

strength is decreased

Increased Sarcomere length

above 120% at resting length

produces reduced tension due to lack of concentraction

Zone of overlap reduces until no overlap occues which results in no tension

Motor Unit Recruitment

Fewer = less force, more control

More = more force, less control

Isotonic Contraction

force generated by changing muscle length

Isometric Contraction

force generated while maintaining muscle length

Concentric contraction

Type of isotonic contraction

muscles shorten

flexion

Eccentric contraction

type of isotonic contraction

muscles lengthen

extension

Fast Twitch

Type IIx

Low efficiency

anaerobic

low ATP

highest speed of shortening

Slow twitch

type I

aerobic

low/slowest speed of shortening

large ATP

high efficiency