Chapter 9 - Muscles and Muscle Tissue

Movement

involuntary or voluntary

Body Posture & Body Position

muscles work to hold us up against gravity

Joint Stability

muscles & tendons reinforce joints

Maintaining Body Temperature

muscle contractions produce heat

• shivering

Excitability

generate action potentials in response to stimulus

Contractability

muscle cells shorten when they contract

Extensibility

muscle cells can lengthen/stretch

Elasticity

healthy muscle cells return to their original shape

Skeletal Muscle Tissue

voluntary muscle tissue

• provides movement of body parts

• striated

• attaches to and uses the skeleton (via tendons)

  • uses them as levers → pulling on the bone

• creates the most force

  • but needs the most rest

• adaptable

Cardiac Muscle Tissue

involuntary muscle tissue

• moves blood through the body

• striated

• found in the heart

Smooth Muscle Tissue

involuntary muscle tissue

• moves fluids and substances through body

• no striations

Innervation of Skeletal Muscle Tissue

each muscle cell (fiber) synapses with 1 motor nerve

• but → each muscle can be served by multiple motor neurons

• function = nerve ending controls activity

Vascularization of Skeletal Muscle Tissue

blood supply brings in nutrients and removes waste

Connective Tissue Sheaths

function = supports muscle, holds muscle together

• 3 layers:

  • endomysium 

  • perimysium

  • epimysium

Endomysium

innermost layer of connective tissue sheaths

• surrounds individual muscle fibers

• preventing them from influencing each other

Perimysium

middle layer of connective tissue sheaths

• discrete bundles of muscle fibers grouped → form fascicles

Epimysium

outermost layer of connective tissue sheaths

• surrounds the entire muscle

• separates individual muscles from each other

Direct Attachment

epimysium of the muscle fuses directly to the bone (or cartilage)

• not a lot of muscles have this attachment

• tears a lot more easily than a tendon

Indirect Attachment

involved tendon

• more common form of attachment because tendons are very thick and tough

Tendon

a band of dense fibrous connective tissue that connects a muscle to a gone

Origin

where the muscle attaches to a less moveable bone

• always proximal

Insertion

where the muscle attached to a moveable bone

• always distal

Sarcolemma

plasma membrane of muscle fibers

Sacroplasm

cytoplasm of muscle fibers

• glycosomes

• myoglobin

Glycosomes

store gylcogen

• glycogen is converted to glucose for ATP production

Myoglobin

stores oxygen

• oxygen is needed for ATP production

Myofilaments

protein filaments in muscle tissue

• types of contractile myofilaments:

  • myosin

  • actin

Myosin & Actin Function

interact during muscle to create tension in the muscle

• responsible for contraction of the skeletal muscle cell

Structure of Myosin Filaments

2 chains 

• myosin heads found at end of each chain

  • each myosin head has 2 binding sites → 1 for ATP, 1 for actin

• myosin head used to link two types of myotilaments during contraction

Structure of Actin Filaments

chains of G actin proteins with myosin binding sites

• myosin head binds to the myosin binding site of actin during muscle contraction

  • regulatory proteins of actin control if/when myosin head can bind

Tropomyosin

arranged along length of thin filament

• blocks myosin binding sites on actin filament when muscle is relaxed

Troponin

globular protein associated with tropomyosin

• binds tropomyosin to position its on the actin filament

Myofibrils

rod-like organelles inside muscle cells

• myofibrils are made up of bands of actin and myosin

  • myofilaments overlap in some regions of the myofibril to produce dark bands

    • this is what creates striations in skeletal muscle

• each muscle fiber has several myofibrils

A band

region of myofibril where actin and myosin filaments overlap

I band

region of myofibril with only actin filament 

• Z disc at center holds the actin filaments in place

Sarcomere

found between neighboring Z discs

• an entire A band and 2 halves of an I band

• the sarcomere is the smallest contractile unit of skeletal muscle tissue

• entire muscle contracts when the sarcomere shortens

T-Tubules

extensions of the sarcolemma that wrap around deeper myofibrils

• increase surface area of sarcolemma

• carry action potential to deeper regions of the cell, ensuring that all the myofibrils get the exact same message at the same time

Sarcoplasmic Reticulum

wraps around myofibrils

• stores and releases Ca2+ for muscle contraction and relaxation

  • releasing Ca2+ is the final action of muscle contraction

• form terminal cisterns around T-Tubules

  • action potentials travel down T-Tubules to stimulate release of Ca2+

The Neuromuscular Junction

site of synapse between a somatic motor neuron and a muscle fiber

• neurotransmitter released → acetylcholine (ACh)

• stimulatory → will contract

Events at the Neuromuscular Junction (step 1)

motor neuron releases ACh at neuromuscular junction

• ACh binds to receptors on sarcolemma

Generation of EPP and action potential across sarcolemma (step 2)

ACh opens ion channels on the sarcolemma to create the end plate potential (EPP)

End Plate Potential (EPP)

a graded potential specific to muscle tissue

• EPP depolarizes sarcolemma

  • if strong enough → action potential is generated and spreads down sarcolemma

Excitation-Contraction Coupling (step 3)

action potential spreads to T-Tubules

• when action potential arrives at T-Tubules → Ca2+ channels in terminal cistern open

  • result = Ca2+ released from sarcoplasmic reticulum

Cross Bridge Formation & Muscle Contraction (step 4)

1. Ca2+ binds troponin and it changes shape

2. change in troponin shape causes tropomyosin rolls to the side

3. when tropomyosin is moved, the myosin binding site on actin is exposed

4. myosin head splits ATP into ADP + P_i → allows myosin head to bind to actin

5. ADP + P_i is released from myosin head = myosin head bends → myosin head “pulls” actin filament toward center of sarcomere

6. myosin head binds to another ATP → myosin head detaches from actin binding site

• Steps 4-6 occurs along the length of the actin filament until muscle contraction ends of ATP/Ca2+ runs short

  • myosin head binds to a new actin binding site with each new ATP molecule

Cross Bridge

the attachment of myosin to actin

Power Stroke

myosin head “pulls” actin filament toward center of sarcomere

4 Steps for Stimulation of Muscle Fiber to Occur

1. events at neuromuscular junction

2. generation of EPP and action potential across sarcolemma

3. excitation-contraction coupling

4. cross bridge formation and muscle contraction

Sliding Filament Model of Muscle Contraction

during contraction, actin filaments “slide” over myosin filaments

• myosin head “slide” thin filaments toward the center of the sarcomere

• effect = when the filaments “slide,” the sarcomere shortens and generates tension in the muscle

Motor Units

a single motor neuron and all the muscle fibers it innervates

• a single motor neuron can innervate multiple muscle fibers

• BUT → a single muscle fiber is innervated by only ONE motor neuron

Motor Unit “Rules”

• when the motor neuron fires → all fibers it innervates will contract

  • fibers innervated by a single motor neuron are spread out over the entire muscle → not clumped together

• number of muscle fibers a motor neuron innervates influences movement

  • motor neuron innervating a few fibers vs motor neuron innervating many fibers

    • ONE motor neuron → 3 muscle cells (fine-tuned control over muscle cells)

    • MORE THAN ONE motor neuron → (more coarse control over larger muscles)

Graded Muscle Contractions

muscle contraction that is modified to produce varying amounts of force

• 2 ways of grading:

  • temporal summation

  • motor unit summation

Temporal Summation

increasing the frequency of stimulation

• increasing the firing rate of a motor neuron can generate more force

• fire stimuli in rapid succession → the second impulse hits the muscle fiber before it has completely relaxed from the first stimulus

Motor Unit Summation

increasing the number of motor units used

• more force by increasing the number of motor units used during contraction

Size Principal of Motor Unit Summation

• motor units with smallest muscle fibers recruited first

• motor units with larger muscle fibers recruited last → create most force

• motor units recruited asynchronously → some contracting, others relaxing

  • DONE to make the contractions last longer; won’t fatigue as fast

Muscle Tone

relaxed muscles are always slightly contracted → created muscle tone

• does not produce movement

• keeps muscle tissue healthy and responsive, stabilizes joints, maintains posture

• loss of muscle tone leads to loss of responsiveness

  • muscle will NOT respond to stimuli

Isotonic Contraction

muscle tension develops to overcome the load and muscle shortening occurs

• 2 subtypes:

  • concentric contraction

  • eccentric contraction

Concentric Contraction

muscle shortens and does work

Eccentric Contraction

muscle lengthens while under tension

Isometric Contraction

tension develops in a muscle, but the length of the muscle does not change

• occurs when the load is not moved

• cross-bridge formation still occurs, but the sarcomeres do not shorten

• ex) muscles in the neck, hold up the head

Energy Needs for Contraction

ATP is the ONLY energy source used directly for contractile activity

• skeletal muscle stores glycogen for ATP production

Direct Phosphorylation

creates ATP from ADP + P_i using creatine phosphate (CP)

• P_i from CP transferred directly to ADP molecule

• 1 ATP produce per CP molecule

• does NOT require oxygen

  • drawbacks:

    • CP has a limited supply

    • only producing 1 ATP per CP; low energy yield

  • benefits:

    • immediate and rapid supply of ATP

Anaerobic Pathways: Glycolysis

glucose broken down to form 2 ATP and pyruvic acid

• in absence of O2 → pyruvic acid converted to lactic acid

  • benefits:

    • does not require oxygen

    • produced ATP quickly

  • drawbacks:

    • low ATP yield (2 ATP per glucose)

    • lactic acid build-up might cause muscle fatigue or soreness

Aerobic Pathway: Cellular Respiration

creates ~95% of ATP used by muscle during rest and light to moderate long-term exercise

• benefit:

  • produce 30-32 ATP

• drawbacks:

  • slow process

  • requires constant O2 and glucose

Muscle Fatigue

ATP is not unlimited

• muscle fatigue occurs → muscle is physiologically incapable of contracting

  • muscle fatigue is important to make sure there is still ATP in the muscle, so it’s not completely depleted

    • if depleted, → can’t form cross-bridges and can’t perform other chemical reactions to keep the cell alive

• rate and duration of fatigue depend on the activity

  • high-intensity exercise vs low-intensity exercise

    • high → weight lifting; fatigue sets in more quickly, but will recover faster

    • low → marathon running; recovery period is longer since it’s being used longer

Muscle Contraction: Force

force of contraction is determined by the number of cross-bridges formed between myosin and actin filaments

• more cross-bridges = more force

Influences of Muscle Contraction: Force

1. frequency of stimulation → temporal summation

2. number of muscle fibers recruited → motor unit summation

3. size of muscle fiber → larger fibers generate more force

4. degree of muscle stretch

Hypertrophy

increase size of muscle fibers in muscle to increase force generated

• rate of hypertrophy dependent on genetics, sex, nutrition, etc.

Length-Tension Relationship

the amount of tension a muscle can produce depends on its length

• if the muscle cell is already contracted, → can’t slide, which means the muscle can’t generate force

• if muscle cells is already stretche,d → can’t form cross-bridges, since the filaments don’t overlap with one another

Muscle Contraction: Velocity and Duration

fiber type influences velocity and duration of muscle contraction

• speed of contraction; dependent on:

  • how fast ATP is split → how fast cross-bridges can form and break

  • electrical activity of motor neurons → fast neurons = fast contraction

• pathway of ATP production

Fast Glycolytic Fibers

contract quickly

• use anaerobic pathways

  • high glycogen content

  • low myoglobin

  • low mitochondria and blood supply

Slow Oxidative Flibers

contract slowly

• use aerobic pathways

  • low glycogen content

  • high myoglobin content

  • lots of mitochondria and blood supply

Fast Oxidative Fibers

contract quickly

• used aerobic pathways

  • has some glycogen

  • lots of myoglobin

  • lots of mitochondria and blood supply

Gross Anatomy of Smooth Muscle

hollow organs in the body and a few other regions (iris, bronchi, etc.) have smooth muscle

• most organs have 2 layers of smooth muscle tissue that never contract simultaneously in the same part of an organ:

  • longitudinal layer = muscle fibers run the length of the organ

    • when contracted, → causes the organ to be shorter and wider

  • circular layer = muscle fibers run the circumference of the organ

    • when contracted, → causes the organ to be longer and narrower

Microscopic Anatomy of Smooth Muscle

• no neuromuscular junctions → innervation forms varicosities

  • wide synaptic cleft that release neurotransmitter to multiple smooth muscle fibers simultaneously

• smooth muscle fibers have gap junctions

  • “spontaneous” depolarization

• smooth muscle fibers have no T-Tubules and less sarcoplasmic reticulum

  • sarcoplasmic reticulum released only a small amount of Ca²⁺

• no striations or sarcomeres

• no troponin

Caveolae

invaginations of the sarcolemma of the muscle fiber

• have Ca²⁺ ion channels

  • most Ca²⁺ come from outside of the muscle cell

Calmodulin

protein that acts as Ca²⁺ binding site to initiate contraction

Unitary Smooth Muscle

everything described so far are characteristics of unitary smooth muscle

• much more common → found in hollow organs

Multi-Unit Smooth Muscle

have no gap junctions

• muscle fibers are structurally independent

• forms motor units

• have graded contractions with recruitment

• found in arrector pili, smooth muscle of airways, and internal eye muscles