chapter 10 - muscle tissue

3 types of muscle

skeletal - moves & positions the body

smooth - pushes fluids & solids along digestive tract & regulates artery (blood vessel) diameter

cardiac - pushes blood through circulatory system

functions of skeletal muscle

skeletal movement - pulls tendons to move bones

maintain posture & body position

support soft tissues - ex. muscles of the abdominal wall & pelvic cavity

guard entrances & exists - openings of the digestive & urinary tracts

maintain body temp

store nutrients - glucose, lipid & even muscle proteins used during fasing

roots associated with muscle

myo, mys = muscle (myoneural junction = muscle nerve junction)

sarco - flesh (Gk) 

sarcolemma - cell membrane of a muscle cell 

sarcoplasm - cytoplasm of a muscle cell

sarcoplasmic reticulum - modified endoplasmic reticulum of a muscle cell

muscle tissue

muscle consists of muscle fibres (myofibers) wrapped in a connective tissue called the endomysium

a number of fibres are wrapped together in periosteum to form a fascicle, while fascicles are bundled together to form a muscle, which is wrapped in epimysium or deep fascia

the 3 connective tissue layers come together to form a tendon

Individual fibres are full of contractile organelles called myofibrils, which fill the cell and displace the nuclei to one side

Myofibrils are a systematic arrangement of proteins (myofilaments) which form the contractile unit of the muscle called a sarcomere, which give muscle its striated apperance

connective tissues around

whole muscle - epimysium

fascicles - perimysium

cells (myofibers) - endomysium

muscle attachments

the collagen fibres of 3 different connective tissue layers come together at the ends of the muscle to form tendons (bundles) or aponeuroses (sheets)

structure - tendons in turn are continuous with matrix of the bone they are attached to

aponeuroses

plural 

layers of flat broad tendons with fewer vessels & nerves 

structure - typically both tendons and aponeuroses attach muscles to bones

nerve and blood supply

nerve and blood vessels penetrate epimysium together and branch through perimysium and endomysium

each cell is

• adjacent to capillaries

• Inverted by nerve fibre axons

skeletal muscle is made up of..

myofibrils - a bundle of smaller rod like structures

Myofilaments - even smaller structures/proteins make up myofibrils

development of skeletal muscle

A few meyoblast cells remain in the tissue as meyosatilite cells

Myoblasts → Fuse together (form multinucleate cells)→ mature into skeletal muscle cells where they start producing proteins and contribute to muscle contraction

tubules surrounding the myofibrils

tiny tubules called transverse tubules or T tubules penetrate the sarcolemma and travel deep into the cell surrounding the myofibrils

t tubules - conduct waves of electrical activity from the sarcolemma deep into the cell, providing the signal for the cell to contract

structure of myofibrils

bundles of thick and thin filaments (protein strands)

sarcoplasmic recticulem (basics)

sarcolemma - plasma membrane of the muscle fiber, has an intricate system of penetrating T - tubules that connect to a specialized smooth ER called sarcoplasmic reticulum (SR)

the SR stores and releases calcium ions which are important for muscle contraction

most SR ca2+ bound to calsequestrin protein

muscle fiber bands/sections

A band - (dark area) corresponds to thick filaments

• M line - line of proteins that connect the thick filaments

• H zone - lighter region contains no overlap

Zone of overlap - thin filaments protrude between thick filaments

I band - (light area) extends from A band to A band

• includes thin filaments, with no overlap with thick filaments

• includes Z lines - anchor thin filaments

sarcomere - extends from Z line to Z line 

• functional unit of the muscle cell

Protein titin anchors thick filaments to Z lines (actin)

sarcoplasmic reticulum (in depth)

• form a tubilar network around each myofibril

• proteins form enlarged sacs adjacent to the T Tubules = terminal cisternae

sarcoplasmic reticulum contains Ca+ ions (some free and some bound to calsequestrin)

the cell contracts when stored Ca+ ions are released into the sarcoplasm

sliding filament mechanism of contraction

H zone - narrower

I band - narrower

A band - stays the same 

Zone of overlap - wider

sarcomeres - contract

sarcomeres along each myofibril

lie end to end along each

all of the sarcomeres shorten during a contraction, the myofibrils shorten

as all the myofibrils in a muscle cell shorten, the entire muscle fiber (cell shortens & pulls on tendon)

skeletal muscle contraction - overview

1 - skeletal muscle cell contracts only when activated by a neuron

2- electrical activity passes over the sarcolemma, and down the T tubules, triggering the release of calcium from the sarcoplasmic reticulum

3 - Ca+ triggers interactions between thick and thin filaments, causing them to contract muscle fiber

4- as the cell shortens it generates active tension

thick filaments

made of myosin 

each myosin molecule consists of 2 tails wrapped into 1 and 2 heads that can swivel

each myosin head has binding sites for - actin and atp

thin filaments consist of __ proteins

actin - globular, strung together like beads on a string each molecule has active binding site for myosis

nebulin - long threadlike protein holding actin in place

tropomyosin - strandlike protein, at rest lies over the active sites on actin molecules

troponin - a globular protein, holds tropomyosin in place

• each troponin has a binding site for calcium

when Ca is present in the cytoplasm..

it binds to troponin

this changes the shape of troponin, causing it to allow tropomyosin to slide off the myosin binding the sites of actin

the actin and myosin are now free to interact = cross bridge cycling

contractile proteins

myosin - 

• largest of the contractile protein 

• double stranded molecule, produces two heads at one end 

• myosin is capable of using ATP to generate force and is called a molecular motor

actin -

• form the thinner of the contractile filaments and are anchoring strand for myosin 

• each actin molecule has a binding site for the myosin heads 

• for contraction to occur the myosin head has to bind to actin

the binding site -

• on each actin mol is protected by a long tropomyosin strand 

• troponin molecules are along tropomyosin strand 

• when calcium binds to troponin, the tropomyosin strand is pulled aside to reveal the myosin binding site on actin

cross section bridge occurs when..

calcium binds to troponin complex,

tropomyosin is moved from its actin blocking position.

Once the myosin binding site is revealed on the actin molecule, the myosin head can bind and initiate the cross bridge cycle

cross bridge cycle

refers to the process of myosin heads binding to actin and the generation force to contract the muscle fibers

• begins when myosin is bound to actin after the myosin head is energized by ATP

• the myosisn head changes shape, pulling on the actin filament and releasing ADP. The results in the shortening of the sarcomere

• the head remains bound to the actin filament until another ATP binds to it, allowing myosin to release the actin

• once released the myosin head is energized when ATP is broken down into ADP and Pi which again provides the energy for subsequent binding and pulling

sarcomere and the cross bridge

when calcium is released into the cytosol, actin & myosin are allowed to interact,

resulting in the myosin heads pulling the actin fibers,

generating tension: if sufficient this force shortens the sarcomere & the muscle fiber

each skeletal muscle cell is controlled by..

a single nerve cell

excitation contraction coupling

1. when an electrical stimulus triggers the release of calcium by the sarcoplasmic recticulem, initiating the mechanism of muscle contraction by the sarcomere

2.as the thick and thin filaments interact, the sarcomeres shorten, pulling the ends of the muscle fiber closer together

2.during the contraction, the entire skeletal shortens and produces a pull, or tension, on the tendons on either end

ACh acetylcholine

chemical message - neurotransmitter

EXCITATION CONTRACTION COUPLING

RELAXATION PHASE

muscle twitch

a single stimulus contraction relaxation

• response of a muscle to a stimulus (action potential)

tension

• active (shortening) force exerted by a muscle when it contracts

to generate useful contractions we must be able to vary the amount of tension generated when a. muscle contracts

• typically a contraction is held for longer than a single twitch

factors that influence the amount of tension developed by each muscle cell

1- length of the cell at the time of contraction

2- frequency of stimulation

how does the length of muscle influence the amount of tension developed by each muscle cell

it’s the degree of stretch when the body is in a resting position : degree of overlap between thick and thin filaments

we can alter the degree of stretch by holding the muscle so that muscle fibers are stretched to a length that is close to “ideal” before they are stimulated to contract 

effects of over stretching/compressing the cell

overstretching - reduces the zone overlap and # of cross bridge interactions

over compressing - reduces the zone of overlap and # of cross bridge interactions

in both cases the amount of tension possible is reduced

how does the frequency of stimulation influence the amount of tension developed by each muscle cell

increased frequency of stimulation —> increased tension

1) treppe 

• subsequent twitches have increased tension

• cause - gradual increase in Ca + ions in sarcoplasm, becasue SR ion pumps dont. have time to capture the Ca ions in between stimuli

2) wave stimulation

• muscle cell stimulated a seccond time before relaxation phase is complete

• result - wave stimulation; muscle is never allowed to relax completely, and tension rises until it is roughly 4x the maximum produced by the treppe

• stimulus frequency >50/second

3) Complete/Unfused tetanus

• cell stimulated repeatedly and periods of relaxation are very brief and muscle reaches a submaximal tension

4) complete/fused tetanus

• cell stimulated a high frequency no relaxation between stimuli and all cross bridges form, so muscle reaches max tension

cause of 3 AND 4 - Ca levels rise in sarcoplasm as SR ion pumps dont have time to recapture Ca between stimulation

tension produced by the muscle depends on

1 - tension generated by individual muscle cells activated depends on

a) tendons resting length when stimulated

b) degree of overlap between thick and thin filaments

c) frequency of stimulation

2 - number of cells in stimulated muscle

3 - number of contractile proteins (myofibrils) in each stimulated muscle cell (differences between slow, intermediate and fast twitch muscle fibers

motor unit

all the muscle fibers controlled by a single motor neuron

number of cell varies from a few to 1000s

how would the nervous system increase muscle tension

the nervous system can activate more motor units - recruitment

sequence of recruitment

a) smallest motor unit containing fewest an slowest muscle fibers

b) larger motor units containing faster and more powerful muscle

c) peak tension production when all motor units are in a state of complete tetanus

asynchronous motor unit

at slightly less than maximal tension because motor units are activated on a rotating basis 

func - limited energy reserves make it necessary for motor units to rest and recover.

This is done on a rotating basis to allow for simultaneous recovery and sustained contraction

*asynchronous motor unit summation

muscle tone

slight level of contraction in muscles that are at rest

caused by - in any skeletal muscle, some motor units are always active

func - does not generate active movement, but tenses and firms the muscle to maintain/stabilize bone positions and also absorb sudden bumps/shocks

heightened muscle tone

accelerates the recruitment process, because some motor units are already stimulated

Isotonic Muscle Contractions

muscle length changes 

a) concentric contractions 

• peak muscle tension exceeds resistance —> muscle shortens

b) eccentric contractions 

• peak muscle tension less than load —> muscle elongates

isometric contrations

tension does not overcome resistance

no change in length of muscle

as load increases

the slower the start of contraction, the slower the speed of contraction and the more tension produces.

A concentric contraction occurs and the muscle will shorten

muscle relaxation

muscles cannot lengthen actively

a) ELASTIC FORCES

• Some of the energy spent stretching tendons and organelles is recovered as they recoil

b) Opposing muscle contractions

• contraction of opposing muscles, return a muscle to resting length quicker than elastic forces can

c) gravity

• may assist (relaxing biceps, involves pulling forearm downward)

• some active tension will be required however, to controlthe rate of movement & prevent damage to joints

polio

attacks neurons in the spinal cord and brain. 

what happens?-motor neurons die and can no longer stimulate myofibers resulting in muscular atrophy (reduction in muscle size) and flaccid paralysis 

Tetanus

suppresses the inhibition of motor neuron activity

what happens - a bacterium, clostridium tetani releases a toxin, tetanospasm, that block inhibitory neurotransmitters glycine and GABA - causing inappropriate activation of motor neurons and thus sustained muscle contractions, “spastic paralysis”

Botulism and Myasthenia gravis

affect neuromuscular communication

what happens. - in botulism, a bacterium clostridium botulinum releases a toxin that blocks the release of neurotransmitter ACh receptors resulting in muscle weakness in many regions of the body

without adequate ATP

calcium ions remain in cytosol and cross bridges cannot detach, causing muscle fibers to lock in contracted state. 

This happens when muscle cells are deprived of O2 and nutrients causing sustained contraction

(the cause of rigor mortis after death - which lasts a few days until decomposition begins)

Muscle’s energy

sources of ATP

1- STORED in muscle fiberes before contraction begins as: 3mmol ATP, 20mmol CP (creatine phosphate) and 100mmol glycogen

2 - GENERATED in 3 ways in muscle cell:

Generated ATP in Muscle Cell (3 ways)

1) direct phosphorylation ADP by creatine phosphate:

              CP + ADP ←→ Creatine + ATP

←→ - CPK (Creatine PhosphoKinase)

at rest - skeletal fiber produces more ATP & CP than it needs

as ATP is used - more atp is made through:

2) aerobic metabolism & 3) anaerobic metabolism

what to do in the absence of O2

anaerobic respiration : glycolysis followed by lactic acid production (can’t do krebs or ETC without O2

anaerobic respiration has limits- 

• lowered pH will disable key enzymes necessary for contraction and decrease Ca+ binding to troponin

• debleation of metabolic reserves within muscle fibers 

• damage to sarcolemma and SR

• Muscle fatigue

advantages of aerobic respiration over anaerobic

1 - produces 32 ATP/glucose molecule instead of just 2

2- no lactic acid produced

limiting factors of aerobic respiration

-availblability of O2 that can diffuse into muscle fiber

energy use of muscular activity - at rest

mostly fatty acids (& some glucose) used as fuel to generate ATP (aerobic resp) ATP used to build reserves of CP and glycogen

resting - fatty acids are catabolized; the ATP produced is used to build energy reserves of ATP,CP, and glycogen

energy use of muscular activity - moderate activity

glucose and fatty acids used as fuel in aerobic respiration —> 32 molecules of ATP/molecule of glucose

energy use of muscular activity - peak activity

most (2/3) ATP produced through glycolysis (anaerobic resp)

some ATP via CP catalysis to creatine

• buildup of H+ ions leads to fatigue once sarcoplasm buffer system reaches its limit (enzymes become less functional)

• when blood pH drops = metabolic acidosis (or lactic acidosis)

peak - most ATP is produced through glycolysis, with lactic acid as a by product.

Mitochondrial activity now provides only about one-third of the ATP consumed

glycolysis

enables a skeletal muscle to continue contracting even when insufficient oxygen is available

post exercise recovery processes

remove lactic acid from muscle & convert to pyruvic acid in liver;

• remove ATP, CP, & glycogen reserves in muscle

the oxygen debt

the above processes all require ATP

• generated by aerobic respiration

• requires O2 above resting rates

O2 debt

the amount of O2 required to restore normal, pre exersion conditions (restore ATP,CP and glycogen reserves) and convert lactic acid to pyruvic acid or glucose

what happens at rest

85% of heat needed to maintain body temp, is produced by skeletal muscles

during aerobic respiration, 58% of released energy warms sarcoplasmm, interstitial fluid and circulating blood

as a consequence - during exercize, body temperature starts to rise, and anaerobic respiration releases 70% of energy as heat. Heat loss is accelerated at the skin level to compensate

psychological fatigue

the desire to discontinue the activity due to the effect of low blood pH & feelings of pain on the brain

causes of actual muscle fatigue & soreness

not well understood

• depletion of glycogen, lipid and amino acid reserves

• accumulation of lactic acid and other metabolites

causes of muscle soreness (DOMS)

• small damage that will be repaired

classification of skeletal muscle

based on 2 criteria

1 - speed of contraction : fast, intermediate, slow

result of differences in number of myofibrils, glycogen reserves & number of mitochondria between fast & slow fibers

2- major pathways used to form ATP

• anaerobic or aerobic respiration

based on these 2 factors there are 3 types of skeletal muscle cells

3 types of skeletal muscle cells

slow oxidative (slow fibers) = tyoe I 

fast oxidative (intermediate fibers) = type IIa

fast oxidative (intermediate fibers) = type IIb