KIN 223 Chapter 10

functions of skeletal muscle

1. move the body

2. maintain posture

3. protect & support

4. regulate elimination of materials

5. produce heat

characteristics of skeletal muscle

1. excitability

2. conductivity

3. contractibility

4. extensibility

5. elasticity

excitability

ability to respond to a stimulus by changing electrical membrane potential

conductivity

involves sending an electrical change down the length of the cell membrane

contractility

exhibited when filaments slide past each other; enables muscle to cause movement

extensibility

ability to be stretched

elasticity

ability to return to original length following a lengthening or shortening

fascile

muscle fibers bundled within a fascicle

consists of many muscle cells (muscle fibers)

epimysium

dense irregular CT wrapping whole muscle

AROUND MUSCLES

perimysium

dense irregular CT wrapping fascicle

houses many blood vessels and nerves

AROUND FASCICLES

endomysium

areolar CT wrapping individual fiber

delicate layer for electrical insulation, capillary support, binding of neighboring cells

AROUND MUSCLE FIBERS (aka muscle cells)

CT attachments

tendons: cordlike structure of dense regular CT

aponeuroses: thin, flattened sheet of dense irregular CT

deep fascia

dense irregular CT superficial to epimysium

separates individual muscles; binds muscles with similar functions

superficial fascia

areolar and adipose CT superficial to deep fascia

separates muscles from skin

blood vessels and nerves

• skeletal is vascularized, has extensive blood vessels

  • deliver oxygen and nutrients, removing waste products

• skeletal muscle is innervated my somatic motor neurons

  • axons of neurons branch, terminate at neuromuscular junctions

  • considered voluntary muscle, because contraction is consciously controlled

sarcoplasm (cytoplasm)

has typical organelles plus contractile proteins and other specializations

multinucleated

• multiple nuclei

• cell is formed in embryo when multiple myoblasts fuse

• satellite cells: myoblasts did not fuse w/ muscle fibers during development & remain in adult skeletal MT

sarcolemma (plasma membrane)

• sarcolemma has voltage-gated ion channels that allow for conduction of electrical signals

• entry/exit of materials

T-tubules (transverse tubules)

deep invaginations of the sarcolemma

transport action potential from sarcolemma throughout entire muscle fiber

contains voltage-sensitive calcium channels

myofibrils

bundles of myofilaments surrounded by sarcoplasmic reticulum

approx 80% of volume

sacroplasmic reticulum

internal membrane complex similar to smooth ER

contains terminal cisternae, calcium pumps, and calcium release channels

terminal cisternae

blind sacs of sarcoplasmic reticulum

serve as reservoirs for calcium ions

two cisternae with T-tubule in between = triad

myofilaments

contractile protein filaments within myofibrils

thick filaments

consist of bundles of 200-50 myosin protein molecules

myosin heads (myosin ATPase) point toward ends of the filament

globular head: binding site of actin and ATPase cite (shortens a sacromere)

thin filaments

two twisted strands of actin (each F-actin composed of G-actin monomers)

G-actin has myosin binding site where myosin heads attach

tropomyosin and troponin = regulatory proteins

tropomyosin

short, thin, twisted, “string like” filament

covers active sites on actin @ rest

troponin

globular “ball like” protein attached to tropomyosin

sacromeres

myofilaments arranged in repeating units called sacromeres

composed of overlapping thick and thin filaments

delineated at both ends by Z discs (specialized proteins perpendicular to myofilaments; anchors thin filaments)

the positions of think and thick filaments give rise to alternating I-bands and A-bands

I-bands

light appearing regions that contain only THIN filaments

bisected by Z disc

get smaller when muscle contracts (can disappear with maximal contraction)

A-band

dark-appearing region that contains thick filaments and overlapping thin filaments

contains H zone and M line

makes up central region of sacromere

does not change length during contraction

H zone

most central part of the A band in a resting sacromere

Thick filaments only!

disappears with maximal muscle contraction

M line

middle of H zone

protein meshwork structure

attachment site for thick filaments

connectin

extends from Z disc to M line

stabilizes thick filaments and has “springlike” properties (passive tension)

Dystrophin

anchors some myofibrils to sarcolemma proteins

abnormalities of this protein cause muscular dystrophy

nebulin

anchored to Z disc and extends the length of the thin filament

funtions in sacromere assembly; role in cross-bridge cycling

mitochondria in skeletal muscle

• muscle fibers have abundant mitochondria for aerobic ATP production

• myoglobin within cells allows storage of oxygen used for aerobic ATP production

• glycogen is stored for when fuel is needed quickly

• creatinine phosphate can quickly give up its phosphate group to help replenish ATP supply

motor unit

• single motor neuron & the skeletal muscle fibers it controls

• the number of fibers a neuron innervates varies

  • small motor units have <5; allow for precise control

  • large motor units have thousands of muscle fibers; large amount of force, but not precise control

• fibers of a motor unit are dispersed throughout the muscle, not just in one clustered compartment

neuromuscular junction

location where motor neuron innervates muscle

usually mid-region of muscle fiber

has a synaptic knob, synaptic cleft, motor end plate

synaptic knob

• expanded tip of the motor neuron axon

• houses synaptic vesicles filled with acetylcholine (ACh)

• has voltage-gated Ca2+ channels in membrane

motor end plate

• specialized region of sarcolemma with numerous folds

• has many ACh receptors

  • allow Na+ entry and K+ exit

synaptic cleft

• narrow fluid-filled space

• separates synaptic knob from motor end plate

• Acetylcholinesterase resides here (breaks down ACh molecules)

Skeletal muscle fiber at rest

exhibit a resting membrane potential (RMP)

fluid inside cell is negative compared to outside fluid

RMP of muscle cell is -90mV

RMP established by leak channels and Na+/K+ pumps (voltage-gated channels are closed)

excitation of skeletal muscle fiber

occurs @ neuromuscular junction and results in ACh release (and binding)

1. calcium entry @ synaptic knob

2. release of ACh from synaptic knob

3. binding of ACh @ motor end plate

excitation-contraction coupling

1. development at end plate potential (EPP) at motor end plate of muscle fiber

2. action potenital is initated and an electrical signal is propogated along sarcolemma and T-tubules

3. Release of Ca2+ from the sarcoplasmic reticulum

-90 mv @ resting membrane potential

-65 mv @ threshold

+30 mv @ depolarization

-90 mv @ repolarization

cross-bridge cycling

calcium binding to troponin

1. crossbridge formation (myosin attaches to actin)

2. power stroke (myosin head pulls thin filament in)

3. release of myosin head (ATP binding to myosin head to release head from actin)

4. resetting of myosin head (ATP split into ADP and phosphate)

results in sacromere shortening; filaments are the same length but just slide past one another

Events in muscle relaxation

• termination of nerve signal and ACh release

• hydrolysis of ACh by acetylcholinesterase

• Closure of ACh receptor terminates end plate potential

• No further action potential generation

• Closure of calcium channels in sarcoplasmic reticulum

• Return of Ca2+ to sarcoplasmic reticulum by pumps

• Return of troponin to original shape

• Return of tropomyosin blockade on actin

• Return of muscle to original position due to its elasticity

Skeletal muscle relaxation and elasticity

Connectin is a cablelike protein associated with the thick filament. Part of the protein has a coiled shape that stretches when the muscle is stretched. When tension decreases, connectin recoils and helps the muscle fiber return toward its normal resting length—sort of like a bungee cord returning to its original length when the tension is removed.

During muscle relaxation, calcium is pumped back into the sarcoplasmic reticulum, cross-bridge cycling stops, and muscle tension decreases. The elasticity provided by connectin then helps the muscle return toward its resting length.

Therefore, relaxation stops the production of active tension, while elasticity helps restore the muscle’s resting length.

Supplying Energy for Skeletal Muscle Metabolism

• muscle cells have only a little ATP storage

• only about 5 seconds and then its rapidly produced by myokinase

• Additionally provided via creatine phosphate, glycolysis, and aerobic cellular respiration

Creatine phosphate

contains high-energy bond between creatine and phosphate

phosphate can be transferred to ADP to from ATP

catalyzed by creatine kinase

10-15 seconds of additional energy

glycolysis

does not require oxygen

glucose is converted to two pyruvate molecules

rapid release of 2 ATP per glucose molecule

occurs in cytosol

aerobic cellular respiration

requires oxygen

occurs within mitochondria

variety of nutrients can be oxidized (pyruvate, fatty acids, amino acids)

slower than glycolysis, but greater amounts of ATP produced

Lactate formation

from pyruvate occurs under conditions of lower oxygen availability

coverted via lactate dehydrogenase

can be used as fuel by skeletal muscle fiber or enter blood and taken up by cardiac muscle or liver

Energy Supply at varying exercise

• for a sprint (<10 seconds) = ATP supplied by phosphate transfer system

• for a 400 meter sprint (<1 minute) = ATP supplied by glycolysis

• for a 1600 meter run (>1 minute) = ATP supplied by aerobic processes

oxygen debt

amount of additional oxygen needed after exercise to restore pre-exercise conditions

additional oxygen required to

• replace oxygen on hemoglobin and myoglobin

• replenish glycogen

• replenish ATP and creatine phosphate

• convert lactic acid back to glucose

skeletal muscle fiber classification

1. type of contraction generated

2. means for supplying ATP

type of contraction generated

• differences in power, speed, and duration

  • power = diameter

  • speed and duration = type of myosin ATPase, quickness of action potential propagation and quickness of Ca2+ release and reuptake

ex. fast twitch fibers are more powerful and have quicker and briefer contractions than slow-twitch fibers

means for supplying ATP

1. oxidative fibers (fatigue resistant) use aerobic cellular respiration

   1. extensive capillaries, many mitochondria, large supply of myoglobin

   2. red fibers

2. glycolytic fibers (fatigable) use anaerobic cellular respiration

   1. fewer capillaries, fewer mitochondria, small supply of myoglobin, large glycogen reserves

   2. white fibers

types of muscle skeletal fibers

1. slow oxidative (SO) fibers (type I)

   1. contractions are slower and less powerful

   2. high endurance since ATP supplied aerobically

   3. about half the diameter of other fibers, red in color due to myoglobin

2. fast oxidate (FO) fibers (type IIa, intermediate)

   1. contractions are fast and powerful

   2. primarily aerobic respiration, but delivery of oxygen lower

   3. intermediate size, light red in color

3. fast glycolytic (FG) fibers (type IIx, fast anaerobic)

   1. contractions are fast and powerful

   2. contractions are brief, as ATP production is primarily anaerobic

   3. largest size, white in color due to lack of myoglobin

distribution of skeletal muscle fiber types

• a single muscle contains a mixture of fiber types

• variations in proportions of fiber types in different muscle types

  • hand muscles have a high percentage of fast glycolytic fibers for quickness

  • back muscles have a high percentage of slow oxidative fibers to continually maintain postural support

  • long distance runners have a higher proportion of slow-oxidative fibers in legs

  • sprinters have a higher percentage of fast glycolytic fibers

• determined primarily by genes, and partially by training

muscle tension

force generated when a muscle is stimulated to contract

muscle twitch

a brief contraction to a single stimulus

the minimum voltage that triggers a twitch is the threshold

periods of a muscle twitch

• latent period

  • time after stimulus but before contraction begins

  • no change in tension

• contraction period

  • time when tension is increasing

  • begins as power strokes pull thin filaments

• relaxation period

  • time when tension is decreasing to baseline

  • begins with release of crossbridges

motor unit recruitment

if stimulating voltage increases as muscle is stimulated repeatedly, more units are recruited to contract

• called recruitment, or multiple motor unit summation

• explains how muscles exhibit varying degrees of force

• recruit few motor units to lift pencil vs many to lift a suitcase

above a certain voltage, all units are recruited, and so maximum contraction occurs (regardless of how much higher voltage is)

recruitment order based on size of motor units (small first, large last)

wave summation (temporal summation)

if stimulation frequency is increased while voltage remains constant, wave summation occurs

contractile forces are added

relaxation is not completed before next stimulus arrives

contractile forces add up to produce higher tensions

occurs if stimulus frequency set at about 20-50 per second

incomplete tetany

if frequency is increased further, myogram exhibits incomplete tetany

tension increases and twitches partially fuse

tetany

if frequency increases further (ex. 40 to 50 seconds), myogram exhibits tetany

tension trace is a smooth line without relaxation

fatigue

high frequncy stimuli leads to fatigue

muscle tone

• resting tension in a muscle

• generated by involuntary nervous stimulation of muscle

• some motor units are stimulated randomly at any time

• change continuously so units not fatigued

• does not generate enough tension for movement

• decreases during deep sleep

isometric contraction

tension is sufficient to overcome resistance

muscle length stays the same (ex. holding a weight while arm doesn’t move

isotonic contraction

muscle tension overcomes resistance resulting in movement

tone stays constant, but length changes

concentric contraction: muscle shortens as it contracts (ex. bicpes brachii when lifting a load)

eccentric contraction: muscle lengthens as it contracts (ex. biceps brachii when lowering a load)

length-tension relationship

tension a muscle produces depends on its length at the time of stimulation

• fiber @ resting length generates maximum contractile force (optimal overlap of thick and thin filaments)

• fiber @ shortened length generates weaker force (filament movement is limited bc already close to Z disc)

• fiber extended length generates weaker force (minimal thick and thin filament overlap for crossbridge formation)

• visualized by length-tension curve

muscle fatigue

reduced ability to produce muscle tension

primarily caused by a decrease in glycogen stores

other causes:

• excitation at neuromuscular junction

• excitation-contraction coupling

• crossbridge cycling

changes from sustained exercise

• endurance exercise leads to better ATP production (ex. more mitochondria)

• resistance exercise leads to hypertrophy

  • muscles increases in size due to increased in synthesis of contractile proteins

  • muscle also increases glycogen reserves and mitochondria

  • limited amount of hyperplasia (increase in number of fibers)

changes from lack of exercise

• atrophy: decrease in size due to lack of use

  • ex. someone wearing a cast

  • initially reversible, but becomes permanent if extreme

effects of aging

• loss of muscle mass with age

  • slow loss typically begins in mid 30s

  • decreased size, power, and endurance of skeletal muscle

  • loss in fiber number and diameter (decrease in myofibrils)

  • decreased oxygen storage capacity

  • decreased circulatory supply to muscles with exercise

• reduced capacity to recover from injury

  • decreased number of satellite cells

  • fibrosis: muscle mass often replaced by dense regular CT; decreased flexibility

cardiac muscle cells

• short, branching fiberes

• one or two nuclei

• striated (contain sarcomeres)

• many mitochondria (use aerobic respiration)

• intercalated discs join ends of neighboring fibers

• contractions started by heart’s auto rhythmic cells (aka pacemaker)

• heart rate and contraction force influenced by autonomic nervous system

smooth muscle locations

• in blood vessels of cardiovascular system

  • helps regulate blood pressure and flow

• in bronchioles of respiratory system

  • controls airflow to alveoli

• in intestines of digestive system

  • mixes and propels materials

• in ureters of urinary system

  • propels urine from kidneys to bladder

• in uterus of reproductive system

microscopic anatomy of smooth muscle

• cells have fusiform shape (wide in the middle with tapered ends)

• smaller than skeletal muscle fibers

• sarcolemma has varied types of Ca2+ channels

• transverse tubules absent (surface area increased by calveolae)

• sarcoplasmic reticulum sparse

smooth muscle contraction

1. opening of voltage-gated Ca2+ channels

2. binding of Ca2+ to calmodulin

3. activation of myosin light-chain kinase (MLCK)

4. activation of myosin head

5. crossbridge formation, power stroke, reattachment

smooth muscle relaxation

• cessation of stimulation

• removal of Ca2+ from sacroplasm

• dephosphorylation of myosin by myosin light-chain phosphatase

• can be slow to relax due to latchbridge mechanism

smooth muscle anatomy differences

latchbridge mechanism: myosin attaches to actin for extended time without using extra ATP

calmodulin: protein that binds Ca2+, triggers contraction

myosin light chain kinase: enzyme that phosphorylates myosin heads when activated by calmodulin

myosin light chain phosphatase: enzyme that dephosphorylates myosin head (required for relaxation)

smooth muscle characteristics

• long laten period

• long duration

• slowness fits its functional requirements

• fatigue resistant

• broad length tension curve

control of smooth muscle

• autonomic nervous system secretes neurotransmitter

  • muscles response depends on neurotransmitter present and muscle’s receptor for it

• response to stretch

  • myogenic response is contraction in reaction to stretch

  • stress-relaxation response is relaxation after prolonged stretch

• other stimulating factors such as hormones, low pH, high oxygen, high carbon dioxide, certain drugs, pacemaker

multiunit smooth muscle

• arranged in units that receive stimulation to contract individually

• found in

  • iris and ciliary muscle of the eye

  • arrector pili muscles in skin

  • larger air passageways in respiratory system

  • walls of larger arteries

• degree of contraction depends on number of motor units activated, similar to skeletal muscle

single-unit smooth muscle

• most common type

• stimulated to contract in unison as cells linked by gap junctions

• form two or three sheets in wall of hollow organ

• locations include

  • walls of digestive, urinary, and reproductive tracts

  • portions of respiratory tract

  • most blood vessels

stimulation of single-unit smooth muscle

• occurs through varicosities (swellings of autonomic neurons)

  • contains synaptic vessels with one type of neurotransmitter (ACh and norepinephrine)

• receptors scattered across the sarcolemma (diffuse junctions)

• numerous smooth muscle cells stimulated simultaneously

• stimulation spread from cell to cell via gap junctions so contraction is synchronous