1- Chapter 10: Muscle Tissue

skeletal, cardiac, and smooth muscle

what are the 3 types of cells in muscle tissue?

skeletal

what muscle type is this?

cardiac

what muscle type is this?

smooth

what muscle type is this?

myocyte

what is the term for a muscle cell?

fiber

what is the term for a skeletal or smooth muscle cell?

sarcolemma

what is the term for a muscle plasma membrane?

sarcoplasm

what is the term for the cytoplasm of a muscle cell?

sarcoplasmic reticulum (SR)

what is the term for a modified endoplasmic reticulum (smooth)?

myo, mys, and sarco

what 3 prefixes refer to muscle?

excitability, contractility, extensibility, and elasticity

what are the 4 functional characteristics of muscle tissue?

excitability

the ability to receive and respond to stimuli

contractility

the ability to shorten forcibly

extensibility

the ability to be stretched or extended

elasticity

the ability to recoil and resume the original resting length

skeletal muscles

what muscle responsible for all locomotion?

cardiac muscles

what muscle is responsible for pumping the blood through the body?

muscles

generally, what is responsible for maintaining posture, stabilizing joints, and generating heat?

smooth muscles

what muscle helps maintain blood pressure, and squeezes or propels substances (ex: food or feces) through organs?

endomysium, perimysium, and epimysium

what are 3 connective tissue sheaths?

endomysium

fine sheath of areolar connective tissue surrounding each muscle fiber

perimysium

fibrous connective tissue that surrounds groups of muscle fibers called fascicles

epimysium

an overcoat of dense irregular connective tissue that surrounds the entire muscle

skeletal muscle: nerve and blood supply

each muscle is served by one nerve, an artery, and one or more veins
each ______ ______ fiber is supplied with a nerve ending that controls contraction
contracting fibers require continuous delivery of oxygen and nutrients via arteries
wastes must be removed via veins

skeletal muscle microscopic anatomy

each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
fibers are 10 to 100 micrometers in diameter, and up to 30 centimeters long
many mitochondria
glycosomes for glycogen storage, myoglobin for O2 storage
fibers are formed by fusion of many embryonic myoblasts giving each fiber multiple nuclei

myofibrils

are densely packed, rodlike contractile elements
make up most of the muscle volume, about 80%
contain sarcomeres- the smallest contractile units

sarcomeres

the smallest contractile unit of a muscle
the region of a myofibril between two successive Z discs (lines)
thin filaments and thick filaments

thin filaments

made of the protein actin, tropomyosin, and troponin

thick filaments

made of the protein myosin, bundles of contractile

z disc or line

separate sarcomeres, coin-shaped sheet of proteins on midline of light I band

a band

darker and denser region, run length of thick filaments

h band

narrower center of each A band

i band

lighter, less dense area with thin filaments only

m band

middle of sarcomere, line of protein (myomesin) that bisects H band vertically

thick, thin, and elastic filaments

myofibrils contain what 3 types of myofilaments?

elastic filaments

single massive, spring like structural protein (titin); stabilizes myofibril structure; resists excessive stretching

myosin

(thick filaments) contains 2 heavy and 4 light polypeptide chains
heavy chains intertwine to form ____ tail
light chains form myosin globular head
____ heads bind to actin
converts chemical energy to mechanical energy
300 per thick filament
shaped like two golf clubs twisted together

actin

(thin filaments)
_____ is polypeptide made up of kidney-shaped G _____ (globular) subunits
G ____ subunits have binding sites for myosin head attachment during contraction
G ____ subunits link together to form long, fibrous F _____ (filamentous)
2 F ____ strands twist together in a helix to form a thin filament

tropomyosin

regulatory protein
wrapped around actin covering myosin binding site

troponin

holds tropomyosin in place when not bound to calcium ions

dystrophin

links thin filaments to proteins of sarcolemma

nebulin, myomesin, and c proteins

bind filaments or sarcomeres together to maintain alignment of sarcomere

sarcoplasmic reticulum

similar to endoplasmic reticulum
wraps around each myofibril
pairs of terminal cisternae form perpendicular cross channels
stores calcium ions which when released triggers contraction of myofibrils

transverse tubules

_____ _____ are continuous with the sarcolemma
they conduct impulses to the deepest regions of the muscle fiber
these impulses signal for the release of Ca2+ from adjacent terminal cisternae
associate with the paired terminal cisternae of SR to form triads that encircle each sarcomere

sliding filament theory of contraction

In the relaxed state, thin and thick filaments overlap only slightly
During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
Thin filaments of sarcomere slide toward M line, alongside thick filaments
The width of A zone stays the same
Z lines move closer together

activation and excitation

for a skeletal muscle to contract there must be ____ and _____

activation

(at neuromuscular junction)
must have nervous system stimulation
must generate action potential in sarcolemma

excitation

contraction coupling
Action potential propagated along sarcolemma
Intracellular Ca2+ levels must rise briefly
Linking the electrical signal to the contraction

motor neuron and skeletal muscle fiber

neuromuscular junction
region synaptic contact between __ ___ and ___ ___ ____

synaptic cleft

neuromuscular junction
Region of communication between neuron and another cell come together
Gap between cells called ____ ____

somatic motor neurons

neuromuscular junction
skeletal muscles are stimulated by ___ ___ ___

axons

neuromuscular junction
(long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle
Each axon divides into many branches as it enters muscle
_____ branches end on muscle fiber, forming neuromuscular junction
-Each muscle fiber has one neuromuscular junction with one motor neuron

axon terminals

neuromuscular junction
neuron ends in ___ ___ (end bulb)
-within ___ ___ are synaptic vesicles containing neurotransmitter acetylcholine (ACh)

motor end plate

neuromuscular junction
Region of sarcolemma adjacent to axon terminals
Contain receptors for neurotransmitter

ion channels

play the major role in changing of membrane potentials
2 classes: chemically gated ion channels and voltage gated ion channels

chemically gated ion channels

opened by chemical messengers such as neurotransmitters
ex: ACh receptors on muscle cells

voltage gated ion channels

open or close in response to voltage changes in membrane potential

step 1 apg

action potential generation
Nerve action potential (nerve impulse) arrives at end bulb opening Ca2+ channels and Ca2+ enters axon

step 2 apg

action potential generation
Ca2+ cause synaptic vesicles to undergo exocytosis

step 3 apg

action potential generation
Acetylcholine released into synaptic cleft and crosses the cleft by diffusion

step 4 apg

action potential generation
ACh binds to ACh receptors on sarcolemma
-Na+ diffuses in and the interior of the sarcolemma becomes less negative – depolarization

step 5 apg

action potential generation
Sodium influx triggers action potential along sarcolemma and down T tubules

step 6 apg

action potential generation
ACh in synaptic cleft broken down by acetylcholinesterase

step 7 apg

action potential generation
Each impulse triggers one muscle action potential

action potential

A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane)

excitation-contraction coupling

Events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)
Once generated, the action potential:
-Is propagated along the sarcolemma
-Travels down the T tubules
-Triggers Ca2+ release from terminal cisternae
Ca2+ binds to troponin and causes:
-The blocking action of tropomyosin to cease
-Actin active binding sites to be exposed thus leading to contraction
Linking the electrical signal to the contraction is excitation-contraction coupling

preparation of cp

contraction phase
Calcium ions released from terminal cisternae of SR bind to troponin
-Troponin changes shape and moves
-Tropomyosin moves with the troponin and active sites of actin are exposed
-Blocking action of tropomyosin ends

step 1 cp

contraction phase
crossbridge cycle
Cross bridge formation – myosin head binds to actin

step 2 cp

contraction phase
crossbridge cycle
power stroke- inorganic phosphate detaches from myosin head and myosin pulls actin (thin) filament toward M line; ADP leaves myosin

step 3 cp

contraction phase
crossbridge cycle
Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches

step 4 cp

contraction phase
crossbridge cycle
Reactivation or “Cocking” (energizing) of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state

during cp

contraction phase
crossbridge cycle
Myosin cross bridges alternately attach and detach
Thin filaments move toward the center of the sarcomere
Hydrolysis of ATP powers this cycling process
ATP-> ADP + Pi

muscle relaxation

Motor neuron action potentials stop signaling for release of acetylcholine from axon terminals
Calcium ions are actively pumped back into SR terminal cisternae

step 1 mr

skeletal muscle relaxation
Acetylcholinesterase degrades remaining Ach; ligand-gated sodium channels close; end plate potential ends; final repolarization begins

step 2 mr

skeletal muscle relaxation
Sarcolemma returns to resting membrane state and Ca++ channels in SR close as T-tubules repolarize

step 3 mr

skeletal muscle relaxation
Calcium ions pumped back into SR; returns cytosol concentration to resting level

step 4 mr

skeletal muscle relaxation
In absence of calcium, troponin and tropomyosin block active sites of actin, and muscle relaxes; myofilaments slide back into original positions

tension production

a muscle fiber is either contracted or relaxed, which depends on:
The number of pivoting cross-bridges
-Amount of overlap between thick and thin fibers
The fiber’s resting length at the time of stimulation
The frequency of stimulation
-A single neural stimulation produces:
-A single contraction or twitch
-Which lasts about 7–100 msec.
Optimum overlap produces greatest amount of tension
-Too much or too little reduces efficiency
Normal resting sarcomere length
-Is 75 to 130 percent of optimal length

latent, contraction, and relaxation

what 3 phases does muscle twitches occur?

latent period (phase)

The action potential moves through sarcolemma
Causing Ca2+ release

contraction phase

Calcium ions bind
Tension builds to peak

relaxation phase

Ca2+ levels fall
Active sites are covered and tension falls to resting levels

treppe

A stair-step increase in twitch tension
Repeated stimulations immediately after relaxation phase
-Stimulus frequency

wave summation

Increasing tension or summation of twitches
Repeated stimulations before the end of relaxation phase
-Stimulus frequency

incomplete tetanus

Muscle produces near-maximum tension
Caused by rapid cycles of contraction and relaxation

complete tetanus

Higher stimulation frequency eliminates relaxation phase
Muscle is in continuous contraction
All potential cross-bridges form

motor unit

is a motor neuron and all the muscle fibers it supplies
the number of muscle fibers per ____ ____ can vary from 4 to several hundred
muscles that control fine movements (fingers, eyes) have small ____ ____
large weight-bearing muscles (thighs, hips) have large ___ ___
Muscle fibers from a ___ ___ are spread throughout the muscle; therefore, contraction of a single ___ ___ causes weak contraction of the entire muscle
____ ____ in a muscle usually contract asynchronously; helps prevent fatigue

recruitment

multiple motor unit summation
In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated

maximum tension

Achieved when all motor units reach tetanus
Can be sustained only a very short time

motor unit recruitment

Contraction is stronger as number of contracting motor units increases
Alternating contraction of motor units
Delays muscle fatigue
Allows for smooth muscle movements
Allows for precision movements
As one unit is turned off another is turned on maintaining tension but allows relaxation of first motor unit

contraction

The generation of force
Does not necessarily cause shortening of the fiber
Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

Isotonic contraction

Skeletal muscle changes length
Resulting in motion
two types: concentric and eccentric

concentric contraction

If muscle tension > load (resistance):
Muscle shortens

eccentric contraction

If muscle tension < load (resistance):
Muscle lengthens

isometric contraction

Skeletal muscle develops tension, but is prevented from changing length
iso- = same, metric = measure

ATP

is the only source used directly for contractile activity
regenerated by: the interaction of ADP with creatine phosphate (CP), anaerobic glycolysis, and aerobic respiration

creatine phosphate

Excess ATP produced during relaxation is used to synthesize creatine phosphate
One of ATP’s high energy phosphate groups is transferred to creatine
Three to six times more plentiful than ATP in sarcoplasm of relaxed muscle fiber
When contraction begins, phosphate transferred back to ADP

anaerobic cellular respiration

Glucose taken up by cells and broken down (glycolysis)
If not enough oxygen is available
Anaerobic processes turn pyruvate into lactate
Lactic acid diffuses into the bloodstream and is picked up and used as fuel by the liver, kidneys, and heart and is converted to pyruvate or glucose

aerobic cellular respiration

Series of oxygen requiring reactions that produce ATP in mitochondria
-Pyruvate from glycolysis enters mitochondria
-Completely oxidized to ~ 34 molecules of ATP, carbon dioxide, water, and heat
Sources of oxygen
-Diffused from blood
-Released by myoglobin in sarcoplasm
Provides enough ATP for prolonged activity as long as sufficient oxygen and nutrients are available
Nutrients include
-Glycogen
-Bloodborne glucose
-Fatty acids (from triglycerides)
-Amino acids
ATP produced by aerobic cellular respiration
-Activities lasting more than 10 minutes, most ATP
-Endurance events 100% of ATP

muscle fatigue

When muscles can no longer perform a required activity, they are fatigued
results- Depletion of metabolic reserves
Damage to sarcolemma and sarcoplasmic reticulum
Low pH (lactic acid)
Muscle exhaustion and pain