chapter eleven - muscular tissues

5 universal properties of muscular tissue

excitability, conductivity, contractility, extensability, and elasticity

excitability

irritability; the ability to respond to a stimulus such as a nerve impulse

conductivity

a muscle can take the electric charge and recieve it, then pass it throughout the muscle

contractility

a muscle's repsonse to an electric charge

extensibility

a muscle's ability to be stretched or lengthened without damage

elasticity

a muscle's ability to return to their original length after being stretched

"sarc" and "myo" are the root words for...

muscle

sarcolemma

plasma membrane of a muscle fiber

nuclei

plural of nucleus

sarcoplasm

cytoplasm of a muscle cell

transverse (T) tubules

Tubules that extend across the sarcoplasm and allow electrical impulses to travel deep into the cell

myofibrils

Microscopic protein filaments that make up muscle cells.

sarcoplasmic reticulum

smooth endoplasmic reticulum of muscle cells

terminal cisternae

enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules that store calcium

why do muscle cells NEED mitochondria?

because they require a huge amount of energy (ATP) to contract and move

which part of the muscle cell receives electrical impulses (the wave of sodium and potassium)?

sarcolemma

do nuclei divide in muscle cells?

NO, they can only enlarge and shrink

where are electrolytes found in the muscle cells

sarcoplasm

what part of muscle cells takes Na+ and K+ into the cell?

transverse tubules

what do myofibrils do?

allow muscle to contract

what creates striations in musclular tissue?

the organized arrangement of actin (thin) and myosin (thick) protein filaments within sarcomeres

A band

made of thick myosin filaments; dark band

I band

made of thin actin filaments; light band

Z disc

coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another

myosin

The contractile protein that makes up the thick filaments of muscle fibers

what does the myosin head do?

binds to actin and facilitates muscle contraction by movement

G-actin

globular actin

where are active sites located?

G-actin active sites

F-actin

A fibrous protein made of a long chain of G actin molecules twisted into a helix; main protein of the thin myofilament

thin filament is made of...

actin, troponin, tropomyosin

tropomyosin

chain that covers myosin binding sites on the actin molecules

troponin

A protein of muscle that together, with tropomyosin, forms a regulatory protein complex controlling the interaction of actin and myosin

if tropomyosin is the "lock" that covers the active sites, what is the KEY?

calcium

what happens when calcium interacts with a cell to facilitate muscle contraction?

floods cell, binds to troponin, leading to a confirmational change. the troponin shifts and changes shape and tropomyosin is moved away from the active site. myosin then moves in and begins the muscle contraction

thick filament

the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs

what is often associated with Z discs?

terminal cisternae

somatic motor neuron

a nerve cell that stimulates the contraction of skeletal muscle fibers

somatic motor nerve fiber

bundle of axons and dendrites that send signals to nerve fibers and tell them to move

motor unit

a motor neuron and all the muscle cells it stimulates

what are large motor units needed for?

needed for powerful movements

what are small motor units needed for?

needed for detailed movements (fingers + vocal chords)

what is the ratio of motor units to neuron in large motor units?

as many as 1000:1

what is the ratio of motor units to neuron in small motor units?

as few as 3:1

synapse

Gap between neurons where neurotransmitters are passed from one neuron to the next

neuromuscular junction (NMJ)

the synapse between a somatic motor neuron and a skeletal muscle fiber

what is the neurotransmitter used in muscle contractions?

acetylcholine (ACH)

acetylcholine estrase

breaks down acetylcholine and clears it out of the cleft

synaptic knob

tiny enlargement at the end of an axon that secretes a neurotransmitter

synaptic cleft

a gap into which neurotransmitters are released from the axon terminal

synaptic vesicles

Tiny pouches or sacs in the axon terminals that contain chemicals called neurotransmitters.

neurotransmitter

chemical messengers that cross the synaptic gaps between neurons

acetylcholine (ACh)

A neurotransmitter that triggers muscle contraction

ACh receptors

Proteins that bind Ach on the motor end plate

junctional folds

folds IN SARCOLEMMA at motor end plate that contain acetylcholine receptors

acetylcholinesterase (AChE)

the enzyme that breaks down acetylcholine in the synaptic cleft

electrophysiology

the study of the electrical activity of cells

resting membrane potential

the electrical charge of a neuron when it is not active

what is the RMP of a neuron?

-75 mV

what is the RMP of a muscle cell?

-90mV

sodium potassium pump

a carrier protein that uses ATP to actively transport sodium ions out of a cell and potassium ions into the cell

how does the action potential process work?

depolarization, repolarization, and action potential

depolarization

The process during the action potential when sodium is rushing into the cell causing the interior to become more positive.

repolarization

Return of the cell to original RMP, caused by reentry of potassium into the cell while sodium exits the cell.

action potential

a quick "spike" of depolarization and repolarization; excitation of a nerve or muscle cell

self propogating

- charge continues to propagate itself in the next region of the axon

- the action potential moves like a wave down the axon, with constant speed and amplitude

what are the 4 stages of muscle action?

1. excitation

2. excitation-contraction coupling

3. contraction

4. relaxation

excitation (muscle action stage)

action potentials in the nerve fiber stimulate action potentials in the muscle fiber (MF recieves impulse)

excitation-contraction coupling (muscle action stage)

events that link the action potentials on the sarcolemma to activation of the myofilaments; preparing them to contract

contraction (muscle action stage)

myofilaments slide over each other and produce tension in muscle cell (usually with shortening)

relaxation (muscle action stage)

action potentials cease and muscle tension declines; muscle might re-lengthen

explain the full process of excitation (step 1)

action potentials travel down the synaptic knob, open voltage-gated Ca2+ channels, and Ca2+ enters the synaptic knob. Ca2+ triggers exocytosis, which leads to the release of ACh. ACh binds to the receptors on the sarcolemma. ACh opens the Na+ and K+ channels on the sarcolemma; Na+ flowing in and K+ flowing out, creating end-plate potation (an action potential). end-plate potential stimulates the opening of nearby Na+ and K+ channels.

explain the full process of excitation-contraction coupling (step 2)

action potentials spead throughout the muscle fiber and down into T tubules. this movement opens Ca2+ channels in the sarcoplasmic reticulum. Ca2+ binds to troponin of the thin filaments. tropomyosin then shifts positions and exposes the active sites on the actin filaments, making it possible for myosin heads to bind to the now-exposed active sites. once calcium binds to the myosin, the contraction begins.

explain the full process of contraction (stage 3)

the myosin head, with an ATP on it, breaks the ATP down to ADP + Pi. Myosin "cocks" into its extended, high-energy position. myosin binds to actin, forming a cross-bridge. myosin then releases ADP + Pi and flexes to its low-energy position, tugging on the actin (power stroke). myosin binds to a new ATP, releases the actin, and hydrolyzes the ATP, and "recocks" for the recovery stroke. REPEAT!

explain the full process of relaxation (stage 4)

the nerve signal ceases, and ACh is released. the existing ACh is broken down by AChE. Ca2+ returns to the sarcoplasmic reticulum by active transport. troponin loses all the Ca2+ that was bound to it. tropomyosin returns to its resting position, blocking the actin's active sites. muscle fiber can no longer maintain its tension.

the length-tension relationship

The resting length of a muscle and the tension the muscle can produce at this resting length.

how does muscle fibers being too short affect contraction?

contraction will be weak because the sliding filaments can't go far before hitting the Z-discs

how does muscle fibers being too long (over-stretched) affect contraction?

contraction is weak because not enough myosin heads can get a good grip on the actin.

what is the optimum resting length for muscle fibers?

2.0-2.25

the muscle twitch

the response of a muscle to a single brief threshold stimulus

threshold in a muscle twitch

minimum voltage that triggers a twitch

"twitch" in a muscle twitch

a brief, involuntary, rapid contraction of a muscle fiber or a motor unit

myograph

instrument used to record muscular contractions

3 phases in a muscle twitch

latent period, period of contraction, period of relaxation

latent period

time between application of a stimulus and the beginning of a response in a muscle fiber

contraction phase

time when muscle generates external tension by the myosin heads grabbing the active sites on the actin

relaxation phase

period after twitch contraction when tension decreases

how long does a typical muscle twitch last?

7-100 msec

what is the first tool the body uses to build strength in order to "do" a movement?

multiple motor unit (MMU) summation

in multiple motor unit summation, how do voltages below the threshold effect the process?

voltages below the threshold have no effect

what happens when a voltage above the threshold is reached in multiple motor unit summation?

above threshold, increasing voltage to sciatic nerve stimulates more nerve fibers, therefore more motor units. for example, as we pick up something heavier, more neurons will turn on and respond

describe the process of multiple motor unit summation

Increasing the voltage to the sciatic nerve stimulates more nerve fibers, therefore more turning on more motor units. twitches will increase in strength up to a macimum where ALL the motor units are active

are muscle twitches important in voluntary or involuntary control of muscle strength?

they are important in VOLUNTARY control of muscle strength.

what happens to the tension of a muscle as more neurons turn "online"?

the tension builds

maximum contraction

the force of contraction produced with a single stimulation when all motor units have been turned on

will a muscle twitch ever surpass the maximum contraction level?

no, the muscle will not surpass the maximum contraction level.

since a muscle will never surpass the maximum contraction level, how does a muscle get stronger?

more contractile proteins can be added within a muscle cell to make the muscle stronger

unnaturally low stimulus frequency

identical twitches, complete relaxation between twitches (complete twitch and complete relaxation between)

temporal summation (wave summation)

twitches get stronger and muscle doesn't completely relax between twitches; produces a state of incomplete tetanus

incomplete tetanus

muscle fibers partially relax between contraction, where they are able to rest and get rid of waste products.

still higher stimulus frequency

temporal summation and incomplete tetanus