Physiology Cumulative- After 1st Midterm. Lecture 14-26

Skeletal shape and appearance

long and cylindrical, striated

skeletal nucleus

multinucleated

skeletal regulation of contraction

voluntary

cardiac shape and appearance

shorter and branched, striated

cardiac nucleus

uninucleated

cardiac regulation of contraction

involuntary

smooth appearance

spindle-shaped, non-striated

smooth nucleus

uninucleated

smooth regulation of contraction

involuntary

which muscles are striated

skeletal and cardiac

which muscles are smooth

smooth

striated

actin and myosin proteins within the fibers (muscle cells) arranged very regularly

smooth

cytoplasm (single nucleus), has a uniform, non-striated appearance because actin and myosin are not in regular arrangement

what muscles are voluntary and involuntary

Voluntary: skeletal

Involuntary: cardiac, smooth

voluntary

action completely depends on signals from nervous system to function

involuntary

can be influenced by nervous system and hormones or other stimuli

What is the skeletal muscle's mass in regard to the body?

makes up all the voluntary muscle and represents greatest amount

where is cardiac muscle found?

only associated with heart

where is smooth muscle found?

most in layers

what are individual muscle fibers

very long cells, in long muscles there is dozens of cm

what are muscle fibers containing?

many fused cells, many nuclei

what does each muscle fiber have

sarcomeres

sarcomeres

basic unit of contratcion

what makes up the striations

boundaries of sarcomeres

A bands

Thick filament, dark because myosin is thick

I bands

Thin filament, lighter because actin is thin

what gives skeletal muscle its appearance

the way the actin and myosin are arranged

Breaking down a muscle into contraction

Tendon- muscle, muscle fascicle, muscle fiber myofibril.

what happens during muscle contraction

actin filaments slide over myosin, muscle shortens.

Beginning of muscle contraction

begins with a nerve impulse travelling down an axon. connects with a muscle fibre at the neuromuscular junction.

Step two of muscle contraction

Impulse reaches the synaptic bulb at the axon terminal.

neuromuscular junction

site at which an axon of a motor neuron contacts the muscle fiber that it innervates.

what does the synaptic bulb contain

synaptic vesicles.

what are the synaptic vesicles filled with

ACh (Acetylcholine)

Step 3 of muscle contraction

Molecules of ACh are released into the synaptic cleft, molecules bind to receptors on the muscle (motor endplate)

Step 4 of muscle contraction

binding of ACh to receptors activates sodium channels. Sodium enters the cell and produces local depolarization which initiates the electrical signal in muscle.

step 5 of muscle contraction

the electrical signal passes to t-tubule of the muscle

step 6 of muscle contraction

signal asses to inside of muscle along T tubules, which results in calcium channels openeing

T tubules

extensions of the sarcoplasmisc reticulum

Step 7 of muscle contraction

Calcium is released to interact with actin and myosin fibers

How does Ca+ initiate contraction

we need to look at myosin and actin in more detail and how they interact to understand

What does each myosin head have

two active regions

two active regions of myosin

actin binding site and ATP binding site

What does each myosin filament have

tail and head

Step 8 of muscle contraction

Calcium ions enable actin and myosin to bind and initiate sarcomere contraction. Causes change in tropomyosin and moves down to expose myosin binding sites.

What does actin have

two proteins bound to it

proteins bound to actin

tropomyosin and troponin

Step 9 muscle contraction

attachment of the myosin head to actin causes bending of myosin to centrre of sarcomere. This shortens the sarcomere segment, and often reffered to as the "power stroke".

Step 2 of power stroke (When the binding is happening)

ADP and phosphate are bound to myosin as myosin head attaches to actin.

Step 3 of power stroke

ADP + P release causes head to change position and actin filament to move.

Step 4 of power stroke

Binding of ATP causes myosin head to return to resting position.

Step 1 of power stroke

ATP is hydrolyzed when myosin head is unattached

what happens when power stroke is over

-muscle has only shortened by 1%

-muscles can shorten up to 60%

-cross-bridges have a "cross-bridge cycle"

-once power stroke is over, ATP binds to myosin head

-myosin head detaches from actin

-new power stroke begins

-therefore, one ATP is required for the breaking up of the cross bridges after the power stroke

What stimulates muscle fibers to contract?

Nerve signals stimulate muscle fibers with acetylcholine (ACh).

What releases calcium during muscle contraction?

The sarcoplasmic reticulum releases calcium.

What does calcium (Ca²⁺) cause in muscle contraction?

Calcium leads to a conformational change of actin.

What forms when the myosin head connects to actin?

A cross bridge is formed.

What happens when the myosin head pulls the actin fiber?

The cross bridge is broken, costing 1 ATP for detachment.

What occurs multiple times during a muscle contraction?

Attachment sites form and break, recycling the cross bridge.

How does contraction occur in muscle cells?

Contraction occurs simultaneously in all sarcomeres in the cell.

What happens to the muscle cell during contraction?

The muscle cell shortens, causing the entire muscle to shorten.

What is required to move the head of myosin?

ATP

Rigor mortis

After death, ATP synthesis stops, Myosin and actin cannot separate, and muscles become stiff.

What do contractions vary in

strength and duration

muscle twitch

All muscle contractions are built on this

What is the muscle twitch

all or none, can be rarely a single cell but usually a group

what does motor units contain

consists of one neuron and all the muscle fibers that to which it connects

what will a signal from the neuron cause

all fibers contracting

how many muscle fibers may be stimulated by one nerve

several hundred or very few

what happens with fewer muscle fibers

more control over movement (fine motor skills)

what happens when a signal is sent

latent period, contraction, relaxation

latent period

first few milliseconds, connections between actin and myosin begin to form

contraction

actin slides over myosin, muscle shortens

relaxation

muscle returns to its initial length longest part

what is the signal?

Highly reproducible, but may vary

muscle twitch graph

- time to reach maximum contraction

- ability to sustain tension

- tonic vs. fast/slow twitch fibers

why can twitches vary

muscle fibers with a large diameter are stronger, and some fibers vary in the speed in whihhc they can activate peak force (slow and fast twitch fibers)

Three factors that modify muscle contraction so that we're not twitchy all the time

Isometric vs isotonic contractions, frequency of stimulation, strength of stimulation

Isometric vs isotonic

does not differ in how force is generated but rather if the muscle gets shorter

Isotonic contraction

muscle gets shorter, shortening brings forearm up. ex: lifting weight

Isometric contraction

muscle stays same length but creating tension. ex: holding up a chain from the ground

what happens if muscle contracts isometrically

sarcomeres still shorten, whole muscle does not, force transmitted through certain cellular components and tendons, good for maintaining posture, etc.

condition of a muscle contraction being reproducible

is a muscle is stomulated at low frequency

what happens by increasing frequency of signal sent to muscle

stronger response, second signal is added to the first

more calcium effect on tension generated

more calcium, more myosin binding sites, more cross bridges, more tension generated

Psychological tetanus

after multiple signals in sequence, muscle relaxation disappears

whats after one twitch

second signal before first twitch ends, multiple signals in sequence, next would be physiological tetanus

what is tetanus caused by

Multiple stimuli to muscles- not to be confused with the disease tetanus. caused by soil bacteria

What does tetanus release

a toxin that interferes with the release of inhibitory neurotransmitter

what does the toxin released by tetanus cause

continuous excitation of muscle fibers.

where do early manifestations of tetanus occur

in masseter muscles (lockjaw)

what happens as more muscle fibers are stimulated

strength of contraction increases

what size muscle fiber is stimulated first

smallest

when are larger fibers stimulated

as much force needed

slow and fast muscle fiber twicthes

some muscles take longer to reach peak tension during contractionn

ex of slow twicth muscle

soleus and gastrocnemius

ex of fast twitch muscles

extraocular (fast eye movements)

what does difference between slow and fast twitch reflect

type of myosin present

what does velocity of muscle contraction depend on

how quickly cross bridges recycle

What is the rate-limiting step in muscle contraction

synthesis of ATP