CHAPTER 46: musculoskeletal systems

What are the three types of vertebrate muscle?

skeletal, cardiac, smooth

What is skeletal muscle function?

Voluntary movement, breathing, and maintaining posture

What is cardiac muscle function?

Beating of heart

What is smooth muscle function?

movement of internal organs; under control of the autonomic nervous system

What are skeletal muscle cells called?

Muscle fibers - large and multinucleate

what is skeletal muscle formed from?

From fusion of embryonic myoblast

What does skeletal muscle consist of?

Many muscle fibers bundled together by connective tissue

What are the contractile proteins?

actin and myosin

what are Actin and myosin?

Long filaments within muscle cells

What happens when muscle contracts?

The Acton and myosin filaments slide past each other in a telescoping fashion

What is actin?

thin filaments

what is myosin?

thick filaments, pulls actin to contract

What does each muscle fiber have?

Many myofibrils

What are myofibrils?

bundles of actin and myosin filaments

What does each myofibril consist of?

sarcomeres

What are sarcomeres?

units of overlapping Actin and myosin filaments bounded by Z lines, which anchor the actin

What is the Z line?

Boundary of sarcomere anchoring actin filaments

What is the A band?

Contains myosin filaments

What is the H zone and I band?

myosin only, no overlap of Actin

What is the M band?

Has proteins that anchor myosin filaments

What is the sliding filament model?

when muscle contracts, sarcomeres shorten as the actin and myosin filament slide past each other

what happens to the banding pattern when the sarcomere shortens?

it changes

how does the myosin filament look in a sarcomere?

Many molecules in parallel with heads projecting sideways

how does the actin filament look?

actin monomers in long twisted molecule

What is tropomyosin and what does it do?

A binding protein that twist around the actin with troponin attached at intervals

myosin heads bind to:

actin molecules to form cross bridges

what happens when the myosin head change confirmation?

the head bends and causes actin filament to slide 5-10 nm

what does the myosin head do in terms of ATP?

hydrolyzes ATP, myosin changes conformation again, and releases the actin

Why does the contraction of sarcomere involve many cycles of interaction between actin and myosin?

so that when one myosin head breaks its contact with actin, the actin filaments don't slip backward

What is rigor mortis?

stiffening of muscles after death but eventually softens

What causes rigor mortis?

-lack of ATP so myosin remains binded to actin and doesn't release contraction

how does Rigor Mortis help medical examiners?

estimates time of death

Are muscle cells excitable?

Yes, the plasma membranes can conduct action potentials

How is contraction initiated?

action potential from a motor neuron at the neuromuscular junction

What is a motor unit?

One motor neuron, and all the muscle fibers it synapses with

What happens to the fibers in a motor unit when the neuron fires?

Contract simultaneously

one muscle has:

many motor units

How to increase strength of muscle contraction:

increase rate of firing of motor neuron or recruit more motor neurons (more motor units activated)

Acetylcholine binds to what?

receptors in the postsynaptic membrane

Receptors in the postsynaptic membrane need to what

Ion channels open in the motor end, sodium flow in, motor end plate is depolarized

What happens after depolarization spread?

threshold is reached and the muscle fiber membrane fires an action potential

how does the action potential in muscle fiber travel in the cell?

by T tubules

T tubules

runs through the sarcoplasm and connected through receptor proteins to the sarcoplasmic reticulum that surrounds the myofribril

Sarcoplasm

cytoplasm of a muscle cell

in resting muscle, what happens with calcium?

calcium pumps result in low calcium concentration in the sarcoplasm, but high concentration in the sarcoplasmic reticulum

What happens once action potential reaches the receptor protein?

they changed confirmation which opens calcium channels, and calcium flows out of the sarcoplasmic reticulum

What happens when calcium after action center reaches the receptor proteins

calcium bonds to troponin on the actin filaments which twist the tropomyosin so that Actin binding sites are exposed, starting the cycle of making/breaking A-M bonds

when calcium pumps in the sarcoplasmic reticulum remove calcium from the sarcoplasm what happens?

Contraction stop

cardiac muscle

striated; cells are smaller than skeletal muscle, and have only one nucleus

cardiac muscle cells

Branch and help with resistant to tearing and can withstand high pressures

intercalated discs

Provide mechanical adhesions between cells

what do the gap junction intercalated discs allow?

cytoplasm continuity and electrical coupling

Pacemaker and conducting cells

specialized to conduct electrical signals; initiate/coordinate heart contractions

Auto rhythmic

A heartbeat; generated by the heart muscle itself

smooth muscle

found in most internal organs; under automatic nervous system control

How are smooth muscles arranged

In sheet and have electrical contact via gap junctions

acetylcholine in digestive tract causes what

Depolarization and action potentials, causing contraction

what happens when there's action potential in one cell of smooth muscle

It spreads to all the others in the sheet

Norepinephrine causes what

The same cells to hyperpolarized leading to fewer contractions

Smooth muscle cell membranes are what

sensitive to being stretched

Stretched cells, depolarized, and

Fire action potential which causes cells to contract

Stretched smooth muscle cells are important for what

Moving food through the digestive tract

smooth muscle contraction

calcium influx to sarcoplasm is stimulated by stretching, action, potentials, or hormones

What happens after calcium enters smooth muscle cell

it binds with Calmodulin

what happens after the calcium bonds with the calmodulin?

Activate myosin

what happens after myosin kinase activated?

phosphorylates myosin heads which can then bind and release actin

What is a twitch?

Minimum unit of contraction in skeletal muscle

How are twitches measured?

in terms of tension or force it generates

What generates a single twitch

A single action potential

What does a force generated depend on?

how many fibers are in the motor unit

Tension generated by entire muscle depend depends on what?

The number of motor units activated/frequency at which motor units are firing

if action potentials are fired rapidly, new twitches are triggered before:

The myofibrils can return to resting condition

What is the result of action potential firing rapidly?

The twitches sum tension increases and becomes more sustained

Why do twitches sum?

because calcium pumps cannot clear calcium from sarcoplasm before the next action potential arrives

What is tetanus?

maximum level of contraction. action potentials are so frequent there's always calcium in the sarcoplasm.

What are the two types of skeletal muscle fibers?

slow twitch and fast twitch

What are slow twitch fibers?

oxidative/Red muscle, low ATPase activity, has myoglobin

what is myoglobin?

an oxygen binding protein, and many mitochondria;well supplied with blood vessels

what does low ATPase activity do?

Can recycle Acton - myosin cross bridges rapidly

what can slow twitch fibers have?

Reserves of glycogen fat, which can produce ATP as long as oxygen is available

What are muscles with a high proportion of slow twitch fiber good for?

long-term aerobic work such as long distance running, cycling

what are fast twitch fibers?

Glycolytic/white muscle, few mitochondria and blood cells, little or no myoglobin

Maximum tension and fast twitch fiber

Fast fibers can develop greater maximum tension, faster, but fatigue more quickly

can fast which fibers replenish ATP?

cannot replenish ATP for prolonged contraction, high ATPase activity

What are fast twitch fiber is good for

Short term work that requires maximum energy

Who have high proportions of fast twitch fibers

legs and arm muscles of weightlifters and sprinters

What do anaerobic activities do?

increase strength (maximum force a muscle can exert) which is the function of muscle volume

What is an example of an anaerobic activity?

weightlifting

What does weightlifting induce?

fatigue, but also causes new actin and myosin filaments to form, and hence the muscles, get larger and stronger

What do aerobic activities do

increase oxidative capacity from increased myoglobin, mitochondria, and enzymes involved in energy use, and density of capillaries that deliver oxygen

What is an example of an aerobic activity?

jogging

What does jogging increase

endurance (how long a given workload can be sustained)

What is myoglobin similar to?

To hemoglobin but has a higher affinity for oxygen

Skeletal systems are

The rigid supports against which muscles can pull

What are hydrostatic skeleton?

fluid enclosed in a body cavity surrounded by muscle

What are examples of a hydrostatic skeleton

cnidarians, annelids, other invertebrates

What happens when muscles oriented in One Direction contract?

The fluid filled body cavity bulges out in the opposite direction

Exo skeleton

Hardened outer surface to which muscles attached

Examples of exoskeleton

mollusks and arthropods

What can muscle contractions do to the exoskeleton?

cause segments of it to move