Anatomy Exam 3 Unit 5

three layers of CT enclose the muscle fibers

epimysium

perimysium

endomysium

epimysium

surrounds entire muscle

perimysium

surrounds groups of 10-100 or more individual muscle \n fibers.

perimysium contains

fascicles

what are fascicles

bundles muscle fibers into units

they contain BV and nerve supply

Endomysium

separates individual muscle fibers

the 3 layers of the connective tissue are

All continuous with one another and may extend beyond muscle fibers to form a tendon that attaches muscle to bone

Nerves and blood vessels usually penetrate

the muscle together

Somatic motor neurons stimulate

the muscle to contract

Muscle contraction demands a great deal of energy, so

muscle tissue is highly vascular

Within the endomysium, arterioles branch off into smaller \n capillaries to

supply each muscle fiber

Skeletal muscle can be very

long and may extend the entire length of the muscle

Skeletal muscle is

Multinucleated due to muscle cell formation during embryonic development

Each muscle cell is made up of

many myoblast cells that fuse together.

Some myoblasts do

not fuse and are called satellite cells.

Satellite cells can divide after injury and

fuse with muscle fibers to repair damaged tissue

Sarcolemma

cell membrane

Sacroplasm

cytoplasm with muscle fiber

a transmembrane potential exists across the membrane and it gives the muscle

the property of electrical excitability

However, muscle cells are very large, so if a muscle cell is stimulated by a motor neuron to contract, it must

be able to relay this message throughout the \n entire cell so that contraction can occur simultaneously

done by transverse tubules or T tubules

T tubules extend

into the muscle cell to form a tunnel of extracellular \n fluid

T tubules ensure that

the entire muscle cell becomes excited by an action \n potential simultaneously.

The sarcoplasmic reticulum (SR) is comparable to

smooth endoplasmic reticulum in other cells

sarcoplasmic reticulum (SR) made up of

interconnecting tubes that encircle each myofibril

sarcoplasmic reticulum (SR)

is used to store calcium which is crucial to muscle cell contraction

terminal cisterns

Enlarged portions of the SR

terminal cisterns form next

to the T tubules.

triad

arrangement of one T tubule, surrounded by two \n terminal cisterns

The sarcomere is the

basic functional unit of the muscle cell

Z discs are a type protein

within the muscle fiber.

Sarcomeres extend from Z disc to Z disc

A sarcomere is made up of various proteins, called

myofilaments

myofilaments aid in

contraction

Together, the myofilaments make up

myofibrils, \n which extend the length of the entire muscle cell

Actin is the

thinner of the two myofilaments;

it makes up the I band (light band) of the sarcomere

Myosin is the

thicker of the two myofilaments;

it makes up the A band (dark band)

Within the A band is a darker region where it contains thick filaments called

the H zone

The structure of myosin is arranged

into a tail portion and a head portion

(ATP can bind to the head portion)

In the center of each sarcomere is

\

M-line helps

to stabilize the myosin filaments.

Alternating A and I bands are what give

muscle tissue its striated appearance

Titin is

another protein that is important for helping the muscle return to its original relaxed state after contraction has occurred.

Titin anchors myosin to the z line and

helps to stabilize its protein

titin prevents

overstretching of the muscle fibers

Actin has

myosin binding sites

myosin binding sites

where the myosin heads can attach.

Two proteins usually cover the locations of attachment for the myosin heads:

troponin and tropomyosin

Ca++ binds to

troponin

Motor neurons stimulate

muscle cells to contract

The neuromuscular junction is where

the motor neuron and muscle fibers meet

The end of the motor neuron enters into the perimysium and branches into

synaptic terminal

synaptic cleft

The space between the synaptic terminals and the sarcolemma of the muscle fiber

The motor neuron releases a neurotransmitter into the synaptic cleft

acetylcholine (ACh)

ACh is released from vesicles within the synaptic terminals when

an action potential travels down the entire length of the neuron

Ligand-gated channels are present on the motor end plate. Once …

ACh binds to these channels they open, allowing Na+ into the muscle cell.

Sodium rushing into the cell generates an action potential along

the entire sarcolemma

The chemical signal (ACh) must be removed from synaptic cleft so that

signaling is precise and not prolonged

So that signaling is precise and not prolonged, one way is to use

An enzyme called acetylcholinesterase (AChE) to break down ACh so that it can no longer bind to receptors.

So that signaling is precise and not prolonged, second way is to use

The motor neuron will reabsorb the components of ACh; a process called reuptake

A single motor neuron can stimulate many muscle fibers at one time (usually around 150). A single motor neuron and all of the cells that it stimulates

motor unit

All or none principle

once a fiber reaches threshold, the cell will contract fully.

Threshold stimulus

The amount of ACh that is required in order for a cell to reach threshold

Different motor units have various levels of threshold meaning…

they require a certain amount of stimulus (ACh) before an action potential will occur.

Excitation – Contraction Coupling

the link between the action potential occurring at \n the sarcolemma and the actual muscle contraction

As the action potential travels across the sarcolemma and into the T tubules,

it causes the release of Ca++ from the SR

Calcium binds to receptors on troponin..

and alters its shape and position.

After the binding of calcium on troponin, it weakens the tropomyosin. Next..

actin bond actually pulls tropomyosin away from \n actin revealing the active site and allowing myosin heads to bind to actin (cross bridges form)

Power stroke

the stored energy in ATP is released as the myosin head moves toward the M line

500x second

Power stroke of the muscle

causes the shortening of each sarcomere and therefore the entire muscle

When ATP binds to myosin again,

it releases from the actin binding site and swivels back into the “cocked” position

If the motor neuron continues to stimulate the muscle with release of ACh,

Ca++ will continue to bind troponin.

In addition, as long as ATP reserves are still \n available,

muscle contraction will continue

How does relaxation occur?

Stimulus from motor neuron ends

\n - ACh broken down or taken up into the motor neuron

\n -Ca++ is resequestered (ACTIVE TRANSPORT) into the SR and some is also pumped out into ECF.

\n - Once Ca++ detaches from troponin, it returns to its original shape and position and tropomyosin returns to its position covering the myosin binding sites.

\n - Muscle cell lengthens with assistance of elastic fibers (titin)

What is Rigor Mortis

ATP cannot continue to be produced after death; so high Ca++concentrations within the SR cannot be maintained.

* Eventually Ca++ will leak out into the sarcoplasm and bind to troponin causing steady contraction. (Usually occurs 3-4 hours after death.)

ATP is obtained from

cellular respiration (the catabolism of glucose or fats)

When muscles are resting,

ATP reserves are established for use during sustained \n activity.

However, even these reserves will be used up within 4- 6 seconds

in Aerobic respiration, glucose is broken down to

ATP in mitochondria- requires oxygen

Aerobic. myoglobin

Oxygen can be obtained from RBCs or from an oxygen binding protein within the muscle cell

Myoglobin binds excess

oxygen when it is plentiful and then releases it in times of need

Glucose can be obtained from the

blood, or from glycogen stores within the muscle cell

Aerobic respiration can provide a continual supply of

ATP during exercise, as long as sufficient oxygen and nutrients are available

Anaerobic Respiration

A series of reactions that produce ATP without the use of oxygen

What happens in Anaerobic respiration?

The process of glycolysis breaks down glucose into two molecules of pyruvic acid to gain 2 molecules of ATP.

The pyruvic acid is then converted into lactic acid

Cori cycle

Lactic acid can then leave the muscle cell and go to the liver where it is converted back into glucose so that it can be reused when oxygen becomes available.

What is the cause of muscle cramping and fatigue

Some lactic acid might accumulate within the muscle cell resulting in a decrease in pH.

Anaerobic respiration provides energy for 30-40 seconds of activity. This allows

muscles cells to continue contraction during peak activity when oxygen demands \n are not being met

Creatine Phosphate

another high energy compound that can store energy for use during sustained activity.

This can allow contraction to occur for another 15-17 seconds

While the muscle is at rest, it builds reserves of creatine phosphate….

o ATP + Creatine → ADP and creatine phosphate \n o ADP + creatine phosphate → ATP + creatine

Creatine phosophokinase (CPK) is the enzyme

that carries out this reaction.

(If the muscle cell is injured CPK leaks across cell membrane. CPK is detected in the blood after a heart attack.)

Creatine phosophokinase (CPK) reaction is

anaerobic

creatine phosphate has

Immediate availability, so ideal for short-term, high intensity activity

The Recovery Period

Conditions within the muscle cell are returned how they were prior to activity

This is called the oxygen debt. It “pays back” the oxygen needed to restore metabolic conditions, such as:

* replenish O2 stores → myoglobin
* replenish ATP store
* replenish glycogen stores
* replenish creatine phosphate
* repairing
* removal of lactic acid

Mitochondria, Myoglobin and Capillaries are the only ones different because

Fast fibers are fewer and Slow Fibers are faster

Fast fibers are

anaerobic

Slow fibers are

aerobic

fast fibers color

white

slow fibers color

dark red

the two things that effect strength of contraction..

the number of muscle fibers activated

the frequency of neural stimulation to the muscle fibers.

The number of motor units there is, the

increase in strength of contraction

For Frequency the process goes by:

1. Twitch
2. Treppe
3. Wave summation
4. Tetany
5. Fatigue

Twitch

single stimulus contraction