A and P Mod5

Muscular System
Muscle Tissue

4 Functions:
1. Produce skeletal movement and
all types of movement
2. Maintain posture and body
position
3. Guard the entrances and exits
of the body
4. Maintain body temperature

Characteristics of Muscles
Muscle cells are elongated

Muscle cell are called muscle
fibers
Contractionof muscles is
due to the movement of
microfilaments in cell

4 major functional characteristics
of muscle are:
1. CONTRACTILITY – CAN
SHORTEN FORCEFULLY

2. EXCITABILITY –
STIMULATED BY HORMONES
AND NERVES

Four major functional characteristics of muscle are:
1. CONTRACTILITY
2. EXCITABILITY

3. EXTENSIBILITY – CAN BE
STRETCHED OUT

4. ELASTICITY – EXTEND
BEYOND NORMAL LENGTH
AND RECOIL

3 basic types of muscle are found in the body :

1. Cardiac muscle
 found only in heart
 Striated (striped in appearance)
 single nucleus
 involuntary


2. Smooth muscle
 found in organs
 nonstriated
 involuntary


3. Skeletal
 Striated
 voluntary
 multinucleated

Muscles can be also categorized by
whether they are:

VOLUNTARY – CONSCIOUSLY
CONTROLLED
Or

INVOLUTARY – NOT
CONSCIOUSLY CONTROLLED

Muscles can be also categorized by
whether they are:

STRIATED – STRIPED
IN APPEARANCE
or
NONSTRIATED – NOT STRIPED
IN APPEARANCE

Skeletal Muscle Characteristics
 multinucleate
– Have more
than one nucleus per cell

 Nucleus is on the outside of
the cell

Skeletal muscle characteristics continued:

Muscle are attached to
bone by tendons.

tendons are dense
regular connective tissue
belly

Muscle itself is
called the belly.

Skeletal muscle characteristics continued:
The belly of the muscle is made
of bundles of muscle fibers (cells).

Bundles of muscle fibers
wrapped together are
called fascicles.

Connective Tissue Wrappings of Skeletal Muscle
1. Endomysium –
wraps a single
muscle fiber
2. Perimysium –
wraps a fascicle
(bundle) of fibers
3. Epimysium–
wraps entire muscle

Connective Tissue Wrappings of Skeletal Muscle

These three
wrappings extend
past the muscle to
make the tendon
and connect to
bone.

Skeletal muscle characteristics continued:

have nerves and blood vessels.
controlled by nervous system
require lots of energy because of
cellular respiration
therefore need lots
of oxygen

Microscopic Anatomy of Skeletal Muscle

Special terms for muscle
fiber organelles:

Sarcosome =
mitochondria

Sarcoplasm = cytoplasm

Microscopic Anatomy of Skeletal
Muscle
Sarcoplasmic reticulum =
smooth endoplasmic reticulum

Sarcolemma = specialized plasma
(cell) membrane

Microscopic Anatomy of Skeletal Muscle

Also has:
T-tubules= tiny tubes sprouting
into the sarcolemma forming pits in the
sarcolemma which allow signals to
travel and stimulate movement.

Also has:

Myofibril – threadlike structures
in muscle fiber

Myofibril made of two types of
filaments:
1.Myosin protein is a thick filament

2. Actin protein is a
Thin filament

Also have:

Sarcomeres
• smaller sections of a myofilament
•around 2 µm in length
•functional unit of
the skeletal muscle

Also have:
 Sarcomeres

• Sarcomere is the repeating units of the
myofibrils

• Sarcomeres give skeletal muscle
its striated appearance

Also have:
 Sarcomeres

Structures of the Sarcomere:

 A bands – dark thick bands made
from myosin protein

 I bands – lighter bands made from
actin protein

H zone – area between the
ends of the actin myofilaments
which point to one another from
opposite ends of the sarcomere

heads
actin
myosin

H zone

Z disk
– where the actin or thin
filaments attach at one end
-Z disk are made out of protein

The myosin proteins have
head-like structures on them

Sliding-Filament Model
Is the model explaining how a
muscle cell or fiber is believed
to contract

Sliding Filament Model

Actin is surrounded by two other
proteins:
1. tropomyosin which winds
around the actin blocking
active sites

2. troponin which lies across
the tropomyosin and keeps
it in place

Sliding filament model
Nerve impulse by neurotransmitter
acetylcholine stimulates the
sarcoplasmic reticulum to release Ca2+
into the sarcoplasm

Ca2+ bind to the troponin and cause
the position of the tropomyosin to
change exposing the active sites on
the actin filament


The heads of the myosin filament
then bind to the active site of the actin
filament forming a cross bridge


The Ca2+ ions are then released

Once cross bridges are formed the
myosin filament heads will shift called
a power stroke pulling the actin
filaments closer together;


This happens simultaneously in all
the sacromeres and muscle fibers
causing the muscle to contract.

During sarcomere contraction:
Reduced are the :
H zone (shortens)
I band (shortens)
Overall length of sarcomere
(reduced)

I

During sarcomere contraction:

Stays same:
actin filament
myosin filament
A band (made by myosin)

Energy Requirements
•Muscle contraction requires lots
of energy provided in the form of
ATP (adenosine triphosphate).

•When the first phosphate bond in
ATP is broken large amounts of
energy is released.

•ATP becomes ADP and P
molecule.

ATP related to sliding filament model:

•ATP bonds to head of myosin
filament
•ATP ADP + P energy
becomes available to the myosin
head to move.
•P released from myosin, myosin
then binds to the actin strongly and
power stroke occurs

ATP related to sliding filament model:

•ADP falls off and is recycled

•New ATP can attach to provide
more energy

•New ATP attached, crossbridge
is broken

What stops the contraction?

Same thing that started it –
Ca ions
2+

When nerve impulse stops, sarcoplasmic
reticulum stops releasing Ca2+ ions which
no longer binds with troponin, and the active
sites are no longer available.

Motor Units

 Motor unit is
 One motor neuron
(nerve cell) and all
the
 Muscle cells
stimulated by that
neuron

Parts of the motor neuron
 Axon – process that extends from the
cell body and carries signal away Note:
may branch

Presynaptic terminal –very end of
the axon neurotransmitter containing
vesicles are found

Parts of the motor neuron

Neuromuscular junction – where the
nerve and muscle fiber meet (don’t touch).

Parts of the neuromuscular junction

Synapse – interface between a nerve cell
and another cell (muscle cell)

Synaptic cleft – gap between nerve and
muscle cells that is filled with interstitial
fluid

Postsynaptic membrane –
membrane of the muscle fiber in the
region of the synapse

Acetylcholine (ACh)– Most common
neurotransmitter (chemical) released by
neuron traveling across the cleft
interacting with the postsynaptic
membrane of muscle cell stimulating it to
contract.

Synaptic vesicles – found inside the
presynaptic terminal containing
neurotransmitters.

Acetylcholinesterase – enzyme that
inactivates ACh to stop muscle
contraction.

How the motor neuron stimulates muscle
contraction:
1.Action potential (electrical
signal) travels down axon
2. Vesicles in the presynaptic
terminal release ACh.
3. ACh travels across the synaptic
cleft.

How the motor neuron stimulates muscle contraction:

4. ACh stimulates a new action
potential in the sarcolemma
through the T-tubules

5. T-tubules stimulate the
sarcoplasmic reticulum to
release Ca+2 ions by
diffusion.

How the motor neuron stimulates muscle contraction:

6. We know Ca+2 ions attach
to troponin changing
arrangement of
tropomyosin exposing
sites on actin-
crossbridges-powerstroke-
contraction

How motor neuron is responsible
for stopping contraction:
1.Action potential stops
2.ACh is no longer released and is
broken down by acetylcholinesterase.
3. Calcium is no longer released but is
pumped back into sarcoplasmic
reticulum.

How motor neuron is responsible for stopping contraction:

5.Troponin releases Ca ions and +2

tropomyosin rearranges covering the
actin active sites again.

6.Crossbridges of myosin/actin are
broken

7.Muscle is relaxed!!

All-or-none law of skeletal
muscle contraction

Individual muscle fibers
contract with equal force in
response to each action
potential

Either a fiber is contracted or
relaxed no inbetween.

How do you get degrees of
muscle movement?

Slight, fine movement
(thread a needle) compared to
coarse, extreme movements
(pick up a 100 lb weight).

How do you get degrees of muscle movement?

1. By the size of the motor unit or
amount of axon branching
A. Large motor unit – axon
branches hundreds of times stimulating
hundreds of muscle fibers at the same
time resulting in coarse movement with
less control
or

Two ways:
1. The size of the motor unit or amount of axon branching

A. Large motor unit – axon branches hundreds of times stimulating
hundreds of muscle fibers at the same time resulting in coarse movement with less control
or

B. Small motor unit – axon
branches only a few times
stimulating only a few muscle
fibers or just one at the time
resulting in fine movement with
more control.


2.Frequency of the action potential
can affect muscle contraction

Stronger the stimulus the higher
the frequency of action potentials
that travel down motor neuron

There are 4 levels of frequency:

A.Subthreshold stimulus – too
small to create an action potential

B.Threshold stimulus – strong
enough to create an action
potential

B. Threshold stimulus -
stimulus is strong enough to create an action potential

C. Submaximal stimuli – stimuli
of increasing strength that create
more action potentials along
more neurons

C. Submaximal stimuli – stimuli of increasing strength that create more
action potentials along more neurons

D.Maximal stimulus – a stimulus
which is strong enough to create
action potentials in all the
neurons innervating a whole
nerve

Innervating = controlling

Nerve stimulates several motor
units together.
The more motor units (multiple
motor unit summation) activated the
more muscle fibers contract the
greater the force of the muscle
contraction.
If motor unit is stimulated it has
been recruited.

•Muscle tone – the state of partial
contraction of the muscle, even
when not being used.

•Less muscle tone when asleep,
than when awake.
•Motor units will take turns
contracting and relaxing to have
muscle tone.

Energy for muscle fibers comes
from one of three ways:

1.Aerobic respiration
 glucose synthesized into ATP
 most efficient; 36 ATP – 1
Glucose
 slow and requires oxygen

2. Creatine phosphate
• gives up a phosphate to ADP
converting it to ATP
• stored in the muscle cells in
only small amounts
• 1 ATP for each creatine
phosphate stored, less efficient
than aerobic

3. Anaerobic respiration
• Glucose broken down to
pyruvic acid to ATP
• No oxygen required
• Produces lactic acid toxic to
muscles causes burning
• Not very efficient, 1 pyruvic acid
yields 2 ATP

If anaerobic is less efficient than
aerobic why use it?
Sometimes body cannot keep the
muscle supplied with oxygen at a
fast enough rate.
Myoglobin molecule stores oxygen in
muscle cell but sometimes not
enough.

A and P Mod5

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