Download Microscopic Anatomy of the Skeletal Muscles

Microscopic Anatomy of
the Skeletal Muscles
Taking a look at the individual
muscle fiber and how it works with
other fibers
I. Microscopic Anatomy organelles
 A.

 B.



Multinucleated
The first nucleus (circular) can be seen just
under the sarcolemma (plasma membrane)
Myofibrils
1. Ribbon-like
2. Almost fill the cytoplasm
3. Have alternating light and dark bands, give
striation look to the fiber
I. Microscopic Anatomy organelles
 B.


Myofibrils (cont.)
4. Myofibrils are made up of myofilaments
5. Myofibrils are chains of sarcomeres
• a. Formation is from z-disc to z-disc
• b. Arranged like boxcars
• c. The lighter/thinner bands are thin filaments
 Light/thinner bands have a mid-line interruption
called the Z-disc
 Z-discs connect the light/thinner bands of
adjacent sarcomeres (brings the sarcomeres
closer together during a contraction)
I. Microscopic Anatomy organelles
 B.
Myofibrils (cont)
• d. The darker bands are thick filaments (myosin
filaments)
 Has a light central area called the H-zone
• M-line in the H-zone has proteins that hold
together adjacent filaments
• e. The I-band


Doesn’t polarize light
Includes the z-disc
• f. The A-band


Polarizes light
Includes the sarcomere
I. Microscopic Anatomy organelles
 B.
Myofibrils (cont.)
• g. Thick filaments run the entire length of the A-band
 The middle of the thick filaments are smooth
(around the M-line)
 The ends of the thick filaments are studded with
small projections (myosin heads)
• The myosin heads are called cross bridges,
when they link the thick and thin filaments
together during contraction
I. Microscopic Anatomy organelles
 B.
Myofibrils (cont)
• h. Thin filaments are composed of contractile proteins
called actin
Also contain other proteins to stop myosin
heads from becoming bound
 Are anchored to the Z-disc

 C.
Sarcoplasmic Reticulum is a specialized
ER
• 1. Surrounds the myofibril
• 2. Stores calcium for contractions
Sarcoplasmic Reticulum in Muscle
Fibers
II. Microscopic Anatomy –
Contraction of a single cell
► A.
General Information
 1. Muscle cells must be stimulated by nerve
impulses
 2. One motor neuron and all of the skeletal
muscle it stimulates is called a motor unit
 3. Nerve extensions (or axons) branch into axon
terminals
►Form
junctions with sarcolemma of different muscle
cells (called neuromuscular junctions )
II. Microscopic Anatomy –
Contraction of a single cell
► A.
General Information (cont.)
 4. The nerves never touch the muscle cells
 5. The gap is called the synaptic cleft
►Filled with interstitial fluid
 6. When an impulse reaches a nerve, a
neurotransmitter is released into the interstitial
fluid
►a. Acetylcholine or ACh is the chemical
released
Synaptic Cleft
Synaptic Cleft
II. Microscopic Anatomy –
Contraction of a single cell
 6. (cont)
►b.
When enough ACh is in the gap, the sarcolemma
increase permeability to Na+ ions and K+ ions
►c. Na+ ions rush into the cells
►d. K+ ions rush out of the cells
 This is why physical activity requires salt and bananas!
►e.
The increased sodium ions creates a greater
positive charge




Called an Action Potential
It is unstoppable
Travels over the entire surface of the sarcolemma
This conducts the electrical impulse over the entire surface
of the muscle cell – resulting in a contraction
II. Microscopic Anatomy –
Contraction of a single cell
 7. Acetylcholine is broken down by enzymes
after the action potential starts acetylcholinase
 8. Since ACh breaks down after the start of the
action potential, only one nerve impulse can
cross the gap at a time
II. Microscopic Anatomy –
Contraction of a single cell
► B.
The Sliding Filament Theory
 1. Contraction causes the shortening of each
individual fiber
 2. The fibers are shortened by the shortening of
each individual myofibril
 3. Myofibrils are shortened by reducing the
length between the Z-discs
 4. The A-bands do not shorten but appear
closer together
II. Microscopic Anatomy –
Contraction of a single cell
► B.
The Sliding Filament Theory (cont.)
 5. I-bands (which represent the distance
between the A-bands) decrease in length
 6. Both the thick and thin filaments remain the
same length
 7. Shortening of the sarcomeres is produced by
sliding of the thin filaments over the thick
filaments
 8. The thin filaments extend deeper toward the
center
II. Microscopic Anatomy –
Contraction of a single cell
► B.
The Sliding Filament Theory (cont.)
 9.The thin fibers increase the amount of
overlap, decreasing the H-zone
► C.
What causes the filaments to slide?
 1. The process is energized by ATP
 2. The myosin heads attach to binding sites on
the thin filaments
 3. The cross bridges (myosin heads) attach and
detach several times during contraction
II. Microscopic Anatomy –
Contraction of a single cell
► C.
What causes the filaments to slide?
(cont)
 4. Cross bridge attachment requires Ca2+ ions
 5. After contraction, the Ca2+ ions are
reabsorbed by the sarcoplasmic reticulum
III. Macroscopic Contraction of the
Muscle
► A.
“All or none” law of muscle physiology
applies to the muscle cell
 1. The muscle cell will contract completely when
properly stimulated
► B.
Muscle cells within the “muscle”, reacts
with a graded response
 1. Two ways of graded muscle reaction
►Changing
the frequency
►Changing the number of muscle cells being
stimulated
III. Macroscopic Contraction of the
Muscle
► C.
Response to increasingly rapid stimulation
 1. A muscle twitch is a brief, single, jerky
contraction
►May
be a result of a nervous condition
►Not a normal muscle operation
 2. Stimuli occur very rapidly, muscle cells do not
have a chance to fully relax between stimuli
 3. Successive contractions are ‘summed’
together
 4. When stimulated where there is no relaxation,
the muscle is fused or complete tetanus
III. Macroscopic Contraction of the
Muscle
► C.
Response to increasingly rapid
stimulation (cont.)
 5. As the stimuli increases, if there still is
relaxation, the muscle is unfused or incomplete
tetanus
IV. The Energy for
Contraction
• A. General Information
• 1. Muscle cells only store 4-6 seconds
worth of ATP
• Enough to start the contraction
• 2. 3 pathways for ATP generation
• a. Direct phosphorylation of ADP by creatin
phosphate
• b. Aerobic respiration
• c. Anaerobic glycolysis and lactic acid
formation
IV. The Energy for
Contraction
• a. Direct Phosphorylation
• Creatin phosphate is a high energy
molecule (called CP)
• Only found in muscle fibers
• Energy is transferred from ADP to
CP after ATP reduction
• This regenerates ATP rapidly
• CP is exhausted in about 20 seconds
IV. The Energy for
Contraction
• b. Aerobic Respiration
• During light activity, 95% of ATP is
created by aerobic respiration
• Occurs in the mitochondria
• Oxygen is necessary
• The pathways are called oxidative
phosphorylation
• Glucose is broken down into ATP
molecules
• Slower than the other 2 forms
IV. The Energy for
Contraction
• c. Anaerobic Glycolysis
• Breakdown of glucose into pyruvic
acid in the absence of oxygen
• Pyruvic acid is converted into
lactic acid (with 4 ATP molecules)
• Occurs in the cytosol of the
mitochondria
• 2 1/2 times faster than aerobic
respiration
V. Muscle fatigue and Oxygen
Debt

A. General Information
1. The muscle is unable to contract
 2. Still being stimulated
 3. Contraction becomes weaker without rest
 4. Oxygen debt is the cause

Occurs during prolonged muscle activity
 Glucose is converted into energy and then lactic
acid
 acid build up causes the muscles to cramp

V. Muscle fatigue and Oxygen
Debt

A. General Information (cont)

5. Oxygen debt is recovered after the
activity is concluded with rapid breathing
and high overall blood pressure and rapid
heart beat
VI. Types of Muscle
Contractions
 A. General Information

1. Isotonic Contractions
 a.
Myofilaments are successful in sliding and
the muscle shortens

2. Isometric contractions
 a.
Myofilaments are “skidding their wheels”
and the tension in the muscle increases
 b. Muscles are trying to contract, but are
moving against an immovable object