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Muscular System
Human Physiology
Muscular System
PCB 3703 Human Physiology
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Muscular System
Muscle Classification
Functionally
1. Voluntarily
2. Involuntarily
Structurally
1. Striated
2. Smooth
Combined
1. Visceral
2. Cardiac
3. Skeletal
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Muscular System
Sarcomere
Z
A
Z
A
Z
A (I)
I
Z
PCB 3703 Human Physiology
Z
Z
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Muscular System
Categories of skeletal muscle actions
Categories
Extensor
Flexor
Abductor
Adductor
Levator
Depressor
Rotator
Sphincter
PCB 3703 Human Physiology
Actions
Increases the angle at a joint
Decreases the angle at a joint
Moves limb away from midline of body
Moves limb toward midline of body
Moves insertion upward
Moves insertion downward
Rotates a bone along its axis
Constricts an opening
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Muscular System
Myofilaments
1. Myosin: 110Å thick; confined to the
A-band. (Mole. wt. 500,000 deltons; 200
molecules/myofilament)
A. Tail- 800Å long, composed of a double helix
B. Head (cross bridges)-600Å terminating in a
globular double structure. Contains binding sites
for actin & ATP
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Muscular System
Myofilaments
2. Actin: 60A thick; runs from Z-line (disc) to just
inside A-band. Mole wt. 60,000 deltons.
G-actin (globular units): contracted form
F-actin (fibrous polymers): relaxed form
Actin associated proteins
A. Tropomyosin
B. Troponin
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Muscular System
Mechanics of Muscle Contraction
1. An action potential is generated by a motor
nerve.
2. This causes the release of acetylcholine from
the axon terminals at the neuromuscular
junctions.
3. This Ach causes an increase in membrane
permeability at the motor-end plate, causing
the production of an end-plate potential (EPP).
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Muscular System
Mechanics of Muscle Contraction
4. The EPP depolarizes the fiber membrane
(sarcolemma) causing a muscle action potential
which spreads over the entire surface of the
fiber membrane.
5. This depolarizes the T-tubules, causing ionic
conduction through their extracellular fluid,
and the release of inositol triphosphate as a
second messenger.
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Muscular System
Mechanics of Muscle Contraction
6. Ca++ is then released from the endoplasmic
reticular fluid of the cisterns (lateral sacs) into
the surrounding myofibril.
7. Ca++ binds to the actin associated protein
troponin, allowing the attachment of actin to
the myosin-ATP complex to form a strong
ATPase.
8. The ATPase splits ATP, releasing the energy
needed for the movement of the myosin cross
bridges.
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Muscular System
Mechanics of Muscle Contraction
9. Energy from creatine phosphate replaces ADP
on the myosin cross bridges, thereby breaking
the A-M bond and allowing the cross bridges to
relax.
10. The Ca++ are forced back into the walls of the
longitudinal tubules by active transport.
11. This restores the inhibitory action of the
troponin-tropomyosin complex.
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A
H
I
Z
Muscular System
Selective Terms
1. Motor Unit: consists of all the muscle fibers
innervated by terminals from a single axon.
(Range from 23 - 2,000 fibers)
2. All or None Law: at or above threshold levels; the
degree of contractile response of a single muscle fiber
(or motor unit) is independent of stimulus strength
3. Tension: force exerted by a contracting muscle
4. Load: force exerted on a muscle by the weight of an
object
5. Isotonic contraction (same tension): the tension
developed by the contracting is greater than the load.
Therefore, the muscle shortens.
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Muscular System
Selective Terms
6. Isometric contraction (same length): the strength of
the load is greater than the tension of the muscle.
Therefore, the muscle remains at the same length.
7. Muscle spindle apparatus: a series of small spindle
shaped fibers within the muscle for detecting changes
in the length (stretch) of the muscle.
8. Golgi tendon organ: tension receptors located in
tendons, and activated by the pull of a contracting
muscle
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Cross
Extensor reflex
Inhibitory
interneuron
inhibited
excited
Flexor
Reflex
From
stimulus
source
Repetitive
after discharge
excited (oscillatory circuit)
Reciprocal
inhibition
Vasodilation
Blood vessel Antidromic
Reflex
Red line
Red flare
wheel
No
motor activity
Renshaw
cell
Impulse Responses of Skeletal Muscles
1) Twitch
Contraction phase (0.04 sec)
Relaxation phase (0.05 sec)
Latent period (0.002 sec)
2) Summation
a) wave (frequency)
Tetanus
fatigue
b) Multiple motor unit (recruitment) intensity
5
3
4
12
1. Subthreshold
2. Threshold
3. suprathreshold
4. Maximal
5. Supramaximal
Voltage
c) Treppe (staircase phenomenon)
2 stimuli/sec
Muscular System
Lactic Acid in Cellular Respiration
(Glycolysis)
Lactic Acid
Glucose
Pyruvate
O2
Acetyl CoA
(LDH: Lactic Acid Dehydrogenase)
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Muscular System
Glycolysis [cont.]
Fate of Lactic Acid
1. Used by the heart for energy: it can convert lactic
acid back to pyruvate.
2. Decarboxylation (buffering action):
 CO2 which  ventilation
3. Converted back to pyruvate then to TCA
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Muscular System
Glycolysis [cont.]
Pain
1. Bradykinin: a peptide released from damaged
tissue. It excites the pain nerve endings.
2. Ischemia: If the brachial artery is occluded & the
muscles of the forearm exercised, pain will begin in
15 seconds. With no exercise it takes 4 minutes.
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Muscular System
Strength-Duration Curve
(Excitability Curve)
Utilization time (nerve)
Utilization time (muscle)
Chronaxie of nerve
2.0
Chronaxie of muscle
1.5
1.0
.5
0.1
0.2
0.3
Duration of stimulus
(seconds)
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Relative tension
2.0m
2.25m
1.65m
1.0
0.5
3.65m
1.25m
0
60
80 100
120
140
160
Percentage rest length
1.65 m
2.25 m
3.65 m