Muscle Physiology:
The Actions of the Sarcomere.
Cardiac
Muscle
Characteristics
 Intercalated disks
 Striated
 Involuntary
 Located in heart
Skeletal Muscle
Characteristics
 Many nuclei per
cell
 Striated
 Voluntary
 Located along
bones
Smooth Muscle
Nonstriated
Involuntary
Located in
digestive tract
Functions of Muscles
Movement: results from muscle contraction,
enables you to respond quickly
Maintains Posture and Joint Stability:
allows you to sit upright; stabilize joints of the body
Support Soft Tissue:
underlying digestive organs.
abdominal muscles protect
Guard Entrances and Exits
Generate Heat: heat is generated as they
work…FRICTION


Maintains body temperature
Skeletal muscles create the most heat
Characteristics of Muscle Tissue
Excitability:
ability to receive and respond to
stimuli…
Contractibility:
ability to shorten quickly and
with force…
Extensibility: ability to be stretched or extended
beyond their resting state…
Elasticity:
ability of a muscle fiber to recoil and
resume its resting length
Organization of Muscle
Muscles are
composed of groups
of fibers called
fasicles.
Fibers are the muscle
cells inside all muscle.
Tendons are bands of
collagen fiber that
attach muscle to
bone.
Skeletal Muscle Striations
Z line
I band
H band
A band
Organization
from the
muscle fiber
to the
sarcomere.
Cross sectional view of Sarcomere.
Differences are
detected in the
sizes of the
myofilaments
Myosin is the
thicker fiber.
Actin is the
thinner fiber.
Striations are seen because of sarcomere
bands.
Sarcolemma
Mitochondrion
Myofibril
Dark A band Light I band Nucleus
(b) Diagram of part of a muscle fiber showing the myofibrils. One
myofibril is extended afrom the cut end of the fiber.
Muscle fiber structure
Muscle cell
Sarcolemma
Sarcoplasm
Sarcoplasmic
reticulum
T tubule
mitochondria
Sliding Filament Theory
Sliding Filament Theory
Actin slides over myosin shortening the
sacromere between the Z lines
Events at the Neuromuscular Junction
1 Action potential
arrives at axon terminal
of motor neuron.
2 Voltage-gated Ca2+
channels open and
Ca2+ enters the axon
terminal.
3 Ca2+ entry
causes some
synaptic vesicles
to release their
contents
(acetylcholine)
by exocytosis.
4 Acetylcholine, a
neurotransmitter, diffuses
across the synaptic cleft
and binds to receptors in
the sarcolemma.
Ca2+
Ca2+
Axon terminal
of motor neuron
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic cleft
Fusing
synaptic
vesicles
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of muscle fiber
Events at the Neuromuscular Junction
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action
potential (AP)
Nucleus
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated
Ca2+
channels
open and Ca2+ enters the axon
terminal.
Axon terminal
of motor neuron
3 Ca2+ entry causes some
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic
cleft
Fusing synaptic
vesicles
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis.
4 Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma.
5 ACh binding opens ion
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber.
6 ACh effects are terminated
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
Ca2+
Ca2+
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Na+ K+
Ach–
Degraded ACh
Na+
Acetylcholinesterase K+
Postsynaptic membrane
ion channel opens;
ions pass.
Postsynaptic membrane
ion channel closed;
ions cannot pass.
Setting the stage
Axon terminal
of motor neuron
Action potential
Synaptic cleft
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Muscle fiber Ca2+
Triad
One sarcomere
Actin
Ca2+
Troponin
Tropomyosin
blocking active sites
Myosin
3 Calcium binds to
Active sites exposed and
ready for myosin binding
Myosin
cross
bridge
The aftermath
troponin and removes
the blocking action of
tropomyosin.
4 Contraction begins
Cross Bridge Cycle (2 of 4)
ADP
Pi
2
The power (working) str
Cross Bridge Cycle (3 of 4)
ATP
3
Cross bridge detachmen
Cross Bridge Cycle (4 of 4)
ADP
PI
4
ATP
hydrolysis
Cocking of myosin head.
What happens at the sarcomere?
Players for the power stroke
Cross bridge
attachment
Power strokes
Cross bridge
detachment
“Cocking” of the
myosin head
Power stroke
Motor Unit: A motor neuron and all the
muscle fibers it stimulates.
Atrophy- when muscle fibers become weaker and
smaller due to lack of stimulation by a motor neuron.
Muscle Tension
The amount of tension produced by a muscle is
determined by:
1. The frequency of muscle stimulation.
2. The number of muscle fibers activated.
3. Degree of stretch by sarcomere. (length-tension
relationship
Myogram – a graph that measures tension developing in
a muscle fiber.
Diagram of a Muscle Twitch
Increase in muscle tension
due to continued stimulation.
Muscle that reaches peak tension during rapid
cycles of contraction and relaxation.
Complete tetanus = relaxation
state is eliminated.
Recruitment –
multiple motor unit
summation
Relationship between stimulus intensity and muscle tension.
Stimulus strength
Maximal
stimulus
Threshold
stimulus
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
Label the
following!
Muscle stores limited reserves of
ATP ~ 4-6 Seconds
3 Pathways for Generating ATP
1. Production of ATP from
Creatine phosphate
2. Aerobic Respiration
3. Anaerobic Respiration
Aerobic Muscle Metabolism
Glycolysis
Aerobic
Respiration


Krebs Cycle
ETC
Anaerobic Muscle Metabolism
Oxygen Debt
Lactic Acid
Fermentation
Muscle Fatigue
Creatine Phosphate
Muscle cells store 2-3 times
creatine as ATP.
Stored ATP and CP provide for
maximum muscle power for 1416s. (100 m dash)
CP + ADP
creatine kinase
Creatine + ATP
3 Pathways for regenerating ATP during muscle activity.
(a)
Direct phosphorylation
(b)
Anaerobic pathway
(c)
Aerobic pathway
Coupled reaction of creatine
phosphate (CP) and ADP
Glycolysis and lactic acid formation
Aerobic cellular respiration
Energy source: CP
Energy source: glucose
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
CP
Glucose (from
glycogen breakdown or
delivered from blood)
ADP
Creatine
kinase
Creatine
O2
Glycolysis
in cytosol
ATP
O2
2 ATP
net gain
Released
to blood
Oxygen use: None
Products: 1 ATP per CP, creatine
Duration of energy provision:
15 seconds
Glucose (from
glycogen breakdown or
delivered from blood)
Pyruvic acid
O2
Lactic acid
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
Pyruvic acid
Fatty
acids
O2
Aerobic
respiration
Aerobic respiration
in mitochondria
in
mitochondria
Amino
acids
CO2
32 ATP
H2O
net gain per
glucose
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
Comparison of energy sources between short term exercise and
prolonged exercise.
Short-duration exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
Prolonged-duration
exercise
ATP is generated by
breakdown of several
nutrient energy fuels by
aerobic pathway. This
pathway uses oxygen
released from myoglobin
or delivered in the blood
by hemoglobin. When it
ends, the oxygen deficit is
paid back.
Isotonic and Isometric Exercise
Isotonic – tension
increases and the
muscle shortens
 Lifting weights
Isometric – muscle does
not shorten, the tension
produced never exceeds
resistanc

Trying to pick up a car
Red (slow) twitch fibers
Aerobic
Slow-acting ATPases (enzymes that break
down ATP)
Large amounts of myoglobin
Red color to cell
Abundant supply of mitochondria
Fatigue resistant-as long as O2 is available
High endurance (jogging, swimming, soccer)
White (fast) twitch fibers
Large pale cells with twice the diameter of
red fibers
Very little myoglobin
Contain fast-acting ATPases and contract
rapidly
Contain few mitochondria, but large glycogen
stores
Depend on anaerobic resp. to make ATP,
therefore fatigues easily
Low endurance, much power….sprints
Pink (intermediate) twitch
fibers
Mixture of red and white fibers
Aerobic mechanisms and fatigue
resistant
Contain fast-acting ATPases
High myoglobin content
Force of Muscle Contraction
Number of Muscle
Fibers Stimulated
Size of the Muscle
Fibers (red vs.
white)
Frequency of
Stimulation
Degree of Muscle
Stretch
Proportion of motor units excited
Factors influencing force of skeletal muscle
contraction.
Large
number of
muscle
fibers
activated
Large
muscle
fibers
High
frequency of
stimulation
Contractile force
Muscle and
sarcomere
stretched to
slightly over 100%
of resting length
Length-tension relationships of sarcomeres in skeletal
muscles.
Sarcomeres
greatly
shortened
Sarcomeres at
resting length
75%
100%
Optimal sarcomere
operating length
(80%–120% of
resting length)
Sarcomeres excessively
stretched
170%