Muscle cell action potential - Peggy Hunter

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6-1
MUSCLE PHYSIOLOGY
SKELETAL MUSCLE MICROANATOMY
muscle cell =
skeletal muscle fibers

long (
embryonic cells)

nerve ending contacts muscle cell at

muscle cell membrane 

cytoplasm 

sarcolemma penetrates cytoplasm to form
tubules)  spread electrical impulse (action potential)

special endoplasmic reticulum 
(stores
ions)

fibers contain
thousands of parallel bundles of
), striated, multinucleated cells (form by fusion of many
organized into hundreds /
= contractile structures
diagram of muscle cell or ‘fiber’
Biology 142 NOTES (2007)
(transverse
6-2
myofibril


of muscle fibers
contains 3 kinds of
________________________ (thin)
________________________ (thick)
________________________ (elastic)

arranged into contractile units 
(sarcomeres shorten when contraction occurs)
NOTE:
I bands =
actin only (thin filaments, light in colour)
A bands =
myosin only (heavy filaments, dark in colour)
Z discs =
delineate endz of sarcomere (attachment of titin here)
Biology 142 NOTES (2007)
6-3
SKELETAL MUSCLE CONTRACTION

skeletal muscle cells contract in response to
from
motor neuron
motor unit
=
NOTE: some motor units have only a few muscle cells (permits fine, precise
motor control); other motor units have many muscle cells (gross motor movement
/ powerful contractions)
Events at neuromuscular junction

motor neuron releases neurotransmitter
(ACh release triggered by
influx into neuron)

ACh molecules bind to

binding triggers
Muscle cell action potential

muscle cells have
membrane potential (def’n)
Biology 142 NOTES (2007)
in muscle cell
6-4
WHY?
pumps
→ makes extracellular fluid (
intracellular fluid (
)
)
and
than
So……. muscle cell membrane is
THINK!
Which direction will Na+ move if Na+ gates are suddenly stimulated to
open as a result of binding of ACh?
action potential (def’n)
RECALL:
NOTE: enzyme
Biology 142 NOTES (2007)
degrades Ach in synapse
6-5
Excitation – contraction coupling

muscle cell excitation triggers contraction
Na influx causes muscle cell
spreads
over sarcolemma to
triggers
from cisternae of
causes
between
filaments
Biology 142 NOTES (2007)
6-6
Sliding of filaments (molecular events)
Biology 142 NOTES (2007)
6-7
muscle cell contraction


contraction continues as long as
muscle cell relaxation

(no action potential)

back into sarcoplasmic reticulum

of muscle cell (Na+/K+ transporter restores
transmembrane potential)

recoil of
(assisted by gravity)
THINK!
Why does rigor mortis occur within a few hours after death?
Botulism toxin prevents ACh release. What effect would this have?
Myesthenia gravis is an autoimmune disorder where the body makes antibodies
to ACh receptors. What effect would this have?
Curare blocks ACh receptors in skeletal muscle cells. What effect would this
have on an animal shot with an arrow coated in curare?
Biology 142 NOTES (2007)
6-8
CONTRACTION OF WHOLE MUSCLE

requires simultaneous stimulation of
muscle twitch = simultaneous contraction of motor units triggered by
1 ____________________ ( t-tubules depolarize; muscle cannot respond to another
stimulus)
2 ____________________ sarcomeres shorten
3 ____________________ Ca++ transported back into cisternae of SR
NOTE:
tension =
affected by



Biology 142 NOTES (2007)
6-9
1. length – tension relationship
moderate stretch
unstretched
overstretched
NOTE: muscles are stretched when they are attached to bone (perfect amount of
stretch to permit generation of maximum tension)
2. strength of stimulus

stronger stimulus →
“
“ tension may increase a little with repeated stimulation even when
stimulus strength remains same →
(increases efficiency of
chemical reactions in muscle cells)
NOTE:
is maintained by small asynchronous contractions of
random motor units due to neural stimulation
THINK! What would happen to muscles in patients with injuries affecting motor neuron
pathways (eg. spinal cord injuries)?
Biology 142 NOTES (2007)
6 - 10
3. frequency of stimulus
“
“ = smooth sustained contraction (fusion of individual
twitches as frequency of stimulation increases)
NOTE: normal muscle contraction is tetanic
Types of contraction
______________________ tension increases but muscle does not change
length
eg.
______________________ tension generated as muscle changes length
eg.
2 phases
phase (as muscle shortens)
phase (as muscle lengthens)
NOTE: ‘braking’ actions (such as running downhill) are also eccentric;
eccentric work causes more muscle damage (and soreness) than
concentric work (it also facilitates ~ 10% greater strength gains in training)
Biology 142 NOTES (2007)
6 - 11
MUSCLE METABOLISM

muscle contraction requires energy (

ATP required for 3 steps
)
1.
2.
3.
3
pathways of ATP synthesis (
1.
)
(amino acid derivative stored in muscle)
creatine – P
creatine
(duration of activity
)
2.
(no oxygen required)
(duration of activity
)
NOTE: lactic acid diffuses into blood →
pyruvate or glucose)
3.
(reconverted to
(requires
(duration of activity
*(muscle
a polymer of glucose)
is
)
)
stored in muscle; glycogen is
NOTE: all 3 metabolic pathways generate
(approx. 85% of heat
required to maintain normal body temperature is generated by muscle contraction)
THINK! Why do you ‘shiver’ when you are cold?
Biology 142 NOTES (2007)
6 - 12
muscle fatigue
def’n

(depletion of glucose, glycogen, creatine phosphate)

build-up (lowers pH in muscle → reduces contraction efficiency)

(especially K+ loss from muscle)

‘derecruits’ motor units? (“
”)
oxygen debt
=

metabolic processes following exercise require O2
eg.
muscle
=

result of resistance training

myofibrils enlarge (

hormone
steroids are forms of testosterone)
)
facilitates muscle hypertrophy (anabolic
NOTE: muscle hyperplasia (increase in number of muscle cells) does not
normally occur in skeletal muscle
What causes DOMS (

DOMS peaks at
Biology 142 NOTES (2007)
)?
6 - 13
NOTE: the speed of contraction, strength of contraction, ability to hypertrophy and
endurance of a muscle are determined by the number, size and type of muscle fibers
making up the muscle (affected by training and genetic limitations)
Muscle fiber types
3 main types
___________________________ (slow oxidative / type I)



(oxygen binding pigment) →
in colour due to
do not fatigue quickly (
)
eg. marathon running / distance cycling, maintaining posture
____________________________ (fast glycolytic / type IIx / type IIb)


in colour (no myoglobin)

eg. hitting baseball, power lifting

undergo
____________________________ (fast oxidative / type IIa)

intermediate in power and endurance

(have some myoglobin)

more fatigue resistant than fast fibers (
)
eg. walking, sprinting
NOTE: type of training / genetics will influence fiber content of muscles
eg. sprinter’s legs
(
marathoner’s legs (
Biology 142 NOTES (2007)
slow fiber,
slow fiber,
intermediate,
intermediate,
fast fiber)
fast fiber)
6 - 14
Comparison of Muscle Fiber Types
SLOW
INTERMEDIATE
FAST
Fiber diameter
Contraction speed
Mitochondria
Capillaries
ATP synthesis
pathway
Myoglobin content
Glycogen stores
Rate of fatigue
Acivites best
suited for
NOTE: all muscles contain all fiber types, but proportions vary and are influenced by
training
Biology 142 NOTES (2007)
6 - 15
effects of exercise on muscle

USE IT OR LOSE IT!
(muscle tissue is lost if muscles are not used
regularly……saves energy; THINK! can muscle ‘turn into fat’????
atrophy (def’n)
strength / resistance training
hypertrophy → increased muscle mass → increased strength / power


(more actin and myosin)


are especially responsive to strength training
males vs. females?
body strength / kg muscle identical BUT amount of muscle tissue differs
females
males
WHY?
endurance training
muscle endurance =

(increased use of
pathway,
increased number of
, increases muscle enzymes,
increased ability to store and use
)

(better
delivery, faster
and
removal


(increased oxygen supply within muscle itself)
especially responsive
lactate threshold training (speedwork!)
Biology 142 NOTES (2007)
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