- A muscle twitch is a brief, weak muscle contraction in response to a single action potential. - There are three phases to a muscle twitch: latent period, contraction period, and relaxation period. - For a muscle to generate force, multiple motor units must contract together through processes like motor unit summation and tetanization which involve increasing stimulation frequency. - Muscle force production depends on factors like the number of motor units recruited, length of the muscle fibers, and whether the contraction is isotonic or isometric.

1.  A muscle twitch is the response of a muscle to
a single, brief threshold stimulus or response
to a single action potential.
 It is too short or too weak to be useful
 E. g. blinking of the eye
 There are three phases to a muscle twitch
 Latent period
 Period of contraction
 Period of relaxation

3.  Latent period – first few
msec after stimulus; EC
coupling taking place
 Period of contraction –
cross bridges form;
muscle shortens
 Period of relaxation –
Ca2+
reabsorbed; muscle
tension goes to zero
 The entire contractile
response last to a
single AP last for about
100msec
Figure 9.14a

4.  Increased
intracellular Ca2+
concentrations (Ca2+
released from
Sarcoplasmic
reticulum and T –
tubules) lasts longer
than action
potentials
 even longer lasts
single muscle twitch
- as skeletal muscle has
short refractory period and
action potential delivered to the sarcolema spreads along
all membrane, T – tubules, and Sarcoplasmic reticulum

5. Figure 9.14b

6.  Muscle consists of groups of muscle fibers
bundled together and attached to bones
 Connective tissue covering muscle divides
muscle internally into bundles
 Connective tissue extends beyond ends of
muscle to form tendons
 Tendons attach muscle to bone

7.  Each person has 600 skeletal
muscles which range in size from
delicate external eye muscles that
control eye movements having few
hundred fibers to the large
powerful leg muscles having several
thousands of fibers

8.  Connective tissue separates
muscle cells - each one must be
stimulated by axons of a
motoneuron
 Motor unit - all muscle cells
innervated by the same
motoneuron – they will contract
at the same time
 Motor units vary in size - mostly
mixture of motor units of
different sizes
large motor units >100
cells (typically slow
postural muscles)
small motor units about
10 cells (precise control
fast acting muscles –
those moving the eye)

10.  Twitch
 Brief, weak contraction
 Produced from single action potential
 Too short and too weak to be useful
 e.g. Blinking of the eye
Contractions of whole muscle can be of
varying strength
 Two primary factors which can be adjusted to
accomplish gradation of whole-muscle tension
 Number of muscle fibers contracting within a
muscle (motor unit recruitment)
 Tension developed by each contracting fiber

11.  Adding together individual twitch contractions to
increase the intensity of overall muscle
contraction.
 Summation occur in 2 ways:
1. By increasing the number of motor units
contracting simultaneously (multiple fiber
summation)
2. By increasing the frequency of contraction which
can lead to tetanization. (frequency summation)

12.  Motor unit
 One motor neuron and the muscle fibers it innervates
 Number of muscle fibers varies among different
motor units
 Number of muscle fibers per motor unit and
number of motor units per muscle vary widely
 Muscles that produce precise, delicate movements
contain fewer fibers per motor unit
 Muscles performing powerful, coarsely controlled
movement have larger number of fibers per motor unit

13.  Muscle fibers from a
motor unit are spread
throughout the muscle;
therefore, contraction of
a single motor unit
causes weak contraction
of the entire muscle

14. Increasing
level of
central
(moto-
neuronal)
activation –
successive
activation
of higher
threshold
motor units

15. When a weak signal is sent by the CNS to contract a muscle, the
smaller motor units, being more excitable than the larger ones,
are stimulated first. As the strength of the signal increases, more
motor units are excited in addition to larger ones, with the largest
motor units having as much as 50 times the contractile strength
as the smaller ones. As more and larger motor units are activated,
the force of muscle contraction becomes progressively stronger.
Also called recruitment which brings more and more muscle
fibers into play
Importance of recruitment: Asynchronous recruitment of motor
units helps delay or prevent fatigue

16. Figure 9.16

17.  A concept known as the size principle, allows for a
gradation of muscle force during weak contraction
to occur in small steps, which then become
progressively larger when greater amounts of force
are required.
 Cause of size principle:
Smaller motor units are driven by small motor
nerve fibers and the small motor units are more
excitable than the larger ones so they naturally
are excited first.

18. Figure 9.17

19.  A single stimulus results in a single contractile
response – a muscle twitch
 Frequently delivered stimuli (muscle does not
have time to completely relax) increases
contractile force – wave summation. Motor unit
stimulated by a high frequency of action
potentials from its motoneuron.
Figure 9.15

20.  More rapidly delivered stimuli result in
incomplete tetanus
 If stimuli are given quickly enough,
complete tetanus results
Figure 9.15

21.  Twitch summation
 Results from sustained elevation of cytosolic
calcium
 Tetanization:
 Occurs if muscle fiber is stimulated so rapidly that it
does not have a chance to relax between stimuli
 Contraction is usually three to four times stronger
than a single twitch

23.  Tension : a force exerted on an object by a
contracting muscle
 Load: force exerted on the muscle by an object
(usually its weight)
 Both are opposing forces. In order for muscle fiber
to shorten and thereby move a load muscle
tension must be greater than the opposing load

24. • DEFINITION:
When a series of maximal stimuli are delivered to the muscle at
a frequency just below tetanizing frequency
(when muscle twitch due to previous stimulus has just
completed), the tension/amplitude developed during each
twitch increases till a max. height is reached & a plateau is
formed. This is called the Treppe/ staircase effect.
Because the tension rises in stages, like the steps in a
staircase, this phenomenon is called treppe, a German
word meaning "stairs."
• CAUSE: The rise is thought to result from a gradual increase
in the concentration of calcium ions in the sarcoplasm, in
part because the ion pumps in the sarcoplasmic reticulum
are unable to recapture them in the time between
stimulations.

25. Figure 9.18

26. • Length of fiber at
onset of contraction
is a very important
factor influencing
extent to which
tension can be
developed in a
muscle

27.  Muscles operate with
greatest active force
when close to resting
length 2.5μm. When
stretched or shortened
beyond this, the
maximum active force
generated decreases
 This decrease is
minimal for small
deviations, but the force
drops off rapidly as the
length deviates further
from the resting state

28.  If muscle is passively
stretched the thin
filaments are pulled out
from b/w the thick
filaments so decreasing
the no. of actin sites
available for cross bridge
binding
so due to un matching
of the two sites , many of
them are unused. So
when less cross bridge
activity so less tension

29.  Less tension will
develop by the
sarcomere because
the actin filament
become overlapped
limiting the opportunity
for the cross bridges to
interact with actin
- The ends of the filament
become forced against
the z-discs so no further
shortening can take
place.

30.  The extremes in muscle length that prevent
development of tension occur only under experimental
conditions when the muscle is removed and
stimulated at various lengths.
 In the body the muscles are so positioned that their
relaxed length is approximately their optimal length.
So they can achieve near maximal tetanic contraction.
 Because attachment to the skeleton imposes
limitations, a muscle cannot be stretched or shortened
more than 30% of its resting optimal length

31.  Tension is produced internally within
sarcomeres(tension generating unit)
 And sarcomeres are not attached to bones
 Tension must be transmitted to bone by means of
connective tissue and tendons before bone can be
moved (series-elastic component which serve as a
stretchy spring b/w the sarcomeres and the bone and
that is to be moved against an external load.
 Shortening the sarcomere stretches the series elastic
components. Muscle tension is transmitted to the bone
by tightening of these series elastic components. This
force applied to the bone moves the bone against the
load

32.  There are two primary types of contraction, depending on
whether the muscle changes length during contraction. They
are
 Isotonic
 Muscle tension remains constant as muscle changes
length
 Isometric
 Muscle is prevented from shortening
 Tension develops at constant muscle length

33. • Isotonic contraction: occurs when muscle
contracts with shortening of length but against a
constant load, thus, the tension on the muscle
remains constant (iso= same, tonic= tension)
OR
A contraction that creates force & moves a load.
Isotonic contractions are used for body movements
and for moving external objects. E.g. picking up a
book, a box.

34.  Concentric = shortening muscles
 Eccentric = lengthening muscles

35.  Isometric contraction: occurs when muscle
contracts without shortening in length.
(iso= same, metric= measure or length)
OR
A contraction that creates force without movement.
Isometric contractions can be seen in 2 cases:
1. If the object you are trying to lift is too heavy.
2. If the tension developed in the muscle is deliberately
less than needed to move the load. E.g. standing for
long time or holding up a glass of water while taking
sips.

36. • shortening
• isometric
• lengthening
(Isotonic: shortening against fixed
load, speed dependent on
M·ATPase activity and load)
Three Potential Actions During Muscle Contraction:
Most likely to cause
muscle injury
Biceps muscle shortens
during contraction
Biceps muscle lengthens
during contraction

37. The same internal events occur in both isotonic and isometric
contractions:
Muscle excitation starts the sliding filament cycling; the cross bridges
start cycling; and filament sliding shortens the sarcomeres, which exert
force on the bone at the site of the muscle’s insertion.
During a given time, a muscle may shift between isotonic & isometric
contractions. E.g. when you lift a book up it is isotonic contraction and
when you keep holding the book up while reading it is isometric
contraction.
NOTE:
Since Work=Distance X Load,
Isotonic contractions do work where as Isometric do not.

38. ISOMETRIC CONTRACTIONS ISOTONIC CONTRACTIONS
1. Length same
2. Tension changes
3. W= FX D since no distance
covered so no work
performed
4. e.g. walking,
1. Length changes
2. Tension is same
3. Work is performed
4. e.g. standing, pushing
against the wall

39.  When the muscle contract against
the load it perform the work and the
energy required to perform the work
is derived from the chemical reaction
in the muscle cells during
contraction

40.  Most of the energy is required for:
1. Walk along theory
2. Ca pump
3. Na K pump

41.  Muscle contraction depends on the energy
supplied by the ATP
 Since ATP is the only source of energy that directly
be used for contractile activity to continue, so
ATP must be constantly supplied
 Only limited stores of ATP are immediately
available in muscle tissue which produces
muscle contraction for 1-2 seconds but 3
pathways supply additional ATP as needed
during muscle contraction

42.  Transfer of high-energy phosphate from creatine
phosphate to ADP
 First energy storehouse tapped at onset of contractile
activity
 Oxidative phosphorylation (citric acid cycle and
electron transport system
 Takes place within muscle mitochondria if sufficient O2
is present
 Glycolysis
 Supports anaerobic or high-intensity exercise

44. Figure 9.20

45. Creatine + ATPCreatine phosphate + ADP

46.  Glycolytic reaction can occur even in the
absence of oxygen so muscle contraction can
be sustained up to a minute when when oxygen
delivery from blood is not available
 Rate of formation of ATP by glycolysis is 2.5
times faster as compared to oxidative
phosphorylation

47.  Large amount of nutrient fuel is used giving
less amount of energy so glycolysis rapidly
depletes the storage pool of glycogen
 Lactic acid production may cause pain and
stiffness in the muscle
 So both factors play a role in the onset of
muscle fatigue

48.  More than 95% of all energy used by muscles for
sustained long term contraction is derived from this
source
 The food stuffs consumed during this process are :
 Carbohydrates
 Fats
 Proteins
For long term maximal contraction (period of
hours) greatest energy production from fats

49.  % of energy input that is converted to work
instead of heat
 It is about 25% with 75% becoming heat
which is required in maintaining body
temperature
 But in isometric contraction the muscle
efficiency is 0 (zero)

50.  Every muscle of the body is composed of a
mixture of fast and slow fibers and other fibers
gradated b/w these two extremes
 Rapidly acting muscles are composed of fast
fibers mainly with small number of other varieties
 Slowly acting muscles but with prolonged
contraction are composed of mainly slow fibers
(soleus muscle)

51.  Classified based on differences in ATP
hydrolysis and synthesis
 Three major types
 Slow-oxidative (type I) fibers
 Fast-oxidative (type IIa) fibers
 Fast-glycolytic (type IIx) fibers

52. Table 9.2

53.  Occurs when exercising muscle can no longer
respond to stimulation with same degree of
contractile activity
 Defense mechanism that protects muscle from
reaching point at which it can no longer
produce ATP
 Underlying causes of muscle fatigue are
unclear

54.  Occurs when CNS no longer adequately
activates motor neurons supplying working
muscles
 Mechanisms involved in central fatigue are
poorly understood

55.  However stimulation of the nerve fiber at rates greater
than 100 times per second for several minutes often
diminishes the no. of acetylcholine vesicles so much
that impulses fail to pass into the muscle fiber and
finally muscle will not contract at all. This is called
fatigue of NMJ
 Cause: inability of the synthesis of acetylcholine to
keep pace with its release and hydrolysis.
 In intact organism it is unlikely to be the site of fatigue.

56.  Lack of oxygen causes ATP deficit
 Lactic acid builds up from anaerobic glycolysis
 A local increase in ADP and inorganic
phosphate from ATP breakdown which will
interfere with cross bridge cycling
 Accumulation of ECF K+ when Na K pump
cannot transport K back into muscle so
decrease in membrane potential so decrease
excitability
 Depletion of glycogen energy reserves

57.  All muscles of the body are continually being
remodeled to match the functions that are
required of them
 Their diameter are altered
 Their lengths are altered
 Their strengths are altered
 Their vascular supplies are altered

58. Definition:
When the total mass of a muscle increases, this is called Muscle
Hypertrophy. The resulting muscle enlargement comes from
an increase in diameter of the muscle fibers. It is in response
to a regular & intensive use of that particular muscle. e.g. body
building or excessive use of steroids
Physiologic Basis:
 ↑in the number of actin & myosin filaments causing increase in
thickness of individual muscle fibers---called fiber
hypertrophy
 Rate of synthesis of actin & myosin far greater

60. Definition:
When the total mass of a muscle
decreases, it is called Muscle
Atrophy. If a muscle is not used, its
actin and myosin content decreases,
its filaments become smaller and the
muscle decreases in mass and
becomes weaker.

61.  Disuse atrophy: When the muscle is prevented from
doing work even though the nerve supply is intact. e.g. in
bed-ridden patients, in a limb in a plaster of Paris cast. This
type is thus called Disuse Atrophy.
1. Denervation atrophy: Atrophy also seen nerve supply
to the muscle is lost. This can be due to an accident or
when motor neurons supplying a muscle are destroyed .e.g.
Poliomyelitis.
• Muscle fiber becomes thin & low in proteins, glycogen and
ATP.
• Most of the fibers are destroyed and replaced by fibrous
tissue which leads to contractures leading to disfiguring

64.  Under rare conditions of extreme muscle force
generation, the actual number of muscle fibers
increase, in addition to the fiber hypertrophy
----This increase in fiber number is called
Muscle Hyperplasia.
Mechanism: Linear splitting of previously
enlarged fibers

66.  Stimulate muscle growth and
hypertrophy
 That’s the reason athletes have
dangerous practice of taking steroid
hormones to increase their athletic
performance

67. It is a fatal muscle-wasting
disease that primarily strikes
boys and leads to their death
before the age of 20.
There is progressive
degeneration of contractile
proteins of the muscle and
their replacement with
fibrous tissue.
It is a genetic X-linked disease.

68. Mutation in the Dystrophin gene located on X-chromosome
↓
Skeletal muscle lacks protein dystrophin (a large protein that provides
structural stability to the muscle cell’s plasma membrane)
↓
Its absence leads to constant leakage of Ca into the muscle cell
↓
Ca activates proteases that start damaging the muscle
↓
Leads to increasing muscle weakness & fibrosis
↓
Symptoms start at 2-3 years, patient wheel-bound at 10-12 years Usually
die at about 25-30 years of age (usually Males)
↓
Death is usually due to respiratory failure or heart failure as the
respiratory or heart muscles become too weak.
↓
Milder disease is Becker’s muscular dystrophy