Lec7
Physiology
Dr.HananLuay
Objectives
1-Recognize the meaning of summation of contraction and its types.
2-detrmine the effect of changing length on skeletal muscle tension.
3-Differntiate between the 2 types of muscle fibers.
4- Describe the motor unit and how its type affects the function of the
muscle?
5- Discuss the energy sources for the muscle and for what it is used?
6-Outline the abnormalities in muscle fiber contractions
The summation of contraction:
The strength of a muscle’s contraction is influenced by a variety of
factors.
These
include
a- the number of fibers within the muscle that are stimulated to
contract.
b- the frequency of stimulation.
c- the thickness of each muscle fiber (thicker fibers have more myofibrils
and thus can exert more power).
d- the initial length of the muscle fibers when they are at rest.It means
the adding together of individual twitch contractions to increase the
intensity of overall contraction. Because the contractile mechanism has
no refractory period, repeated stimulation (i.e. increase the frequency)
before relaxation can produce additional activation of the contractile
elements and a response will be added to that already present, this is
called "summation of contraction". It depends on the frequency of
stimulation it occurs in 2 ways:
1 – Multiple fiber summation (increasing the number of motor units
contracting at the same time).When the central nervous system sends a
weak signal to contract a muscle, the smaller motor units of the muscle
may be stimulated in preference to the larger motor units. Then, as the
strength of the signal increases, larger and larger motor units begin to be
excited as well.
2- Frequency summation and tetanization, with rapid repeated
stimulation, activation of the contractile mechanism occurs repeatedly
before any relaxation occurs and the response fuses into one continuous
contraction and the whole contraction appears to be smooth called
Tetanus (by increasing the frequency of contraction).
During tetanus the tension developed is 4 times than the individual
contraction.
At slightly higher frequencies, the strength of the muscle contraction
reaches maximum, so any additional increase in frequency beyond that
point has no further effect in increasing the force of contraction, this is
because enough Ca ions are maintained in the sarcoplasm ,even
between action potentials so will not allow relaxation to happen.
But if a lower frequency is used, there will be a period of incomplete
relaxation between the summated stimuli; this condition is called
incomplete tetanization or clonus.
Treppe
(staircase effect):
It is another type of graded response occurs when a muscle begins to
contract after a long period of rest.
When a series of maximal stimuli is
delivered to skeletal muscle at a low frequency (just below the
tetanizing frequency), there is an increase in the tension developed
during each twitch (the second more than the first and the third more
than the second etc.) until, after several contractions, a uniform tension
per contraction is reached. It is believed that
Treppe may represent a
warm-up effect
, to be due to increased availability of Ca
2+
for binding to
troponin C , which is needed for muscle contraction.
Effect of muscle length on the force of contraction:
When the length of the unstimulated muscle fiber is changed (the
muscle stretched but not stimulated) this is called passive tension.It
exists because the muscle and its connective tissues have some
elasticity.
Passive tension curve is the curve plotted to include the changes in
tension against the changes in the muscle length. It shows that, as the
length increases, it rises sharply.
The total tension curve shows the total tension against the passive
tension, we will see a sharp increase up to a maximum value, then the
curve declines.
If we measure the distance between the two curves we will see the
active tension curve which is similar in shape with the total tension
curve but has a lower peak.So the active tension recorded when the
muscle contracts.
From these curves we will see that the maximum tension occurs when
the muscle length is at its resting length i.e. the sarcomere length is
about 2 micrometer (relaxed state). Shortening or contraction of the
muscle will reduce this tension. This is because the maximum number of
interaction between actin and myosin occurs at the resting length
.Stretching of muscle fiber (increase in its length) will cause less actin to
myosin interaction i.e. bigger sarcomere with a large gap in the middle
.Shortening of the muscle fiber, causes smaller sarcomere and actin
filament overlap, so less actin to myosin interaction sites.
Energy sources and metabolism:
Contraction of the muscle depends on energy supplied by ATP.
In general the source of energy is the metabolism of carbohydrates and
lipids.
Most of the energy required for Physical activity (contraction, relaxation)
or walk along mechanism, and small amounts are required for:
1- Pumping of Ca ions from the sarcoplasm to the sarcoplasmic
reticulum after the contraction is over.
2- Regeneration of ATP.
ATP split to ADP, then ADP rephosphorlated to ATP.
3- Removal of lactic acid.
4- Heat production.
Sources of energy for the re phosphorlation:
1- Substance called phosphocreatine (high energy phosphate bond)
ADP+P→ATP
This compound synthesized during resting conditions .During exercise
this compound hydrolyse at the junction between actin and myosin
releasing energy ,this reaction is catalyzed by the enzyme
phosphorylcreatinine at the mitochondria and myosin heads.
ADP+Phosphocreatinin→creatinine + ATP
2- Glucose: it is supplied by the blood and undergoes series of reactions
forming finally Co2, H2o and Energy.
3- Glycolysis of glycogen stored in the muscle cells: enzymatic break
down of glycogen to pyruvic which has 2 pathways in the presence of
O2, it enters citric acid cycle (crips cycle), then the respiratory chain to
form co2, H2O and large amount of energy, (aerobic Glycolysis). But in
the absence of O2(like in prolong contraction), pyruvate is reduced to
lactate and lactic acid and small amount of energy.
4- The free fatty acids(FFA) (gives double the energy that glucose gives)
skeletal muscles take the FFA in the blood and oxidized to give Co2, H2o
and ATP (the use of FFA mainly at rest and during recovery after
contraction).
The oxygen debt mechanism:
During exercise the blood vessels dilates to provide enough O 2 for the
muscle, the energy is supplied by aerobic glycolysis ,but if the exercise is
sever or continues for longer periods, the anaerobic glycolysis contribute
also to provide energy ,but it is self limiting ,because lactic acid will
diffuse to the blood it will lower the PH ,accumulate in the muscle
causing muscle exhaustion .So after the exercise there will be a period of
hyperventilation to produce the extra amount of O2 in order to remove
the lactic acid and to rebuild the storage of ATP and phosphorylcreatinin
. This extra amount of O2 taken to replace the demand required more
than that supplied by the aerobic glycolysis during exercise is called the
oxygen debt mechanism.
Trained persons need less period of hyperventilation because they have
smaller oxygen debt mechanism (i.e. he has endurance, he can use the
muscle to perform the job better). They have a lactate threshold that is a
higher than untrained. These athletes thus produce less lactic acid at a
given level of exercise than the average person, and therefore they are
less subject to fatigue than the average person. They have adaptation
that spares muscle glycogen will improve physical endurance. By an
increased proportion of energy that is derived from the aerobic
respiration of fatty acids, resulting in a slower depletion of their muscle
glycogen.
Heat production in the muscle:
This is produced as:
1- Resting heat: liberated during resting stage.
2- Initial heat which is produced during activity and it is divided into:
a- Activation heat: produced during contraction, from the actin –myosin
interaction.
b - Shortening heat: produced during shortening (isotonic contraction),
due to the changes in muscle fiber structure during shortening.
In isotonic contraction there are both activation and shortening heat but
in isometric there is only activation heat.
3- Recovery heat: liberated by the metabolic processes that restore the
muscle fiber to its precontraction state and it is rather equal to the initial
heat.
4- Relaxation heat: liberated because work should be done to return the
muscle to its original length (after isotonic contraction) in addition to the
recovery heat.
Types of muscle fibers
:
3 types according to the differences in enzyme activity, metabolism and
contractile properties
:
1- Type I fibers: these are darker than other muscles called Red muscles
or the slow fibers, they response slowly and have longer duration of
action (resist fatigue). They are specialized for long slow sustained
contraction, supplied by slow conducting fibers. e.g. muscles in the back
and in the lower limbs which are used to maintain posture.They are
small fibers, have more extensive blood supply to supply high oxygen,
and high number of mitochondria and large number of myoglobulin.
2- Type II b: called the white muscles or the fast fibers, they are
innervated by large mylinated fibers, they have short duration of
action(fatigue quickly) .so they are specialized for fine skilled movement
e.g. muscles of the hand and the extra-ocular muscles.They are large
fibers and have extensive sarcoplasmic reticulum for rapid release of Ca
ions, large amounts of glycolytic enzymes for rapid release of energy,
they have less extensive blood supply and fewer mitochondria and less
myoglobulin
.
3- Type II a: this is rare type in human, has properties similar to type I,
and other properties to type II.
The motor units:
The motor unit means all the muscle fibers innervated by a single nerve
fiber .i.e. the axon of a single motor neuron divides to supply many
muscle fibers. There are 2 types of motor units:
1- Small motor units: contain 3-6 muscle fibers, concerned with fine
graded, precise movement, like movements of the hand. They are Small
muscles that react rapidly and whose control must be exact have more
nerve fibers for fewer muscle fibers (2-3 muscle fibers for each motor
unit, e.g. laryngeal muscles).
2- large motor units :contain usually 120-165 muscle fibers ,like muscles
of the back ,for the sustained form of activity. These are large muscles
that do not need fine control (e.g.
soleus
muscle), may have several
hundreds of muscle fibers in the motor unit.
Each motor unit is of one type i.e. innervates one type of muscle, but
when a nerve to slow muscle is cut and replaced by a nerve to fast
muscle ,the slow muscle after a period of time becomes fast.
The muscle fibers in each motor unit overlap other motor units in
microbundles of 3-15 fibers. This interdigitation allows the separate
motor units o contract in support of one another rather than as
individual segment.
Denervation :
Means the deprivation of muscles from the nerve supply, the following
effects will happen:
1- Immediate loss of muscular activity called flaccid paralysis.
2- Abnormal excitability of muscle fiber with increase sensitivity to
circulate Ach (deneravation hypersensitivity), this result in fine irregular
contraction of individual muscle fiber (fibrillation).
Up to these 2 points, if the nerve fiber regenerates these 2 effects will
disappear.
3- Atrophy of the muscle: prolong denervation results in degeneration of
the muscle fibers and replacement by fibrous tissue, this result in
reduction of muscle size called wasting (because of decay of contractile
proteins). If after 2 months the nerve supply back, full return of function
.but after 3 months the return back of muscle function is less, and after
1-2 years, no return of muscle function.
All these features are of lower motor neuron lesions which is the effect
from the spinal cord to the muscle. Above that is called upper motor
neuron lesion.
Contracture:
This is persistent sustained contraction of the muscle without an action
potential, causing no relaxation. It occurs due to the increase level of
extracellular calcium.
Rigor:
When the muscle fiber is completely depleted of ATP and
phosphorylcreatinin, they develop a state of complete contraction
causing increase rigidity after death because no ATP available for
relaxation ,in this state Rigor mortis ,almost all the myosin heads
attached to the actin in an abnormal fixed and resistant way.(it occurs
more rapidly in high temperature).
Muscle fatigue:
Prolonged and strong muscle contraction lead to muscle fatigue. This
results from:
-Depletion of glycogen stores in the muscle (inability of the contractile
and metabolic processes to continue supplying the same work).
-Diminished transmission in the NMJ.
- Interruption of blood supply or flow results in nutrient and O2
deficiency.