Effect of increasing strength of stimulus on skeleton muscle contraction

This a fully prepared depth ppt by Dr Kavish Jain

2. To study the effect of increasing strength of
stimuli on performance of muscle nerve
preparation

3. When a muscle is stimulated with stimuli of
different strength the force of contraction
also differs.
With increasing strength of stimuli,
amplitude of contraction increases due to
recruitment of MOTOR UNITS[it is
defined as a single nerve fibre and all its
branches supplying the muscle fibres]

4. ALL OR NONE LAW
It states that when the tissue is stimulated
with threshold or more than threshold
strength the amplitude response will remain
the same but for a stimuli of less than
threshold strength there will not be any
response .
However it must be noted that this law is
applicable to strength of stimuli only but
not to whole skeletal muscle.

5. Dissecting instruements
Amphibian ringer solution
Kymograph with drum
Myographboard with stimulating electrodes
Weights
Tap key
Du Boys Reymond Inductorium
Low resistance wire
Thread
Pins
Tuning fork[100Hz]

7. Intensity or strength of a stimulus is of 5 types
Subminimal/ Subthreshold–This stimulus is
less than minimal strength and therefore doesn’t
evoke response
Minimal/threshold-It is minimum strength of
stimulus which is just sufficient to cause
contraction of minimal number of motor units to
produce a muscle twitch
Submaximal-It is more than minimal but less
than submaximalstrength of stimuli which can
produce a greater force of contraction as
compared to minimal but less when compared to
minimal but less when compared to maximal
strength of stimulus

8. Maximal-This type of stimulus produces
maximum response. Amplitude of contraction
increases due to recruitment of more number
of motors unit.
Supramaximal-This stimulus is stronger than
the maximal stimulus but it doesn’t changes
magnitude of contraction because all the
motor units of muscle contract with the
maximal stimulus.

10. No contraction is observed with sub minimal
stimuli beacusea subthresholdstimulus fails to
excite enough motor units that could give
response.
With threshold stimuli, contraction is observed
only with the break stimuli as it excites a few
motor units .
With increase in strength of stimuli ,there is a
graded increase in the amplitude of contraction
with both make and break stimuli this is called
as QUANTAL SUMMATION

11. A greater height is observed in break stimuli till the
stimuli reach submaximalbecause the strength of
induced current is greater of break stimuli
With maximal stimuli height of contraction is maximum
irrespective of make or break stimuli because all the
motor unit are activated with the maximum stimulus
With supramaximalstimuli no further increase in height
of contraction is observed because maximum number of
motor unit has been recruited with maximum
stimulation

12. Exclude drum from the circuit
Record observation both during make and
break stimuli
Start from the sub minimal to supramaximal
with sufficient time between two successive
stimuli

13. Q.1 What is a motor unit?
A single anterior horn cell (alpha motor neuron), its axon and
all its branches, and all the muscle fibersinnervated by this
neuron is called a motor unit
. Q.2 What are the different grades or degrees (in terms of strength)
of stimuli?
Subminimal
Minimal
Submaximal
Maximal
Supramaximal
. Q.3 Which stimulus is the threshold stimulus in your experiment?
The “break” induced shock with the secondary coil at 18 cm is
the threshold stimulus, i.e. the minimum
intensity of stimulation that would activate enough motor
units to cause contraction of the muscle. It is also obvious that
this stimulus is stronger than the “make” induced shock

14. . Q.4 Why does the force of contraction increase when the
strength of stimuli, in the submaximalrange, is gradually
increased?
The sciatic nerve contains motor fibersof varying
excitability. Subthresholdstimuli fail to excite any, while a
threshold stimulus excites a few motor units and the
muscle gives a weak contraction. As the strength of the
stimuli is increased, more and more fibersare recruited
into activity and the force of contraction goes on
increasing. This phenomenon is called quantalor
multifibersummation
. Q.5 Why does the force of contraction not increase after
the strength of stimulus is increased beyond the maximal
level?
A maximal stimulus is that which excites all the motor
fibers, and therefore all the motor units are already
contracting to their maximum extent (all-or-none law; see
below). As a result, any further increase in the strength of
the stimuli (supramaximalstimuli) has no effect in
increasing the force of contraction.

15. THANK YOU