Definition of MuscleTone Significance of Muscle Tone Spinal & Supraspinal Mechanism of Muscle Tone Decerebrate Rigidity

1. Stretch Reflex
Dr Mrs Padmaja R Desai
Professor & Head
Department of Physiology
D Y Patil Medical College Kolhapur

2. Specific Learning Objectives
• Definition of MuscleTone
• Significance of Muscle Tone
• Spinal & Supraspinal Mechanism of Muscle
Tone
• Decerebrate Rigidity

3. Muscle Spindle

6. Myotatic reflex or stretch reflex:-

8. • When the muscle is stretched, stretch receptors i.e.
muscle spindles are stretched and therefore
stimulated. From stretch receptors afferent impulses
are carried by Group Ia and group II fibres and they
directly synapse with the alpha motor neurons without
interposition of any interneurons.Efferents from alpha
motor neuron carry impulses to the extrafusal fibres of
the muscle from where the muscle spindle fibres
originated and cause its contraction.
• This is a monosynaptic reflex.

9. • Tone:-
• It is a sustained partial state of
contraction of skeletal muscle at rest.
• Importance of tone:-
• It is important for maintenance of posture.
• To perform various voluntary movements.
• It is important for lifting of load.

10. • Tone of skeletal muscles is maintained by two
fundamental mechanisms.
• The spinal mechanism i.e. stretch reflex.
• The supraspinal mechanism i.e. the
descending tracts both pyramidal and extra
pyramidal which influence the stretch reflex.

11. Spinal Mechanism
• Tone is maintained reflexely by monosynaptic
stretch reflex.
• The central nuclear bag region of the
muscle spindle is stretched either due to
• Stretching of the extrafusal fibres when the
muscle is stretched.
• OR
• When the intrafusal fibres contract

12. • Though this reflex is spinal but the supraspinal
pathways modify this reflex.

13. Supraspinal mechanism
• The descending tracts i.e. both the Pyramidal
and extra pyramidal terminate on α or r motor
neurons either directly or through
interneuron’s and modify their activity. Thus
these tracts influence the stretch reflex.
Mainly the extrpyramidal tract is responsible
for maintaining the tone.

14. • It consists of Basal ganglia motor nuclei of
reticular formation of brainstem, vestibular
nuclei and descending fibers converging
impulses to spinal cord.

15. • Normal muscle tone is due to continuous
impulses send by facilitatory areas to spinal
motor neurons (i.e. α and r motor neurons).

17. Decerebrate rigidity
• When the brain stem is transected
at the intercollicular level i.e. between the
superior and inferior colliculus there is
increase in the muscle tone and this rigidity
produced is called as decerebrate rigidity

18. References
1. Text book of Medical Physiology
-Guyton & Hall, 12th edition.
2. Ganong’s review of Medical Physiology
- 23rd edition.
3. Text book of Medical Physiology
- 2nd edition
4. Net sources ( Acknowledge for all online source)
5. Text book of Medical Physiology
- Prof. A.K.Jain