Structure Of Muscle Spindle Mechanism of Stretch Reflex Functions of Stretch reflex Negative Stretch Reflex Inverse Stretch Reflex

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

2. Specific Learning Objectives
• Structure Of Muscle Spindle
• Mechanism of Stretch Reflex
• Functions of Stretch reflex
• Negative Stretch Reflex
• Inverse Stretch Reflex

3. Muscle Spindle

6. • It is a stretch receptor or Mechanoreceptor present in
every skeletal muscle. It is fusiform in shape so called
as muscle spindle. Every skeletal muscle contains
variable number of muscle spindles. These spindles are
found in the belly of the muscle. There are many
muscle spindles in the belly. Small muscles of the hand
which do the delicate jobs and the antigravity muscles
have more number of muscle spindles. The individual
muscle fibre of the belly is called as Extrafusal fibre.
•

7. • Each spindle contains a few (3-10) delicate
striated muscle fibre called intrafusal fibres
within the fibrous capsule of the spindle. The
intrafusal fibres are attached with the fibrous
capsule of the spindle and the capsule of the
spindle in turn is attached with the extrafusal
fibres or tendon of the muscle.

8. • The muscle spindle is tapering at its both ends
which are called the polar regions and the broad
central region is called equatorial zone.
• Polar region contains actin and myosin filaments.
Equatorial zone is a receptor portion ,there are
no actin and myosin filaments.
• The intrafusal fibres lie parallel to the extrafusal
fibres.
•

9. • The intrafusal fibres are of two types-
• Nuclear bag fibres.
• Nuclear chain fibres.

10. • Nuclear bag fibres:-
• These fibres extend from one end to other
end of the muscle spindle. There are 1-3 nuclear
bag fibres.At the equatorial zone or central
portion the nuclear bag fibres contains large
number of nuclei hence they are called as nuclear
bag fibres.This portion does not contain
contractile elements i.e. actin and myosin but it is
the sensory portion.
•

11. • Nuclear chain fibres:-
• There are 3-9 nuclear chain fibres.They
are attached with nuclear bag fibres at its
polar regions. They do not have central bag of
nuclei but the nuclei are aligned in a chain
throughout the polar regions hence they are
called as Nuclear chain fibres

12. • Muscle spindles are the stretch receptors. They are
stimulated when the central receptor portion of the
intrafusal fibre is stretched .Therefore muscle spindles are
stimulated when
•
• The entire muscle is stretched. The intrafusal fibres are
parallel to extrafusal fibres.Capsule of the spindle is
attached to the extrafusal fibre.Therefore when the entire
muscle i.e. the extrafusal fibres are passively stretched the
stretch is transmitted to the intrafusal fibres. This leads to
the stimulation of Ia fibres which causes stimulation of Aα
motor neurons supplying the extrafusal fibres.This causes
reflex contraction of extrafusal fibres.This is called as
stretch reflex or Myotatic reflex.

13. • Gamma motor fibres are stimulated ,the
intrafusal muscle fibres contract shortening
occurs at the polar ends but there is stretching
of the equatorial zone ---->stimulation of Ia
fibres which causes stimulation of Aα motor
neurons. This causes reflex contraction of the
extrafusal fibres
• (Stretch Reflex) --->shortening of main
muscle belly`.

15. • Conversely when the extrafusal fibres shorten due to
contraction the nuclear bag relaxes and the Ia stops to fire
.This leads to relaxation of extrafusal fibres.
•
• (Muscle spindle thus is the length (of the extrafusal fibre)
detecting device. Stretch reflex maintains the constancy of
the length of the extrafusal fibres.Thus muscle spindle is a
homeostatic organ).
•
• Thus the simplest manifestation of muscle spindle function
is the muscle stretch reflex also called as Myotatic reflex.
•

16. Myotatic reflex or stretch reflex:-

18. • 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.
•

21. • The stretching of the nuclear bag region is of two
types
•
• Sudden or dynamic:-Impulses are carried by Ia
afferent.
•
•
• Sustained or Static:-Impulses are carried by
group II afferent fibres.
•

22. • Negative stretch reflex:-
• When the muscle is suddenly
shortened exactly opposite effects occur
because of decreased nerve impulses from the
spindles. There is reflex inhibition of the
muscle .There is dynamic as well as static
reflex causing such muscle inhibition. This is
called as Negative stretch Reflex.
•

23. Functions of stretch Reflex:-
1. Muscle spindle acts as length detector and
comparator . when the length of the muscle
spindle is more the muscle spindle is
stimulated and the extrafusal fibres contracts.
2. Important function of the stretch reflex is its
ability to prevent oscillation or jerkiness of
body movements. This is a damping or
smoothing function of muscle spindle.

24. 3. When the signals are transmitted from the motor
cortex to the α motor neurons, the r motor
neurons are also stimulated simultaneously. This
is called as α, r co- activation. This causes both
extrafusal fibres and intrafusal muscle fibres to
contract at the same time. This causes stretching
of the nuclear bag region and keeps the muscle
fibre sensitive for stretch reflex.
4. Maintenance of tone:-Tone is a partial sustained
state of muscle contraction at rest. This is the
action of Myotatic reflex.

25. 5 Maintenance of posture :-Static component of
the myotatic reflex is the fundamental postural
mechanism.eg When a person is standing
upright gravity tends to stretch the quadriceps
muscle .This stretching elicits stretch reflex
resulting in sustained contraction as long as
stretch is there. This maintains extension around
the knee joint and upright posture.

26. Applied
• Hypotonia: - The r motor fibres receive
impulses from the descending fibres from the
brain. Damage to the descending fibres leads
to alteration of r motor activity and decreased
tone in the muscle called as hypotonia.
•

27. • Hypertonia: - Means increase in the tone of the
muscle. When the animal is made decerebrate by
taking a section between superior and inferior
colliculus there is total loss of communication
between cerebral hemisphere and brain stem.
This removes major inhibitory influence on r
motor neuron. This causes the over activity of the
facilitatory areas causing stimulation of the r
efferent and the muscle tone increases.

28. Golgi tendon reflex or lengthening
reaction OR inverse stretch reflex

29. • When the muscle tension increases, signals from
Golgi tendon organ are transmitted through
group Ib fibres to the spinal cord. These signals
excite the inhibitory interneuron’s which is in turn
inhibit the α motor neuron which inhibit the
muscle from which impulses have originated.
Thus Golgi tendon organ causes reflex inhibition
i.e. Relaxation of the muscle. This is Golgi tendon
reflex or lengthening reaction OR inverse stretch
reflex.
•

31. • Function:-
• It is an inhibitory reflex, protective in
nature. It prevents excessive contraction of
muscle and there by prevents the
development of too much tension on the
muscle. It prevents the tearing of the muscle
or avulsion of the tendon from its attachment
to the bone.

32. 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