Muscle spindles are proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath (spindle). They run parallel to the extrafusal muscle fibers and act as receptors that provide information on muscle length and the rate of change in muscle length. The spindles are stretched when the muscle lengthens. This stretch causes the sensory neuron in the spindle to transmit an impulse to the spinal cord, where it synapses with alpha motor neurons. This causes activation of motor neurons that innervate the muscle. The muscle spindles determine the amount of contraction necessary to overcome a given resistance. When the resistance increases, the muscle is stretched further, and this causes spindle fibers to activate a greater muscle contraction.

1. MUSCLE SPINDLE & MUSCLE
TONE
FATIMA WAHID
MANGRIO

2. PROPRICEPTORS
• Proprioceptors are specialized sensory receptors
that are located within joints, muscles, and
tendons.
• Proprioceptors provide Proprioceptive sensations
• Proprioceptive sensations allow us to recognize
that parts of our body belong to us (self).
• They also allow us to know where our head and
limbs are located and how they are moving even if
we are not looking at them, so that we can walk,
type, or dress without using our eyes.

3. • Proprioception, also referred to as Kinesthesia
(perception of body movements).
• Proprioceptors also allow weight
discrimination, the ability to assess the weight
of an object. This type of information helps you
to determine the muscular effort necessary to
perform a task.
• For example, as you pick up a shopping bag,
you quickly realize whether it contains books
or feathers, and you then exert the correct
amount of effort needed to lift it.

4. PROPRIOCEPTORS
1. Muscle Spindles
Provide sensation about muscle length.
2. Tendon organs
Provide sensation about muscle tension.
3. Joint kinesthetic receptors
Provide sensation about muscle tension Joint
position and movement.

5. Muscle spindles
• Muscle spindles are skeletal muscle sensory
receptors within the body of a muscle that
primarily detect changes in the length of this
muscle contributing to fine motor control and
providing position information to the central
nervous system. The responses of muscle
spindles to changes in length also play an
important role in regulating the contraction of
muscles, by activating motor neurons via
the stretch reflex to resist muscle stretch.

6. FUNCTIONS OF MUSCLE SPINDLE
• Muscle spindle gives response to change in
the length of the muscle. It detects how much
the muscle is being stretched and sends this
information to central nervous system (CNS)
via sensory nerve fibers. The information is
processed in CNS to determine the position
of different parts of the body. By detecting the
change in length of the muscle, the spindle
plays an important role in preventing the
overstretching of the muscles.

7. STRUCTURE OF MUSCLE SPINDLE
• Each muscle spindle is formed by intrafusal
muscle fibers. All these fibers are enclosed by a
capsule, which is formed by connective tissue.
• The capsule is attached to either side of
extrafusal fibers or the tendon of the muscle.
Thus, intrafusal fibers are placed parallel to the
extrafusal fibers. Intrafusal fibers are thin and
striated

8. • Central portion of the intrafusal fibers
does not contract because it has only few
or no actin and myosin filaments. So, this
portion acts only as a receptor.
• Only the end portion of intrafusal fibers
can contract.
• The discharge from gamma motor
neurons causes the contraction of
intrafusal fibers.

9. Types of Intrafusal Fibers
• Muscle spindle is formed by two types of
intrafusal fibers:
1. Nuclear bag fiber
Central portion of this fiber is enlarged like a bag
and contains many nuclei. Hence, it is called the
nuclear bag fiber.
2. Nuclear chain fiber
In nuclear chain fiber, central portion is not
bulged and the nuclei are arranged in the center in
the form of a chain.

10. MUSCLE SPINDLE

11. NERVE SUPPLY TO MUSCLE SPINDLE
• Muscle spindle is innervated by both sensory and
motor nerves. It is the only receptor in the body,
which has both sensory and motor nerve supply.

12. Sensory Nerve Supply
1. Primary sensory nerve fiber
Primary sensory nerve fiber belongs to type Iα
(Aα) nerve fiber. Each sensory (afferent) nerve
fiber has two branches. One of the branches
supplies the central portion of nuclear bag fiber.
The other branch ends in central portion of the
nuclear chain fiber. These branches end in the form
of rings around central portion of nuclear bag and
nuclear chain fibers. Therefore, these nerve endings
are called annulospiral endings.

13. Nerve supply to muscle spindle

14. 2. Secondary sensory nerve fiber
• Secondary sensory nerve fiber is a type II
(Aβ) nerve fiber. It innervates only the
nuclear chain fiber and ends near the end
portion of nuclear chain fiber like the petals
of the flower. So, this nerve ending is called
flower spray ending.

15. Motor Nerve Supply
• Motor (efferent) nerve fiber supplying the
muscle spindle belongs to gamma motor
neuron (Aγ) type.
Motor nerve supply to nuclear bag fiber
• Gamma motor nerve fiber supplying the
nuclear bag fiber ends as motor end plate.
This nerve ending is called plate ending.

16. Motor nerve supply to nuclear chain fiber
• Gamma motor nerve fiber supplying the
nuclear chain fiber divides into many
branches, which form a network called
trail ending.

17. Muscle spindle has two functions:
1. It forms the receptor organ for stretch
reflex.
2. It plays an important role in
maintaining muscle tone.

21. The Stretch Reflex
• A stretch reflex causes contraction of a skeletal
muscle (the effector) in response to stretching of
the muscle. This type of reflex occurs via a
monosynaptic reflex arc. The reflex can occur by
activation of a single sensory neuron that forms
one synapse in the CNS with a single motor
neuron. Stretch reflexes can be elicited by tapping
on tendons attached to muscles at the elbow,
wrist, knee, and ankle joints. An example of a
stretch reflex is the patellar reflex (knee jerk).

22. 1 Slight stretching of a muscle stimulates
sensory receptors in the muscle called
muscle spindles. The spindles monitor
changes in the length of the muscle.
2 In response to being stretched, a muscle
spindle generates one or more nerve
impulses that propagate along a somatic
sensory neuron through the posterior root of
the spinal nerve and into the spinal cord.

23. 3 In the spinal cord (integrating center), the
sensory neuron makes an excitatory synapse
with, and thereby activates, a motor neuron in
the anterior gray horn.
4 If the excitation is strong enough, one or more
nerve impulses arises in the motor neuron and
propagates, along its axon,which extends from
the spinal cord into the anterior root and
through peripheral nerves to the stimulated
muscle. The axon terminals of the motor
neuron form neuromuscular junctions (NMJs)
with skeletal muscle fibers of the stretched
muscle.

24. 5 Acetylcholine released by nerve
impulses at the NMJs triggers one or
more muscle action potentials in the
stretched muscle (effector), and the
muscle contracts. Thus, muscle stretch
is followed by muscle contraction,
which relieves the stretching

26. MUSCLE TONE
Muscle tone is defined as the state of
continuous and passive partial contraction
of muscle with certain vigor and tension. It
is also called tonus.

27. MAINTENANCE OF MUSCLE TONE
In Skeletal Muscle
• Maintenance of tone in skeletal muscle is
neurogenic. It is due to continuous
discharge of impulses from gamma motor
neurons in anterior gray horn of spinal cord.
The gamma motor neurons in spinal cord
are controlled by higher centers in brain

28. In Cardiac Muscle
• In cardiac muscle, maintenance of tone is
purely myogenic, i.e. the muscles
themselves control the tone. The tone is
not under nervous control in cardiac
muscle.
In Smooth Muscle
• In smooth muscle, tone is myogenic. It
depends upon calcium level and number
of cross bridges.

29. Significance of Muscle Tone
• Muscle tone plays an important role in
maintenance of posture. Change in muscle
tone enables movement of different parts of
the body. Muscle tone is present in all the
skeletal muscles. However, tone is more in
antigravity muscles such as extensors of lower
limb, trunk muscles and neck muscles.

30. Development of Muscle Tone
• Gamma motor neurons and muscle spindle
are responsible for the development and
maintenance of muscle tone. Muscle tone is
purely a reflex process. This reflex is a
spinal segmental reflex. It is developed by
continual synchronous discharge of motor
impulses from the gamma motor neurons
present in the anterior gray horn of the spinal
cord

31. Sequence of events
1. Impulses from the gamma motor neurons
cause contraction of end portions of intrafusal
fibers (stimulus)
2. This stretches and activates the central
portion of the intrafusal fibers, which initiates
the reflex action for development of muscle
tone by discharging the impulses

32. 3. Impulses from the central portion of intrafusal
fibers pass through primary sensory nerve
fibers (afferent fibers) and reach the anterior
gray horn of spinal cord.
4. These impulses stimulate the alpha motor
neurons in anterior gray horn (center).
5. Alpha motor neurons in turn, send impulses
to extrafusal fibers of the muscle through spinal
nerve fibers (efferent fibers)

33. 6. These impulses produce partial
contraction of the muscle fibers resulting in
development of muscle tone (response).
When the frequency of discharge from
gamma motor neurons increases, the
activity of muscle spindle is increased and
muscle tone also increases. Stimulation of
gamma motor neurons increases the muscle
tone. Lesion in gamma motor neurons leads
to loss of tone in muscles.

36. Regulation of Muscle Tone
• Though the muscle tone is developed by
discharges from gamma motor neurons, it is
maintained continuously and regulated by
some supraspinal centers situated in different
parts of brain. Some of these centers increase
the muscle tone by sending facilitatory
impulses while other centers decrease the
muscle tone by inhibitory impulses.

37. • Supraspinal facilitatory centers
Supraspinal centers, which increase the muscle
tone:
1. Motor area 4 in cerebral cortex
2. Cerebellum
3. Descending facilitatory reticular system
4. Red nucleus
5. Vestibular nucleus.

38. • Supraspinal inhibitory centers
Supraspinal centers, which decrease the
muscle tone:
1. Suppressor areas of cerebral cortex
2. Basal ganglia
3. Descending inhibitory reticular system.

39. Role of motor area of cerebral cortex –
coactivation
• Motor area of cerebral cortex influences the
activity of lower motor neurons by sending motor
impulses through the pyramidal tract fibers. Motor
impulses from cerebral cortex stimulate both
αmotor neurons and γ motor neurons
simultaneously. This type of simultaneous
stimulation is called coactivation. It is also called
α-γ coactivation.

40. • Stimulation of α-motor neurons causes
contraction of extrafusal fibers. Stimulation
of γ-motor neurons causes contraction of
intrafusal fibers, which leads to increase in
muscle tone.

41. Role of cerebellum and basal ganglia
• It is interesting to find that cerebellum
and basal ganglia influence the muscle
tone without sending direct fibers to γ
motor neurons. These parts of brain
influence the muscle tone indirectly
through brainstem centers.

42. Role of brainstem centers
• Brainstem centers which influence the γ
motor neurons are in reticular formation,
red nucleus and vestibular nucleus. These
centers modulate the discharge from γ
motor neurons by receiving signals from
cerebral cortex, cerebellum and basal
ganglia.