Muscle Spindles

Muscle Spindles


MUSCLE SPINDLES

  • Are “Stretch receptor” located within a muscle.
  • Yet found, intermingling with muscle fibers.
  • ‘Intrafusal fibers’ – Specialized muscle fibers seen within spindle 
  • (Note: – Fusus refers to spindle).
  • Distinguished from fibers of muscle itself (“Extrafusal fibers”) 

DIVISIONS OF INTRAFUSAL FIBERS:

  • Central/equatorial part.
  • Peripheral part.

Central/equatorial region:

  • Does not contain any contractile protein.
  • Hence, non-contractile portion of muscle spindle.
  • Are more sensitive to stretch & acts as “Stretch receptors”.

Peripheral region:

  • Contain contractile proteins.
  • Hence, contractile part of muscle spindle.
TYPES OF INTRAFUSAL FIBERS:
  • 2 types

1. Nuclear bag fibers:

  • Have central bulge due to presence of nuclei bunch.
  • Usually, per spindle.

Types:

Nuclear bag fiber 1 – 

  • With low level of myosin ATPase activity.

Nuclear bag fiber 2 – 

  • With high level of myosin ATPase activity.

2. Nuclear chain fibers:

  • Are thinner & shorter
  • Do not have central bulge.
  • Four or more fibers per spindle (more than nuclear bag fibers)

RATE OF MUSCLE SPINDLE CONCENTRATION:

  • Greater muscle spindles concentration seen with,
  • Muscles with more refined function.

Eg:

  • Lumbrical muscles of hand.
  • Suboccipital muscles.
  • Extraocular muscles.

INNERVATION OF MUSCLE SPINDLE:

1. Sensory/afferent innervation:

  • Two afferent fiber types originate intrafusally, 

Annulospiral/primary endings:

  • Found wound around central region of both nuclear bag & nuclear chain fiber.
  • Are “Aα”/”Ia” fibers.
  • Flower-spray/secondary endings:
  • Innervate peripheral parts/ends of nuclear chain fiber.
  • Are “Aβ”/”II” fibers.

2. Motor/efferent innervation:

  • Innervates peripheral parts of both nuclear bag & nuclear chain fibers.
  • Mainly by “Aγ” type/”γ-motor neurons”/”Fusimotor neuron”/”Small motor nerve system of Lekcell efferents”.
  • Because of their characteristic smaller size.
  • Aγ fibers alter spindle sensitivity to stretch.

Types of γ fibers:

  • 2 types – 

γ1/dynamic γ-fibers:

  • Innervates nuclear bag fiber.
  • Their terminals are called ‘Plate ending’.
  • Alter dynamic sensitivity of stretch reflex.

γ2/static γ-fibers:

  • Innervates nuclear chain fibers.
  • Their endings are called ‘Trail ending‘.
  • Alter static sensitivity of stretch reflex.

TYPES OF STRETCH RESPONSES: 

  • On intrafusal fiber stretching,
  • 1° & 2° afferents respond differently. 
  • Two types of responses
  • Static & Dynamic response.

Dynamic response:

  • With afferent originating from nuclear bag fibers 

Static response:

  • With afferent originating from nuclear chain fibers.
  • Seen with flower-spray/secondary endings with II (Aβ) fibers.

Both static & dynamic response:

  • Annulospiral/primary endings with Ia (Aα) fibers
  • Since origination is from both nuclear bag & chain fibers.
Exam Question
 

MUSCLE SPINDLE

  • Are “Stretch receptor” located within muscle.
  • ‘Intrafusal fibers’ – Specialized muscle fibers seen within spindle .
TYPES OF INTRAFUSAL FIBERS:

1. Nuclear bag fibers – 

2 per spindle.

Nuclear bag fiber 1 – 

  • With low level of myosin ATPase activity.

Nuclear bag fiber 2 – 

  • With high level of myosin ATPase activity.

2. Nuclear chain fibers:

  • Are thinner & shorter.
  • Four or more fibers per spindle (more than nuclear bag fibers).

DIVISIONS OF INTRAFUSAL FIBERS:

Central/equatorial region:

  • Does not contain contractile protein.
  • Hence, non-contractile portion of muscle spindle.
  • Are more sensitive to stretch & acts as “Stretch receptors”.

Peripheral region:

  • Contain contractile proteins.
  • Hence, contractile part of muscle spindle.

Greater concentration of spindles found in –

  • Lumbrical muscles of hand.
  • Suboccipital muscles.
  • Extraocular muscles.

Two types of sensory/afferent innervation of muscle spindle:

Annulospiral/primary endings:

  • Found wound around central region of both nuclear bag & nuclear chain fiber.
  • Are “Aα” / “Ia” fibers.

Flower-spray/secondary endings:

  • Innervates peripheral parts/ends of nuclear chain fiber.
  • Are “Aβ” / “II” fibers.

Motor innervation/efferent of muscle spindle:

  • Innervates peripheral parts of both nuclear bag & nuclear chain fibers.
  • Mainly by “Aγ” type.
  • Also referred “γ-motor neurons”/”Fusimotor neuron” /”Small motor nerve system of Lekcell efferents”.
  • Because of their characteristic smaller size.

γ1/dynamic γ-fibers:

  • Innervates nuclear bag fiber.
  • Their terminals are called ‘Plate ending’.

γ2/static γ-fibers:

  • Innervates nuclear chain fibers.
  • Their endings are called ‘Trail ending’.
Don’t Forget to Solve all the previous Year Question asked on Muscle Spindles

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