- 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.
- Does not contain any contractile protein.
- Hence, non-contractile portion of muscle spindle.
- Are more sensitive to stretch & acts as “Stretch receptors”.
- Contain contractile proteins.
- Hence, contractile part of muscle spindle.
- 2 types
1. Nuclear bag fibers:
- Have central bulge due to presence of nuclei bunch.
- Usually, 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
- 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.
- Lumbrical muscles of hand.
- Suboccipital muscles.
- Extraocular muscles.
INNERVATION OF MUSCLE SPINDLE:
1. Sensory/afferent innervation:
- Two afferent fiber types originate intrafusally,
- 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 –
- Innervates nuclear bag fiber.
- Their terminals are called ‘Plate ending’.
- Alter dynamic sensitivity of stretch reflex.
- 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.
- With afferent originating from nuclear bag fibers
- 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.