Stretch Reflex Components Quiz

Stretch Reflex

• The stretch reflex is a -ve feedback reflex that returns the muscle to its normal length when stretched
• It has four components: receptor, afferent, center, and efferent

Components of Stretch Reflex

• Receptor: Muscle spindle (central portion)
• Afferents: Thick myelinated fibers (primary Ia & secondary II)
• Center: Alpha motor neuron in spinal cord (AHCs)
• Efferent: Thick myelinated A α fibers
• Response: Muscle contraction

Muscle Spindles

• They are encapsulated mechanoreceptors
• They monitor muscle length or rate of change of muscle length
• They are spindle shaped and encapsulated
• Each muscle contains many spindles, distributed throughout the belly of the muscle

Structure of Muscle Spindles

• Each muscle spindle contains 2-10 intrafusal muscle fibers
• There are two types of intrafusal fibers: nuclear bag (1-3) and nuclear chain (3-9)
• The central part of the intrafusal fibers is the receptor area, which detects stretch of the muscle
• The peripheral part of the intrafusal fibers is contractile and receives output from γ fibers from the spinal cord

Types of Muscle Spindles

• Nuclear bag fibers: have a dilated central area filled with nuclei
• Nuclear chain fibers: have multiple nuclei arranged as a chain in the receptor area

Innervation of Muscle Spindle

• Type Ia nerve fibers: arise from the receptor areas of both nuclear bag and chain muscle fibers
• Type II nerve fibers: arise from the sides of the primary endings in the nuclear chain fibers
• Gamma efferent nerves: supply the peripheral (contractile) parts of the intra-fusal fibers

Alpha Gamma Linkage

• Alpha motor neurons are activated simultaneously with gamma motor neurons
• The role of gamma efferent co-activation is to prevent relaxation of muscle spindles during muscle contraction
• It increases sensitivity to the stretch reflex