spinal cord

Muscle Sensory Receptors
?Proper control of muscle function requires:
?Excitation by anterior motor neurons
?Continuous sensory feedback information from each muscle – muscle status - that is:
A.What is the length of the muscle?
B.What is its tension?
C.How rapidly length or tension changing?
?Muscle spindles & Golgi tendon organs.
Length-Monitoring System & Stretch Reflex
?Changes in muscle length monitored by stretch receptors embedded in them.
?2 receptor types respond to:
•How much the muscle stretched.
•Magnitude of stretch and speed with which it occurs.
?Receptors consist of nerve fiber endings wrapped around modified muscle fibers.
Length-Monitoring System & Stretch Reflex
?Several muscle fibers enclosed in a connective tissue capsule “muscle spindle”.
?Each spindle is 3-10 mm long.
?Build around 3-12 very small modified intrafusal fibers attached to glycocalyx of surrounding large "extrafusal fiber“.
Intrafusal Skeletal Muscle Fibers
?Small skeletal muscle fiber
I.Central region - no or few actin and myosin filaments ? does not contract when ends do (Function as sensory receptors).
II.End portions that do contract - excited by small gamma motor neurons.
?2 types
Intrafusal Skeletal Muscle Fibers
(1)Nuclear-bag fibers
?1-3/spindle
?Central portion - Large number of nuclei congregated in an expanded bag.
?Innervated by primary (Ia) nerve endings.
Intrafusal Skeletal Muscle Fibers
(2) Nuclear-chain fibers
?3-9/spindle.
?1/2 in diameter & 1/2 as long as first type.
?Nuclei aligned in a chain.
?Innervation: 1ry & 2ndary nerve endings (II).
?Ends connected to nuclear-bag fibers.