Spinal Cord

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:
What is the length of the muscle?
What is its tension?
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.

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
Central region - no or few actin and myosin filaments ? does not contract when ends do (Function as sensory receptors).
End portions that do contract - excited by small gamma motor neurons.
2 types