Skeletal Muscle Control • Draw or describe the control pathway from Sensory Information to response for different types of reflexes. • apparatus detects • Explain the components and mechanism of muscle length (stretch) action for spindles fibers and Golgi organs. • Golgi tendon organs detect tension the • State the role of different CNS integrating muscle exerts on its tendons (stretch centers in control of and contraction) function.

Muscle Spindle Apparatus

• Contains thin muscle cells: intrafusal fibers: • (Other muscle fibers are called extrafusal) • Contractile apparatus absent in center • Stretching a muscle causes spindles to stretch, stimulating sensory

Motor Neurons

• Alpha: neurons that innervate extrafusal fibers; cause muscle contraction • Gamma:innervate the intrafusal fibers; cause muscle spindle fibers to shorten

1 Muscle spindles and the simple stretch reflex

A sensory connected to a muscle spindle detects muscle stretch. A somatic alpha causes muscle twitch. alpha, gamma coactivation

Muscle spindles ¾ specialized muscle fibers (intrafusal fibers) + + sensory endings Reflex ¾ monitor rate and changes in muscle fiber length ¾ stretching of muscle spindles causes reflex stimulation causing agonist muscle fibers to contract and resist stretch ¾ functions to oppose sudden changes in muscle length • ‘involuntary motor acts’ - not originated Golgi tendon organs from cortex ¾ sensory receptors in muscle tendons • (but can be modulated by cortex) ¾ respond to tension, stretch or contraction ¾ stimulation causes reflex inhibition of agonist muscles and • rapidly executed stimulation of antagonist muscles • stereotyped ¾ function as an inhibitory reflex for injury prevention and for fine motor control

THE STRETCH REFLEX Reflex - Examples (KNEE JERK REFLEX) • stretch reflex (Ex/knee jerk reflex) • TAP PATELLAR TENDON BELOW KNEE • Golgi tendon r. (inverse myotatic reflex) • SENSORY SIGNALS FROM MUSCLE SPINDLE INDICATE FLEXION AT KNEE • withdrawal reflex • FLEXOR IS INHIBITED • crossed extensor reflex • EXTENSOR ACTIVATED • control of posture • LEG EXTENDS (typically, agonists and antagonists coordinated)

2 THE INVERSE MYOTATIC REFLEX • DETECTION OF EXCESS WEIGHT BY GOGLI TENDON ORGAN

• INHIBIT AGONIST MUSCLE

• STIMULATE ANTAGONIST

INVERSE MYOTATIC REFLEX INVERSE MYOTATIC REFLEX

WEIGHT TOO ARM FLEXED ARM EXTENDS HEAVY

BICEPS & SYNERGYSTIC MUSCLE WEIGHT BICEPS & SYNERGYSTIC MUSCLE CONTRACTED EXTENSOR RELAXED RELEASED RELAXEDEXTENSOR CONTRACTED TENDON STRETCHED

3 THE FLEXOR WITHDRAWAL REFLEX Flexor • PAINFUL STIMULUS DETECTED • IPSILATERAL EXTENSORS INHIBITED withdrawal • IPSILATERAL FLEXORS EXCITED • LIMB IS WITHDRAWN reflex

THE CROSSED EXTENSOR REFLEX • PAINFUL STIMULUS DETECTED • IPSILATERAL EXTENSORS INHIBITED • IPSILATERAL FLEXORS EXCITED • LIMB IS WITHDRAWN

• * WHILE STANDING, CONTRALATERAL SIDE EXTENSORS ACTIVATED AND FLEXORS INHIBITED TO SUPPORT WEIGHT

4