Neuroanatomy Lecture 19

Spinal Reflexes

Neuronal Circuit for the Stretch Reflex

three types of neurons involved, two main muscles (agonist doing most of moving and antagonist), a type of deep tendon reflex

Stretch Reflex Step 1

monosynaptic activation of the agonist muscle

Stretch Reflex Step 2

disynaptic inhibition of the antagonist muscle, slight delay because traveling, synapses onto an inhibitory interneuron

Summary of Stretch Reflex

primary sensory afferent (Ia) from muscle spindle in agonist muscle excites the alpha motor neuron for the agonist muscle which causes that muscle to contract and excites the inhibitory interneuron which inhibits alpha motor neuron for the antagonist muscle and that muscle relaxes

Feedback Correction for an Increased Load

when there is an increase in the pull on a muscle, the stretch reflex allows the muscles to adjust so they can maintain length (agonist muscles contracts)

Feedback Correction for a Decreased Load

when there is a decrease in the pull on a muscle, the stretch reflex allows the muscles to adjust so they don’t stretch and the agonist muscle is more relaxed

Alpha-gamma Co-Activation: Adjusting Muscle Spindle Sensitivity

when a muscle gets shorter during contraction, gamma motor neurons contract intrafusal fibers in muscle spindles with a normal signal from Ia afferents, or alpha motor neuron is stimulated and the muscle contracts so there is slack but no signal from Ia afferent

Stretch Reflex Postural Control

surface suddenly moves backwards, stretch reflex corrects posture, alpha and gamma motor neurons active

Golgi Tendon Reflex

activation of Ib fibers in the GTO excites Ib inhibitory interneurons in the spinal cord and these interneurons inhibit the alpha motor neurons to the same muscle, causing it to relax

Functions of the Golgi Tendon Reflex

helps resist increased muscle tension, protects muscles from injury when the stretch on them is too great

Basic Withdrawal Reflex

helps move limb away from a painful or dangerous stimulus, A gamma nociceptive sensory afferents innervate interneurons in spinal cord that excite ipsilateral flexor motor neurons and inhibit ipsilateral extensor motor neurons, this can be suppressed by descending control of the interneurons

Crossed-Extensor Reflex

helps move limb away from a painful or dangerous stimulus while keeping upright, signals sent to both ipsilateral and contralateral extensors and flexors, no activation of motor neurons, contract hamstring and prevent quad from being activated