Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 1 of 6
MUSCLES REFLEXES & SPINAL REFLEXES
CRITICAL FACTS
1.
The final common pathway in the motor system is the α motoneuron,
whose output is regulated by spinal reflexes, which are, in turn,
modified by descending control. The actions of spinal reflexes are
often revealed when there are upper motor neuron lesions.
2.
Muscle spindles are responsible for the stretch reflex, where a muscle contracts in
response to moderate stretch. Coactivation of α and γ motorneurons maintains
activity in the spindle and tone in the muscle. Golgi tendon organs initiate the
inverse stretch reflex, where strong contraction is followed by muscle relaxation.
Relaxation occurs because GTO afferents activate an inhibitory interneuron.
3.
Muscle spindles and Golgi tendon organs have opposing effects on the α motor
neurons. The lengthening reaction, which combines the two responses of the two
receptor types, can be observed clinically under hypertonic conditions as the “clasp
knife reflex”.
ESSENTIAL MATERIAL FROM OTHER LECTURES
1.
Skin and muscle receptors (Dr. Downing, Skin/Musculoskeletal System)
2.
Sensory physiology (Neurological Medicine)
LECTURE OBJECTIVES
At the end of this lecture, you should be able to:
1.
List the sequence of events that occurs during the stretch reflex, and outline the role of coactivation of alpha and gamma motoneurons in maintaining the sensitivity of spindle afferents.
Explain the role of gamma motorneurons in maintaining muscle tone.
2.
Describe the role of Golgi tendon organs in the inverse stretch reflex. Define the lengthening
reaction, and the physiology of the circuit that explains why it is typically observed clinically under
hypertonic conditions.
3.
List the components of a reflex arc. Identify ways that a reflex can be modulated, and understand
(in general terms) how adding or removing descending inputs can change reflex functions
(particularly the stretch reflex).
4.
Understand the physiology of the flexor withdrawal and crossed extension reflexes. Be able to
distinguish between recurrent and reciprocal inhibition.
Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 2 of 6
MUSCLE RECEPTORS
Anatomy Review
Muscle spindles are responsible for the stretch reflex, where a muscle
contracts in response to moderate stretch. Coactivation of alpha- and
gamma- motorneurons maintains activity in the spindle and tone in
the muscle. Golgi tendon organs initiate the inverse stretch reflex,
where strong contraction is followed by muscle relaxation. Relaxation
occurs because GTO afferents activate an inhibitory interneuron.
Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Physiology of muscle spindles
Physiology of Golgi tendon organs
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 3 of 6
Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
SPINAL REFLEXES
Final Common Pathway
Definitions
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 4 of 6
Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Lengthening reaction
Withdrawal reflex
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 5 of 6
Med 6573 Neurological Medicine
Dr. Janet Fitzakerley (jfitzake@d.umn.edu)
http://www.d.umn.edu/~jfitzake/Lectures/Teaching.html
Spinal Integration
Spring 2013
Muscle Receptors & Spinal Reflexes
Page 6 of 6