SPINAL REFLEXES

                                                                                                                                                                                © 2006 zillmusom

I. DEFINITION AND OVERVIEW OF REFLEXES

 

A. Definition of a reflex - stereotyped motor response to a specific sensory stimulus.

 

            B. Typical reflex arc consists of:

 

                        1. sensory receptor - detects stimulus (termed: afferent arm of reflex)

                        2. interneurons - receive inputs from sensory receptors and synapse on motor neurons; effects on motor neurons can be excitatory or inhibitory; not present in monosynaptic reflexes

                        3. motor neurons - (termed efferent arm of reflex) produce muscle contraction, motor response. 

 

Note: Reflexes often have effects in groups of motor neurons to different muscles; sometimes at different joints in same limb or in opposite limb. 

 

C. Reflexes are valuable tools for clinical evaluation of nervous system function.  For reflex to occur, all elements must be functional and pathways must be intact.  If reflexes are absent, physician can diagnose where pathway is interrupted; if reflexes are abnormal, can diagnose where function is compromised. Reflexes are evaluated according to 1) amount (size, magnitude) of motor response and 2) latency (time to elicit motor response); in some disease processes, damage can enhance motor responses

 

 

D. Some reflexes are protective and relatively constant; ex. Pupillary light reflex - Light shone in eye causes pupil to constrict; sensory neurons are photoreceptors in retina; afferent signals in Optic Nerve (Cranial Nerve II); efferent signals in Oculomotor Nerve (Cranial Nerve III); functions to limit amount of light, protects photoreceptors in retina. 

 

E. Other reflexes are relatively constant under same controlled circumstances; ex. Monosynaptic stretch reflex - can be elicited from a variety of skeletal muscles; response is consistent if patient is relaxed.  Reflexes can be modulated by the central nervous system - reflexes can be changed or blocked in some behaviors. 

 

            F. Functions of many reflexes are complexReflexes can be incorporated as elements into automatic reactions. Examples: 1) maintaining balance when standing and walking, 2) regulation of muscle tensions, 3) avoiding painful stimulus (stepping on a nail). 

 

            G. Other 'reflexes' actually represent triggering of more complex behaviors by sensory signals.  Some behaviors are produced by activities in groups of interneurons in the CNS that form pattern generating circuits (ex. stepping 'reflexes' in infants).

 

II. THREE CLASSIC SPINAL REFLEXES - Each reflex has a specific sensory stimulus and motor response

 

Note: Terminology - In describing a reflex:

 

            Homonymous muscle - muscle that contains or is associated directly with sense organ producing the reflex

            Synergist muscle - muscle that produces a similar motor action (movement)

            Antagonist muscle - muscle that produces the opposite motor action (movement)

            Contralateral muscle - muscle of opposite limb (leg or arm).

 

            A. Stretch reflex (also termed: Myotatic Reflex, Deep Tendon Reflex)

 

                        1. Stimulus - fast stretch of muscle; clinically this is produced by a brief sharp tap to a muscle tendon (this results in sudden small lengthening of muscle, not in stimulation of tendon receptors).

 

                        2. Sense organ excited - this strongly excites muscle spindle Primary (Group Ia) afferents; can also produce much weaker discharges of muscle spindle  Secondary  (Group II) afferents.  

 

                        3. Primary response - muscle that is stretched contracts rapidly

 

                                    a. Synapses - Group Ia muscle spindle sensory neurons make strong monosynaptic excitatory connections with alpha (a) motor neurons of homonymous muscle (same muscle in which spindle is located).   Group II muscle spindle sensory neurons have 1) weaker monosynaptic and 2) stronger polysynaptic (through interneurons) excitatory effects on same motor neurons. 

 

Note: Monosynaptic reflex is the fastest reflex known with a delay of about 1 msec at the synapse. 

 

                        4. Other effects

 

                                    a. Excite synergist muscles - muscle spindle afferents also make excitatory monosynaptic connections with synergist muscles (ex. in arm - biceps spindle sensory neurons excite motor neurons to brachialis muscle). 

 

                                    b. Inhibit antagonist muscles (RECIPROCAL INHIBITION) - Spindle sensory neurons also produce inhibition of motor neurons to antagonist muscles (ex. biceps spindle neurons produce inhibition of triceps motor neurons); these connections are polysynaptic. The spindle afferent excites interneurons, which then fire and produce inhibitory synaptic potentials in motor neurons to the antagonist muscle.          

 

                        5. Muscle Tonus - The ongoing activity in muscle spindles is important in maintaining the desired activity of motor neurons to muscles (because the connection is monosynaptic). Eliminating the sensory activity can cause a decrease in muscle tonus (measured by resistance to slow stretch of the muscle).  Increased activity can increase muscle tonus.

 

                        6. Clinical Testing of stretch reflex - A rapid tap to the tendon produces a very quick, small stretch of the muscle.    Most, if not all of the spindles in the muscle, are excited simultaneously, producing a discharge of sensory neurons that act convergently upon the motor neurons resulting in a brief and rapid muscle twitch.

 

                        7. Modification of reflexes - Even monosynaptic reflexes can be changed.

Reflexes can be altered by mechanisms of 1) pre-synaptic inhibition (decrease effectiveness of spindle sensory discharges) and 2) modulation of motor neuron activities.  Some of these changes are produced by activities in neurons of descending motor tracts. Changes in stretch reflexes are also symptomatic: In general, Decrease stretch reflexes can indicate Lower Motor Neuron Disorders, Increase Stretch reflexes can indicate Upper Motor Neuron Syndromes.

 

                        8. Renshaw cells - Alpha motor neurons have recurrent processes (axon collaterals); these branches synapse in the central nervous system; some branches make excitatory synapses upon interneurons (Renshaw cells).  Renshaw cells make inhibitory synapses upon the same motor neurons.  These circuits can limit motor neuron firing and the extent of inhibition can change reflexes.  Renshaw cells receive inputs from descending motor tracts. 

 

                        9. Functions of stretch reflex - Countering perturbations of balance when standing (for example: maintaining balance when standing on one foot or when standing on a moving bus). Perturbations (or fatigue) can produce stretch of muscles, generating discharge of muscle spindle afferents. Reflex connections cause contraction of the stretched muscle and synergist muscles and inhibit antagonist muscles to aid in bringing the body back to the original position. These types of circuits are called Negative feedback systems: the stimulus causes a response that acts to decrease the stimulus (stretch causes shortening of muscle, which decreases Ia discharge). The same mechanism can work in any motor action if unexpected loads are encountered that produce stretch of muscles. While stretch reflexes contribute to these responses,   Postural reactions differ from reflexes in that they are 1) longer in duration than stretch reflexes, 2) also involve contractions of muscles not in the limb (ex. extensor muscles of the back), 3) can be adjusted by other sensory inputs (ex. wearing a back pack).  

 

            B. Autogenic Inhibition (also termed: Inverse Myotatic Reflex, Tendon Organ Reflex or Clasped-Knife Reflex)

 

                        1. Stimulus - large force exerted by pulling on muscle tendon (muscle is strongly contracted)

 

                        2. Sense organ excited - Golgi tendon organ (Ib afferent)

 

                        3. Primary response - muscle attached to tendon relaxes

 

                                    a. Synapses - polysynaptic; Ib afferent makes an excitatory synapse upon an interneuron; the interneuron makes an inhibitory synapse upon the motor neuron from the same muscle in which the tendon organ is located.

 

                        4. Other effects

 

                                    a. Inhibits synergist muscles - synaptic connections are also polysynaptic; the GTO sensory neuron makes an excitatory synapse upon an interneuron that inhibits motor neurons to the synergist muscles.

 

                                    b. Excites antagonist muscles - synaptic connections are also polysynaptic; the GTO afferent makes an excitatory synapse upon an interneuron that excites motor neurons to the antagonist muscles.

           

                        5. Function of Autogenic inhibition - Regulating muscle tensions - The force developed by contractions of muscles are automatically controlled so that they do not cause damage to tendons (example: lifting a very heavy object). 

 

Note: The connections for autogenic inhibition are inactivated during walking; Effects of Golgi tendon organs then become excitatory (through other interneurons). 

 

                        6. Clinical significance - Clasped knife reflex: In Upper Motor neuron lesions, tonus may increase and resistance of muscle to stretch increases; if sufficient force is applied, limb resistance suddenly decreases (like a pocket knife snapping shut); thought to be mediated by reflexes of Golgi tendon organs. 

 

            C. Flexor reflex - excitation of flexor motor neurons to withdraw from painful or noxious stimulus; can take different forms (exciting muscles with other actions, ex. abductor muscles that pull limb away from midline)

 

                        1. Stimulus - noxious or painful stimulus to skin

 

                        2. Sense organs excited - Cutaneous afferents, pain receptors (nociceptors)

 

                        3. Primary response - protective withdrawal of limb (often by exciting flexor muscles)

 

                                    a. Synapses - polysynaptic; cutaneous afferents make excitatory synapses upon interneurons; the interneurons (one or more in pathway) make excitatory synapses upon motor neurons to flexor muscles.

 

                        4. Other effects

 

                                    a. Excite synergist muscles (polysynaptic) - Cutaneous, pain afferents also make excitatory synapses upon interneurons that excite motor neurons to other flexor muscles in the same limb (often at different joints).

 

                                    b. Inhibit antagonist muscles (polysynaptic) - Cutaneous, pain afferents make excitatory synapses upon interneurons that inhibit motor neurons to extensor muscles in the same limb.

           

                                    c. Crossed Extension reflex - Flexor reflexes can also have effects in the contralateral leg in standing.  These effects are opposite those seen in the same leg (called opposite sign of reflex); connections via commissural interneurons excite extensor motor neurons and inhibit flexor motor neurons of muscles in opposite leg. 

 

                        5. Function of flexor reflexes - Protective (example: stepping on a nail).  The net effect of these connections is that very rapid adjustments are made so that one leg is lifted rapidly and the other supports the weight of the body. 

 

                        6. Clinical - Flexor Reflexes can change after lesions, disease processes; ex. Babinski sign - seen after Upper Motor neuron lesion; normal response - stroking sole of foot normally results in flexion of toes (not strictly a withdrawal reflex) ; Babinski sign - direction of movement changes from flexing toes, to extending toes. 

 

III. PATTERN GENERATORS IN SPINAL CORD AND BRAINSTEM

 

            A.  Spinal cord contains networks of interneurons that generate patterned motor activities (networks are called Pattern Generators). 

 

            B. ex. Walking - Walking is thought to be produced by pattern generators. In animals, rhythmic activities can be recorded in ventral roots similar to that seen in walking, even after complete removal of spinal cord (ex. lampreys, neonatal rats, cats).   After spinal cord lesion, rear limbs of animals and legs of humans can walk on treadmills (if body weight is supported).  

 

                        1.  There is thought to be a pattern generator for walking movements of each leg; the pattern generator produces leg lifting (swing) and leg placement (stance) in a single limb;  pattern generators for right and left legs have inhibitory connections (MUTUAL INHIBITION) so that when one leg is lifted, other leg remains on ground. 

 

                        2. Stepping reflexes in infants probably represent activation of pattern generator for walking.

 

SUMMARY OF SPINAL REFLEXES

                                                                                                                                                                               

REFLEX

STIMULUS (CLINICAL TEST)

RESPONSE

SENSORY RECEPTOR

SYNAPSES

EFFECT ON MUSCLE

OTHER EFFECTS

FUNCTION

Stretch (Myotatic) Reflex

Rapid Stretch of muscle (test: tap on muscle tendon)

 

Stretched muscle contracts rapidly (ex. knee jerk)

Muscle Spindle Primary (Ia) and Secondary (II) sensory neurons

 

Ia: Mono-synaptic

 

II: Mono-synaptic  (weak) and Poly-synaptic

Excite Homon-ymous (same muscle)

Also Excite synergist muscles; Inhibit antagonist muscles (Reciprocal Inhibition)

Aid in maintaining posture, counter sudden loads

Autogenic Inhibition (Inverse Myotatic Reflex)

Large force on tendon (pull on muscle when resisted)

 

Muscle tension decreases (Clasped knife reflex)

Golgi Tendon Organ (Ib)

Poly-synaptic (via interneuron)

 

Inhibit Homon-ymous (same muscle)

Also Inhibit synergist muscles; Excite antagonist muscles

Protective, prevent damage to tendon

Flexor Reflex

Sharp, painful stimulus (as in stepping on nail)

Limb is rapidly withdrawn from stimulus

Cutaneous (skin) and pain receptors

Poly-synaptic (via interneuron)

 

Excite Flexor muscle

Also Inhibit extensor muscle of same limb; Excite extensor  muscles and Inhibit flexors of opposite limb (Crossed Extensor Reflex)

Protective, withdraw from painful stimulus; Cross extension aids in maintaining posture when leg is lifted