Reflex:  Common misnomer: an involuntary reaction to an external stimulus  Monosynaptic (one central synapse)  Oligosynaptic (a few central synapses; usually, 2 to 3)  Polysynaptic (many central synapses)  Tonic (slow, steady-state, maintained)  Phasic (fast, transient, in response to a change in the stimulus) Lecture 8: Monosynaptic Reflexes

A reflex arc consists of a sensory element (receptor), an afferent (sensory) nerve, a central processing unit, an efferent (command) nerve, and an effector (for example, a muscle). A Scheme of a Reflex Afferent nerve Efferent nerve Receptor Muscle Central processing unit

Components of the reflex latency: Reflex Latency Afferent nerve Efferent nerve Muscle spindle Muscle Central processing unit Reaction Time Stim Latency ∆T a e c a + ∆T c e  Afferent conduction delay  Central processing delay  Efferent conduction delay

Monosynaptic reflexes involve one central synapse. In humans, they originate from Ia spindle afferents and induce responses in the same muscle or in muscles in the vicinity. Monosynaptic Reflexes Ia afferents Efferent nerve Muscle spindle Muscle  -motoneuron

A scheme of experiments with an electrical stimulation of a muscle nerve. Note that the stimulus is applied to both afferent and efferent fibers. A Scheme for H-Reflex Experiments Afferent nerve Efferent nerve Muscle spindle Muscle Central processing unit Stim

Afferent fibers are the first to react to a slowly increasing electrical stimulus. They induce a reflex muscle contraction (H-reflex). Later, efferent fibers become excited and induce a direct muscle contraction (M-response). H-Reflex and M-Response (I) St EMG Time St EMG Time H-reflex St EMG Time H-reflex M-response

Further increase in the strength of the stimulation leads to an increase in the M-response and suppression of the H-reflex. H-Reflex and M-Response (II) St EMG Time H-reflex M-response St EMG Time H-reflex M-response St EMG Time M-response

A H,M A ST Threshold H M This figure shows how the peak-to-peak amplitude of the H-reflex and the M- response depends on the strength of the stimulation applied to a muscle nerve (A ST ). Note the nonmonotonic H-curve and a monotonic increase in the M-response. Changes in the Amplitude of the H-Reflex and M-Response With the Amplitude of the Stimulus

When an afferent fiber delivers a presynaptic action potential to an  -motoneuron whose axon hillock has just responded to an antidromic efferent action potential, the motoneuron is unable to generate another efferent action potential because of the refractory period. AP Collision  -motoneuron Axon hillock Orthodromic action potential Efferent fiber Antidromic action potential Afferent fiber

Successive stimuli at a high frequency induce similar M responses but progressively smaller H-reflexes. Time scales are certainly different in the lower graphs as compared to the upper panel. Effects of High Frequency Stimulation on H-Reflex EMG Time H-reflex M response EMG Time EMG Time St 1 St 2 St 3

A tendon tap excites spindle endings and may induce a monosynaptic reflex contraction (T-reflex). Its reflex pathway is the same as for the H-reflex. Tendon Tap (T-Reflex) EMG Time T-reflex Tap Tendon Muscle Spindle  -motoneuron

Voluntary muscle activation increases the amplitude of the H-reflex in the activated muscle through an excitation of the motoneuronal pool. H-Reflex (and T-Reflex) Under Voluntary Muscle Activation Ia afferents Efferent nerve Muscle spindle Muscle  -motoneuron Voluntary activation Time St H-reflex M response EMG St Without voluntary activation With voluntary activation

Monosynaptic Reflexes in Humans  Electrical stimulation of Ia afferents  Excitation of alpha-MNs through a central synapse  Efferent command to the target muscle  Twitch muscle contraction  Fast stretch of a muscle, leading to activation of primary muscle spindle afferents  Then same as H-reflex H-reflex: T-reflex:

An antidromic action potential in an efferent fiber, induced by an electrical stimulus, can induce an orthodromic action potential, leading to a muscle contraction called an F-wave. F-Wave Muscle  -motoneuron F-wave M-response EMG Stim Antidromic action potential Orthodromic action potential