Download (H-reflex). - Human Kinetics

Lecture 8:
Monosynaptic Reflexes
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)
A Scheme of a Reflex
Central
processing
unit
Efferent
nerve
Muscle
Afferent
nerve
Receptor
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).
Reflex Latency
Central processing unit
Efferent
nerve
∆T c
∆T e
Afferent
nerve
Components of the reflex
latency:
 Afferent conduction delay
∆T a
 Central processing delay
Muscle
spindle
Muscle
 Efferent conduction delay
Reaction
∆T a + ∆T c + ∆T e
Stim
Time
Latency
Monosynaptic Reflexes
a-motoneuron
Efferent
nerve
Muscle
Ia afferents
Muscle
spindle
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.
A Scheme for H-Reflex Experiments
Central processing unit
Stim
Efferent
nerve
Muscle
Afferent
nerve
Muscle
spindle
A scheme of
experiments with an
electrical stimulation
of a muscle nerve.
Note that the
stimulus is applied to
both afferent and
efferent fibers.
H-Reflex and M-Response (I)
EMG
Time
St
EMG
H-reflex
Time
St
EMG
H-reflex
M-response
Time
St
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 (II)
EMG
H-reflex
M-response
Time
St
EMG
M-response
H-reflex
Time
St
EMG
M-response
Time
St
Further increase in
the strength of the
stimulation leads to
an increase in the
M-response and
suppression of the
H-reflex.
Changes in the Amplitude of the H-Reflex and
M-Response With the Amplitude of the Stimulus
AH,M
M
H
AST
Threshold
This figure shows how the
peak-to-peak amplitude of
the H-reflex and the Mresponse depends on the
strength of the stimulation
applied to a muscle nerve
(AST). Note the
nonmonotonic H-curve
and a monotonic increase
in the M-response.
AP Collision
a-motoneuron
Orthodromic
action potential
Axon
hillock
Antidromic
action potential
Afferent
fiber
Efferent
fiber
When an afferent fiber
delivers a presynaptic action
potential to an a-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.
Effects of High Frequency
Stimulation on H-Reflex
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EMG
H-reflex
M response
EMG
EMG
St1
St2
Time
Time
Time
St3
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.
Tendon Tap (T-Reflex)
a-motoneuron
Tap
Muscle
Spindle
Tendon
EMG
T-reflex
Time
Tap
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.
H-Reflex (and T-Reflex)
Under Voluntary Muscle Activation
Voluntary
activation
Efferent nerve
Muscle
EMG
a-motoneuron
Ia afferents
Muscle spindle
M response H-reflex
Without
voluntary
activation
St
With
voluntary
activation
St
Time
Voluntary muscle
activation increases
the amplitude of the
H-reflex in the
activated muscle
through an
excitation of the
motoneuronal pool.
Monosynaptic Reflexes in Humans
H-reflex:
 Electrical stimulation of Ia afferents
 Excitation of alpha-MNs through a central synapse
 Efferent command to the target muscle
 Twitch muscle contraction
T-reflex:
 Fast stretch of a muscle, leading to activation
of primary muscle spindle afferents
 Then same as H-reflex
F-Wave
a-motoneuron
Antidromic
action potential
Orthodromic
action potential
Stim
Muscle
EMG
M-response
F-wave
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.