The H-reflex
Named after Paul Hoffman who in 1926 discovered that stimulation of nerves can result in a reflex contraction
with low stimulus intensities, Ia fibers are first to be activated - why?
 since they are larger they have lower threshold
 at high stimulus intensities H-reflex disappears

The H-reflex
with increased stimulation alpha MNs fibers are activated - this produces a direct muscle response (M wave)
 H-reflex disappears because antidromic firing of motor fibers makes MNs refractory to Ia & antidromic motor volley collides with orthodromic reflex volley
- antidromic - towards
- orthodromic - away
this evidence indicates axons can transmit bidirectionally

Reflex pathways from Ia afferents - excitatory input
Ia fibers provide homonymous (autogenic) excitation to MNs (motor neurons) innvervating parent muscle and
heteronymous (heterogenic) excitatation to MNs supplying other muscles
homonymous Ia input is > than other inputs
muscles other than mechanical synergists may receive heteronymous excitation from Ia fibers

Reflex pathways from Ia afferents - disynaptic inhibition
Ia afferents produce disynaptic inhibition of antagonists MNs
Ia inhibitory postsynaptic potential (IPSP) lags behinds the Ia excititory postsynaptic potential (EPSP) by 0.8 ms.  This is evidence that the signal must synapse with an interneuron before synapsing with the antagonist MN - (ex. of an oligosynaptic reflex- 2-3 synapses)

Reflex pathways from Ia afferents - disynaptic inhibition
a motor nucleus receives Ia disynaptic inhibition primarily from antagonists - known as - reciprocal inhibition
spinal interneuron imposed between the Ia and MN is - Ia inhibitory interneuron
Ia inhibitory interneuron receives multiple inputs - corticospinal tract, FRAs cutaneous, Ia and Ia inhibitory input from antagonists

Reflex pathways from Ia afferents - disynaptic inhibition
a functional unit in the spinal cord is formed between the alpha (a), gamma (y), and Ia inhibitory interneurons
inputs to the unit produce a-y coactivation and reciprocal inhibition of antagonists
What are the advantages of having such a ìhard-wiredî reflex pathway in the spinal cord? (page 136)

Reflex pathways from Ib afferents
Evidence for Ib inhibition -
when a stimulus applied to dorsal root of synergist reached a particular level, investigators detected an inhibition in agonist 0.5-1 ms after the Ia facilitation - this was attributed to disynaptic inhibition produced by Ib neurons (GTOs) - ex. oligosynaptic reflex
stimulation of antagonist nerves produced facilitation of agonist - inverse myotatic reflex

Reflex pathways from Ib afferents
Ib projections more extensive than Ia
projections may span more than one joint
Ib interneurons facilitated by low threshold cutaneous and joint afferents - functional role? (page 139)

Group II afferents and FRAs
stimulation of group II fibers produces excitation of flexors & inhibition of extensors regardless of which muscles are stimulated
evidence suggests that more than one reflex pathway for group II fibers and which pathway is facilitated may be ëselectedí by supraspinal inputs

group II afferents and FRAs
group II muscle afferents often function as part of a larger reflex system - flexor reflex afferent (FRA) system
stimulation of group II & III muscle afferents, cutaneous & joint afferents produce ipsilateral flexor excitation & ipsilateral extensor inhibition
weaker, opposite effects are observed contralaterally - functional role?

group II afferents and FRAs
stimulation of receptors that are part of FRA system do not always result in the ëclassicí response - FRA system appears to be one that can be utilized as a functional unit when the circumstances call for the behavior

Renshaw cells
Motoneuronal pool - all alpha & gamma MNs that invervate a particular muscle

Renshaw cells are interneurons that are responsible for recurrent inhibition - postsynaptic inhibition of a motoneuronal pool
Renshaw cells
Renshaw cells receive monosynaptic input from alpha motor neuron axon collaterals and the activation of the Renshaw cell monosynaptically inhibits the MN pool

this is a negative feedback system - the more excited the MNs the more inhibited they are

Renshaw cells
Renshaw cells are subject to cortical & spinal influences - what is functional value of this?

Reflexes
Phasic reflexes - short duration in response to change in the level of stimulation to a particular receptor - all monosynaptic reflexes are phasic

Tonic reflexes - are long lasting (i.e. > than phasic) - can lead to sustained contraction or inhibition - always polysnaptic
Reflexes
Tonic vibration reflex - vibration can drive primary afferents - driving is when an action potential is induced in response to every cycle of the stimulus

when a muscle is vibrated it produces a tonic contraction

Reflexes
TVR is unique for a variety of reasons
1) subjects can consciously inhibit the TVR

2) monosynaptic reflexes are inhibited during TRV - monosynaptic inputs are inhibited presynaptically but polysynaptic inputs remain excitatory - hence tonic muscle contraction

Reflexes
3) muscles not subject to vibration display reflex responses (responses can be intersegmental)

4) vibration produces illusions - why

Long latency stretch reflexes
stretch of actively contracting muscle produces both a tendon jerk reflex & a long latency response (LLR)
LLR has a latency about twice as long as M1 (tendon jerk) response & may last up to 100 ms
a.k.a. M2 response

Long latency stretch reflexes
M2 is probably associated with a transcortical pathway
patients with lesions at any point in the hypothesized transcortical pathway (dorsal columns, sensorimotor cortex, corticospinal tract) have absent or reduced amplitude M2 responses although M1 remain intact.
an alternative explanation involves supraspinal facilitation of polysynaptic spinal pathways

Tonic vibration reflexes
vibration at 50-150 Hz produces a slow developing reflex contraction of the vibrated muscle which is sustained throughout the vibration.
reflex pathway probably involves both mono and polysynaptic pathways
TVR seems to require supraspinal facilitation since no reflex is observed in paraplegics with complete spinal transection.

TVR
TVR can inhibit both monosynaptic reflex & H-reflex
inhibitory mechanisms may include presynaptic inhibition of Ia terminals and
post-activation depression of transmitter release.  Thus, when an H-reflex stimulus is given during vibration less transmitter is available for release - smaller response

The Servo Hypothesis
proposed by Merton in 1953 - offered an explanation for role of gamma MNs
proposed as a control mode for slow movements
steps involved in generating slow movements
 - activation of gamma MNs
 - contraction of intrafusal muscle fibers
 - stretch of sensory endings

Servo hypothesis
 - discharge of spindle afferents
 - discharge of alpha MNs
 - limb motion
advantage of this control mode would be to maintain a relationship between length of extrafusal fibers & sensory region of spindle

Servo hypothesis
problems with the servo hypothesis -
 - it was hypothesized that spindle discharge   must always begin prior to extrafusal     electrical activity - evidence is that spindle discharge always begins after muscle activity
for a servo mechanism to function effectively ìgainî of the reflex must be high

Servo hypothesis
What is high gain?
 - the output of the system is proportionally higher than the input - ex. for a small stretch of the spindle there is a large muscle force produced
evidence indicates that the gain of the stretch reflex is fairly low, thus disturbances of a limb during a movement cannot be effectively corrected through the servo loop

Alpha-gamma coactivation
a-g coactivation suggests that both a & g are activated together
Figure 6.6 (p. 166) shows that during a muscle contraction the spindle is also firing, this, despite the fact the muscle is shortening
this should only happen if the gammas are firing causing the contraction of the intrafusal fibers keeping the sensory region taut

a-g coactivation
what is functional role? - by keeping sensory region taut the stretch reflex loop can correct for movement disturbances.
however, the gain of the stretch reflex is low thus ability to compensate may be limited.
the gain of the stretch reflex for small disturbances is greater than it is for large disturbances (around the limit of perception)

a-g coactivation
it is proposed that the stretch reflex can support voluntary movement because the reflex continues to operate during a contraction
During local anesthetic block voluntary strength is reduced.  The block interrupts the conduction of the gamma fibers thus impacting the stretch reflex.