Classification & anatomy of nerve fibers
cutaneous afferents - classification based on activation peaks in compound action potential
A fibers - myelinated, fast conducting
B fibers - myelinated, slower conducting
C fibers - unmyelinated, slowest conducting
in humans fastest conduction - 50-70 ms-1

Classification & anatomy of nerve fibers
Horseradish peroxidase (HRP) used to determine localization of Ia afferent terminals within spinal cord (SC)
after injection HRP is transported anterogradely and retrogradely
anterogradely - toward axon terminals
retrogradely - towards cell body

Classification & anatomy of nerve fibers
after entry into SC, axon splits into ascending and descending branches in dorsal column
every millimeter axon gives off collaterals which terminate into specific lamina in the dorsal horn of the SC (Figure 5.2, page 130)

Reflex pathways from Ia afferents - excititory input
reflex triggered by Ia electrical stimulation is monosnyaptic (one synapse)
evidence is that > stimulation of dorsal root > size of response in ventral root but doesn’t < latency of first response (1.5 ms)

Reflex pathways from Ia afferents - excititory input
Ia fibers provide homonymous excitation to MNs (motor neurons) innvervating parent muscle and
heteronymous 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 - (disynaptic inhibition)

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 “hard-wired” 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)
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 ‘selected’ 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 ‘classic’ response - FRA system appears to be one that can be utilized as a functional unit when the circumstances call for the behavior
FRA circuitry may aid in alternating stepping pattern seen in locomotion - how?

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

Presynaptic inhibition
Presynaptic inhibition is the result of axo-axonic synapses on the afferent fibers
Release of neurotransmitter (GABA) produces depolarization of the afferent terminal - results in decrease in number of quanta of transmitter released per nervous impulse