Spatial and temporal summation and occlusion can be utilized to illustrate the interactions between spinal cord neurons and reflex circuits. To assess the reflex excitability of a group of α motor neurons, a monosynaptic reflex discharge can be evoked by stimulating the group Ia afferent fibers in a muscle nerve. By choosing the appropriate muscle nerve, the discharges of either extensor or flexor α motor neurons can be recorded. To examine changes in the response to group Ia stimulation, other types of afferent fibers are stimulated along with the homonymous group Ia afferent fibers from the muscle. Inhibition of the response to homonymous group Ia stimulation can be observed when group Ia afferent fibers in the nerve to the antagonist muscles are stimulated, mediated by the reciprocal group Ia inhibitory interneuron.

Another example involves stimulating the small afferent fibers of a cutaneous nerve to elicit a flexion reflex. This leads to inhibition of the responses to group Ia stimulation in the α motor neurons innervating extensor muscles, while the responses in α motor neurons innervating flexor muscles are potentiated.

Stimulation of a ventral root results in the inhibition of group Ia responses and the reciprocal group Ia inhibition. This suggests the presence of axon collaterals within the ventral root that excite inhibitory interneurons, which then provide feedback to the same motor neuron population known as Renshaw cells. Ventral root stimulation uniquely inhibits reciprocal group Ia interneurons, while activating group Ia stimulation. These experiments have contributed to a comprehensive understanding of the spinal cord circuitry.

Descending motor pathways have traditionally been classified into pyramidal and extrapyramidal pathways. This terminology stems from the clinical differentiation between pyramidal tract disease and extrapyramidal disease. Pyramidal tract disease involves the interruption of the corticospinal (pyramidal) tract. Although the signs of this disease were initially attributed to the loss of pyramidal tract function, in many cases, the functions of other pathways are also affected. Most signs of pyramidal tract disease can be attributed to alterations in other pathways as well.
The phenomena of spatial and temporal summation and occlusion can also be used to demonstrate interactions between spinal cord neurons and the various reflex circuits. To start, a monosynaptic reflex discharge can be evoked by stimulation of the group Ia afferent fibers in a muscle nerve. This is a test of the reflex excitability of a population of α motor neurons. The discharges of either extensor or flexor α motor neurons can be recorded if the proper muscle nerve to be stimulated is chosen. Other kinds of afferent fibers •Fig. 9.11 Spatial Facilitation. A, Arrangementforusingelectricallyevokedafferentvolleysandrecordingsfrommotoraxonsinaventralroottostudyreflexes.B, Experimentinwhichcombinedstimulationofafferentfibersintwomusclenervesresultedinspatialsummation(RAandRB).Thedischargezones(pink areas) encloseα motorneuronsthatareactivatedabovethresholdwheneachnervebranchisstimulatedseparately.InC, thecombinedvolleyscausedocclusion.(RedrawnfromEyzaguirreC,FidoneSJ.Physiology of the Nervous System. 2nded.Chicago:Mosby–YearBook;1975.) are then stimulated along with the homonymous group Ia afferent fibers from the muscle to demonstrate whether the response to the group Ia stimulation changes. For example, stimulation of group Ia afferent fibers in the nerve to the antagonist muscles produces inhibition of the response to the homonymous group Ia stimulation (which is mediated by the reciprocal group Ia inhibitory interneuron described previously).
As another example, if the small afferent fibers of a cutaneous nerve are stimulated to evoke a flexion reflex, the responses to group Ia stimulation of the α motor neurons that innervate the extensor muscles are inhibited (and those of α motor neurons that innervate flexor muscles are potentiated).
As a final example, stimulation of a ventral root causes inhibition of group Ia responses and inhibits the reciprocal group Ia inhibition. Because the ventral root contains only motor neuron axons, this result implies the presence of axon collaterals that excite inhibitory interneurons that feed back onto the same motor neuron population (
Fig. 9.12 ). These interneurons are named Renshaw cells. Because ventral root stimulation also inhibits the group Ia inhibition of antagonist motor neurons, but no other classes of interneurons, the reciprocal group Ia interneurons are uniquely inhibited by ventral root stimulation (and activated by group Ia stimulation). Experiments like these described have been used to provide a detailed knowledge of the circuitry of the spinal cord.
•Fig. 9.12 Renshaw Cell (RC) Connections With Motor Neurons and Group Ia Inhibitory Interneurons. ThecircuitsshownmediategroupIareciprocalinhibitionofantagonistmuscles(inthiscase,anextensor)andinhibitionofthisreciprocalinhibitionbyRenshawcells.NotethatequivalentnumbersofRenshawcellsandgroupIainhibitoryinterneuronsareassociatedwithextensormotorneuronsandgroupIainputfromspindlesinextensormuscles,buttheyarenotshownforsimplicity.Orange cells areinhibitory,andblue andgreen cells areexcitatory.
Classification of Descending Motor Pathways
Descending motor pathways were traditionally subdivided into pyramidal and extrapyramidal pathways. This terminology reflects a clinical dichotomy between pyramidal tract disease and extrapyramidal disease. In pyramidal tract disease, the corticospinal (pyramidal) tract is interrupted. The signs of this disease were originally attributed to the loss of function of the pyramidal tract (so named because the corticospinal tract passes through the medullary pyramid). However, in many cases of pyramidal tract disease, the functions of other pathways are also altered, and most signs of pyramidal tract disease (see the later section “