Lizard Descending Pathways 
117 
descending fibers to the MLF (Goldby and 
Robinson, 1961; Nieuwenhuys, 1964; Don- 
kelaar, 1976). Behavioral observations sug- 
gest that the MLF carries fibers mediating 
rhythmic respiratory movements of thoracic 
musculature (Ebbesson, 1967; Cruce, 1975). 
The MLF also continues as a large bundle 
below thoracic levels, but the function of 
these fibers is unknown. 
Lizard supraspinal pathways may influ- 
ence medial motoneurons either through di- 
rect monosynaptic connections (since the 
medial lamina IX group is filled with axon 
terminals) or through connections to inter- 
neurons in lamina VIII which impinge on 
medial motoneurons. The termination of de- 
scending pathways is almost bilaterally sym- 
metrical with respect to the medial moto- 
neurons, although this is not to imply that 
the terminals on each side are functionally 
equal. 
The behavioral effects of hemisection in 
the lizard are most dramatic in the trunk 
musculature (innervated by medial moto- 
neurons) : There is a loss of the ability to 
flex the trunk ipsilaterally. Thus the major 
net effect of fibers descending on one side 
of the spinal cord is to facilitate ipsilateral 
trunk flexion. This presumably occurs 
through a combination of facilitatory and 
inhibitory actions which reciprocally affect 
agonistic and antagonistic muscle groups on 
both sides of the body (since ipsilateral 
fibers project contralaterally as well as 
ipsilaterally). 
Such a pattern of descending synaptic 
activity would be ideal for initiating or 
facilitating a horizontal wave of flexion in 
the trunk, exactly as has been observed dur- 
ing normal locomotion (Snyder, 1952; Bel- 
lairs, 1969). 
Cord hemisection has little effect on limb 
movements; some effects which do appear 
(inability to support body weight) are prob- 
ably due to weakness in shoulder and hip 
musculature, which is more related to trunk 
abnormalities; other effects (passive drag- 
ging of limb) might be due to sensory losses 
caused by cutting the ascending sensory 
pathways. 
It has been hypothesized (Kuypers, 1973) 
that fibers in the dorsolateral funiculus 
(DLF) of mammals exert a descending con- 
trol of limb musculature through synapses 
on interneurons in laminae V-VIII which 
are involved in integrating the activity in 
lateral motoneurons. The sparse termina- 
tion of DLF fibers in the spinal gray of liz- 
ards correlates well with the minimal effects 
which lesions of the pathway have on coor- 
dinated limb movement. 
The lateral motoneurons still may come 
under supraspinal influence via the medial 
motoneuronal dendrite which extends into 
lamina VIII where it might receive terminals 
of medial descending fibers. It is also possi- 
ble that DLF pathways may influence the 
lateral motoneurons directly via synapses 
on their lateral dendrites, though these 
would probably be sparse. The lateral den- 
drite is much more likely to be influenced by 
dorsal root fibers which have profuse termi- 
nations in the lateral parts of laminae V, VI, 
and VII — exactly where the lateral moto- 
neuronal dendrite is located (Joseph and 
Whitlock, 1968; Cruce, 1978). Since fibers 
of the DLF pathway terminate in a region 
overlapping with part of the terminal field 
of dorsal root afferents, the possibility also 
exists that DLF fibers exert brainstem con- 
trol over incoming sensory activity at spinal 
levels. 
Recently, Shapovalov (1975) reported 
that, in the turtle, stimulation of the medul- 
lary reticular formation generates EPSP’s 
in lumbar lateral motoneurons, whereas 
stimulation of the red nucleus fails to gener- 
ate even polysynaptic EPSS’s. While this 
physiological evidence may not apply to the 
lizard, since turtles lack both trunk muscula- 
ture and any significant number of medial 
motoneurons (Ariens Kappers et al., 1936; 
Nieuwenhuys, 1964), it provides a parallel 
to the anatomical findings in the lizard that 
the medullary reticulospinal pathway could 
terminate on the proximal as well as distal 
dendrites of lateral motoneurons (laminae 
VII and VIII). It is far less likely that the ru- 
brospinal pathway can influence lateral moto- 
neurons to any great extent, both due to 
