54 
Northcutt 
obviously patterned with regard to both on- 
set and duration of contraction. 
The primary pathways, excluding the de- 
scending vestibulo-spinal paths, linking the 
brain to the spinal cord are the descending 
reticular pathways (Fig. 20). Cerebellar 
efferents (Senn and Goodman, 1969), tectal 
efferents (Foster and Hall, 1975), and tel- 
encephalic efferents (see telencephalic sec- 
tion for summary of studies) do not ter- 
minate on cranial nerve motor nuclei or 
motor neurons of the spinal cord. All of 
these higher neural centers funnel into the 
reticular formation of the brainstem. This 
pattern of connectivity places the reticular 
brainstem in an ideal position to function 
as a set of pattern generators. The reticular 
formation consists of both ipsilateral and 
contralateral pathways projecting to most 
portions of the spinal cord (Ten Donkelaar, 
1975; Cruce, Newman, and Stefi, 1976). Such 
reticular neurons, projecting over wide seg- 
ments of the spinal cord and motor nuclei 
of the medulla, could produce a wide range 
of output signals depending on the temporal 
and spatial nature of the arriving afferent 
signals from such diverse neural centers as 
the cerebellum, tectum, hypothalamus, and 
telencephalon. Such an hypothesis is con- 
sistent with electrical stimulation studies, as 
well as with available anatomical informa- 
tion. The majority of reported stimulation 
sites from which recognizable motor be- 
haviors can be stimulated are found within 
the reticular core or its derivatives (Distel, 
1973; Sugarman, 1974; Kennedy, 1975). 
In mammals the cerebellum can be di- 
vided into three longitudinal zones : a medial 
or vermal zone, primarily related to axial 
muscles; an intermediate zone, coordinating 
whole limb movements; and a lateral zone, 
primarily concerned with movements involv- 
ing distal appendicular muscles. It is this 
lateral cerebellar zone with its well-devel- 
oped isocortical channels (brachium conjunc- 
tivum, cortico-olivo-cerebellar, and cortico- 
ponto-cerebellar pathways) that becomes 
greatly developed in the cerebellum of mam- 
mals. 
Goodman (1964) has identified vermal and 
intermediate cerebellar zones in Caiman, but 
believes that lateral cerebellar zones exist 
only in birds and mammals. To date, neither 
pontine nuclei nor telencephalo-bulbo-cere- 
bellar pathways have been identified in rep- 
tiles, suggesting that behaviors such as nest 
building, limb manipulation of prey, and 
social signaling are motor events coordinated 
primarily at cerebellar and reticular levels 
with little, if any, direct cortical control. 
The nature of telencephalic involvement in 
motor sequences is one of the many unan- 
swered questions regarding CNS organiza- 
tion in reptiles. Hoogland (1975) reported 
a direct pathway from the striatum to the 
lateral cerebellar nucleus in Tupinamhis. At 
present, this pathway and the strio-bulbar 
pathways are the only known direct connec- 
tions with the medulla. While lizards with 
complete telencephalic ablations do not feed, 
they show no obvious locomotor deficits 
(Goldby, 1937). 
While the striatum is traditionally thought 
of as a motor center, it may possess a far 
more important role as a sensory integration 
center related to species-typical behaviors 
(Greenberg, 1977). The striatum of rep- 
tiles receives inputs from midbrain auditory 
and visual centers (torus and tectum) via 
thalamic relays, and probably receives so- 
matic input as it does in other tetrapods. In 
mammals the striatal neurons are known to 
be multimodal (Laursen, 1963), as is likely 
the case in reptiles. Such multimodal units 
might serve as a filter to facilitate or inhibit 
species-typical behaviors organized at the 
reticular level. The possibility of such a 
striatal role is particularly attractive con- 
sidering the midbrain afferents to the stri- 
atum and its position intermediate to the 
sensory areas of the dorsal ventricular ridge. 
The mammalian striatum may possess a sim- 
ilar function as suggested by the studies of 
MacLean (1972). 
The midbrain roof areas of vertebrates 
possess auditory, visual, and probably 
somatotopic maps that are organized to per- 
form sensory elicited orienting movements. 
In this sense the midbrain roof consists of a 
series of centers that function to tell an 
