Corticospinal Tract & Corticonuclear (Corticobulbar) Fibers Study Guide
Corticospinal Tract
Two neuron pathways:
UMN – pyramidal cells of cerebral cortex
LMN – neurons in anterior horn of spinal cord
Neurotransmitter:
Glutamate (+)
Function:
Lateral
Voluntary motor control of extremities
* Fractionation
Particularly important in fine control of digits of hand
Anterior
Voluntary motor control of axial musculature
Origin
Cerebral Cortex
Primary motor (Both Corticospinal tracts), Premotor (Ant Corticospinal tract
only)
Course
Cerebral Cortex (primary motor and/or premotor cortex)  corona radiata & posterior limb of
internal capsule  cerebral peduncles of midbrain  anterior pons  pyramids of medulla 
lateral funiculus of SC (lateral corticospinal tract) or anterior funiculus of SC (anterior
corticospinal tract)  lower motor neurons in ventral horn of SC
Termination
Spinal Cord (primarily laminae VI-IX)
Corticonuclear (Corticobulbar) Fibers
Two neuron pathway:
UMN – cells of cerebral cortex
LMN – motor cranial nerve nuclei in brainstem
Fibers influence motor nuclei of oculomotor, trochlear, trigeminal, abducens,
facial, glossopharyngeal, vagus, accessory, & hypoglossal nerves
Function
Controls muscles of face, head & neck
Initiation and control of voluntary & fine movements
Neurotransmitter
Glutamate (+)
Innervated by the cranial motor nuclei (V, VII, XII), nucleus ambiguus (IX and X) & by
accessory nucleus
DOES NOT innervate nuclei for nerves III, IV, and VI
Fibers do NOT project to the oculomotor, trochlear, and abducens DIRECTLY
(from frontal eye fields to terminate in mesencephalon [sup colliculus] and then
relay input to the motor nuclei
Origin
Precentral gyrus - somatomotor cortex, area 4 (primary motor cortex)
Frontal eye fields - areas 6 and 8
Postcentral gyrus - areas 3, 1, 2 (some)
Course
Precentral gyrus  descends through corona radiata  genu of internal capsule (few fibers in
the posterior limb of the internal capsule)  crus cerebri  brainstem
Major Structures
Precentral gyrus, corona radiate, genu of internal capsule, posterior limb of internal capsule,
cerebral peduncles (midbrain), brainstem (medulla & pons), cranial nerve nuclei
Termination
Rostral Interstitial Nucleus of the MLF
Paramedian Pontine Reticular Formation
Precentral gyrus
Brainstem motor nuclei: CN V (bilateral), CN VII (contralateral), CN XII
(contralateral tongue)
Nucleus Ambiguus (bilateral): CN IX, CN X (soft palate, pharynx, larynx)
Accessory Nucleus (bilateral): CN XI
Upper half of the facial motor nucleus receives BILATERAL corticonuclear
projections
Lower half of the facial motor nucleus receives only CONTRALATERAL
projections