Neuroanatomy Ch 6 224-240
Corticospinal Tract Anatomy
Motor Cortex, Sensory Cortex, and Somatotopic Organization –
The primary motor cortex is in the precentral gyrus just anterior to the central sulcus, while the
primary somatosensory cortex is in the postcentral gyrus behind the central sulcus
-just anterior to the primary motor cortex, two other areas exist for motor function: called the
premotor cortex and the supplemental motor area; both involved in higher motor planning
-just posterior to the primary somatosensory cortex, we have a somatosensory association
cortex in parietal lobe meant to process high-order sensory function
-primary motor and somatosensory cortices are somatotopically organized: regions on cortex
correspond to adjacent areas on body surface
-can be depicted by a motor homunculus and a sensory homunculus
Basic Anatomy of the Spinal Cord – cord contains central gray matter surrounded by ascending
and descending white matter columns
-DRG sensory neurons have axons that bifurcate: one branch conveys sensory info from
periphery and the other carries this info through dorsal nerve root filaments into dorsal cord
-the dorsal horn for sensory processing, the intermediate zone contains interneurons
-the ventral horn contains motor neurons with axons leaving through ventral nerve roots
-white matter consists of dorsal columns, lateral columns and ventral columns
-white matter is thicker in cervical region (all ascending fibers entered cord and
descending have not yet terminated) and sacral cord is mostly gray matter
-cervical enlargement and lumbar enlargement give rise to plexuses for arms and legs
-spinal cord has more gray matter at cervical and lumbosacral levels than thoracic
-in thoracic cord, lateral horn is present containing intermediolateral column
Spinal Cord Blood Supply – blood supply arises from vertebral arteries and spinal radicular
arteries. Vertebral arteries give rise to anterior spinal artery along ventral surface
-two posterior spinal arteries arise from vertebral or posterior inferior cerebellar arteries that
form a spinal arterial plexus surrounding the cord
-31 segmental arterial branches enter the spinal canal along its length; most from AORTA and
supply the meninges
-only 6-10 of these reach spinal cord as radicular arteries
-one prominent radicular artery called great radicular artery of Adamkiewicz usually
between T9-T12 but can be T5-L3, and supplies the lumbar and sacral cord
-*the mid-thoracic region (T4-T8) is between lumbar and vertebral arterial supplies and is called
the vulnerable zone of decreased perfusion, most susceptible to infarction
-ant spinal artery supplies anterior 2/3rds of cord (anterior horns and anterior/lateral columns)
-posterior spinal arteries supply posterior columns and dorsal horns
-venous Batson’s plexus does not contain valves
General Organization of Motor Systems – cerebellum and basal ganglia involved in feedback
loops, projecting back to cerebral cortex through thalamus and do not themselves project to
lower motor neurons
-inside cerebral cortex, lots of circuits exist for motor control, such as using association cortex
regions like the supplementary motor area, premotor cortex, and parietal association cortex for
planning and formulating motor activities
-lesions of regions of association cortex can cause apraxia (deficit in high-order motor planning)
-sensory inputs play a role in motor control and motor circuits/feedback loops
-upper motor neurons carry motor system outputs to lower motor neurons in spinal cord and
brainstem, which then project to muscles of the periphery
-descending upper motor neuron pathways arise from cerebral cortex and brainstem
-descending motor pathways can be divided into lateral motor system and medial motor
system based on location in the spinal cord
-lateral motor systems travel in lateral columns of spinal cord and synapse on lateral
groups of ventral horn motor neurons
-divided into lateral corticospinal tract and rubrospinal tract
-medial motor system travel in anteromedial spinal columns and synapse on medial
ventral horn; descend IPSILATERALLY or bilaterally, terminate on interneurons that
project to both sides of the spinal cord to control movements involving bilateral spinal
segments (unilateral lesions produce no deficits)
-divided into anterior corticospinal tract, vestibulospinal tracts, reticulospinal
tracts, tectospinal tracts
Lateral Motor Systems (basic overview)
1. Lateral Corticospinal Tract – originates in primary motor cortex, crosses at the
pyramidal decussation at cervicomedullary junction, and terminates in the entire cord
a. Movement of contralateral limbs; rapid dexterous movements of digits
2. Rubrospinal Tract – originates in the red nucleus, crosses in the ventral tegmental
decussation in midbrain, and ends in cervical cord
a. Movement of contralalteral limbs
Medial Motor Systems (basic overview)
1. Anterior Corticospinal Tract – originates in primary motor cortex and terminates in
cervical and upper thoracic cord
a. Control of bilateral axial and girdle muscles
2. Vestibulospinal Tracts – originates in the medial/lateral vestibulospinal tracts and
terminates in the medial VST of C and T cord, and lateral VST in entire cord
a. Medial VST: positioning of head/neck; Lateral VST: balance
3. Reticulospinal Tracts – originate in pontine and medullary reticular formation and
terminates in the entire cord
a. Automatic posture and gait-related movements
4. Tectospinal Tracts – originate in superior colliculus and terminate in cervical cord
a. Coordination of head and eye movement
Lateral Corticospinal Tract – most important descending pathway controlling movements of
extremities, and lesions produce characteristic deficits enabling clinical localization
-greater than 50% of fibers originate in primary motor cortex of precentral gyrus, the rest from
premotor and supplementary motor areas or from parietal lobe
-primary motor cortex neurons contributing to the corticospinal tract are located in cortical
layer 5; layer 5 pyramidal cell projections synapse directly onto motor neurons in ventral horn of
spinal cord
-3% of corticospinal neurons are giant pyramidal cells called Betz cells
-axons from cerebral cortex enter upper portions of cerebral white matter (corona radiata) and
descend toward internal capsule; cerebral white matter also conveys bidirectional information
between different cortical areas, and between cortex and deep structures such as basal ganglia,
thalamus, and brainstem forming fanlike pathways as they enter internal capsule
Internal capsule – thalamus and caudate nucleus are always medial to the internal capsule,
globus pallidus and putamen always on the outside
-internal capsule is composed of 3 parts: anterior limb, posterior limb, and genu
-Anterior limb separates head of caudate from globus pallidus and putamen
-Posterior limb separates thalamus from globus pallidus and putamen
-Genu is transition between anterior and posterior limbs at foramen of monro
-**the lateral corticospinal tract lies IN the posterior limb of the internal capsule
-the somatotopic map is preserved in the internal capsule so that motor fibers for face are most
anterior, followed by arm  trunk  leg more posterior
-fibers projecting from cortex to brainstem, including motor fibers for face are called
corticobulbar instead of corticospinal because brainstem is “bulb”
-lesions at internal capsule cause weakness of entire contralateral body (face, arm, leg)
-the internal capsule continues into the midbrain cerebral peduncles (feet of brain) where white
matter is located ventrally on peduncles and is called the basis pedunculi
-middle 1/3rd of basis pedunculi contains corticobulbar and corticospinal fibers with
face, arm, and leg axons arranged from medial to lateral
-corticospinal tract fibers descend through the ventral pons and collect on ventral medulla to
form medullary pyramids, giving corticospinal tract the name pyramidal tract
-transition of medulla to spinal cord is called cervicomedullary junction and occurs at the level
of the foramen magnum
-at this point, 85% of pyramidal tract fibers cross over at the pyramidal decussation to enter the
lateral white matter columns of the spinal cord, forming the lateral corticospinal tract
-lesions above pyramidal decussation cause contralateral weakness; lesions below cause
ipsilateral weakness
-axons of the lateral corticospinal tract enter spinal cord central gray matter and synapse on
anterior horn cells
-remaining 15% of coticospinal fibers continue into the cord ipsilaterally without crossing and
enter the anterior white matter columns to form the anterior corticospinal tract
Autonomic Nervous System – controls automatic and visceral bodily functions
-in somatic efferents, anterior horn cells or cranial nerve motor nuclei project directly from CNS
to skeletal muscle
-Autonomic Efferents – there is a peripheral synapse in a ganglion interposed between CNS and
effector gland or smooth muscle
-sensory inputs to ANS exists, but the ANS itself is only efferent pathways
-the sympathetic division (thoracolumbar) arises from T1 to L2/3 levels and is involved in
fight/flight functions like increasing heart rate and blood pressure, bronchodilation, and
increasing pupil size
-the parasympathetic (craniosacral) arises from cranial nerve nuclei and from S2-4; involved in
increasing gastric secretions, slowing heart rate, and decreasing pupil size
-the enteric nervous system consists of a neural plexus in the gut and controls GI secretions
Sympathetic Nervous System
-preganglionic neurons are located in the intermediolateral cell column in lamina VII of spinal
cord levels T1-L3
-two sets of sympathetic ganglia:
1. paravertebral ganglia in a chain called sympathetic trunk running from cervical to sacral
levels on each side of the cord
a. allows sympathetic efferents exiting only at thoracolumbar levels to each other
parts of body: sympathetics to head and neck provided for by sympathetic chain
ganglia called superior, middle, and inferior cervical ganglia
2. prevertebral ganglia are other, unpaired ganglia located in celiac plexus surrounding
aorta and include the celiac ganglion and superior/inferior mesenteric ganglia
-axons of the preganglionic sympathetics have a short distance to travel, while
postganglionic axons have a long distance to travel to reach effector organs
Parasympathetic Nervous System – preganglionic fibers must travel a long distance to reach
terminal ganglia located within or near effector organs
-parasympathetic preganglionic fibers arise from cranial nerve parasympathetic nuclei and from
sacral parasympathetic nuclei (in gray matter of S2, 3, 4)
-Sympthetic and parasympathetic systems differ in the neurotransmitters they secrete:
1. Sympathetic Postganglionic Neurons : norepinephrine
2. Parasympathetic Postganglionic Neurons : acetylcholine  activate muscarinic receptors
3. Preganglionic Neurons in BOTH systems: acetylcholine  activate nicotinic receptors
-Sympathetic and parasympathetic outflow are controlled by higher centers including
hypothalamus, amygdala, and limbic cortex