Descending Motor Pathways

General Scheme

motoneurons (a MN) located in brainstem and spinal cord control contraction of skeletal muscle

receive inputs from sensory fiber, interneurons and descending axons from cerebral cortex and brainstem

inputs to a motoneurons: receive and integrate inputs from multiple sources Þ Convergence

(1) Segmental sensory input: Þ stretch reflex; Group 1A afferent from muscle spindles synapse directly on a MN

(2) Segmental interneurons: Þ interneuron connection

(3) Propriospinal interneurons: Þ central pattern generators for rhythmic repetitive movements

(4) Descending pathways: Þ direct input to motoneurons and indirect input to motoneurons via interneurons

Origin of Descending Motor Pathways – cell bodies located in cerebral cortex and brainstem

Corticospinal Pathways – Neurons located in cortex and terminate in spinal cord

Corticobulbar Pathways – Neurons located in cortex and terminate in brainstem – cranial n. motor nuclei
in either case, named for cell group from which they originate: (tectospinal, rubrospinal, reticulospinal, vestibulospinal)

Termination of Descending Pathways:

motoneuronal pool: motoneurons innervating a single muscle; lie in longitudinal columns in ventral horn

axial muscles: medial ventral horn
distal muscles: lateral ventral horn
extensor muscles: ventral portion of ventral motor horn
flexor muscles: dorsal portion of ventral motor horn

Corticospinal Tract Pathway

Somatotopic Organization: Legs lateral everywhere except in cortex

(1) Origin: Layers 3 and 5 in Primary Motor Cortex, Premotor Cortex and Primary Sensory Cortex

Somatotopic Organization of Primary Motor Cortex: "Homunculous" Þ legs are located medially on the medial surface of the cortex; trunk, arms, hand and face are located more laterally in the order listed

(2) Chorona Radiata: fibers entering and exiting the cortex

(3) Internal Capsule (IC): corticospinal tract within posterior limb

Horizontal section: IC has "V" shape with genu at the point of the "V" being medial
Thalamus is medial and the globus pallidus and putamen are lateral

Somatotopic Organization: – legs lateral, thorax and then arms most medial
Corticobulbar Tract (cortical control of fascial muscles): located most medially at the genu; bilateral innervation to cranial nuclei (exceptions: contralateral to lower facial, contralateral to genioglossus of XII, ipsilateral to XI)

(4) Crus Cerebri: located on anterior surface of midbrain; coronal section: anterior to substantia nigra

Corticospinal tract located in middle third of fiber bundle

Somatotopic Organization: legs lateral, thorax and then arms most medial
Corticobulbar tract is located medial to corticospinal tract

(5) Pyramids: located on anterior surface of medulla; coronal section: anterior to inferior olive

corticospinal tract forms the pyramids Þ corticospinal tract is also referred to as the pyramidal system

Somatotopic Organization: legs lateral, thorax and arms most medially

(6) Pyramidal Decussation: located at caudal end of medulla; coronal section: crossing fibers start ventromedially and go to the dorsolateral aspect of the medulla

75-90% of fibers cross and form lateral corticospinal tract after the pyramidal decussation

10-25% of fibers do not cross and form medial corticospinal tract

Somatotopic Organization: legs lateral, arms medial in lateral corticospinal tract

Lateral corticospinal tract Þ innervates distal musculature

Medial corticospinal tract Þ innervates axial musculature; bifurcate at segment level and innervate bilateral motoneuronal pools

Three Sets of Pathways

(1) Lateral Pathways

Lateral Corticospinal Tract – innervates motoneurons in lateral aspect of ventral horn

Rubrospinal Tract – located just anterior to lateral corticospinal tract and targets upper limb flexor motoneurons

Origin: magnocellular subdivision of the red nucleus in the midbrain

Fibers cross at level of midbrain and project caudally in lateral brainstem; (not distinctive fiber tract in brainstem)

(2) Medial Pathways

Medial Corticospinal Tract – innervates motoneuron in medial aspect of ventral horn

Tectospinal Tract – innervates motoneuronal pools that innervate axial muscles of the neck (cervical spinal cord)

Origin: superior and inferior colliculi

Fibers cross at level of midbrain and project caudally in the medial brainstem; located just anterior to medial longitudinal fasciculus (MLF) in the brainstem and near medial corticospinal fibers in the spinal cord

(3) Anterior Pathways (stay ipsilateral)

Vestibulospinal Tract – innervates motoneuronal pools that innervate extensor muscles (lower limbs)

Origin: lateral vestibular nuclei

Fibers do not cross; form vestibular nuclei in brainstem; fibers travel in anterior aspect of the spinal cord

Reticulospinal Tract – innervates motoneurnal pools that innervate axial muscles of neck (cervical spinal cord)

Origin: reticular formation
minor innervation: extensor of all limbs and neck

Fibers do not cross; within brainstem fibers do not form fascicles; fibers travel in anterior aspect of spinal cord

Decorticate – loss of cortex control, but midbrain and below are intact – flexed arms (rubro-), extended legs (vestibulo-)

Decerebrate – loss of cortex AND midbrain (red nucleus) – extended legs (vestibulo-), extended arms (reticulo-)

Summary

Rubrospinal – mostly limb flexors (Decorticate state will have flexed arms)

Vestibulospinal – mostly extensors of lower extremity (Decorticate state will have extended legs), minor role is axial extensors (so when you fall arms shoot out for balance)

Tectospinal – axial muscles of neck

Reticulospinal – minor extension of upper and lower extremities, also minor innervation of axial muscles of neck (so loss of vestibulospinal and rubrospinal will result in function being taken over by reticulospinal. Extension!!)

Decerebrate state (midbrain and red nucleus is gone) will have extended arms (reticulospinal) and extended legs (vestibulospinal)