Descending Tracts Flashcards
What are the different types of descending tracts?
Cortical descending tracts:
- Originates in the motor areas of the cerebral cortex
- From primary motor cortex
Non-cortical descending tracts:
- Originate in the sub-cortical areas of the brain
- Midbrain, Brainstem, Pons, Medulla
What are the two varieties of cortical descending tracts?
Neurones of the pre-frontal, supplementary motor cortex & somatosensory cortex
Terminate on the basal ganglia or cerebellum
Neurones of the motor strip (pre-central gyrus of the frontal cortex)
Terminate on cell bodies of lower motorneurones of muscles of the contralateral side of the body
Tell me about the cortical descending tracts terminating in the basal ganglia.
They account for the majority of purposeful motor outputs of the brain
They carry motor programs of the brain’s intended motor output to the body’s musculature
Disorders of degeneration of the brain result in disturbance of this motor system
What is the further division of cortical descending tracts that terminate on LMNs?
Cortico-bulbar tracts
- Terminates in cranial nerve motor nuclei of the
opposite side
Corticospinal tracts
- Terminates on the cell of spinal motor nuclei of the contralateral side
Tell me about the cortico bulbar tracts.
Originate in the motor strip
Targets are cell bodies of cranial nerve motor nuclei
Tell me about the corticospinal tract.
Travel from motor strip to spinal cord without terminating
2 classes:
- Lateral corticospinal tract
- Ventral corticospinal tract
They are mainly inhibitory but the ones going to hand and fingers are excitatory
Tell me about the lateral corticospinal tract.
Originates in the motor cortex
They constitute 75-85% of CST fibres
Axons travel via internal capsule
Axons decussate at the medulla
They dive to the base of the medulla where they form the medullary pyramids
They then travel in the lateral funiculus of the spinal cord
In the lateral CST
Tell me about the anterior corticospinal tract.
Originates in the motor cortex
They constitute between 15-25% of CST fibres
Axons travel via the internal capsule
They join medullary pyramids
Axons stay on same side of origin until they reach the spinal neural segments of their intended target where they decussate to the opposite side
They travel in the ventral funiculus
In the anterior CST
What are some brainstem motor nuclei that project to the spinal cord.
Red nucleus
- Rubrospinal tract
Reticular formation
- Reticulospinal tract
Rectum
- Tectospinal tract
Vestibular nuclei
- Vestibulospinal tract
(These do not project directly onto the cell bodies of spinal motorneurones, instead they project onto interneurons)
Their main action is to modulate spinal reflex circuits
Tell me about the Rubrospinal tract.
Originates from the red nucleus of the midbrain
Its fibres decussate in the midbrain
Travels in lateral funiculus in close association to the lateral corticospinal tract
Most of its fibres terminate in the cervical spine
Its main targets are motorneurones of the flexors of the upper limb
It is small and rudimentary in humans
Cell of red nucleus are red due to high iron
Tell me about the Reticulospinal tract.
Originates from the reticular formation (tegmental fields of the brainstem) 2 origins: Pontine reticular formation (in pons) - The medial reticulospinal tract Medullary reticular formation - The lateral reticulospinal tract
Normally under inhibition from corticospinal tract
It’s release from inhibition results in decerebrate rigidity
- Unopposed extension of the head & limbs
- Indicative of damage to the brain at or below red
nucleus
Tell me about the Tectospinal tract.
Originates from the tectum of the mesencephalon
- Inferior Colliculus (Auditory tectum)
- Superior Coliculus (Visual tectum)
Decussates in the midbrain
Terminates in the cervical cord of the contralateral side
Mediates postural reflex movements of the head to auditory and visual signals
Tell me about the Vestibulospinal tract.
Originates from vestibular nuclei such as:
- Deiter’s nucleus
- Nucleus of Bechterew
Some elements remain uncrossed
Others project to the cord bilaterally
Responsible for - Upright posture - Maintenance of balance - Posture of the body and head It has a major influence on muscle tone
Damage leads to:
- Loss of righting reflex
- Ataxia of gait
- Postural instability
What are the upper motorneurone signs?
Spastic paralysis Hypertonia (Spasticity) Cog-wheel rigidity Hyperflexia Clonus No fasciculations Babinski sign (rub foot) Choreoforms
