Pyramidal and Extrapyramidal Pathways Flashcards
Part 2 of Tractology: For extrapyramidal pathways, their roles are more important than the course.
Briefly discuss upper motor neurons.
- they arise from higher centres of CNS, and are entirely found within the CNS
- decussate to the opposite site during their descent
- synapse either directly or indirectly (via an interneuron) with the lower motor neuron
Briefly discuss lower motor neurons.
- arise from either the brainstem or spinal cord
- comes out of the CNS to the PNS
- forms (part of) either a cranial nerve or a spinal nerve
- does not decussate except that of the trochlear nerve (which happens within the midbrain)
Based on the origins of the upper motor neurons, there are 2 categories of the descending pathways. State these 2 categories.
Pyramidal tracts: arise from the cerebral cortex; terminate in the brainstem (corticonuclear) or the spinal cord (corticospinal).
Extrapyramidal tracts: arise from motor nuclei within the brainstem; these nuclei receive input from the basal ganglia and the cerebellum.
NB: Corticopontine fibres are neither pyramidal nor extrapyramidal.
The upper motor neurons (UMN) of the pyramidal pathways arise from the ____________________________, in the inner pyramidal layer (layer V) of the cerebral cortex.
giant pyramidal cells of Betz
State the cortical areas of origin of upper motor neurons of the pyramidal pathway.
- primary motor cortex
- premotor area
- supplementary motor cortex
- Broca’s area
- frontal eye field
- somatosensory cortices
State the course of the upper motor neurons of pyramidal pathways within the cerebrum.
☛ after arising from the cerebral cortex, their axons descend through the subcortical white matter of the cerebrum
☛ these fibres form part of the corona radiata
☛ the fibers then converge to pass through the internal capsule to reach the brainstem
☛ corticonuclear tracts occupy the genu of the internal capsule
☛ corticospinal tracts occupy the anterior aspects of the posterior limb of internal capsule
Further notes:
☛ The corona radiata is a bundle of projection fibers connecting the cortices of the brain with the brainstem via the internal capsule.
☛ The internal capsule is a collection of projection fibres on one side of the hemisphere. [Parts of the internal capsule]
Describe the course of the pyramidal pathway within the brainstem.
- from the internal capsule the pyramidal tract enter the brainstem
- corticospinal fibers traverse whole brainstem, while corticonuclear fibres terminate on the motor nuclei of cranial nerves
- midbrain: occupy middle 2/3 of the crus cerebri; corticonuclear (CN) projection to nucleus of III & IV
- pons: scattered within the basal pons (scattered because the decussating transverse pontocerebellar fibres tend to disperse the corticospinal and CN tracts); nuclei of V, VI and VII receive CN projection
- medulla oblongata: fibres converge to form the medullary pyramids, after which most of the fibers decussate to form the lateral corticospinal tract; CN projection to nuclei ambiguus and XII
Describe the origin, course and termination of corticospinal tract.
- Origin: primary motor (30%), premotor (30%), primary
sensory (40%). Layer 5 major output, giant pyramidal cells of Betz - Course: corona radiata, posterior limb of internal capsule, crus cerebri, ventral pons, medullary pyramids (decussation occurs)
- 85% of the fibres decussate (lateral corticospinal) and 15% uncrossed (anterior corticospinal)
- Termination: synapse with anterior horn (alpha and gamma motor neurons) while some synapse with interneurons. The fibres of anterior corticospinal decussate at level of termination.
- [Diagram 1] [Diagram 2: parts of the internal capsule] [Diagram 3]
Some UMN of cranial nerves give bilateral projection i.e. goes contralaterally and ipsilaterally. Give examples of cranial nerve nuclei receiving bilateral projection.
Facial Nucleus: The facial nucleus, which is associated with the facial nerve (Cranial Nerve VII), receives bilateral upper motor neuron (UMN) input for the muscles of the upper face. However, the lower face muscles receive predominantly contralateral UMN input.
Nucleus Ambiguus: The nucleus ambiguus, which gives rise to efferent motor fibers of the vagus nerve (Cranial Nerve X) and the glossopharyngeal nerve (Cranial Nerve IX), receives bilateral UMN input.
Hypoglossal Nucleus: The hypoglossal nucleus, associated with the hypoglossal nerve (Cranial Nerve XII), receives bilateral UMN input for the genioglossus muscle, which is responsible for protruding the tongue.
Motor Nucleus of Trigeminal Nerve: The motor nucleus of the trigeminal nerve (Cranial Nerve V) receives bilateral UMN input.
Nucleus of Oculomotor and Trochlear Nerves: The oculomotor (Cranial Nerve III) and trochlear (Cranial Nerve IV) nuclei receive bilateral UMN input.
What is worth noting about extrapyramidal pathways?
- the UMNs arise from various nuclei in the brainstem
- the nuclei are interconnected with basal ganglia, cerebellum, vestibular nuclei & reticular nuclei
- extrapyramidal system provides indirect control of the anterior horn cells (LMNs) of the spinal cord [most synapse with interneurons]
Discuss the rubrospinal tract. (Hint: origin, decussation or not, course in spinal cord, extent within spinal cord, function)
- arise from the red nucleus in the mesencephalic tegmentum
- fibers decussate and descend contralaterally
- occupy the lateral funiculus of the spinal cord white matter
- functions are poorly defined in man, projecting only to upper cervical cord (while in animals it extends to lumbosacral levels), but the tract is facilitatory to flexors of upper limb
- lesions are similar to those of corticospinal in terms of clinical presentations
Discuss the tectospinal tract.
- arise from the superior colliculus of midbrain
- fibers decussate and descend contralaterally
- they occupy the ventral funiculus of the spinal cord white matter
- the tract terminates in the upper cervical segments of the spinal cord
- facilitate visual reflexes; neck movements in response to light signals
Discuss the vestibulospinal tracts.
- consist of the lateral and medial vestibulospinal tracts
- arise from the vestibular nuclear complex, at the level of pontomedullary junction (lateral from lateral vestibular nucleus, medial from medial vestibular nucleus)
- lateral vestibulospinal tract descend ipsilaterally, occupy the ventral funiculus of spinal cord, and excite extensor muscles - both axial and limb musculature
- this tract is an important efferent path for equilibrium
- medial vestibulospinal tract descends through the ventral funiculus of the spinal cord.
- this medial vestibulospinal tract is a downward continuation of medial longitudinal fasciculus and terminates in the upper cervical spinal cord
- this tract is responsible for the neck righting reflex in response to stimulus received from vestibular apparatus
Discuss the reticulospinal tracts.
- arise from the brainstem reticular formation
- consist of lateral and medial reticulospinal tracts
- display bilateral projection and high degree of collateralization
- medial/pontine reticulospinal tract arises from the medial part of the reticular formation of pons
- These fibres descend in the ventral funiculus.
- The tract is facilitatory to the extensor muscles of the trunk and limbs. [It synapses with interneurons that inhibit the flexors and stimulate the extensors of the axial and proximal limb musculature.]
- FUNCTIONS: the tract is concerned with postural adjustments of the head, trunk and limbs. Control of bladder and bowel is also done through this tract.
- lateral/medullary reticulospinal tract arises from reticular formation of the medulla oblongata
- These fibres descend in the lateral funiculus.
- The tract is facilitatory to the flexor muscles of the trunk and limbs. [It simulates flexors and inhibits extensors.] Automatic breathing is also controlled by lateral reticulospinal tract.
- it is also concerned with control of pain perception (efferent modulation).
- [Diagram]
Click here to check out a description of the gate theory of pain modulation.
