Pyramidal and Extrapyramidal Pathways

Question 1

[8-minute video]: Pyramidal and Extrapyramidal pathways

Answer 1

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Question 2

Briefly discuss upper motor neurons.

Answer 2

• 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

Question 3

Briefly discuss lower motor neurons.

Answer 3

• 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)

Question 4

Based on the origins of the upper motor neurons, there are 2 categories of the descending pathways. State these 2 categories.

Answer 4

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.

Question 5

The upper motor neurons (UMN) of the pyramidal pathways arise from the ____________________________, in the inner pyramidal layer (layer V) of the cerebral cortex.

Answer 5

giant pyramidal cells of Betz

Question 6

State the cortical areas of origin of upper motor neurons of the pyramidal pathway.

Answer 6

• primary motor cortex
• premotor area
• supplementary motor cortex
• Broca’s area
• frontal eye field
• somatosensory cortices

Question 7

State the course of the upper motor neurons of pyramidal pathways within the cerebrum.

Answer 7

☛ 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]

Question 8

Describe the course of the pyramidal pathway within the brainstem.

Answer 8

• 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

Question 9

Describe the origin, course and termination of corticospinal tract.

Answer 9

✪ 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]

Question 10

Some UMN of cranial nerves give bilateral projection i.e. goes contralaterally and ipsilaterally. Give examples of cranial nerve nuclei receiving bilateral projection.

Answer 10

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.

Question 11

What is worth noting about extrapyramidal pathways?

Answer 11

• 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]

Question 12

Discuss the rubrospinal tract. (Hint: origin, decussation or not, course in spinal cord, extent within spinal cord, function)

Answer 12

• 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

Question 13

Discuss the tectospinal tract.

Answer 13

• 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

Question 14

Discuss the vestibulospinal tracts.

Answer 14

• 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

Question 15

Discuss the reticulospinal tracts.

Answer 15

☑ 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]

Question 16

Discuss the solitariospinal tract

Answer 16

• a small group of mostly crossed fibres arising from the nucleus solitarius of the medulla oblongata
• concerned with regulation of respiration
• the axons terminate on:
  ~ phrenic motor neurons - innervates thoracic diaphragm
  ~ thoracic motor neurons - supply the intercostal muscles

Question 17

Discuss the hypothalamospinal tract.

Answer 17

• arise from the hypothalamus and descend ipsilaterally
• terminate in the intermediolateral nucleus of the spinal cord
• controls both sympathetic and parasympathetic preganglionic autonomic neurons

Question 18

The lower motor neurons are _______________. (according to neurotransmitter)

Answer 18

cholinergic

Question 19

Discuss upper motor neuron lesions. (clinic presentations, causes)

Answer 19

These are lesions of neural pathways above the anterior horn of the spinal cord or motor nuclei of the cranial nerves
Clinical presentations include:
1. spastic paralysis of muscle groups or entire limbs
2. exaggerated deep tendon reflexes [Video]
3. absent superficial reflexes such as abdominal and cremasteric reflexes
4. Babinski sign positive [Video]
5. fasciculations and fibrillations do not occur
6. muscle atrophy may not be marked (may occur late and will be due)

[NB: It is more helpful to mention the clinical presentations that you actually observe with regards to UMN lesions. However, the features here that are regarded as absent are important for LMN lesions.]

Causes: strokes, traumatic brain injury, traumatic spinal cord injury, multiple sclerosis

Further notes:
(1) Babinski’s sign, also known as the Babinski reflex or plantar reflex, is a response to stimulation of the bottom of the foot. It can help doctors evaluate a neurological problem in people over age 2.

To perform the Babinski test, the doctor will use their finger or a tool like a stick or a hammer to stimulate the bottom of the patient’s foot. They’ll run it firmly from the outer edge of the foot at the heel to the bottom of the big toe.

In older children and adults without neurological problems, the foot will remain still, or the toes may curl downward during the Babinski test. If you do have a Babinski sign, your big toe will bend back, and your other four toes will spread out like a fan.

The Babinski reflex is never a normal finding in adults. If the Babinski sign is still noticeable beyond that, it likely indicates neurological problems.

(2) Fasciculations refer to small, involuntary muscle twitches under the skin. They are visible and can last for several seconds, minutes, or even hours. On the other hand fibrillations involve the rapid, irregular, and often unsynchroized contraction of muscle fibres. Unlike fasciculations, fibrillations are not visible to the naked eye.

Question 20

Discuss lower motor neuron lesions. (clinical presentation, examples)

Answer 20

✪ these lesions may involve cranial nerve motor nucleus, anterior horn of the spinal cord, a peripheral nerve, or the neuromuscular junction
✪ clinical presentations:
(1) flaccid paralysis (muscle tone)
(2) individual muscles paralysed
(3) absent deep tendon reflexes
(4) absent superficial reflexes
(5) no plantar response
(6) muscle atrophy will be early and severe and due to denervation
(7) fasciculations and fibrillations are common

Question 21

Click on Answer for notes on the Corticorubral tract.

Answer 21

The corticorubral tract is a neural pathway that connects the primary motor and sensory areas of the cerebral cortex to the red nucleus, which is located in the midbrain. It is involved in motor coordination, particularly the excitation of flexor muscles and inhibition of extensor muscles, The tract originates mainly from Brodmann areas 4 and 6, which are associated with motor functions, and descends through the posterior limb of the internal capsule, terminating on the ipsilateral aspect of the red nucleus. The red nucleus then processes this information and conveys it down the rubrospinal tract, which continues through the spinal cord. This tract plays a role in voluntary movement control and is also associated with certain reflexive movements.