Six Flashcards
What is the pyramidal system? What is its functional importance? What is its clinical importance? What is another name for it?
- Anatomical Neurons in Cerebral Cortex whose Axons Project Directly or Indirectly to LMNs
- Functional Importance Motor Command Pathway for All Movements and Solely Highly Skilled Movements
- Medical Importance Pathway Present in All Levels of the CNS
- Upper Motor Neurons = Pyramidal System
Where specifically are most upper motor neurons located? What part of the primary motor cortex controls musculature of the head? The upper limb? The lower limb?
Most upper motor neurons are located in the precentral gyrus and anterior part of the paracentral lobule, which comprise the primary motor cortex. Within the primary motor cortex movements of the musculature of the head are represented ventrally and of the upper limb dorsally. Movements of the lower limb are represented in the anterior part of the paracentral lobule.
What are axons destined for the brainstem motor nuclei called? What are those destined for the spinal motor nuclei called?
Axons of the pyramidal system destined for brainstem motor nuclei form the corticobulbar or corticonuclear tract, whereas those destined for the spinal motor nuclei form the corticospinal or pyramidal tract.
Where do corticospinal fibers descend (5 steps)? Where do they decussate? What percent decussate? What do those that decussate form in the spinal cord? How far does it descend? What do those that don’t decussate form in the spinal cord? How far do they descend?
The corticospinal fibers descend through the corona
radiata, the posterior limb of the internal capsule, the cerebral crus, the basilar part of the pons, and the medullary pyramid. Ordinarily about 85-90% of the corticospinal axons in the medullary pyramid decussate before entering the spinal cord. These form the lateral
corticospinal tract which descends through all spinal cord levels. The uncrossed axons form the ventral corticospinal tract which rarely extends below the upper thoracic
segments. Most of these fibers cross at their levels of terminations. Those lower motor neurons whose muscles act in unison with homologous contralateral muscles are innervated bilaterally by corticospinal fibers.
Where do the corticobulbar fibers descend? Where is the last point where you’ll usually see them descend (a lesion at which point in the pyramidal system will still affect them? What is the only exception to their reaching their target nuclei bilaterally? How can this aid clinically? Which nuclei are not under direct influence of the cerebral cortex?
The corticobulbar fibers descend in the corona radiata and the posterior limb of the internal capsule, and then may follow the corticospinals through the midbrain and pons, or they may descend in the tegmentum of the pons.
They travel together only through midbrain….
The corticonuclear fibers pass to their target nuclei bilaterally in almost all cases (except lower part of facial nerve…this can be used to differentiate between UMN and LMN).
The nuclei innervating the external ocular muscles (III, IV and VI nuclei) are not under direct influence of the cerebral cortex. Voluntary eye movements are so intricate that they are controlled by cortical centers which influence specialized gaze centers in the brainstem as
will be described with the ocular motor system.
What are the functions of the Pyramidal System? What do they command? What do they command indirectly?
Commands
- All Movements
- Exclusively Highly Skilled Movements
- Most Rapid Movements by Direct Corticomotoneuronal Connections
Indirect Commands
• Some Supraspinal Motor Centers
What are some characteristic signs of upper motor neuron syndrome? What is the result of a unilateral lesion of the pyrimidal tract proximal to decussation? Distal? What facial movements are affected in a unilateral corticobulbar tract lesion? How and why? What are two important features of upper motor neuron lesions?
Damage to the pyramidal tract neurons or to their axons results in a number of characteristic signs that comprise the upper motor neuron syndrome. The essential features of this syndrome include paralysis and increased resistance to passive stretch, referred to as spastic paralysis. In addition, exaggerated myotatic reflexes and an abnormal plantar reflex, the extensor plantar or Babinski response, occur.
Unilateral lesions of the pyramidal tract proximal to its decussation result in contralateral hemiplegia (limbs on one side). In contrast, unilateral lesions of the lateral corticospinal tract in the spinal cord result in ipsilateral symptoms in the limb(s) below the level of the lesion.
Due to the bilateral representation in most of the corticobulbar system, usually only contralateral lower facial movements exhibit permanent weakness following a unilateral corticobulbar tract lesion.
Two important features of upper motor neuron lesions are: 1) they occur only in the CNS, never in the PNS; and 2) the distribution of the paralysis is always in groups of
muscles, never in individual muscles.
