How commands from the cortex and the brainstem reach the spinal motor neurons, and what happens when the pathway is cut above the anterior horn cell or at it. Tracing the corticospinal tract with a diagram, together with the effects of a lesion at the internal capsule, is the long question examiners set most; UMN vs LMN paralysis is the favourite short note.
Topics, in the book’s order
- Corticospinal (pyramidal) tract · pp. 1061–1064 · asked 8 times
- Upper vs lower motor neuron lesions · pp. 1064–1068 · asked 8 times
- Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts · pp. 1065–1068
When you’ve read them: Quiz 129 · Descending Pathways, 15 questions.
After this chapter you should be able to
- classify the descending pathways into lateral and medial systems, and say why the pyramidal–extrapyramidal split misleads
- draw and label the corticospinal tracts
- describe the origin, course and functions of the corticospinal tracts, and the effects of a lesion at the internal capsule
- list the differences between UMN and LMN paralysis and explain the physiological basis of each
- give the course and functions of the other descending pathways
Going further: the extrapyramidal (brainstem) pathways in detail, and the finer reasoning behind each UMN and LMN difference.
Viva checklist
- The lateral and medial descending systems: their tracts, and what each controls
- Why “pyramidal vs extrapyramidal” is a poor classification
- Where CST fibres arise, and why Betz cells alone can’t supply them (30,000 cells, 1 million axons)
- The course of the CST, and the share that crosses (80%)
- Where the lateral and anterior CSTs end
- Why the internal capsule is the commonest site of a CST lesion; Charcot’s artery
- Why a capsular lesion gives contralateral hemiplegia of the “complete UMN” type
- Why tone rises in a UMN lesion (corticoreticular fibres, pontine reticulospinal tract)
- Why tendon reflexes are brisk and superficial reflexes lost in a UMN lesion
- Babinski’s sign: what it looks like and why it appears
- At least six differences between UMN and LMN paralysis, and the one sign they share
- Monoplegia, hemiplegia, paraplegia and quadriplegia
- Which descending tracts cross to the opposite side
- The lateral vestibulospinal tract and decerebrate rigidity