How each segment of the spinal cord runs its own motor circuit: the muscles, their afferents, the upper and lower motor neurons, the motoneuron pools and the interneurons. Long questions are rare here; upper and lower motor neurons, γ motor neurons, the motor unit and the motoneuron pool are the usual short notes.
Topics, in the book’s order
- Segmental organization of the motor system · pp. 1044–1047
When you’ve read them: Quiz 126 · Segmental Organization of Motor System, 14 questions.
After this chapter you should be able to
- describe how the motor system is organized at each spinal segment
- classify muscles on physiological grounds
- define upper and lower motor neurons and say what each does
- explain motor unit recruitment and why it matters
- describe how the motoneuron pool and the spinal interneurons are arranged topographically
Viva checklist
- Segmental versus suprasegmental organization
- The five components of segmental organization
- Physiological classification of muscles; the antigravity muscles
- What the proximal and the distal muscle groups do
- UMN and LMN: definition, where each arises, examples
- The final common path
- Definition of a motor unit; how many fibres one α motor neuron supplies
- Small versus large motor units
- Motor unit recruitment: the size principle and its significance
- Why antigravity muscles are mostly slow-twitch
- γ motor neurons: their share, what they supply, what drives them
- The fusimotor system
- How the motoneuron pool is arranged, and why the lateral pool is large in the brachial enlargement
- Where interneurons sit, and why they are the main target of descending tracts