Posture and movement are put together at six levels of the nervous system, and the classic way to find out what each level does is to cut the neuraxis below it and watch what survives. That gives four preparations — spinal, decerebrate, midbrain and decorticate — and the examiner’s favourite long question is exactly that: how the cord and the medulla hold posture. Spinal shock, the mechanism of decerebrate rigidity, the righting reflexes and the motor areas of the cortex with the homunculus are the short notes that come round again and again.
Topics, in the book’s order
- Postural reflexes, spinal animal and spinal shock · pp. 1071–1074 · asked twice
- Medullary integration and decerebrate rigidity · pp. 1074–1076
- Midbrain integration and righting reflexes · pp. 1077–1078
- Cortical integration and motor cortex · pp. 1078–1080 · asked twice
When you’ve read them: Quiz 130 · Regulation of Posture and Movement, 15 questions.
After this chapter you should be able to
- sketch how motor control is organized, and name the six levels at which posture and movement are integrated
- classify the postural reflexes, and say which level integrates which
- describe the spinal, decerebrate, midbrain and decorticate preparations and what each one can still do
- describe the three stages of a spinal animal and the features of each
- explain how decerebrate rigidity comes about, and why it picks out the extensors
- name the cortical motor areas with their Brodmann numbers and their jobs
- explain how the body is laid out in the motor homunculus, and why the hand and face take up so much of it
- put the whole chapter together: what each part of the neuraxis contributes to posture and movement
Going further: the full mechanism of postural and movement regulation, and what goes wrong when each level is damaged.
Viva checklist
- The six levels at which posture and movement are integrated
- Release phenomenon: what it is and its two causes
- Classification of postural reflexes, with two examples of each class
- Where the section is made in a spinal animal, and why below C5 or below mid-thoracic
- The three stages of a spinal preparation, and the leading feature of each
- Duration of spinal shock in frog, cat, monkey and man, and what decides it
- Mechanism of spinal shock, and the evidence for it
- Paraplegia in flexion versus paraplegia in extension: when each appears
- Mass reflex, coitus reflex and the Babinski sign
- Positive and negative supporting reactions; the magnet reaction
- Central pattern generator for locomotion: where the spinal ones sit and what drives them
- Where the cut is made for a decerebrate preparation, and why there is no shock
- Mechanism of decerebrate rigidity: the two reticular areas and the three inhibitory brain areas
- Antigravity muscles; which tract works through γ and which through α motor neurons
- Tonic labyrinthine and tonic neck reflexes: receptors, pathway, response
- Name the righting reflexes; which one is not integrated in the midbrain
- Features of a decorticate animal; why decorticate rigidity is so mild
- The cortical motor areas with their area numbers, and the motor homunculus