In one breath
Cut the brainstem between the superior and inferior colliculi (a midcollicular section) and the animal stiffens at once into extension: that is decerebrate rigidity, and unlike the spinal animal there is no stage of shock. The cut takes the cortex and basal ganglia away from the inhibitory reticular area in the medulla, while the facilitatory area in the pons goes on discharging by itself — so the net reticulospinal output swings towards facilitation and γ motor neuron discharge rises. What the medulla holds are the static (tonic) postural reflexes: extensor tone in the antigravity muscles, the tonic labyrinthine reflexes and the tonic neck reflexes.
Builds on: Postural reflexes, spinal animal and spinal shock · Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts · Inverse stretch reflex and muscle tone · Leads to: Midbrain integration and righting reflexes · Cerebellar functions, lesions and tests
Medullary integration
The medulla’s business is the tonic reflexes that hold posture. Its role is studied by making a midcollicular lesion — a section between the superior and the inferior colliculi. The animal is a decerebrate preparation, and the procedure is decerebration. Sherrington devised it, so it is also called Sherringtonian decerebration.
Features of the decerebrate preparation
- No shock. Nothing like spinal shock occurs.
- Severe spasticity of the extensors, immediately. This is decerebrate rigidity, and it is extreme: every limb locks out straight and the back arches backwards.
- Righting reflexes are absent, so the animal simply lies where it is put.
- Tonic reflexes are prominent.
Mechanism of decerebrate rigidity
Rigidity is the result of motor neurons becoming more active and so facilitating the stretch reflexes. Two things do that:
- The motor neuron pool becomes more excitable in general.
- γ motor neurons discharge faster.
The general rise in motor neuron excitability comes from facilitation of the descending inputs converging on the anterior horn cells.
The two reticular areas
For motor control, the brainstem reticular nuclei split into two:
| Area | Where | Its tract | Behaviour |
|---|---|---|---|
| Facilitatory — large | Pons | Pontine reticulospinal | Discharges spontaneously |
| Inhibitory — small | Medulla | Medullary reticulospinal | Needs driving from above |
- The facilitatory area fires on its own; the inhibitory area does not. To discharge effectively it needs input from the cortex, basal ganglia and cerebellum.
- Basal ganglia have no direct line to the brainstem reticular formation. They project to the motor cortex, and through the cortex’s projections to the brainstem they get at medullary reticular activity.
- Normally the medullary reticulospinal fibres inhibit spinal motor neurons. Because the cortex drives that inhibitory area, cortex and basal ganglia count as inhibitory brain areas.
- The cerebellum is classed with them for the same reason: it too drives the inhibitory area. So three areas — cortex, basal ganglia, cerebellum — drive the medullary inhibitory centre.
- The vestibulospinal output, by contrast, is facilitatory.
What the midcollicular cut does
The cut removes two of the three inhibitory drives (cortex and basal ganglia); only the cerebellar drive survives. Then:
- The medullary reticulospinal output becomes less inhibitory, while the pontine facilitatory area keeps firing — it never needed a drive.
- So the net reticulospinal discharge shifts towards facilitation.
- γ motor neuron output is governed mainly by the reticulospinal tract, so γ discharge rises after decerebration.
- Raised γ discharge → more spindle sensitivity → facilitated stretch reflexes → rigidity.
Draw it: control of reticulospinal output
Two boxes down the middle of the page, one above the other: facilitatory area (pons) and inhibitory area (medulla), each with an arrow down to a single spinal motor neuron — label the upper arrow ”+” and the lower one ”−”. Put a small circular arrow on the pontine box for “discharges spontaneously”. Down the left side stack three boxes — cortex, basal ganglia, cerebellum — and draw an arrow from each into the medullary box, marking the trio “inhibitory brain areas (−)”. Route the basal ganglia arrow through the cortex box, not straight to the brainstem. Add a vestibular nucleus box on the right with a ”+” arrow to the motor neuron. Now draw a scissors line across the midbrain between the two colliculi, and score out the cortex and basal ganglia arrows: what is left is one weak inhibitory drive (cerebellum) against a facilitatory area still firing on its own — write “net output → facilitation, γ discharge ↑, rigidity”.
Importance of decerebrate rigidity
The rigidity picks out the extensors, and that is the point of it.
- Extensors are the key part of the posture-regulating machinery: they hold the body upright by keeping the limbs extended.
- Their tone is a static postural reflex, and it is what holds the animal up against gravity — hence antigravity muscles.
- In man these are chiefly the lower limb extensors.
- So extensor rigidity appearing after decerebration is the proof that antigravity tone is set from the medulla.
Medullary reflexes
The reflexes integrated in the medulla for motor control are essentially static postural reflexes, and a midcollicular lesion accentuates all of them:
- Extensor rigidity
- Tonic labyrinthine reflexes
- Tonic neck reflexes
Extensor rigidity
Tone in the extensors (the antigravity muscles) is the outstanding static postural reflex, and it is what keeps posture against gravity.
- This extensor tone comes mainly from motor nuclei in the pons and medulla, carried by the reticulospinal and vestibulospinal tracts.
- The reticulospinal tract works on the antigravity muscles through γ motor neurons; the vestibulospinal tract works through α motor neurons.
- γ motor neurons set muscle tone by tuning spindle sensitivity.
- So γ discharge — and therefore reticulospinal activity — is the chief regulator of muscle tone.
Applied: what deafferentation proves
Dorsal rhizotomy cuts the dorsal roots and so deafferents the segment: the Ia input from the spindle to the cord is abolished. Since the γ route to tone runs through the spindle, cutting Ia should abolish anything that acts through γ — and it does: after rhizotomy the reticulospinal tract loses its influence on tone. The vestibulospinal influence survives, because it ends on α motor neurons that reach the extrafusal fibres directly. That is the experiment behind “reticulospinal → γ, vestibulospinal → α”.
Don't swap the two reticular areas
Facilitatory = large, in the pons, fires on its own. Inhibitory = small, in the medulla, has to be driven. Get them the wrong way round and the whole mechanism inverts. Two more traps: the cerebellar drive to the inhibitory area survives a midcollicular cut, and the basal ganglia reach the brainstem only via the cortex, never directly.
Worth knowing, though not in your pages: because it depends on raised γ discharge and so on an intact spindle loop, classical decerebrate rigidity is called γ rigidity, and dorsal rhizotomy abolishes it. If the anterior lobe of the cerebellum is then removed, the rigidity returns as α rigidity, driven directly on the α motor neurons and no longer abolished by rhizotomy.
Tonic labyrinthine reflexes
A decerebrate animal cannot right itself, so it stays in whatever position you put it in — and the pattern of rigidity changes with the position. That change is the tonic labyrinthine reflex. Rigidity is maximum supine, minimum prone.
| Part of the reflex | Tonic labyrinthine |
|---|---|
| Receptors | Otolith organs of the vestibular apparatus |
| Stimulus | Gravity acting on the otolith organs, as body position alters it |
| Pathway | Vestibulospinal tracts |
| Response | On its back: rigidity maximal, all four limbs fully extended. Turned to either side: rigidity falls. Prone: least of all |
| Importance | Helps hold muscle tone, especially in the erect posture |
Tonic neck reflexes
Here the pattern of rigidity changes when the head moves in relation to the body.
| Part of the reflex | Tonic neck |
|---|---|
| Receptors | Proprioceptors in the upper neck |
| Stimulus | Change in head position, which stimulates those proprioceptors |
| Pathway | Reticulospinal and tectospinal |
| Importance | Lets the animal hold a posture suited to where its head is pointing |
Response, by head position:
| Head is | Forelimbs | Hind limbs |
|---|---|---|
| Ventroflexed (bent down) | Flex | Extend |
| Extended (raised) | Extend | Flex |
| Turned to one side | Limb on that side (jaw limb) extends | Opposite limb (occipital limb) flexes |
Why that is useful: an animal looking up into a tree extends its head, which extends the forelimbs and flexes the hind limbs — exactly the posture for reaching upward. Looking down at the ground flexes the head, which extends the hind limbs and flexes the forelimbs — the posture for reaching down.
Tonic versus righting
Tonic labyrinthine and neck reflexes are static, integrated in the medulla, and they only change the pattern of rigidity — the decerebrate animal still cannot get up. The righting reflexes are phasic, integrated in the midbrain, and they actually restore position (see Midbrain integration and righting reflexes). Both use the otolith organs, so name the level, not just the receptor.
Exam-answer skeleton: "Explain the mechanism of decerebrate rigidity" (short note)
- Define: severe extensor spasticity appearing at once after a midcollicular section, with no stage of shock.
- It is facilitation of the stretch reflexes, by a generally more excitable motor neuron pool and a faster γ discharge.
- The two reticular areas: large facilitatory in the pons (spontaneous), small inhibitory in the medulla (needs driving).
- Three inhibitory brain areas drive the medullary area — cortex, basal ganglia (through the cortex) and cerebellum.
- A midcollicular cut removes cortex and basal ganglia; only cerebellar drive remains, so inhibition weakens while facilitation carries on.
- Net reticulospinal output becomes facilitatory → γ discharge ↑ → spindle sensitivity ↑ → stretch reflexes facilitated → rigidity.
- Why extensors: they are the antigravity muscles, so the medulla is shown to control antigravity tone.