In one breath
Section at the upper border of the midbrain and the animal can get up again: the midbrain integrates the phasic postural reflexes. Extensor rigidity is still there, but only at rest — it vanishes the moment a phasic reflex such as walking takes over. Its reflexes are the righting reflexes, the grasp reflex and the vestibular placing reaction, and the midbrain also holds the mesencephalic locomotion generator that drives the spinal pattern generators.
Builds on: Medullary integration and decerebrate rigidity · Postural reflexes, spinal animal and spinal shock · Leads to: Cortical integration and motor cortex · Cerebellar functions, lesions and tests
Midbrain integration
The midbrain’s job is the phasic postural reflexes: righting reflexes, the vestibular placing reflex and the grasp reflex. The central pattern generator for locomotion at brainstem level also sits here — the mesencephalic generator for locomotion, which drives the spinal CPGs.
The midbrain preparation
The section is made at the superior border of the midbrain, and the animal is called a midbrain animal. Compare it with the decerebrate animal, whose cut is lower down, between the colliculi.
Features of the midbrain animal
- No features of shock.
- Extensor rigidity is present only at rest. It is the same rigidity as in the decerebrate animal, but it disappears whenever a phasic postural reflex is activated — when the animal walks, for instance.
- The animal can right itself — the righting reflexes are preserved — and it can shift its posture at will.
- The grasp reflex and the vestibular placing reaction both work.
Rigidity at rest only
Decerebrate rigidity is continuous; midbrain rigidity comes and goes — it is there at rest, gone during movement. The reason is the returning phasic reflexes, which override the static extensor drive. The same “only at rest” rigidity, weaker still, is what you see after decortication (see Cortical integration and motor cortex).
Righting reflexes
A series of reflexes that correct body position and then hold it, by keeping the head upright. A steady head position is part of the posture machinery: the righting reflexes keep the head stable and the eyes fixed on visual targets however the body moves, and they restore the body’s position whenever it is disturbed.
- They work by stimulating proprioceptors, tactile receptors and vestibular receptors.
- Four things trigger them: vestibular stimulation, stretch of the neck muscles, pressure applied to the flank or to a limb, and visual input.
- They are integrated in the midbrain — with one exception below.
| Righting reflex | Stimulus | Receptor | Response |
|---|---|---|---|
| Labyrinthine | Head tilted (gravity) | Otolith organ | Head kept level |
| Neck | Neck muscles stretched | Muscle spindle | Shoulders, then thorax, then pelvis righted |
| Body on head | Flank pressed while lying on it | Exteroceptors | Head righted |
| Body on body | Flank pressed, head held still | Exteroceptors | Body righted even with the head held sideways |
| Limb | Limb muscles stretched | Muscle spindle | Body righted |
| Optical | Visual cues | Visual receptors | Head righted |
Centres: all of them are integrated in the midbrain, the limb righting reflex through the red nucleus — except the optical righting reflex, whose centre is the visual cortex.
Labyrinthine righting reflex
Hold the animal by the body and rock it sideways: the head stays level. The otolith organs sense the tilt and the neck muscles contract to keep the head where it belongs.
Body on head righting reflex
Lay the animal on its side and flank contact by itself rights the head. It still works after labyrinthectomy — which is how you know it is not the labyrinthine reflex doing the work.
Neck righting reflex
If the first two reflexes have righted the head but the body is still tilted, the neck muscles are put on stretch. They contract, and that rights the thorax and abdomen.
Body on body righting reflex
Press on the flank and the trunk comes round even if the head is held still and not allowed to right itself.
Limb righting reflex
Press on the limbs and the body rights itself. Of all of them, this is the one whose centre is named: the red nucleus.
Optical righting reflex
The righting reflexes work best with the eyes open, even when there is no labyrinthine or body stimulation at all — that extra contribution is the optical righting reflex. Its centre is not the midbrain but the cortex, so it needs cortical function intact.
The odd one out
Every righting reflex in the list is a midbrain reflex except the optical one, whose centre is the visual cortex. That is the single most asked fact on this page, and it is why a decorticate animal keeps its righting reflexes but loses the visual contribution to them.
Draw it: the righting reflexes on one animal
Draw the same cat four times across the page, going from lying on its side to standing. (1) Held by the body and tipped: draw the head level while the trunk is tilted, arrow to the inner ear, label “labyrinthine RR — otolith organ”. (2) Lying on its side: arrow pressing on the flank, head lifting level, label “body on head RR — exteroceptors, works after labyrinthectomy”. (3) Head level but trunk still sideways: mark the stretched neck muscles with a spring symbol and show shoulders → thorax → pelvis turning in that order, label “neck RR — muscle spindle”. (4) Standing: label “limb RR (red nucleus)” at the paws and draw eyes with dashed sight lines to the horizon, labelled “optical RR — centre is visual cortex, not midbrain”. Write “all midbrain except optical” underneath.
Grasp reflex
Hold a stick or any object near a midbrain animal’s limbs and it grasps the object, with the limbs extended. The grasp is primitive, and its use is to help the animal rise and prop its posture. In man it is best seen in infancy and early childhood.
Applied: the grasp reflex in a baby
The palmar grasp is one of the newborn reflexes tested on the paediatric ward, and it should be gone by a few months of age. Worth knowing, though not in your pages: if a grasp reflex reappears in an adult it points to a frontal lobe lesion — the cortex that normally suppresses the primitive reflex has been lost, which is the release phenomenon again.
Vestibular placing reaction
Bring a blindfolded animal down from a height quickly and its forelimbs extend and the toes spread, ready for the ground.
| Part of the reflex | Vestibular placing reaction |
|---|---|
| Receptors | Vestibular receptors |
| Stimulus | Linear acceleration — the body dropping |
| Pathway | Vestibulospinal tract |
| Response | Forelimbs extend, toes spread |
| Importance | Lets the animal land steadily on the ground |
The blindfold matters: it proves the reaction is vestibular, not visual.
Mesencephalic locomotion generator
The brainstem generator for locomotion sits in the midbrain: the mesencephalic locomotion generator, also called the midbrain locomotion centre.
- It drives the spinal locomotion generator (see Postural reflexes, spinal animal and spinal shock).
- It takes the command signals arriving from the motor cortices along the corticobulbar fibres, organizes them, and acts on spinal motor neurons mainly through the reticulospinal tract.
- Locomotion is not only started this way but also altered fast during quick activities such as running or swimming, by adjusting the phasic postural reflexes — and all of those are integrated in the midbrain.
Draw it: the two locomotion generators
Three boxes in a vertical chain. Top: motor cortices, with an arrow down labelled “corticobulbar”. Middle: mesencephalic locomotion generator (midbrain). Bottom: split it into two boxes side by side — cervical CPG (upper limbs) and lumbar CPG (lower limbs) — joined to the midbrain box by one arrow each. From each spinal box run an arrow to a limb, and beside it write “alternate flexor and extensor contraction”. Mark the midbrain-to-cord arrow “mainly reticulospinal”. Then draw the spinal animal’s cut across below the medulla and note “MLG drive lost — CPG now needs an outside stimulus”.
Exam-answer skeleton: "Righting reflexes" (short note)
- Define: a series of phasic postural reflexes that correct body position and hold it by keeping the head upright; integrated in the midbrain.
- Why a steady head matters: it keeps the eyes on target and gives posture its reference point.
- Receptors used: vestibular, proprioceptive, tactile and visual; the four kinds of stimulus.
- Name all five and their responses, in sequence — labyrinthine (head level), body on head (head righted, survives labyrinthectomy), neck (thorax and pelvis follow), body on body, limb.
- Optical righting reflex: works with eyes open, centre is the visual cortex, not the midbrain.
- Centres and receptors as a table; limb righting reflex through the red nucleus.
- Evidence for the level: absent in the decerebrate animal, present in the midbrain animal, still present after decortication.