In one breath
The brainstem sends its own tracts to the spinal motor neurons: the rubrospinal tract helps the lateral corticospinal tract with skilled distal movement, while the vestibulospinal, reticulospinal and tectospinal tracts keep posture through the proximal and axial muscles. The lateral vestibulospinal and pontine reticulospinal tracts excite the extensors, and the medullary reticulospinal tract inhibits them. All the medial tracts stay on their own side except the tectospinal, and two monoaminergic tracts, the raphespinal and ceruleospinal, damp pain and tune motor neuron excitability.
Builds on: Corticospinal (pyramidal) tract · Segmental organization of the motor system · Leads to: Medullary integration and decerebrate rigidity · Midbrain integration and righting reflexes · Endogenous pain control and gate control theory
These are the tracts once called “extrapyramidal”. Why that name misleads is on the corticospinal tract page.
Rubrospinal tract
- Origin: the red nucleus in the midbrain. It receives strong excitatory input from the motor cortex and the cerebellum.
- Course:
- the fibres cross at once, at the level of the nucleus;
- they descend through the opposite half of the brainstem into the lateral column of the cord;
- they end on the lateral group of motor neurons, which supply the distal limb muscles.
- Action: excites flexors and inhibits extensors.
- Function: skilled voluntary movement, alongside the lateral CST.
- Both lateral-system tracts, the lateral CST and the rubrospinal, cross to the opposite side.
Applied: rubrospinal lesion
In experimental animals, cutting the tract weakens the distal limb muscles, the flexors most of all. If the lateral CST is intact the deficit is only temporary. It works the other way too: after a lateral CST lesion, skilled movement recovers because the rubrospinal tract is still there.
Vestibulospinal tracts
- They arise from the vestibular nuclei, of which there are four: lateral, medial, superior and inferior.
- The nuclei receive input from the otolith organs and the semicircular canals, so these tracts adjust posture to head position and to acceleration of the body.
- The nuclei are also linked both ways with the superior colliculi, cerebellum and reticular formation. Through these links they hold the eyes steady during head movement and keep the body balanced while it moves.
| Lateral VST | Medial VST | |
|---|---|---|
| Origin | Deiters’ nucleus | Medial vestibular nucleus |
| Side | Uncrossed | Uncrossed |
| Reaches | Whole cord | Midthoracic cord |
| Main input | Canals and otoliths | Canals |
| Job | Extensors ↑, posture | Head position |
Lateral vestibulospinal tract
- Origin: the lateral vestibular nucleus, also called Deiters’ nucleus.
- Course: uncrossed, through the brainstem and down the ventral funiculus of the cord. It runs the full length of the cord and ends on the medial group of interneurons and motor neurons.
- Function:
- it excites the motor neurons of proximal muscles, especially the limb extensors, and so maintains posture;
- its input comes mainly from the canals and otoliths, so it adjusts posture to both angular and linear acceleration of the head.
Applied: decerebrate rigidity
After a midcollicular section in animals, the lateral vestibulospinal tract loses the inhibitory control of the cortex and becomes overactive. Extensor rigidity is therefore the hallmark of the decerebrate animal (Medullary integration and decerebrate rigidity). In humans, a brainstem stroke or injury releases the same tract, and the neck, arms and legs become rigid.
Medial vestibulospinal tract
- Origin: the medial vestibular nucleus.
- Course: uncrossed, in the brainstem and the ventral funiculus, only as far down as the midthoracic cord. It ends on the medial group of interneurons and motor neurons.
- Function: posture, especially the position of the head, in response to angular acceleration. Its input comes mainly from the semicircular canals.
Reticulospinal tracts
- The reticular formation fills the core of the brainstem.
- Inputs: spinal cord, vestibular nuclei, cerebellum, hypothalamus, tectum and cortex.
- Outputs: mainly to the cortex, thalamus and spinal cord.
- Its spinal projection strongly changes how excitable the motor neurons are, especially the γ motor neurons.
- Two nuclei matter here: nucleus reticularis pontis in the pons and nucleus gigantocellularis in the medulla. They give the pontine and medullary reticulospinal tracts (RSTs).
- The RSTs are the most important medial-system pathways for posture.
| Pontine RST | Medullary RST | |
|---|---|---|
| Origin | N. reticularis pontis | N. gigantocellularis |
| Funiculus | Medial | Ventral |
| Side | Uncrossed | Uncrossed |
| Ends on | Medial group | Medial group |
| Effect | Excites extensors | Inhibits extensors |
Pontine reticulospinal tract
- Origin: nucleus reticularis pontis oralis and caudalis, in the pontine reticular formation.
- Course: uncrossed, in the medial funiculus. It ends on the medial group of interneurons and motor neurons that supply the proximal and axial muscles.
- Function: it works like the lateral vestibulospinal tract, driving the motor neurons of the proximal extensors that hold posture.
Medullary reticulospinal tract
- Origin: nucleus gigantocellularis, in the reticular formation of the medulla.
- Course: uncrossed, in the ventral funiculus, to the medial group of interneurons and motor neurons that supply the proximal muscles.
- Function: mainly inhibitory. It inhibits the motor neurons of the extensor muscles.
Pontine excites, medullary inhibits
Pontine RST → extensors ↑, like the lateral vestibulospinal tract. Medullary RST → extensors ↓. The cortex normally restrains the pontine tract through corticoreticular fibres; a UMN lesion removes that restraint, and spasticity follows (Upper vs lower motor neuron lesions).
Tectospinal tract
- Origin: the deep layers of the superior colliculus (the tectum).
- Course:
- the fibres cross at once, just below the periaqueductal grey;
- they descend in the ventral funiculus and end on the medial group of interneurons and motor neurons;
- it is the smallest descending tract, reaching only the midcervical cord.
- Function: the superior colliculus receives mainly visual input, so this tract turns the head towards the opposite side in response to what is seen.
Which tracts cross?
- Cross: the lateral CST (in the medulla), the rubrospinal and the tectospinal (both at once, at their own level).
- Stay on their own side: the vestibulospinal and reticulospinal tracts, and the raphespinal and (mostly) ceruleospinal tracts. The anterior CST crosses only in the cord, through interneurons.
- Rule of thumb: both lateral-system tracts cross; of the medial ones, only the tectospinal does.
Monoaminergic pathways
Two more descending tracts carry monoamines, and they act on sensation as well as movement.
Raphespinal tract
- Origin: nucleus raphe magnus, in the medulla.
- Course: uncrossed.
- Most fibres end on dorsal-horn interneurons that block transmission in the pain pathway.
- A few end on ventral-horn interneurons that excite the motor neurons.
- Function: the fibres are serotonergic. They are part of the endogenous pain-inhibiting system (Endogenous pain control and gate control theory), and through the motor neurons they help sensory–motor coordination in the cord.
Ceruleospinal tract
- Origin: the locus ceruleus and the nucleus subceruleus.
- Course: mostly uncrossed, to spinal interneurons and motor neurons.
- Function: the fibres are noradrenergic. They inhibit the nociceptive afferents and the motor neurons, change how excitable the motor neurons are to different inputs, and so help sensory–motor coordination in the cord.
All the tracts at a glance
| Tract | Starts in | Crosses? | Main effect |
|---|---|---|---|
| Rubrospinal | Red nucleus | Yes | Flexors ↑; skilled movement |
| Lateral VST | Deiters’ nucleus | No | Extensors ↑; posture |
| Medial VST | Medial vestibular n. | No | Head position |
| Pontine RST | N. reticularis pontis | No | Extensors ↑ |
| Medullary RST | N. gigantocellularis | No | Extensors ↓ |
| Tectospinal | Superior colliculus | Yes | Head turns to a visual stimulus |
| Raphespinal | Raphe magnus | No | Pain ↓ (serotonin) |
| Ceruleospinal | Locus ceruleus | Mostly no | Pain ↓ (noradrenaline) |
Draw it: where the descending tracts run in the cord
- Draw a transverse section of the cord: the grey H with its ventral horns below, and the white matter divided into lateral and anterior funiculi on each side.
- In the ventral horn, mark two clusters of motor neurons: a lateral group (“distal muscles”) and a medial group (“axial and proximal muscles”).
- In the lateral funiculus, draw the lateral CST and the rubrospinal tract, with arrows to the lateral group.
- In the anterior and medial funiculi, draw the anterior CST, the vestibulospinal, reticulospinal and tectospinal tracts, with arrows to the medial group.
- Label the two sides of the drawing “lateral system: skilled movement” and “medial system: posture”.
Exam-answer skeleton: "Vestibulospinal and reticulospinal tracts: origin, course and functions" (short note)
- Place them: medial-system tracts that keep posture through the proximal and axial muscles.
- The four vestibular nuclei and their inputs (otoliths, canals), and their links with the colliculi, cerebellum and reticular formation.
- Lateral VST: Deiters’ nucleus, uncrossed, whole cord, excites limb extensors.
- Medial VST: medial nucleus, uncrossed, to the midthoracic cord, head position in angular acceleration.
- The reticular formation: inputs, outputs, and its strong effect on γ motor neurons.
- Pontine RST (excites extensors) vs medullary RST (inhibits extensors), with origin and funiculus.
- Applied: decerebrate rigidity from the released lateral VST; spasticity from the released pontine RST in UMN lesions.