All parts of the brain › Reticular formation

Where is the reticular formation?

The reticular formation is a net-like set of interconnected nuclei in the core of the brainstem, running from the lower end of the medulla oblongata through the pons to the upper end of the midbrain. Its upward-projecting part is the reticular activating system, which keeps the brain awake. [1]

The 3D brain needs JavaScript. The text below still says where the reticular formation is and what it does.

Lit: the whole midbrain, pons and medulla oblongata.

Slice

The midbrain, pons and medulla oblongata are lit together; the reticular formation is not drawn on its own. The side (sagittal) slice button cuts through them. Drag to turn, pinch or scroll to zoom, tap any part to name it; .

Not a separate shape in this atlas; lit: the part that holds it. Lit: the whole midbrain, pons and medulla oblongata. The reticular formation is a diffuse network of cells in the core of the midbrain, pons and medulla oblongata, not a compact shape; it has no shape of its own in the BodyParts3D model, so the three parts that hold it are lit and nothing is drawn at a guessed point. [1][2]

Lit here: the midbrain and the pons and the medulla oblongata, the parts that hold the reticular formation — see each on its own page.

Next to it, each lit on its own page: raphe nuclei, locus coeruleus.

Where it is

It spans the brainstem from the lower end of the medulla oblongata to the upper end of the midbrain, filling the core between the more compact, named nuclei; it can be seen as all the scattered cells of the brainstem between those structures. [1]
It is traditionally divided into three columns: the raphe nuclei on the midline, the gigantocellular (large-cell) nuclei in a medial column, and the parvocellular (small-cell) nuclei in a lateral column. By level it is named the midbrain, pontine and medullary reticular formation. [1]
The lateral column sits close to the motor nuclei of the cranial nerves, and is most pronounced in the upper medulla and lower pons. [1]

What it does

Wakefulness: the ascending reticular activating system sends projections through the thalamus and hypothalamus to the cortex and acts like an on/off switch for the cortex, keeping it awake and alert; during sleep its neurons fire much more slowly. [1]
Its overall functions are modulatory and premotor: control of body movement and posture, of heart rate and blood pressure, modulation of pain signals, sleep and consciousness, and habituation (tuning out repeated, unimportant input). [1]
The descending part sends the reticulospinal tracts to the spinal cord, which take part in posture and movement; the parvocellular nuclei help regulate breathing out. [1]
A small part of it, the paramedian pontine reticular formation beside the abducens nucleus in the pons, is the horizontal gaze centre that turns both eyes left or right together. [1][3]

Connections

It receives input from many sources and projects to the forebrain, brainstem and cerebellum; its ascending fibres reach the cortex directly and through relays in the thalamus, and its arousal projections reach the prefrontal cortex most of all. [1]
The arousal network uses several transmitters, each from its own group of nuclei: noradrenaline (the locus coeruleus), serotonin (the raphe nuclei), dopamine, histamine, acetylcholine and glutamate; orexin neurons of the lateral hypothalamus reach every part of it. [1]

Other names

“Reticular activating system” (RAS), in full the ascending reticular activating system (ARAS), is the reticular formation’s upward arousal system rather than another name for the whole: it is a network of more than 20 nuclei on each side in the upper brainstem, pons, medulla and back of the hypothalamus, not one nucleus. [1]
“Reticular” means net-like (Latin reticulum, a little net), after the way its cells and fibres are woven through the brainstem; in all, it is made of almost 100 nuclei. [1]

Size

The model has no shape for the reticular formation itself, so it is not measured here. The midbrain that holds it measures about 29 mm front to back, 17 mm top to bottom and 29 mm side to side on this model (the box it fits in, not its volume). [4]
The model has no shape for the reticular formation itself, so it is not measured here. The pons that holds it measures about 34 mm front to back, 29 mm top to bottom and 42 mm side to side on this model (the box it fits in, not its volume). [4]
The model has no shape for the reticular formation itself, so it is not measured here. The medulla oblongata that holds it measures about 33 mm front to back, 41 mm top to bottom and 27 mm side to side on this model (the box it fits in, not its volume). [4]

Sources

Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.