Your Brain, Region by Region

Chapter 2 The back and the base of the brain

The cerebellum and the brainstem

The cerebellum corrects movements you have already started. The brainstem runs the things you never think about. Look at both from the midline.

7 min read · 5 interactive figures

The schematic brain cut down the middle and seen from the left, so the cerebellum, the three levels of the brainstem and the thalamus show in section. Tap a part to name it; drag to turn the model; choose another view under View from.

The cerebellum ("little brain") is the folded structure tucked under the back of the cerebrum. It compares the movement you meant to make with the movement you are actually making, and corrects the difference, which is why it matters for balance, coordination and learning new movements[1][2]. The brainstem is the stalk that joins the brain to the spinal cord. From top to bottom it is the midbrain, the pons and the medulla oblongata, and between them they run breathing, heart rate and blood pressure, sleep and waking, and most of the nerves of the head[3][4].

The figure at the top cuts the brain down the middle, the clearest way to see both. The cerebellum is in section at the back; the brainstem runs down from under the thalamus to the point where it narrows into the spinal cord.

The cerebellum: weight versus neurons

The cerebellum looks like a smaller copy of the cerebrum: two hemispheres, covered in tight, parallel folds called folia, with gray matter outside and white matter inside[3]. It accounts for about 10 percent of the brain's mass[1]. That makes the next question worth a guess.

Question 1 Prediction: where the neurons are#
Figure 1 The whole brain as 100 squares, split three ways: cerebral cortex, cerebellum and the rest (brainstem, thalamus and the other deep structures). It opens counted by weight; switch to Neurons to recount it by number of nerve cells. Shares by weight from OpenStax and Azevedo et al. (2009); neuron counts from Azevedo et al. (2009).#

The answer surprises nearly everyone. In 2009 Frederico Azevedo, Suzana Herculano-Houzel and their colleagues counted the cells of four adult male brains by dissolving tissue into a suspension of nuclei and counting those that carry a neuron-specific marker. They found 86.1 billion neurons on average, not the round 100 billion often quoted. Only 16.3 billion were in the cerebral cortex, which makes up 82 percent of the brain's mass; 69.0 billion were in the cerebellum[5]. established

What the cerebellum does: correcting movements

Walking across a room seems effortless because something is checking it. OpenStax describes the cerebellum's job as a comparison: it receives a copy of the motor command the cerebrum sends to the muscles and, through the spinal cord, sensory feedback from the moving body. When the two disagree, because the ground is uneven or a gust of wind pushes you, it sends a correction[1]. It also helps keep balance and posture using signals from the inner ear[3], and it is thought to be important for procedural memory, the kind involved in learning how to do things[2].

The clearest way to see that correction at work is to give someone a fixed error and watch them learn to cancel it. In experiments published in 1996, Thomas Martin, W. Thomas Thach and their colleagues at Washington University in St. Louis had people throw balls of clay at a target while wearing wedge-prism spectacles, which bend the view sideways. At first the throws went where the bent gaze pointed and missed the target to that side[6].

Question 2 Prediction: after the prisms come off#
Figure 2 Each dot is one throw, plotted as how far it missed the target. Press Throw or Throw 10 with the prisms on, watch the misses shrink, then press Take the prisms off. Switch the thrower to Cerebellar damage to compare. The learning rule is a simple illustrative model, not a fit to the 1996 data.#

The pattern is what Martin and colleagues saw: within 10 to 30 throws people adapted and hit the target, and when the prisms came off they missed to the opposite side until they adapted back[6]. Patients with damage to the cerebellum or to the pathways that feed it had "impaired or absent prism adaptation"[7]. Some of them could hardly adapt at all while showing little clumsiness in ordinary movement, and others were clumsy but still adapted, which tells you that different parts of the cerebellar system handle different parts of the job[7]. established

The brainstem, level by level

The brainstem is small, but almost every signal between the brain and the body passes through it, and ten of the twelve cranial nerves attach to it[3][4]. A loose network of gray matter running through all three levels, the reticular formation, is involved in sleep and wakefulness, arousal and attention[1][2].

Figure 3 The brainstem seen from the side, front to the left, drawn from the same outline as the 3D model. Choose a level, or tap it, to read what it does and which cranial nerves attach there. Nerves I and II (smell and sight) attach to the forebrain instead.#

Midbrain

The top of the brainstem. Two pairs of bumps on its back, the colliculi, do early processing for vision (superior colliculi) and hearing (inferior colliculi). The oculomotor (III) and trochlear (IV) nerves leave it to move the eyes and eyelids. Deeper inside are the red nucleus, which helps coordinate movement, the periaqueductal gray, involved in suppressing pain, and the substantia nigra, whose dopamine-making neurons are the ones lost in Parkinson's disease[4]. OpenStax adds that the midbrain helps line up maps of visual, auditory and touch space, so that the eyes can turn toward a sound[1].

Pons

The name is Latin for "bridge." The pons is the main connection between the brainstem and the cerebellum: descending signals enter the cerebellum through the large white-matter bulge of the pons[1]. It holds four cranial nerves: the trigeminal (V, feeling in the face and chewing), the abducens (VI, eye movement), the facial (VII) and the vestibulocochlear (VIII, hearing and balance)[3]. It also helps regulate brain activity during sleep[2].

Medulla oblongata

The lowest part of the brainstem, only about 3 cm long, where the brain becomes the spinal cord[3]. It contains control centers for heart rate, blood pressure and breathing, and for reflexes such as swallowing and sneezing[3]; OpenStax describes the pons and medulla together as regulating the cardiovascular and respiratory systems[1]. The last four cranial nerves leave here: glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal (XII), which moves the tongue[3]. established

These are the "vital" functions in the literal sense, which is why injury low in the brainstem is so much more dangerous than an injury of similar size in the cortex. A person can lose a large part of one frontal lobe and live for years, as the case of Phineas Gage shows; the medulla has no such reserve.

How the two work together

The cerebellum and the brainstem are wired into each other. Commands from the motor cortex reach the cerebellum by way of the pons; the cerebellum's corrections leave through the midbrain, which sends them down the spinal cord to the muscles[1]. That loop is what Martin's prism experiments interrupted: damage at several points in the brainstem pathways that feed the cerebellum impaired adaptation as surely as damage to the cerebellum itself[7].

This page describes normal anatomy and published experiments for education. It is not medical advice; anyone worried about balance, coordination, swallowing or breathing should see a doctor.