Know your brain: Pons


Where is the pons?

The pons is a major division of the brainstem. It is found above the medulla and below the midbrain, and is anterior to (in front of) the cerebellum. Pons is Latin for "bridge"; the structure was given its name by the Italian anatomist Costanzo Varolio, who thought that the most conspicuous portion of the pons resembled a bridge that connected the two cerebellar hemispheres. This part of the pons is now referred to as the basal or basilar pons; not only is it the most distinct area of the pons, it is also one of the more recognizable areas of the brain. Posterior to (behind) the basal pons is an area sometimes called the dorsal pons or pontine tegmentum. Much of this area is also considered part of the reticular formation. The pontine tegmentum includes the tissue between the basal pons and the fourth ventricle; the pons makes up the floor of the fourth ventricle.

What is the pons and what does it do?

Instead of attempting to identify one overall function (or even a short list of functions) for the pons, it is better to think of the structure as a collection of various tracts and nuclei, all with their own functions. Although describing the pons in this way may make it sound like the pons is involved with a confusing hodgepodge of activities, it is a more accurate approach than attempting to summarize the functions of the pons in just a few actions. 

The most prominent feature of the pons is the bridge-like portion of it from where its name is derived. Despite appearing like a bridge, however, the basal pons is not a direct connection between the two cerebellar hemispheres. Instead, fibers that travel down from the cortex (i.e. corticopontine fibers) synapse on a variety of nuclei here called pontine nuclei. Then, groups of fibers project from the pontine nuclei on one side of the pons, cross to the other side of the pons, and come together to form the middle cerebellar peduncles. The middle cerebellar peduncles are large bundles of fibers that connect the pons to the cerebellum, which thus make up the connecting portions of the "bridge." They represent one of the major pathways for information to travel from the brain and brainstem to the cerebellum.

The pons is home to several cranial nerve nuclei and fibers. These include the main sensory nucleus of the trigeminal nerve and the motor nucleus of the trigeminal nerve---a nerve responsible for sensory and motor functions of the head and face. The abducens nucleus, which controls lateral movements of the eye, is also found in the pons. The facial nucleus, which gives rise to the facial nerveinnervates muscles involved in facial expression and carries sensory information from the mouth. The vestibulocochlear nerve, which carries information about hearing and vestibular senses, enters the brainstem at the junction of the pons and medulla to synapse on various nuclei in these two areas.

The pons also contains groups of neurons that are important to major neurotransmitter systems in the brain. For example, the locus coeruleus (Latin for "blue place" and named for the pigment that gives these neurons a blue-black color in unstained brain tissue) is found in the pons. The locus coeruleus is the largest collection of norepinephrine-containing (aka noradrenergic) neurons in the central nervous system. Noradrenergic neurons leave the locus coeruleus and project throughout the brain and spinal cord. Activity in the locus coeruleus is low during sleep and high during states of arousal (e.g. acute stress like a threatening situation). Projections from the locus coeruleus to a nearby region (sometimes called the subcoruleus region) of the pons also help to regulate rapid eye movement (REM) sleep. Indeed, the subcoeruleus region of the pons is considered to be the most critical region for REM sleep in the brain, and damage to this area has been shown to eliminate REM sleep in experimental animals. The raphe nuclei, clusters of cells that contain serotonin, are also found in the pons (and throughout much of the brainstem).

Due to its central location between the brain and spinal cord, the pons also serves as a conduit for many tracts passing up and down the brainstem. Tracts like the corticospinal tract for voluntary movement, medial lemniscus for tactile and proprioceptive sensations, and anterolateral system for painful sensations, all pass through the pons. 

Thus, due to the diversity of tracts and nuclei found within the pons, the structure is involved with a long list of functions ranging from facial expressions to sleep. The pons is therefore not only one of the most visibly distinct parts of the brain due to the bridge-like appearance of the basal pons, it is also one of the most important.

Haines DE. Fundamental Neuroscience for Basic and Clinical Applications. 4th ed. Philadelphia, PA: Elsevier; 2013.

Vanderah TW, Gould DJ. Nolte's The Human Brain. 7th ed. Philadelphia, PA: Elsevier; 2016.

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2-Minute Neuroscience: The Brainstem

Know your brain: Brainstem

2-Minute Neuroscience: Nucleus Accumbens

In this video, I discuss the nucleus accumbens. The nucleus accumbens is located in the basal forebrain, and is the major component of the ventral striatum. Although it is best known as a key structure in the reward system, the role of the nucleus accumbens in reward is still not fully understood. This is due in part to the fact that the nucleus accumbens also seems to be activated in response to aversive stimuli, and thus some have suggested that it is involved in responses to all motivationally-relevant stimuli---whether positive or negative.

Know your brain: Suprachiasmatic nucleus

Where is the suprachiasmatic nucleus?

the suprachiasmatic nucleus is represented by a small green area within the hypothalamus (indicated by red arrow).

the suprachiasmatic nucleus is represented by a small green area within the hypothalamus (indicated by red arrow).

The suprachiasmatic nuclei are two small, paired nuclei that are found in the hypothalamus. Each suprachiasmatic nucleus only contains approximately 10,000 neurons. The nuclei rest on each side of the third ventricle, just above the optic chiasm. The location provides the rationale for the naming of the structure, as supra means above and chiasmatic refers to its proximity to the optic chiasm. 

What is the suprachiasmatic nucleus and what does it do?

Circadian rhythms are biological patterns that closely follow a 24-hour cycle. The term circadian comes from the Latin for around (circa) and day (diem), and circadian rhythms govern a large number of biological processes including sleeping, eating, drinking, and hormone release. In the 1960s, researchers noticed that damage to the anterior hypothalamus of the rat caused a disruption in the animal's circadian rhythms. Several years later, the specific nucleus in the hypothalamus whose integrity was necessary for maintaining circadian rhythms was identified as the suprachiasmatic nucleus.

We now know that the suprachiasmatic nucleus houses a type of biological clock that is able to keep our circadian rhythms on close to a 24-hour cycle, even without the help of external cues like daylight. Thus, if you were to lock someone in a room with no external light and no other way of telling the time, her body would still maintain a circadian rhythm of around 24 hours. The mechanism that regulates this biological clock was first elucidated in Drosophila, more commonly known as the fruit fly.

In Drosophila, the timekeeping of the circadian clock appears to be controlled by a cycle of gene expression that has an ingenious negative feedback mechanism built into it. Cells in the suprachiasmatic nuclei of Drosophila produce two proteins called Clock and Cycle. Clock and Cycle bind together and act to promote the expression of two genes called period (per) and cryptochrome (cry). The protein products of these two genes, Per and Cry, then bind together and proceed to inhibit the actions of Clock and Cycle, an action which in turn suppresses the production of Per and Cry. Gradually, however, the Per and Cry proteins begin to break down. When Per and Cry degrade fully, Clock and Cycle are free to act again; they go back to promoting the expression of per and cry, starting the cycle anew. The process consistently takes around 24 hours to complete before it repeats. Thus, it is thought that this cycle of gene expression is what acts as the molecular clock in Drosophila suprachiasmatic nucleus cells.

This understanding of the timekeeping mechanism in Drosophila laid the foundation for elucidating the process in mammals, where it is thought to be similar but more complex. The mammalian version of Cycle is called BMAL1 (which stands for brain and muscle ARNT-like 1). When CLOCK and BMAL1 bind together, they enhance the transcription of a family of genes that includes multiple period (Per1, Per2, and Per3) and cryptochrome (Cry1 and Cry2) genes. The resultant PER and CRY proteins form complexes along with other proteins to inhibit the activity of CLOCK and BMAL1 until the PER and CRY proteins degrade (as above). 

We know that this process of gene expression and inhibition acts to keep a 24-hour clock within the neurons of the suprachiasmatic nucleus, but less is known about how the timekeeping within these neurons leads to the regulation of rhythmic activity throughout the body. It is believed, however, that the molecular clocks found within the neurons of the suprachiasmatic nuclei regulate neural activity within the nuclei, which in turn coordinates the activity of multiple signaling pathways as well the stimulation of projections to neuroendocrine neurons in the hypothalamus involved with hormone release.

Additionally, the suprachiasmatic nucleus helps to maintain circadian rhythms by coordinating the timing of billions of other circadian clocks found in cells throughout the rest of the brain and body. Not long after the discovery of the suprachiasmatic nucleus, it was also learned that similar types of molecular clocks exist in most other peripheral tissues and in many areas of the brain. These clocks, sometimes called slave oscillators (while the suprachiasmatic nucleus is considered the master oscillator) appear to depend on signals generated by the suprachiasmatic nucleus to synchronize their time-keeping with that of the suprachiasmatic nucleus. These signals can be associated with rhythms that the suprachiasmatic nucleus helps to establish, like feeding patterns, rest and activity behaviors, etc., or by direct neuronal or hormonal output from the suprachiasmatic nucleus.

Although the suprachiasmatic nucleus is capable of maintaining circadian rhythms independently of any environmental signals (e.g. daylight), it does rely on cues from the environment to make adjustments to the circadian clock. For example, when you fly across multiple time zones, your body's circadian clock becomes significantly out of sync with the timing of the day (e.g. your body might be preparing for sleep when it is still light out). To make adjustments to the circadian clock in such instances, the suprachiasmatic nucleus relies on information it receives from the retina about light in the environment. Such information travels from the retina to the suprachiasmatic nucleus along a path called the retinohypothalamic tract. Additional inputs to the suprachiasmatic nucleus provide more information about light in the environment and other non-photic information about time of day to help to adjust the circadian clock.

Due to the importance of our circadian rhythms to normal functioning, the integrity of the suprachiasmatic nucleus is essential to health. Disrupted function of the suprachiasmatic nucleus is being explored as a potential influence in a variety of psychiatric disorders as well as a factor in age-related decline in healthy sleep. Thus, although we have much more to learn about the suprachiasmatic nucleus, it is clear that it plays a very critical role in healthy brain and bodily function.

Colwell, C. (2011). Linking neural activity and molecular oscillations in the SCN Nature Reviews Neuroscience, 12 (10), 553-569 DOI: 10.1038/nrn3086

Dibner, C., Schibler, U., Albrecht, U. (2010). The mammalian circadian timing system: organization and coordination of central and peripheral clocks Annual review of physiology, 72 (1), 517-549.

2-Minute Neuroscience: Ventral Tegmental Area

In this video, I discuss the ventral tegmental area, or VTA. The VTA is one of the two largest dopaminergic regions of the brain (the other being the substantia nigra). Dopamine neurons leave the VTA in several different pathways and project throughout the brain. The mesocortical pathway projects from the VTA to widespread areas of the cerebral cortex and has diverse functions including motivation, emotion, and executive functions. The mesolimbic pathway projects from the VTA to several limbic structures; the largest projection is to the nucleus accumbens. The mesolimbic pathway also has diverse functions but is best known for its role in processing rewarding stimuli.

2-Minute Neuroscience: Medulla Oblongata

In this video, I discuss the medulla oblongata. The medulla is part of the brainstem and is responsible for a number of important functions. It is involved with regulating cardiovascular and respiratory functions as well as a variety of reflexive actions like swallowing, coughing, and vomiting. It is also home to an assortment of important nuclei and cranial nerve nuclei. Finally, a number of tracts like the corticospinal and corticobulbar tracts pass through the medulla on their way from the brainstem to the spinal cord and vice versa.