Know your brain: Septum

Where is the septum?

the septum, with separate indicators for the septum pellucidum and septal nuclei.

The term septum, when used in reference to the brain (it is a common anatomical term used to refer to a partition), indicates a subcortical structure in the forebrain that is found near the midline of the brain. The septum in humans can be separated into two structures: the septum pellucidum and septum verum. Each of these is sometimes referred to on is own as the "septum," which can make references to the structure a bit confusing.

The septum pellucidum, which is Latin for “translucent wall,” is a thin, almost transparent membrane that runs down the middle of the brain from the corpus callosum to the area of a large fiber bundle called the fornix. The septum pellucidum acts as a partition between a portion of the lateral ventricles, forming part of the walls of the anterior region of the lateral ventricles. It is made up of a thin two-layered structure that consists of white matter, some neurons, fiber bundles, and blood vessels. The septum pellucidum is surrounded by neurons that make up the septum verum, which consists of assorted nuclei commonly referred to as the septal nuclei. The septal nuclei themselves are often categorized based on location and are split up into lateral and medial (and sometimes additional caudal and ventral) divisions.  

What is the septum and what does it do?

Little is known about the functional role of the septum pellucidum, and it is often treated as simply an anatomical barrier in many discussions of the septum. However, its connections with the hippocampus and hypothalamus suggest a role at least as a relay station between these structures. Although abnormalities of the septum pellucidum are associated with several neurological conditions, it is at this point unclear what role, if any, the septum pellucidum plays in directly generating the symptoms of such disorders.

A bit more is known about the actions of the septal nuclei, which seem to be involved in a variety of functions, although their exact role in many of these functions is still relatively poorly understood. Additionally, most of what we do know about the septal nuclei comes from animal studies, as there is little research available on the functions of the septal nuclei in humans.

The septal nuclei are considered part of the limbic system, a group of subcortical structures that are often linked to emotion but are really involved in a long list of functions in the human brain. The septal nuclei receive afferent (i.e. incoming) connections from other limbic structures like the hippocampus, amygdala, and hypothalamus, as well as the dopamine-rich ventral tegmental area. The septal nuclei also send projections to the hippocampus, habenula, thalamus, ventral tegmental area, and hypothalamus.

One of the first functional roles to be linked to the septal nuclei was an involvement in processing rewarding experiences. In a now-famous group of experiments, researchers James Olds and Peter Milner found that electrical stimulation of the septal nuclei and several other areas of the brain seemed to be rewarding to rats. Rats, in fact, responded more strongly to stimulation in the septal region than any other part of the brain studied, leading Olds and Milner to hypothesize the septal region was perhaps the locus of the reward system.

Although our understanding of the reward system has since expanded and less importance has been placed on the septal nuclei in favor of other structures like the ventral tegmental area and nucleus accumbens, the septal nuclei are still thought to potentially play a role in reward processing. Neurons in the septal nuclei give rise to axons that travel in the medial forebrain bundle, a collection of fibers that connects the nuclei with the hypothalamus and ventral tegmental area. The medial forebrain bundle is an important part of the reward system, thought to stimulate dopamine neurons in the ventral tegmental area in response to rewarding stimuli.

The septal nuclei also are densely interconnected with the hippocampus, and through these connections may play a role in learning and memory. The septal nuclei and hippocampus are sometimes referred to as the septo-hippocampal complex, and projections to the hippocampus (which travel in a fiber bundle called the fornix and are often called septohippocampal fibers) are some of largest projections from the septal region. Although the precise role of the septal nuclei in memory functions is not yet clear, the hippocampus receives the majority of its acetylcholine projections from the septal region. These neurons are activated during learning and degenerate in conditions like Alzheimer's disease that are characterized by disruptions in memory processes.

The septal nuclei have been implicated in a number of other roles such as social behavior and the expression of fear, and abnormalities in septal functioning have been linked to a variety of disorders ranging from depression to schizophrenia. The septal area, however, including both the septum pellucidum and septum verum/septal nuclei, is still relatively poorly understood; it will take more research to fully elucidate its functions and influence on behavior.

Sheehan, T., Chambers, R., & Russell, D. (2004). Regulation of affect by the lateral septum: implications for neuropsychiatry Brain Research Reviews, 46 (1), 71-117 DOI: 10.1016/j.brainresrev.2004.04.009

2-Minute Neuroscience: Primary Somatosensory Cortex

In this video, I discuss the primary somatosensory cortex. The primary somatosensory cortex is responsible for processing somatic sensations, or sensations from the body that include touch, proprioception (i.e. the position of the body in space), nociception (i.e. pain), and temperature. The primary somatosensory cortex is generally divided into 4 areas: area 3a, 3b, 1, and 2. In the video, I discuss the relative functions of each of these areas. 

Read more - Know your brain: Primary somatosensory cortex

2-Minute Neuroscience: Suprachiasmatic Nucleus

The suprachiasmatic nuclei (SCN) are thought to be involved with maintaining circadian rhythms, or biological patterns that follow a 24-hour cycle. To accomplish this, the cells of the SCN contain biological clocks. In this video, I discuss the molecular mechanism driving the biological clocks in the cells of the mammalian SCN, and how a cycle of gene expression allows the activity of these cells to follow a 24-hour pattern. 

You can read more about the suprachiasmatic nucleus in this Know Your Brain article.

Know your brain: Pons

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.

Learn more:

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.