Diencephalon

The diencephalon may be divided into four strata: the subthalamus (light tan), the epithalamus (dark tan), the thalamus (red), and the hypothalamus (green).

The subthalamus is a continuation of the anterior midbrain.

It is located inferior to the thalamus, lateral to the hypothalamus, and anterior to the epithalamus.

Subthalamus It contains nuclei of the extrapyramidal motor system and

may be regarded as the motor zone of the diencephalon. It is believed to by the body’s pacemaker and has been

implicated in Parkinson’s disease. Deep brain stimulation of the subthalamus is so dramatic

in reversing the symptoms of PD that it has altered the thinking on Parkinson’s disease.

Surgical destruction or stimulation of the subthalamus with deep brain stimulation has led to the disappearance of the involuntary, writhing movements (dyskinesias) that result from prolonged and excessive levodopa use.

Deep brain stimulation of the subthalamus has been found to reverse the slowness and poverty of movement of PD.

Subthalamus

It is now believed that PD may begin in the subthalamus and not the substantia nigra

In those without PD, damage to the subthalamic nucleus produces involuntary movements which may increase during seizures to violent of the arms or of the entire contralateral side of the body, a disorder known as hemiballismus.

Epithalamus

The epithalamus, posterior to the subthalamus, contains the pineal gland (epiphysis) and the habenular nuclei.

The pineal gland is an endocrine gland shaped like a pine cone and attached to the roof of the third ventricle.

Epithalamus

The gland consists of masses of glial cells and secretory cells called pinealocytes.

The pinealocytes synthesize the hormone melatonin and secrete it directly into the cerebrospinal fluid, which takes it into the blood.

Melatonin affects reproductive development and daily physiologic cycles.

Synthesis and release of melatonin is stimulated by darkness and inhibited by light.

Even without visual cues, the level of melatonin in the blood rises and falls on a daily (circadian) cycle with peak levels occurring in the wee hours of the morning.

Epithalamus

Because the pineal gland lies near the center of the brain, it obtains information about light in the environment through nerve pathways originating in the eyes.

In general, light slows and darkness stimulates the pineal gland's production of melatonin.

Therefore, the gland tends to secrete small amounts of melatonin during the day and larger amounts at night.

In human beings, melatonin has been linked to the onset of puberty.

Studies have shown that the pineal gland's nightly secretion of melatonin decreases when a boy or girl reaches puberty.

Epithalamus

Other studies have indicated that melatonin may help regulate menstrual cycles in women and sperm production in men.

In addition, researchers have suggested a connection between melatonin levels and certain mental illnesses and changes in metabolism and body adiposity in middle age.

Thalamus

The thalamus is an oval structure found above the midbrain that measures about an inch in length.

It constitutes about 4/5 of the diencephalon.

Thalamus

Each thalamus consists of two masses of grey matter organized into nuclei.

The white matter portions of the thalamic masses divide the grey matter into different nuclear groups.

The two thalami are connected to each other by a bridge of grey matter that crosses the third ventricle.

It is called the intermediate mass.

Thalamus

Each group of nuclei assumes a particular “relay” role.

The medial geniculate bodies (dark purple) are the relay stations for auditory stimuli coming into the CNS.

The lateral geniculate bodies (green) are the relay stations for visual stimuli coming into the CNS.

Thalamus

The ventral posterior nuclei (fuchsia) are the relay stations for information about general sensation and taste coming into the CNS.

The outgoing relays in the thalami are the ventral lateral nuclei (tan) for voluntary motor actions and ventral anterior nuclei (red) for involuntary motor actions and arousal.

Hypothalamus

The hypothalamus forms the lowest layer and floor of the diencephalon.

From it the hypophyseal stalk protrudes.

The hypothalamus is the highest regulatory center for the vegetative nervous system.

Hypothalamus

It influences all processes important for the maintenance of homeostasis and regulates the functions of the organs responding to momentary bodily stress: warmth, water and electrolyte balance, cardiac function, circulation and respiration, metabolism, and the sleeping/waking rhythm.

Eating, defecation, gastrointestinal activity, fluid intake, urination, procreation, and sexuality are regulated from this region.

Affective components of pleasure, displeasure, happiness, fear, or anger can be aroused through hypothalamic stimulation.

The hypothalamus is also the conductor of the hormonal symphony.

It kicks the gonads into gear, and starts pumping earlier at puberty in females.

Hypothalamus

The hypothalamus is divided into major regions containing several nuclei.

The infundibulum, contains neurons that transport regulating hormones to the blood vessels and then into the anterior lobe of the pituitary gland.

Hypothalamus

Oxytocin and vasopressin are two closely related hormones made largely in the hypothalamus which are transported by neuronal fibers in the infundibulum to the posterior lobe of the pituitary gland.

Vasopressin regulates the body's retention of water.

It is released when the body is dehydrated and causes the kidneys to conserve water, thus concentrating the urine, and reducing urine volume.

Hypothalamus

It also raises blood pressure by inducing moderate vasoconstriction.

Social attachment behavior is controlled by the neurohormones vasopressin and oxytocin made in the pituitary and the hypothalamus, as well as in the ovaries and testes.

Vasopressin is nature’s cocktail for male attachment and paternal instincts.

The male brain uses vasopressin mostly for social bonding and parenting, whereas the female brain uses primarily oxytocin and estrogen.

Hypothalamus

Unlike vasopressin, oxytocin is released in all female mammals, including women, during the birthing process.

It initiates contractions of the uterus and stimulates the mammary glands to produce milk.

Oxytocin also stimulates bonding between a mother and her infant.

Oxytocin brings out the fluffy, purring kitty; the cuddly, nurturing earth mother; the good with Glinda in The Wizard of Oz.

Hypothalamus

Recent scientific advances have led us to understand that oxytocin is also involved in the feelings of adult-male-female attachment.

In fact, vasopressin and oxytocin are often referred to as the two “satisfaction hormones. “

They are secreted during sexual intercourse during stimulation of the genitals and/or nipples and during orgasm.

Sexual coupling releases large amounts of oxytocin in the female’s brain and vasopressin in the male’s brain.

Hypothalamus

These “cuddle chemicals “ are thought to contribute to that sense of fusion, closeness, and attachment you feel after sweet sex with a beloved (Fisher, 2004).

In both males and females, oxytocin causes relaxation, fearlessness, bonding, and contentment with each other.

And to maintain its effects long-term, the brain’s attachment system needs repeated, almost daily activation through oxytocin stimulated by closeness and touch.

Males need to be touched two to three times more frequently than females to maintain the same level of oxytocin.

Hypothalamus

Without frequent touch, for example, when mates are apart, the brain’s dopamine and oxytocin circuits and receptors can feel starved.

Couples may not realize how much they depend on each other’s physical presence until they are separated for a while.

The oxytocin in their brains keep them coming back to each other, again and again, for pleasure, comfort, and calm.

Hypothalamus

The mammillary region consists of the two small mammillary bodies that serve as relay stations for olfactory neurons involved in reflexes related to the sense of smell.

Afferent projections from the external special sense organs as well as from internal autonomic visceral receptors keep the thalamus apprised of changes occurring in both external and internal environments.

Hypothalamus

Continual monitoring of internal and external events allows the hypothalamus to make changes in vegetative functions.

The dorsal premammillary nucleus is larger in males than in females and contains special circuits to detect territorial changes by other males, making men more sensitive to potential turf threats.

It contains the circuitry for a male’s instinctive one-upmanship, territorial defense, fear, and aggression.

Cerebral Hemispheres

 The cerebrum is supported on the brainstem and forms the bulk of the brain.

Its surface, referred to as the cerebral cortex, is composed of grey matter 2-4 mm thick.

The term cortex (p. cortices) refers to a layer or sheet of nervous tissue made up of neuronal cell bodies.

In most areas of the cortex, there are six layers of nerve cell bodies.

White Matter

Beneath the cortex (stained purple) lies the cerebral white matter.

Nuclei and cortices are interconnected by the axons of projection neurons which are the white matter (tracts) of the CNS.

Convolutions and Fissures

During embryonic development, there is a rapid increase in brain size.

The grey matter of the cortex enlarges out of proportion to the underlying white matter, causing it to roll and fold in upon itself.

The folds are called gyri (s. gyrus) if they are small or convolutions if there are big.

Fissures are deep grooves between the folds, whereas sulci (s. sulcus) are shallow grooves between the folds.

Cerebral Landmarks

Renaissance anatomists found that the most prominent infoldings were invariant from one human brain to the next, so they used these features as landmarks to separate the brain into distinct regions.

The most prominent landmark of the cerebrum is median longitudinal fissure.

It separates the cerebrum along the midline into two fairly symmetrical halves.

Cerebral Hemispheres

These two halves are referred to as the cerebral hemispheres.

They are connected internally by a large bundle of transverse white matter fibers called the corpus callosum.

Central Sulcus

A cleft called the central sulcus (aka Rolandic fissure) separates the cerebral hemisphere into anterior and posterior portion.

The anterior portion is known as the frontal lobe (pink) the posterior portion as the parietal lobe (blue).

All lobes are named for the overlying bones of the skull.

Landmark Gyri

Within each one of these lobes are important gyri, that also serve as landmarks for important processing regions of the lobes.

Immediately anterior to the central sulcus is the precentral gyrus, the primary motor area (M1) of the cerebral cortex of the frontal lobe.

Landmark Gyri

 Immediately posterior to the central sulcus is the postcentral gyrus, the primary sensory area (S1) of the cerebral cortex of the parietal lobe.

Lateral Sulcus

Another consistent cleft within each cerebral hemisphere divides the hemisphere into upper and lower portion.

This cleft is termed the lateral sulcus (aka Sylvian Fissure).

The temporal lobe lies beneath the lateral sulcus.

Heschl’s Gyrus

Heschl’s gyrus is located on the superior temporal gyrus, and extends into the lateral sulcus.

It is the site of the primary auditory area (A1).

Parietal Gyri

Within the parietal lobe are two other prominent gyri.

Curving above the end of the lateral sulcus is the supramarginal gyrus (SmG).

Curving off the end of the lateral sulcus is the angular gyrus (AnG).

Both of these gyri, because of their location, where sensory, auditory, and visual information come together, have important roles in multi-modal processing tasks, such as reading, writing, arithmetic, and construction.

Occipital Lobe

 On the lateral surface of the cerebral hemisphere, there is no clear landmark demarcating the occipital lobe.

However, it is the most posterior lobe that can be visualized on the lateral surface of the cerebrum.

The primary visual area (V1) covers the entire occipital pole.

Calcarine Fissure

On the mesial surface of the hemisphere, however, the primary visual cortex is visible on the banks of the calcarine fissure.

Insula

An additional area of the cerebral cortex became buried in the depths of the lateral sulcus during development of the telecephalon.

This cortex, called the insula, can be revealed by prying open the lateral sulcus.

The portions of the frontal (5), parietal (6), and temporal (7) cortices that cover the insula are referred to as the operculum.

The insula as well as the frontal-parietal operculum constitute the gustatory (taste) cortex.

White Matter

The white matter underlying the cortex consists of myelinated axons running in three principle directions.

Association fibers connect and transmit nerve impulses between gyri in the same hemisphere.

Association Fibers

The arcuate fasciculus is an important bundle of association fibers that links the temporal lobe with the frontal lobe, through the parietal lobe.

It has been implicated in some of the expressive output difficulties of individuals with aphasia.

Commissural Fibers

Commissural fibers transmit impulses from the gyri in one hemisphere to the corresponding gyri in the opposite cerebral hemisphere.

Examples of commissural fibers include the corpus callosum, the anterior commissure, and the posterior commissure.

Corpus Callosum

The corpus callosum consists of cables of tissue that connect the two hemispheres.

It is composed of some two hundred million fibers that link the two brain halves from the forehead to the back of the head.

Anterior Commissure

At least one section of the corpus callosum is somewhat thicker in women than in men.

The anterior commissure is the second tissue bridge connecting brain hemispheres.

It is 12% larger in women than in men.

These thicker connections are believed to allow for greater communication between the two brain hemispheres.

Anterior Commissure

In males, the two brain halves are less in touch; each side operates more independently.

Because the male brain is more lateralized, with each hemisphere more rigidly dedicated to doing one task or another, this set up may enable men to focus their attention more intensely than women.

Women’s well-connected brains may facilitate their ability to gather, integrate, and analyze more diverse kinds of information—an aspect of web thinking.

Women’s less lateralized, more left/right integrated brain probably helps then to embrace the larger view.

They don’t see things as cut and dried, the way that men do (Schultz, cited in Fisher, 1999).

Projection Fibers

 Projection fibers form ascending and descending tracts that transmit impulses from the cerebrum to other parts of the brain and spinal cord.

These diffuse tracts form the corona radiata (#1).

Basal Ganglia

 The basal ganglia consist of several large subcortical masses of grey matter (nuclei) buried deep within the subcortical white matter.

They include the caudate nucleus and the lenticular (lentiform) nucleus.

Caudate Nucleus

The caudate nucleus is a C-shaped structure with an anteriorly located head deep in the frontal lobe.

Its body stretches in a superior and posterior direction following the lateral ventricle around.

Its inferiorly directed tail ends in the temporal lobe, with the amygdala attached.

The lenticular nucleus is comprised of the putamen and globus pallidus.

Lenticular Nucleus

The lenticular nucleus is separated from the caudate nucleus by the internal capsule.

The internal capsule is a compact bundle of fibers through which most of the neural traffic to and from the cortex passes.

Basal Ganglia

The caudate and putamen receive most of their input from the cerebral cortex.

In this sense they are the doorway into the basal ganglia.

The medial caudate receives its input from frontal cortex and limbic areas, and is implicated more in thinking and schizophrenia than in moving and motion disorders.

Indeed, the body and tail of the caudate become particularly active in the brain’s reward system—the mind’s network for general arousal, sensations of pleasure, and the motivation to acquire rewards.

Basal Ganglia

The caudate helps us detect and perceive a reward, discriminate between rewards, prefer a particular reward, anticipate a reward, and expect a reward.

It produces motivation to acquire a reward and plans specific movements to obtain a reward.

We will talk about the motivation and reward system in romantic love when we explore the reticular networks!

The caudate is also associated with the acts of paying attention and learning.

Basal Ganglia

The caudate and putamen are reciprocally interconnected with the substantia nigra, but send most of their output to the globus pallidus.

Although there are many different neurotransmitters used within the basal ganglia the overall effect of the basal ganglia is inhibitory.

Brake Hypothesis The function of the basal ganglia is often described in

terms of a "brake hypothesis.” To sit still, you must put the brakes on all movements

except those reflexes that maintain an upright posture.

To move, you must apply a brake to some postural reflexes, and release the brake on voluntary movement.

In such a complicated system, it is apparent that small disturbances can throw the whole system out of whack, often in unpredictable ways.

Basal Ganglia Defects

The deficits resulting from damage or malfunction of the basal ganglia tend to fall into one of two categories: the presence of extraneous unwanted movements or an absence or difficulty with intended movements.

Huntington's disease, or chorea, is a hereditary disease of unwanted movements.

It results from degeneration of the caudate and putamen, and produces continuous dance-like movements of the face and limbs.

The Limbic System

If you look at the mesial surfaces of the hemispheres, you see an arch-like pattern of cortex surrounding the non-convoluted central portions of the brain.

The cortical gyri that form a ring around the diencephalon include the subcallosal gyrus, the cingulate gyrus, and the parahippocampal gyrus.

Broca referred to this ring of cortex as the limbic lobe.

Limbic Structures

Much later, Papez (1937) proposed that limbic circuits were the basis of emotion.

He hypothesized that emotion was not a function of any specific brain center but of a circuit that interconnected four basic structures: the hypothalamus with its mammillary bodies, the anterior thalamic nucleus, the cingulate gyrus, and the hippocampus.

More recently, Paul MacLean, accepting the essential bases of the Papez proposal, created the denomination limbic system and added new structures to the circuit: the orbitofrontal and medial frontal cortices (prefrontal area), the parahippocampal gyrus, the amygdala, the medial thalamic nucleus, and the septal area.

Limbic Operations

Limbic operations tend to fall into the emotional-affective realm.

In short, the limbic system is responsible for seven functions—the four “Ms”—mating, mood, motivation, and memory; and the three “Fs”-- fear, fighting, food.

The septal region is thought to be involved in feeling and expressive states conducive to sociability and the procreation of the species.

The Septal Region

Parts of the septum are associated with the processing of and registering of emotion.

Individual functions within the region have been difficult to tease apart, because of the many interconnections with other limbic and brainstem regions.

The Cingulate Gyrus

The cingulate gyrus also connects many key sites.

In this region, emotions, attention, and working memory interact.

It is thought to help us to personally animate our attention and to attach it to things in external space.

The Cingulate Gyrus

Some parts are associated with happy states.Others involve awareness of one’s own emotional

state and the ability to assess other people’s feelings during social interactions.

Some other areas are associated with split-second emotional reactions to a win or loss, thereby judging a reward's value.

The Cingulate Gyrus

Damage to an area of the cingulate gyrus could affect outflow throughout the network, affecting the way a person melds affect and higher functions.

The result could be the emotional devaluation or depersonalization of events taking place in the opposite half of space.

The Amygdala

The amygdala or “little almond” lies buried deep within the antero-inferior region of the temporal lobe.

It has connections to the hippocampus, the septal nuclei, the prefrontal area and the medial dorsal nucleus of the thalamus.

If you remember only one word about the amygdala, the word is FEAR.

The Amygdala

The amygdala is the nucleus responsible for the lurch you feel in your stomach when you turn around in a dark alley and notice someone following you.

It couples a learned sensory stimulus (man in ski mask in alley = danger) to an adaptive response (fight or flight).

The amygdala must get sensory input, and fairly highly processed input, to recognize the elements of a scene that signal danger.

The association areas of visual, auditory, and somatosensory cortices are the main inputs to the amygdala.

The Amygdala

The main outputs of the amygdala are to the hypothalamus and brainstem autonomic centers, including the vagal nuclei and the sympathetic neurons.

The amygdala are thought to have a role in controlling the autonomic system to provoke such an instant sympathetic response.

When triggered, it gives rise to fear and anxiety which leads to a stage of alertness, getting ready to flight or fight.

The Amygdala

It is like the brain’s Homeland Security Alerting and Coordinating system, switching on the rest of the body systems—the gut, skin, heart, muscles, eyes, face, ears, and adrenal glands, to look out for incoming emotional stimuli (Brizendine, 2006).

The first relay station from the amygdala to the body is the hypothalamus.

Like the Joint Chiefs, it’s responsible for coordinating the launch of systems that raise blood pressure, heart rate, and breathing rate, and stimulate the fight-or-flight reaction after receiving reports from the body.

The Amygdala

The amygdala also alerts the cortex, the brain’s Intelligence Branch, which sizes up the emotional situation, analyzes it, and determines how much attention it deserves.

If it senses enough emotional intensity, the cortex cues the amygdala to alert the conscious brain to pay attention.

This is the moment when we’re flooded with conscious emotional feeling.

Before this point, all the brain processing is happening behind the scenes.

The Amygdala

A woman’s amygdala is more easily activated by emotional nuance.

The stronger the amygdala response to a stressful situation, such as an accident, an unpleasant event, the more details the hippocampus will tag for memory storage about the experience.

“Scientists believe that because women have a relatively larger hippocampus, they have better memories for the details of both pleasant and unpleasant emotional experiences—in a detailed three-dimensional sensory snapshot (Brizendine, 2006, p. 128).

The Amygdala

With respect to differences in anger processing, the expression of anger and aggression is greater in men.

It’s easier to push a man’s anger button.Women have a much less direct relationship to

anger.When a woman “bites her tongue” to avoid

expressing anger, it’s not all socialization—it’s a lot of brain circuitry hijacking the anger so she will reflect on it first out of fear and anticipation of retaliation, even loss of the relationship.

The Amygdala

Though a woman is slower to act out of anger, once her faster verbal circuits get going, they can cause her to unleash a barrage of angry words that a man can’t match.

Typical men speak fewer words and have less verbal fluency than women so they are handicapped in angry exchanges with women.

Men’s brain circuits and bodies may readily revert to a physical expression of anger fueled by frustration at not being able to match women’s words.

The Amygdala

Experimental destruction of both amygdala in animals makes them tame and sexually non-discriminative, and indifferent to danger.

The electrical stimulus of these structures elicits crises of violent aggressivety.

Humans with marked lesions of the amygdala lose the affective meaning of the perception of outside information, like the sight of a well known person.

The subject knows exactly who the person is, but is not capable to decide whether he likes or dislikes him (or her).

The Hippocampus

If the amygdala is FEAR, then the hippocampus is MEMORY.

The hippocampus is particularly involved with memory phenomena, specially with the formation of long-term memory.

Long-term or declarative memory is composed of all the facts, figures, and names you have ever learned.

All of your experiences and conscious memory fall into this category.

The Hippocampus

It is analogous to the hard drive of a computer. Although no one knows exactly where this enormous

database is stored, it is clear that the hippocampus is necessary to file away new memories as they occur.

The significance of the hippocampus is driven home by a famous patient named H.M.

As part of an epilepsy surgery, doctors removed most of his medial temporal lobes.

Since that surgery, in 1953, he has formed no new memories.

The Hippocampus

He can remember his childhood and everything before the surgery; you can have a normal, lucid conversation with him, but if you leave the room for a moment, when you return he will not remember you or the conversation.

He has completely lost the ability to lay down declarative memory.

Therefore, the hippocampus is critical in laying down declarative memory, but is not necessary for working memory, procedural memory, or memory storage.

The Hippocampus

Damage to the hippocampus will only affect the formation of new declarative memories.

In the case of Alzheimer’s disease, the slow destruction of the hippocampus results in problems with consolidating new memories.

The subject quickly forgets any recently received message, in spite of retained long-term and procedural memory ability.

The hippocampus is also the hub of memory formation related to emotion.

The Hippocampus

The hippocampus in women is larger in women than in men.

This means that, on average, women are better at expressing emotions and remembering the details of emotional events.

Men, by contrast, have two and a half times the brain space devoted to sexual drive as well as larger brain centers for action and aggression.

Sexual thoughts float into a man’s brain many time each day as compared to only once a day for women.

The Hippocampus

The hippocampus in women is larger in women than in men.

This means that, on average, women are better at expressing emotions and remembering the details of emotional events.

If we could construct a map showing the areas for emotion in the brains of the two sexes, the connecting routes between these areas in a man’s brain would be country roads; in the woman’s brain, they’d be superhighways.

“Research has shown that women typically remember emotional events—such as first dates, vacations, and big arguments—more vividly and retain them longer than men” (Brizendine, 2006, p. 127).

The Hippocampus

Men, by contrast, have two and a half times the brain space devoted to sexual drive as well as larger brain centers for action and aggression.

Sexual thoughts float into a man’s brain many time each day as compared to only once a day for women.

So a woman will remember what he said, what they both age, if it was cold outside or it rained on their anniversary, while a man may forget everything except whether or not she looked sexy!

Summary

As we have seen, the limbic system, with its interconnectivity to the cortex, diencephalon, and midbrain, functions to set the emotional tone of the mind.

It filters external events through internal states (emotional coloring) and tags events as internally important.

It stores highly charged emotional memories, modulates motivation, and promotes bonding.

Summary

As it directly processes the sense of smell, it has a role in hunger drive and modulating libido.

When the limbic circuit is damaged, moodiness, irritability, clinical depression may be seen.

There is increased negative thinking, a flood of negative emotions, and perception of events in a negative way.

Accompanying decreased motivation, there may be appetite and sleep problems and decreased or increased sexual responsiveness