15.3: Structures Involved in Emotion

Emotions are one of the most holistic functions of the brain, incorporating neural circuitry from across several different structures, ranging from the phylogenetically older to the most advanced frontal cortical areas. The structures of the Papez circuit are not a comprehensive list of brain structures involved in emotional processing but offer a good starting place for describing the anatomy of emotion.

Amygdala

A limbic structure called the amygdala contributes heavily to processing the valence of emotional experiences. Roughly the shape and size of an almond, the amygdala is part of the temporal lobe. Generally, the amygdala is subdivided into several nuclei, including the basolateral amygdala, central nucleus, and cortical nucleus.

As a temporal lobe structure, amygdala is strongly implicated in emotional memory formation. Emotional memories can be either positive valence (such as the happiness you may have experienced at a birthday party when younger) or negative valence (such as childhood trauma or being teased as a child). Of the forms of declarative memory (chapter 13), autobiographical memories more often have emotional content compared to semantic memories.

Figure 15.10 The amygdala (red) contribute to emotional processing.

Amygdala lesions have been used as a last resort treatment for patients with temporal lobe epilepsy or psychiatric conditions with pathological and dangerous aggressiveness. These psychosurgery strategies have been variably successful, but they often have high complication rates and upwards of a 4% mortality rate. These treatments are rarely used today. Deep brain stimulation may offer a less intrusive and therefore less risky therapeutic approach.

Monkeys with amygdala lesions made up the earliest descriptions of the emotional disruption condition later described as KluverBucy syndrome. These monkeys did not exhibit the traditional fear response when presented with the hissing of a large snake. They also showed an absence of anger, such as when the animal is exposed to provocative novel stimuli.

A useful nonhuman model for studying emotional learning is the fear conditioning paradigm. In this test, a rodent is put into a cage. Occasionally, an innocuous tone and light would activate. Shortly after, an unpleasant foot-shock would be delivered to the animal. Over time, the healthy animal learns that the tone and light precede the shock to the foot, and upon future presentation of these stimuli, they will freeze. If the amygdala is lesioned, however, they freeze less, meaning they do not acquire the emotional memory associated with the stimuli. This paradigm is used as a model for post-traumatic stress disorder (PTSD), a psychiatric condition associated with the recurring recall of negative memories.

Hypothalamus

One of the major output signaling pathways of the amygdala is the hypothalamus, an almond-shaped nervous system structure found at the base of the brain. The hypothalamus is often seen as the neural structure that initiates endocrine responses in the rest of the body, such as hormone production. Several different behaviors, ranging from homeostasis, hunger, and circadian regulation are modulated by hypothalamic signals.

Figure 15.11 The hypothalamus is a brain structure that initiates hormonal changes through influencing the pituitary gland.

Pituitary gland

Downstream from the hypothalamus is the pituitary gland, a pea-sized endocrine organ that protrudes from the base of the brain. It is strongly involved in the production and release of neurohormones, signaling molecules produced by nerve cells that travel throughout the bloodstream to influence the activity of several organs throughout the entire body. The pituitary gland is subdivided into two regions with distinct anatomical and functional differences.

Posterior pituitary gland

The posterior pituitary gland (also called the neurohypophesis) does not synthesize any neurohormones. Instead, the axonal projections of the magnocellular neurosecretory cells of the hypothalamus run through the posterior pituitary, where the hormones are secreted into a special anatomical feature called the hypophyseal portal system, a series of leaky capillaries densely wrapped around the area, allowing for the hormones to easily diffuse into the bloodstream. There are predominantly two main hormones released at the posterior pituitary.

Oxytocin (OT) plays a significant role in the development and maintenance of prosocial behaviors, acts such as trust, compassion, and empathy, all actions that enhance interpersonal relationships. For example, increased blood levels of OT is seen in new couples compared to unattached singles, and OT release happens during orgasm, which may contribute to romantic attachment. OT signaling increases dramatically during childbirth and triggers milk letdown in lactation, which strengthens the motherchild relationship. Interestingly, while OT generally strengthens the social bonds between people, it promotes antisocial behaviors against those not perceived to be within one’s own social group.

Disorders of the OT system are believed to contribute to autism spectrum disorder and psychopathy, two complex conditions characterized partly by social impairment. Some studies have examined the therapeutic use of nasal OT for a variety of psychiatric conditions, but the studies have been unable to demonstrate strong clinical effects despite success in nonhuman animal models.

Vasopressin (AVP; or antidiuretic hormone, ADH) like OT, also contributes to social behaviors. Biochemically, AVP is very similar to OT, as they are both nine amino acid peptides differing by only two residues. AVP additionally regulates osmolarity, increasing water retention by the kidneys, which returns the body from a hypertonic state back to homeostasis. AVP also constricts blood vessels, raising blood pressure. AVP is also used therapeutically to raise blood pressure for patients in shock and to treat diabetes insipidus, a fluid dysregulation disorder (unrelated to diabetes mellitus, the much more common blood glucose disease).

Anterior pituitary

The anterior pituitary (or the adenohypophesis) is capable of both synthesizing and secreting a variety of neurohormones, which contribute to several functions such as the stress response, growth, sexual development, circadian rhythms, and more. These hormones are collectively called the trophic hormones. The neurosecretory cells of the anterior pituitary are sensitive to signals from the hypothalamus.

Figure 15.12 The hypothalamus sends a variety of hormonal signals, which influence several different physiological features.

Within the anterior pituitary are cells called somatotrophs, which produce and secrete growth hormone (GH). GH is a signal that promotes anabolism, the buildup of larger molecules through biochemical reactions. Anabolic processes increase cellular repair and protein synthesis, which promote growth. Most of the time in the day, GH production is at a steady low level, but it increases at specific events, such as after meals or during slow wave sleep. The biochemical signal from the hypothalamus is growth-hormone-releasing hormone (GHRH).

The anterior pituitary releases adrenocorticotropic hormone (or ACTH). Once ACTH is released into the bloodstream, it travels to the adrenal cortices, a pair of organs that sit on the anterior surface of the kidneys. Once there, ACTH triggers production of cortisol, a glucocorticoid hormone. Cortisol is best known for its initiation of the stress response, which is characterized by mild sympathetic nervous system activity. ACTH is synthesized downstream from the hypothalamic signal corticotropin releasing hormone (CRH). The series of organs that result in the stress response is called the hypothalamic-pituitary-adrenal axis, or the HPA axis for short.

The anterior pituitary also synthesizes and releases the gonadotropins, hormones that are important for regulation of puberty, sperm / egg production, the release of sex hormones by the testes / ovaries, and menopause. The two main gonadotropins in humans are luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These are produced in response to the hypothalamic signal gonadotropin-releasing hormone. This signaling cascade is called the hypothalamic-pituitary-gonadal axis, or the HPG axis.

Insula

The insula (or insular cortex) is the lobe of the cortex buried deep within the lateral fissure. Although not visible from a side view, the insula is often considered to be the fifth lobe of the telencephalon. Early studies by Wilder Penfield where he directly stimulated the brain during open brain surgery led researchers to suggest that the insula contributes to interoception, detecting the internal state of the body and conveying that information for processing.

Figure 15.13 The insular cortex is only visible in a cut-away view of the brain.

In functional imaging studies, the insula is involved in the recall or many different emotional stimuli, especially those emotions that have a sensory component.

Notably, the insula is strongly implicated in the emotion disgust. For example, a patient is placed in an fMRI scanner while breathing through a mask, which allows the experimenters to change the smells that are perceived. Patients are then given pleasant smells (such as passion fruit, pear, or mint), a neutral smell, or unpleasant smells (like ethyl-mercaptan or isovaleric acid, which smells like skunk or body odor, respectively). There is increased activity of the anterior insula in response to the unpleasant smells but not the pleasant smells.

The insula also responds to social cues related to disgust as well. When a patient in an fMRI sees a video of a person smelling something unpleasant and reacting with a “repulsed” face (the closing of the nostrils and curling of the upper lip), their anterior insula likewise increases in activity just as if they had smelled it themselves.

In addition to sensory stimuli, feelings of social repugnance (unwarranted violence, murder) or moral disgust (incest) also increase insula activity.

Atypical insula activity is implicated in behavioral disorders. For example, insensitivity to disgust can lead to squalor-dwelling conditions (sometimes seen in excessive hoarding or late cognitive decline), which puts those people at heightened health risks due to regular exposure to unsanitary conditions. Substance use disorders, PTSD, and suicide attempts have all been associated with atypical insula activity.