midbrain

Fear and the amygdala

Posted on by Iasmina Hornoiu

What is fear? Why are we sometimes afraid? Can fear be inhibited? What produces fear – the brain or the heart?

It is definitely the brain! More exactly, something in the brain – a tiny, almond-shaped structure, which sits anteriorly to the hippocampus, called the amygdala. This small part of our brain is to blame for  the perception of fearful stimuli and the physiological responses (increased heart rate, electrodermal responses etc.) to fearful stimuli.

As part of the FEAR system, the amygdala connects to the medial hypothalamus and the dorsal periaqueductal grey matter (in the midbrain), which is important in pain modulation in the dorsal horn of the spinal cord, as well as to sensory and association cortices. The lateral nucleus of the amygdala receives inputs from different brain regions, thus allowing the formation of associations, required for aversive conditioning. Following the processing in the lateral nucleus, information about the stimulus is, then, projected to the central nucleus of the amygdala, where an appropriate response to the stimuli is initiated, provided that the stimuli are detected as threatening or potentially dangerous.

The amygdala is involved in emotional learning and memory, modulating implicit learning, explicit memory, attention, social responses, emotion inhibition and vigilance.

You can find the article on memory here, to brush up a bit.

  • Implicit memory is a type of learning, which cannot be voluntarily reported or remembered. It includes the memories for skills and habits, for procedural knowledge, grammar and languages, priming, simple forms of associative learning and classical conditioning. The latter is particularly important for fear. It involved a conditioned stimulus (CS) and an unconditioned, painful/fearful stimulus (US), with US preceding CS and determining a fearful response to CS. This type of fear learning is adaptive and is known as sensitisation or acquisition.

There are two different pathways in the amygdala, important for fear conditioning. The “low road” pathway: sensory information projects to the thalamus, which directly communicates with the amygdala; this pathway is fast, modulating rapid responses of the amygdala to different types of fearful stimuli. The “high road” pathway is an indirect pathway: sensory information projects to the thalamus and from there, it is conveyed to the sensory cortex for a finer analysis; the sensory cortex, then, communicates the processed information to the amygdala. This pathway ensures that the sensory stimulus is the conditioned stimulus. So the responses of amygdala to threatening stimuli are both rapid and sure.

  • Explicit memory refers to the memory of facts and events; in the case of fear is means the processing and retention for a long time, of emotional events and information. For this type of fear learning, the amygdala interacts with the hippocampus. There is a distinction between the formation of memory for aversive experience (fear conditioning), which is based on previous experience, and explicit learning (in the hippocampus), which involves learning and remembering aversive properties of different threatening stimuli. The memory in the hippocampus is enhanced by arousal produced in the amygdala. The activation of the amygdala can make different cortical areas, not just the hippocampus, more receptive to stimuli that are adaptively important, thus ensuring that unattended, but important stimuli gain access to consciousness.
  • Social responses involve the ability to recognise a stimulus as good, bad, neutral or arousing. This ability, however, does not depend on the amygdala. There is one exception here, otherwise we wouldn’t be talking about it in the context of fear mediated by amygdala – fearful facial expressions. According to Darwin, social species, like humans, use facial expressions to detect internal emotional states of other members of the group. This function, mediated by the amygdala, is crucial in the emotional regulation of human social behaviour. Damage to amygdala has been demonstrated to result in impairment of the patient to identify fearful faces correctly and, therefore, react to them, accordingly. It should also be noted there is no need for the subject’s awareness of the fearful stimulus, for the amygdala to respond. In other words, when a fearful facial expression is presented subliminally, the amygdala will still show activation.
  • Inhibition of fear – it is actually very difficult to escape your own fears, as fear proves to be resistant to voluntary control. However, there is a process called extinction, a method of classical conditioning, where a CS previously associated with an aversive US in presented alone for a number of times, until the CS no longer signals the fearful stimulus. If the US is presented again, after the passage of time, it will evoke fear responses, but in different brain areas. So, the learned fear has been retained in memory, but extinction learning eliminates the response to fear. This mechanism of extinction relies on the activity of NMDA glutamate (excitatory) receptors in the amygdala. When these receptors are blocked, extinction is inhibited (so you will react to fearful stimuli) and when they are active, extinction is augmented (you no longer respond to aversive stimuli).
  • Vigilance – the amygdala is not necessary for the conscious experience of emotional states, but it plays a major role in increasing the vigilance of cortical response systems to emotional stimuli.

Memories about fearful events, just like other types of long-term memory, become permanent through a process of gene expression and novel protein synthesis, which is known as consolidation. Upon retrieval, the memories become susceptible to change and alteration, before they are reconsolidated, which involves additional protein synthesis.

The fact that humans (and probably other animals as well, I am sure, although not widely proven) have the ability to distinguish between emotional information and unemotional information is regarded as an evolutionary advantage. Emotional stimuli signal dangers and the ability to detect and appropriately react to them increases the chance of survival. However, exacerbated fear is detrimental to the individual who experiences it and is a sign of pathology. For instance, in atypical monopolar depression, which includes anxiety as one of the main symptoms, the amygdala is overactive and it determined lowering the threshold for emotional activation and abnormal reactions to stressful stimuli. A similar pattern can be seen as a result of partial chronic sleep deprivation or complete acute sleep deprivation.

References

 Beatty, 2001. The Human Brain – Essentials of Behavioural Neuroscience, Sage Publications Ltd., pg. 293-296

Bernard et al., 2007. Cognition, Brain and Consciousness – Introduction to Cognitive Neuroscience. 1st edition. Elsevier Ltd., pg. 373-383

Gazzaniga et al., 2002. Cognitive Neuroscience – The Biology of Mind. 2nd edition. W.W Norton & Company, Inc., pg. 553-572

Image by  Saya Lohovska. You can find her arts page here.

Mechanisms of schizophrenia

Posted on by Iasmina Hornoiu

It took me a while to figure out whether to divide this article into two parts or to sum up everything in one long, possibly tedious reading. Honestly, I still don’t know, so I’ll just start writing and we shall see what it turns out to be.

I’m sure you’ve all heard of schizophrenia – the disease of thought disorder, or know people who suffer from it. But only a few actually understand what it is about.

No wonder scientists have been struggling to develop efficient treatments for schizophrenia; not only is it largely uncommon (1% of the world’s population is affected), but also its causes are usually unknown. Scientists generally refer to schizophrenia as a psychiatric disease involving a progressive decline in functioning, which begins in early adolescence and persist throughout the patient’s life. Due to its heterogenous symptoms and multiple possible causes, there are many hypotheses that intend to explain what triggers schizophrenia and how it develops.

In spite of the fact that is it a genetic disorder, the environment and external factors (such as viral infections during the intrauterine and infant period) may be crucial to the development of schizophrenia. The symptoms have been divided into two categories. The positive symptoms include thought disorder, hallucinations, delusions, disorganised speech etc., whereas the negative symptoms are characterised by poverty of speech, reduced expression or emotion, memory impairment, anergia, abulia etc. In addition, the brains of schizophrenics show structural macroscopic abnormalities (for instance, the enlarged ventricles and the shrinkage of the surrounding brain tissue), as well as microscopic changes, such as the dysregulation of dysbindin gene in the formation of abnormal dendritic filopodia. There are three types of schizophrenia, according to its symptoms: paranoid schizophrenia – auditory hallucinations, delusions, strong belief of being chased by powerful people; disorganized schizophrenia – reduced emotions and lack of emotional expressions, incoherent speech (mostly negative symptoms); catatonic schizophrenia – impairment of movement, usually immobility and catatonia, bizarre grimacing (this is similar some of the symptoms of hysteria, which has been described as a sexually related and later on, as a psychiatric disorder up until the beginning of the 20th century).

But enough with the boring general details! Let’s get to the fun part: The monoamine hypothesis of schizophrenia! Here we are going to talk about two very important neurotransmitters in the central nervous system: dopamine and glutamate. The second one is the main excitatory neurotransmitter in the brain. There are four main types of glutamate receptors: AMPA, NMDA, kainate and mGluRs. It has been demonstrated that reduced activity of the NMDA receptors can result in some of the negative symptoms of schizophrenia (lack of social behaviour, catatonia).

Dopamine is the metabolic precursor of another neurotransmitter, noradrenaline (norepinephrine). But there is a lot more to dopamine and its roles in the brain than this. There are four main dopaminergic pathways: the mesolimbic pathway – related to the “reward” system and significance; it has its roots in the ventral tegmental area and projects to the nucleus accumbens (in the ventral striatum) and the limbic system; the mesocortical pathway – involved in cognition and motivation; the tuberoinfundibular pathway – roles in lactation; these dopamine neurones originate in the hypothalamus; the nigrostrial pathway – involved in movement planning and connects the substantia nigra (midbrain) to the striatum.

Schizophrenia and another mental illness, a neurodegenerative one, Parkinson’s disease, are also linked to dopamine. When it comes to schizophrenia, it seems that the mesocorticolimbic pathways have more influence on its onset: the ‘positive’ symptoms appear to be triggered by dopaminergic hyperactivity in the mesocorticolimbic system. At the same time, hypoactivity of dopamine is this region is the cause of ‘negative’ symptoms. Nevertheless, it has been discovered that overexpression of the dopamine receptor D2 (DRD2) gene in the striatum also reduces motivational behaviour in mice, therefore mimicking psychotic ‘negative’ symptoms. Similar findings show that increased density of dopamine D2 receptor in the striatum, along with lower thalamic density of this receptor appear to induce divergent thinking, which is associated with schizophrenia.  

All these changes may account for the abnormalities that we see in “mad” people. It seems that we are so fragile, given that often small chemical and physical disruptions can trigger something as big and terrifying as schizophrenia. Imagine hearing, seeing, feeling, smelling things everyone says are not real (schizophrenics often have multiple hallucinations: auditory, visual, gustatory, tactile, olfactory). But to you they are so real and disturbing! Many schizophrenics even hear their own thoughts as if they are coming from the outside and therefore believe that everyone knows what’s in their heads. Imagine having the constant feeling that someone is after you (paranoia) or being certain that you are dead (the Cotard’s Syndrome) or that your husband has an affair (the Othello Syndrome).

I think this topic can never be fully covered and we would spend days talking about schizophrenia, so this article should better come to an end. As I am sure you have lots of questions and comments, don’t be shy and post anything you think it’s relevant to what has been discussed above. Hope you enjoyed this reading.

For further information: 

Bear et al., 2006. Neuroscience – Exploring the Brain. s.l.:Lippincott Williams & Wilknins, pp. 679-684

de Manzano et al., 2010. Thinking Outside a Less Intact Box; Thalamic Dopamine D2 Receptor Densities Are Negatively Related to Psychometric Creativity in Healthy Insividuals. Public Library of Science

Jia et al.,  2014. The Schizophrenia Susceptibility Gene Dysbindin Regulates Dendritic Spine Dynamics. The Journal of Neuroscience, Oct.pp. 34-41

Kandel et al., 2011. Modeling Motivational Deficits in Mouse Models of Schizophrenia: Behavior Analysis as a Guide for Neuroscience. Behavior Processes, pp. 149-156

Kolb et al., 1996. Fundamentals of Human Neuropsychology. 4th Edition ed. s.l.:W.H. Freeman and Company

Image by Damaris Pop