2.2: The Biological Model

Learning Objectives

• Describe how communication in the nervous system occurs.
• List the parts of the nervous system.
• Describe the structure of the neuron and all key parts.
• Outline how neural transmission occurs.
• Identify and define important neurotransmitters.
• List the major structures of the brain.
• Clarify how specific areas of the brain are involved in mental illness.
• Describe the role of genes in mental illness.
• Describe the role of hormonal imbalances in mental illness.
• Describe the role of bacterial and viral infections in mental illness.
• Describe commonly used treatments for mental illness.
• Evaluate the usefulness of the biological model.

Proponents of the biological model view mental illness as being a result of a malfunction in the body to include issues with brain anatomy or chemistry. As such, we will need to establish a foundation for how communication in the nervous system occurs, what the parts of the nervous system are, what a neuron is and its structure, how neural transmission occurs, and what the parts of the brain are. All while doing this, we will identify areas of concern for psychologists focused on the treatment of mental disorders.

Brain Structure and Chemistry

2.2.1.1. Communication in the nervous system. To truly understand brain structure and chemistry, it is a good idea to understand how communication occurs within the nervous system. See Figure 2.1 below. Simply:

1. Receptor cells in each of the five sensory systems detect energy.
2. This information is passed to the nervous system due to the process of transduction and through sensory or afferent neurons, which are part of the peripheral nervous system.
3. The information is received by brain structures (central nervous system) and perception occurs.
4. Once the information has been interpreted, commands are sent out, telling the body how to respond (Step E), also via the peripheral nervous system.

Figure 2.1. Communication in the Nervous System

Please note that we will not cover this process in full, but just the parts relevant to our topic of psychopathology.

2.2.1.2. The nervous system. The nervous system consists of two main parts – the central and peripheral nervous systems. The central nervous system(CNS) is the control center for the nervous system, which receives, processes, interprets, and stores incoming sensory information. It consists of the brain and spinal cord. The peripheral nervous system consists of everything outside the brain and spinal cord. It handles the CNS’s input and output and divides into the somatic and autonomic nervous systems. The somatic nervous system allows for voluntary movement by controlling the skeletal muscles and carries sensory information to the CNS. The autonomic nervous system regulates the functioning of blood vessels, glands, and internal organs such as the bladder, stomach, and heart. It consists of sympathetic and parasympathetic nervous systems. The sympathetic nervous system is involved when a person is intensely aroused. It provides the strength to fight back or to flee (fight-or-flight instinct). Eventually, the response brought about by the sympathetic nervous system must end. The parasympathetic nervous system calms the body.

Figure 2.2. The Structure of the Nervous System

2.2.1.3. The neuron. The fundamental unit of the nervous system is the neuron, or nerve cell (See Figure 2.3). It has several structures in common with all cells in the body. The nucleus is the control center of the neuron, and the soma is the cell body. In terms of distinctive structures, these focus on the ability of a neuron to send and receive information. The axon sends signals/information to neighboring neurons while the dendrites, which resemble little trees, receive information from neighboring neurons. Note the plural form of dendrite and the singular form of axon; there are many dendrites but only one axon. Also of importance to the neuron is the myelin sheath or the white, fatty covering which: 1) provides insulation so that signals from adjacent neurons do not affect one another and, 2) increases the speed at which signals are transmitted. The axon terminals are the end of the axon where the electrical impulse becomes a chemical message and passes to an adjacent neuron.

Though not neurons, glial cells play an important part in helping the nervous system to be the efficient machine that it is. Glial cells are support cells in the nervous system that serve five main functions:

1. They act as a glue and hold the neuron in place.
2. They form the myelin sheath.
3. They provide nourishment for the cell.
4. They remove waste products.
5. They protect the neuron from harmful substances.

Finally, nerves are a group of axons bundled together like wires in an electrical cable.

Figure 2.3. The Structure of the Neuron

2.2.1.4. Neural transmission. Transducers or receptor cells in the major organs of our five sensory systems – vision (the eyes), hearing (the ears), smell (the nose), touch (the skin), and taste (the tongue) – convert the physical energy that they detect or sense and send it to the brain via the neural impulse. How so? See Figure 2.4 below. We will cover this process in three parts.

Part 1. The Axon and Neural Impulse

The neural impulse proceeds across the following steps:

• Step 1 – Neurons waiting to fire are said to be in resting potential and polarized, or having a negative charge inside the neuron and a positive charge outside.
• Step 2 – If adequately stimulated, the neuron experiences an action potential and becomes depolarized. When this occurs, voltage-gated ion channels open, allowing positively charged sodium ions (Na+) to enter. This shifts the polarity to positive on the inside and negative outside. Note that ions are charged particles found both inside and outside the neuron.
• Step 3 – Once the action potential passes from one segment of the axon to the next, the previous segment begins to repolarize. This occurs because the Na channels close and potassium (K) channels open. K+ has a positive charge, so the neuron becomes negative again on the inside and positive on the outside.
• Step 4 – After the neuron fires, it will not fire again no matter how much stimulation it receives. This is called the absolute refractory period. Think of it as the neuron ABSOLUTELY will not fire, no matter what.
• Step 5 – After a short time, the neuron can fire again, but needs greater than normal levels of stimulation to do so. This is called the relative refractory period.
• Step 6 – Please note that this process is cyclical. We started at resting potential in Step 1 and end at resting potential in Step 6.

Part 2. The Action Potential

Let’s look at the electrical portion of the process in another way and add some detail.

Figure 2.4. The Action Potential

• Recall that a neuron is usually at resting potential and polarized. The charge inside is -70mV at rest.
• If it receives sufficient stimulation, causing the polarity inside the neuron to rise from -70 mV to -55mV (threshold of excitation), the neuron will fire or send an electrical impulse down the length of the axon (the action potential or depolarization). It should be noted that it either hits -55mV and fires, or it does not fire at all. This is the all-or-nothing principle. The threshold must be reached.
• Once the electrical impulse has passed from one segment of the axon to the next, the neuron begins the process of resetting called repolarization.
• During repolarization the neuron will not fire no matter how much stimulation it receives. This is called the absolute refractory period.
• The neuron next moves into a relative refractory period, meaning it can fire but needs higher than normal levels of stimulation. Notice how the line has dropped below -70mV. Hence, to reach -55mV and fire, it will need more than the normal gain of +15mV (-70 to -55 mV).
• And then we return to resting potential, as you saw in Figure 2.4

Part 3. The Synapse

The electrical portion of the neural impulse is just the start. The actual code passes from one neuron to another in a chemical form called a neurotransmitter. The point where this occurs is called the synapse. The synapse consists of three parts – the axon of the sending neuron, the space in between called the synaptic space, gap, or cleft, and the dendrite of the receiving neuron. Once the electrical impulse reaches the end of the axon, called the axon terminal, it stimulates synaptic vesicles or neurotransmitter sacs to release the neurotransmitter. Neurotransmitters will only bind to their specific receptor sites, much like a key will only fit into the lock it was designed for. You might say neurotransmitters are part of a lock-and-key system. What happens to the neurotransmitters that do not bind to a receptor site? They might go through reuptake, which is the process of the presynaptic neuron taking up excess neurotransmitters in the synaptic space for future use or enzymatic degradation when enzymes destroy excess neurotransmitters in the synaptic space.

2.2.1.5. Neurotransmitters. What exactly are some of the neurotransmitters which are so critical for neural transmission, and are essential to our discussion of psychopathology?

• Dopamine – controls voluntary movements and is associated with the reward mechanism in the brain
• Serotonin – regulates pain, sleep cycle, and digestion; leads to a stable mood, so low levels lead to depression
• Endorphins – involved in reducing pain and making the person calm and happy
• Norepinephrine – increases the heart rate and blood pressure and regulates mood
• GABA – blocks the signals of excitatory neurotransmitters responsible for anxiety and panic
• Glutamate – associated with learning and memory

The critical thing to understand here is that there is a belief in the realm of mental health that chemical imbalances are responsible for many mental disorders. Chief among these are neurotransmitter imbalances. For instance, people with Seasonal Affective Disorder (SAD) have difficulty regulating serotonin. More on this throughout the book as we discuss each disorder.

2.2.1.6. The brain. The central nervous system consists of the brain and spinal cord; the former we will discuss briefly and in terms of key structures which include:

• Medulla – regulates breathing, heart rate, and blood pressure
• Pons – acts as a bridge connecting the cerebellum and medulla and helps to transfer messages between different parts of the brain and spinal cord
• Reticular formation – responsible for alertness and attention
• Cerebellum – involved in our sense of balance and for coordinating the body’s muscles so that movement is smooth and precise. Involved in the learning of certain kinds of simple responses and acquired reflexes.
• Thalamus – the major sensory relay center for all senses except smell
• Hypothalamus – involved in drives associated with the survival of both the individual and the species. It regulates temperature by triggering sweating or shivering and controls the complex operations of the autonomic nervous system
• Amygdala – responsible for evaluating sensory information and quickly determining its emotional importance
• Hippocampus – our “gateway” to memory. Allows us to form spatial memories so that we can accurately navigate through our environment and helps us to form new memories about facts and events
• The cerebrum has four distinct regions in each cerebral hemisphere. First, the frontal lobe contains the motor cortex, which issues orders to the muscles of the body that produce voluntary movement. The frontal lobe is also involved in emotion and in the ability to make plans, think creatively, and take initiative. The parietal lobe contains the somatosensory cortex and receives information about pressure, pain, touch, and temperature from sense receptors in the skin, muscles, joints, internal organs, and taste buds. The occipital lobe contains the visual cortex for receiving and processing visual information. Finally, the temporal lobe is involved in memory, perception, and emotion. It contains the auditory cortex which processes sound.

Of course, this is not an exhaustive list of structures found in the brain but gives you a pretty good idea of function and which structure is responsible for it. What is important to mental health professionals is some disorders involve specific areas of the brain. For instance, Parkinson’s disease is a brain disorder that results in a gradual loss of muscle control and arises when cells in the substantia nigra, a long nucleus considered to be part of the basal ganglia, stop making dopamine. As these cells die, the brain fails to receive messages about when and how to move. In the case of depression, low levels of serotonin are responsible, at least partially. New evidence suggests “nerve cell connections, nerve cell growth, and the functioning of nerve circuits have a major impact on depression… and areas that play a significant role in depression are the amygdala, the thalamus, and the hippocampus.” Also, individuals with borderline personality disorder have been shown to have structural and functional changes in brain areas associated with impulse control and emotional regulation, while imaging studies reveal differences in the frontal cortex and subcortical structures for those suffering from OCD.

Check out the following from Harvard Health for more on depression and the brain as a cause: https://www.health.harvard.edu/mind-and-mood/what-causes-depression

Genes, Hormonal Imbalances, and Viral Infections

2.2.2.1. Genetic issues and explanations. DNA, or deoxyribonucleic acid, is our heredity material. It exists in the nucleus of each cell, packaged in threadlike structures known as chromosomes, for which we have 23 pairs or 46 total. Twenty-two of the pairs are the same in both sexes, but the 23rd pair is called the sex chromosome and differs between males and females. Males have X and Y chromosomes while females have two Xs. According to the Genetics Home Reference website as part of NIH’s National Library of Medicine, a gene is “the basic physical and functional unit of heredity” (https://ghr.nlm.nih.gov/primer/basics/gene). They act as the instructions to make proteins, and it is estimated by the Human Genome Project that we have between 20,000 and 25,000 genes. We all have two copies of each gene, one inherited from our mother and one from our father.

Recent research has discovered that autism, ADHD, bipolar disorder, major depression, and schizophrenia all share genetic roots. They “were more likely to have suspect genetic variation at the same four chromosomal sites. These included risk versions of two genes that regulate the flow of calcium into cells.” Likewise, twin and family studies have shown that people with first-degree relatives suffering from OCD are at higher risk to develop the disorder themselves. The same is true of borderline personality disorder.

WebMD adds, “Experts believe many mental illnesses are linked to abnormalities in many genes rather than just one or a few and that how these genes interact with the environment is unique for every person (even identical twins). That is why a person inherits a susceptibility to a mental illness and doesn’t necessarily develop the illness. Mental illness itself occurs from the interaction of multiple genes and other factors–such as stress, abuse, or a traumatic event–which can influence, or trigger, an illness in a person who has an inherited susceptibility to it” (https://www.webmd.com/mental-health/mental-health-causes-mental-illness#1).

For more on the role of genes in the development of mental illness, check out this article from Psychology Today:

https://www.psychologytoday.com/blog/saving-normal/201604/what-you-need-know-about-the-genetics-mental-disorders

2.2.2.2. Hormonal imbalances. The body has two coordinating and integrating systems, the nervous system and the endocrine system. The main difference between these two systems is the speed with which they act. The nervous system moves quickly with nerve impulses moving in a few hundredths of a second. The endocrine system moves slowly with hormones, released by endocrine glands, taking seconds, or even minutes, to reach their target. Hormones are important to psychologists because they manage the nervous system and body tissues at certain stages of development and activate behaviors such as alertness or sleepiness, sexual behavior, concentration, aggressiveness, reaction to stress, and a desire for companionship. The pituitary gland is the “master gland” which regulates other endocrine glands. It influences blood pressure, thirst, contractions of the uterus during childbirth, milk production, sexual behavior and interest, body growth, the amount of water in the body’s cells, and other functions as well. The pineal gland helps regulate the sleep-wake cycle while the thyroid gland regulates the body’s energy levels by controlling metabolism and the basal metabolic rate (BMR). It regulates the body’s rate of metabolism and so how energetic people are.

Of importance to mental health professionals are the adrenal glands, located on top of the kidneys, and which release cortisol to help the body deal with stress. Elevated levels of this hormone can lead to several problems, including increased weight gain, interference with learning and memory, reduced bone density, high cholesterol, and an increased risk of depression. Similarly, the overproduction of the hormone melatonin can lead to SAD.

For more on the link between cortisol and depression, check out this article:

https://www.psychologytoday.com/blog/the-athletes-way/201301/cortisol-why-the-stress-hormone-is-public-enemy-no-1

2.2.2.3. Bacterial and viral infections. Infections can cause brain damage and lead to the development of mental illness or exacerbate existing symptoms. For instance, evidence suggests that contracting strep throat, “an infection in the throat and tonsils caused by bacteria called group A Streptococcus” (for more on strep throat, please visit https://www.cdc.gov/groupastrep/diseases-public/strep-throat.html), can lead to the development of OCD, Tourette’s syndrome, and tic disorder in children (Mell, Davis, & Owens, 2005; Giedd et al., 2000; Allen et al., 1995; https://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/syc-20351719), have also been linked to schizophrenia (Brown et al., 2004; McGrath and Castle, 1995; McGrath et al., 1994; O’callaghan et al., 1991) though more recent research suggests this evidence is weak at best (Selten & Termorshuizen, 2017; Ebert & Kotler, 2005).

Treatments

2.2.3.1. Psychopharmacology and psychotropic drugs. One option to treat severe mental illness is psychotropic medications. These medications fall under five major categories.

Antidepressants are used to treat depression, but also anxiety, insomnia, and pain. The most common types of antidepressants are SSRIs or selective serotonin reuptake inhibitors and include Citalopram, Paroxetine, and Fluoxetine (Prozac). Possible side effects include weight gain, sleepiness, nausea and vomiting, panic attacks, or thoughts about suicide or dying.

Anti-anxiety medications help with the symptoms of anxiety and include benzodiazepines such as Clonazepam, Alprazolam, and Lorazepam. “Anti-anxiety medications such as benzodiazepines are effective in relieving anxiety and take effect more quickly than the antidepressant medications (or buspirone) often prescribed for anxiety. However, people can build up a tolerance to benzodiazepines if they are taken over a long period of time and may need higher and higher doses to get the same effect.” Side effects include drowsiness, dizziness, nausea, difficulty urinating, and irregular heartbeat, to name a few.

Stimulants increase one’s alertness and attention and are frequently used to treat ADHD. They include Lisdexamfetamine, the combination of dextroamphetamine and amphetamine, and Methylphenidate. Stimulants are generally effective and produce a calming effect. Possible side effects include loss of appetite, headache, motor or verbal tics, and personality changes such as appearing emotionless.

Antipsychotics are used to treat psychosis or “conditions that affect the mind, and in which there has been some loss of contact with reality, often including delusions (false, fixed beliefs) or hallucinations (hearing or seeing things that are not really there).” They can be used to treat eating disorders, severe depression, PTSD, OCD, ADHD, and Generalized Anxiety Disorder. Common antipsychotics include Chlorpromazine, Perphenazine, Quetiapine, and Lurasidone. Side effects include nausea, vomiting, blurred vision, weight gain, restlessness, tremors, and rigidity.

Mood stabilizers are used to treat bipolar disorder and, at times, depression, schizoaffective disorder, and disorders of impulse control. A common example is Lithium; side effects include loss of coordination, hallucinations, seizures, and frequent urination.

For more information on psychotropic medications, please visit:

https://www.nimh.nih.gov/health/topics/mental-health-medications/index.shtml

The use of these drugs has been generally beneficial to patients. Most report that their symptoms decline, leading them to feel better and improve their functioning. Also, long-term hospitalizations are less likely to occur as a result, though the medications do not benefit the individual in terms of improved living skills.

2.2.3.2. Electroconvulsive therapy. According to Mental Health America, “Electroconvulsive therapy (ECT) is a procedure in which a brief application of electric stimulus is used to produce a generalized seizure.” Patients are placed on a padded bed and administered a muscle relaxant to avoid injury during the seizures. Annually, approximately 100,000 undergo ECT to treat conditions such as severe depression, acute mania, suicidality, and some forms of schizophrenia. The procedure is still the most controversial available to mental health professionals due to “its effectiveness vs. the side effects, the objectivity of ECT experts, and the recent increase in ECT as a quick and easy solution, instead of long-term psychotherapy or hospitalization” (https://www.mhanational.org/ect). Its popularity has declined since the 1960s and 1970s.

2.2.3.3. Psychosurgery. Another option to treat mental disorders is to perform brain surgeries. In the past, we have conducted trephination and lobotomies, neither of which are used today. Today’s techniques are much more sophisticated and have been used to treat schizophrenia, depression, and some personality and anxiety disorders. However, critics cite obvious ethical issues with conducting such surgeries as well as scientific issues.

For more on psychosurgery, check out this article from Psychology Today:

https://www.psychologytoday.com/articles/199203/psychosurgery

Evaluation of the Model

The biological model is generally well respected today but suffers a few key issues. First, consider the list of side effects given for psychotropic medications. You might make the case that some of the side effects are worse than the condition they are treating. Second, the viewpoint that all human behavior is explainable in biological terms, and therefore when issues arise, they can be treated using biological methods, overlooks factors that are not fundamentally biological. More on that over the next two sections.

Key Takeaways

You should have learned the following in this section:

• Proponents of the biological model view mental illness as being a result of a malfunction in the body to include issues with brain anatomy or chemistry.
• Neurotransmitter imbalances and problems with brain structures/areas can result in mental disorders.
• Many disorders have genetic roots, are a result of hormonal imbalances, or caused by viral infections such as strep.
• Treatments related to the biological model include drugs, ECT, and psychosurgery.

Review Questions

1. Briefly outline how communication in the nervous system occurs.
2. What happens at the synapse during neural transmission? Why is this important to a discussion of psychopathology?
3. How is the anatomy of the brain important to a discussion of psychopathology?
4. What is the effect of genes, hormones, and viruses on the development of mental disorders?
5. What treatments are available to clinicians courtesy of the biological model of psychopathology?
6. What are some issues facing the biological model?