The Biological Approach to Behavior: Neurotransmitters

Introduction to the Nervous System

The first principle of the biological approach to behaviour is that behaviour is the product of physiology. The term physiology is an umbrella term that encapsulates the structure and function of the nervous and endocrine systems. However, to fully understand this principle, one has to consider not only the structure of the nervous system but its function as well. How does it work? What processes are used to transfer information in the nervous system? What are the behavioural correlates of these processes?

The Structure of the Nervous System

The nervous system can be divided into two parts:

• The central nervous system (CNS) which is made up of the brain and spinal cord
  • It is called the central system because the brain is the control centre for your body, and the spinal cord relays messages between your brain and your body.
• The peripheral nervous system (PNS) comprises nerves that branch off from the spinal cord and extend to all body parts.
  • The peripheral nervous system has two types of neurons: Sensory neurons and motor neurons. Sensory neurons relay information from your senses, such as pain or temperature, to the brain. While motor neurons relay information from the brain to your muscles, telling your body to move.
  • The peripheral nervous system in turn is divided into the somatic nervous system – which controls voluntary movements – and the autonomic nervous system – which controls involuntary movements and reflexes.

The neuron, or nerve cell, is the basic unit of the nervous system. The human brain contains about 100 billion of them, all of which transmit messages called impulses. There are two main types of neurons: sensory neurons receive information and transmit it to the brain or spinal cord; interneurons relay the brain's response to motor neurons, which further relay the message to the glands and muscles.

A neuron consists of three parts: the body (soma), dendrites and axon. Dendrites and axons are filaments that extrude from the soma, there are typically multiple dendrites but always a single axon. To paint a clearer picture, dendrites resemble tiny branches; Their function is to receive signals from other neurons, while the function of the axon is to transmit the signals further. When the axon of one neuron approaches a dendrite or soma of another neuron, a synapse is formed, connecting the two neurons. On average, each neuron has about 15 000 connections with other neurons, making the nervous system a very elaborate network.

The nature of information transmission between nerves is partly electrical and partly chemical. Every neuron has a certain threshold of excitation received from the other neurons, and if the sum excitation exceeds this threshold, the neuron generates a brief pulse called action potential that travels along the axon to other neurons, which then passes the excitation further. The so-called pulse reaches the end of the axon and to the synaptic gap, where a neurotransmitter is released from the axon terminal into the synaptic gap. (At which point, the mechanism of transmission becomes chemical.)

Neurotransmitters

Neurotransmitters are chemical messengers, constantly synthesized in the neuron and stored in the axon terminal. A released neurotransmitter is available in the synaptic gap only for a short period during which it may be metabolized, pulled back into the pre-synaptic axon terminal through reuptake, or reach the post-synaptic membrane and bind to one of the receptors on its surface. This process changes the membrane potential and contributes to activating an electric pulse in the post-synaptic neuron. Here the chemical mechanism becomes electrical again.

All neurotransmitters are divided into two groups: excitatory and inhibitory. Excitatory neurotransmitters allow the impulse to cross the synapse. They produce stimulating effects on the brain. While inhibitory neurotransmitters stop the impulse, preventing it from crossing the synapse. They produce calming effects on the brain. When excitatory or inhibitory neurotransmitters are out of their optimal ranges in the brain, this may cause various behavioural malfunctions such as mental disorders.

Neurotransmitters themselves are affected by agonists and antagonists. Agonists are chemicals that enhance the activity of a neurotransmitter. Antagonists are chemicals that counteract a neurotransmitter and so prevent a signal from being passed further. Many drugs function as agonists or antagonists.

Agonists fall into two types:

• Direct binding - one that attaches directly to the receptor sites and acts like a neurotransmitter.
  • One example of a direct binding agonist is the drug Apomorphine, which binds to dopamine receptors, causing the user to experience the same effects as if dopamine were released in the brain.
• Indirect binding - increases and enhances the amount of neurotransmitters affected, however, has no specific activity at the receptor.
  • An example of an indirect agonist is Cocaine.

Antagonists also fall into two types:

• Direct acting - binds to and blocks neurotransmitter receptors, preventing the neurotransmitters themselves from attaching to the receptors.
  • One example is the drug Atropine.
• Indirect acting - prevents the production or release of neurotransmitters.
  • One example is the drug Reserpine which treats psychotic symptoms and high blood pressure.

Areas that have been shown to be affected by neurotransmitters have included mood, memory, sexual arousal and mental illness.

Effect of Dopamine on Romantic Love

In 2005, Fisher, Aron and Brown conducted a study of the neural mechanisms involved in romantic love. They hypothesized that dopamine plays a central role in the brain's response to loved ones. Dopamine, simply put, is an excitatory neurotransmitter that is involved in our desire to get things done, in controlling the brain's reward and pleasure centers and in regulating emotional responses.

During the study, ten men and seven women who were considered to be "intensely in love" (but not with each other) were placed in a functional magnetic resonance imaging (fMRI) scanner and engaged in standardized procedure such as looking at photographs while their brain were being scanned. The mean age among the participants was 21 years and the mean reported duration of their romantic relationships 7 months.

The investigation comprised four stages:

1. Each participant viewed a photograph of his or her romantic partner for 30 seconds.
2. Participants were given a 40-second filler activity which was to count back from a given number.
3. For 30 more seconds, participants viewed a photograph of an acquaintance who they shared no particular emotions for.
4. The final stage was another 20 seconds of counting back from a number.

These four steps were repeated six times, meaning that the total procedure lasted 12 minutes.

The results showed that the activation was observed in dopamine-rich neural systems in the brains of participants in response to the photographs of their loved ones. One notable area was the ventral tegmental area (VTA) and caudate nucleus. Both of which are rich in dopamine and form the key part of something called the dopaminergic pathway which is a system that generates and transmits dopamine and increases dopamine-related activity in the brain. Dopaminergic activity is associated with motivation and feelings of pleasure, leading researchers to believe that it does in fact play a role in romantic love.

The Role of Serotonin in Depression

Another area of interest is the role of serotonin in depression. Serotonin has been shown to be involved in the symptoms of major depressive disorder. The serotonin hypothesis states that low levels of serotonin in the brain play a causal role in developing depression.

Previous studies have involved clinical trials with two groups of patients. The experimental group would be given a drug that affects levels of serotonin in the brain while the control group would be given a placebo. The symptoms of depression would be compared after the fact. If a drug that is known to affect serotonin, like an SSRI, leads to a reduction of depressive symptoms in the experimental group, it is concluded that the level of serotonin is the cause of depression.

However, such a conclusion has several limitations.

Firstly, while neurotransmitters are affected by drugs within minutes, the behavioural effects do not manifest immediately. It could take weeks or even longer. This implies that the influence may be indirect or there could exist a longer path oof recovery where changing levels of serotonin is just one stage. For instance, one theory suggests that increased stress can damage neurons in the hippocampus and lead to depression, whereas SSRIs restore the damaged neurons gradually, thus alleviating the symptoms.

Secondly, not all patients benefit from drugs. Which suggests that the link between serotonin and depression is not universal.

Conclusions

There is a lot left unexplored when it comes to neurotransmitters and their effect on human behaviour. Sure, some aspects of human behaviour can be explained by chemistry – but two what extent can we use chemistry to control people's behaviour in constructive ways? Can we invent a drug that will make people fall in love with each other? Can we invent a drug for depression that is universally applicable?

But most importantly, how can we study the effects of neurotransmitters on behaviour, given the large number of interacting factors and contradicting variables?