Biological Bases of Human Behaviour Stage 1 Lecture 6

PSY101: Biological Bases of Behaviour - Lecture 6

Neurotransmission & Psychopharmacology

Lecture Overview

This lecture delves into the intricate world of neurotransmission and psychopharmacology, focusing on the chemical messages that play a pivotal role in the nervous system. The discussion encompasses the various types of neurotransmitters, their specific systems, the dynamics of how psychoactive substances interact with synapses, and a comprehensive classification of drugs used in psychological treatment.

Neural Communication

Neural communication is fundamental to how the nervous system functions, and a seminal experiment by Otto Loewi in 1921 laid the groundwork for our understanding of this process. In his experiment, Loewi electrically stimulated a frog's heart while it was submerged in a salt bath, leading to the observation that a chemical message, specifically Acetylcholine (ACh), was transmitted and had the power to influence a second heart. This pivotal discovery highlighted the role of neurotransmitters, including Acetylcholine which is known to inhibit heartbeat; Norepinephrine (NE) which excites the heartbeat; and Epinephrine (Adrenaline), which is related to NE and produced by the adrenal glands during stress responses. These neurotransmitters together demonstrate the biochemical foundation underlying physiological responses and behaviors.

The Synapse & Neurotransmission

The process of neurotransmission involves several well-orchestrated mechanisms. Neurotransmitters are synthesized and stored in axon terminals, ready to be released upon the generation of an action potential at the presynaptic membrane. Once released, they traverse the synaptic cleft and bind to specific receptors on the postsynaptic membrane, triggering a cascade of responses. Following their action, neurotransmitters must be inactivated or allowed prolonged action to ensure proper signaling.

Key terminology included in this discussion comprises:

  • Synaptic Vesicle: A membrane-bound organelle that stores neurotransmitters, ensuring their availability for transmission.

  • Presynaptic/Postsynaptic Membranes: These membranes are crucial for the neurotransmission process; the presynaptic membrane releases neurotransmitters while the postsynaptic membrane detects these signals and initiates responses in the receiving neuron.

Types of Neurotransmitters

Neurotransmitters can be identified by specific criteria: they must be synthesized and remain located within the neuron, produce a recognizable response upon release, generate the same response in experimental applications, and have a defined mechanism for removal following their action. Neurotransmitters can be classified into three principal groups based on their biochemical composition:

  1. Small-Molecule Neurotransmitters: Simple molecules like ACh, dopamine, and serotonin, which exert rapid effects on synaptic transmission.

  2. Neuropeptides: Larger molecules composed of chains of amino acids, such as endorphins, that often modulate synaptic transmission over longer durations.

  3. Gaseous Neurotransmitters: These include nitric oxide and carbon monoxide, which are unique because they can diffuse across membranes and are involved in retrograde signaling.

The mechanism of action of neurotransmitters varies widely, particularly concerning their receptor types. Ionotropic receptors facilitate the direct movement of ions across cellular membranes, resulting in immediate and localized effects. In contrast, metabotropic receptors are coupled with G proteins and activate a series of intracellular events, leading to more prolonged and complex outcomes in neuronal signaling.

Psychopharmacology Overview

Psychopharmacology examines the relationship between drugs and their effects on the nervous system and behavior. An essential component of this field is understanding psychoactive drugs, which are substances that influence mood, perception, cognition, and behavior, often employed therapeutically for mental health management. The routes by which these drugs enter the central nervous system (CNS) may differ significantly, as drug absorption is affected by its inherent properties and the method of administration. The blood-brain barrier (BBB) serves as a selective filter, restricting factors such as molecular size and lipid solubility that can influence drug access to the brain, adding an additional layer of complexity to psychopharmacological treatments.

Drug Classes and Effects

Various classes of drugs target specific neurotransmitter systems, producing diverse effects based on their mechanisms of action:

  • Antianxiety Agents and Sedative Hypnotics: Commonly used medications include alcohol and benzodiazepines (e.g., Valium). These substances can lead to tolerance, where increased dosages are required to achieve the same level of anxiety reduction or sedation.

  • Antipsychotic Agents: These drugs are primarily used to manage symptoms of schizophrenia by blocking dopamine receptors. They can be classified into typical antipsychotics, which often come with more significant side effects, and atypical antipsychotics, which often have a more favorable side effect profile.

  • Antidepressants: Categories include Monoamine Oxidase Inhibitors (MAOIs), Tricyclic antidepressants, and Selective Serotonin Reuptake Inhibitors (SSRIs). These medications work predominantly by preventing the degradation or reuptake of key neurotransmitters like serotonin and norepinephrine, leading to elevated mood and reduced depressive symptoms.

  • Narcotic Analgesics: Derived from opium, drugs like morphine are effective in providing pain relief. However, they also come with significant risks, and opioid antagonists are critical in reversing opioid overdoses.

Key Concepts

Understanding key concepts such as tolerance and sensitization is essential in the study of psychopharmacology. Tolerance refers to the phenomenon where an individual exhibits a diminished response to a drug over time, necessitating increased doses to achieve the same effect. Conversely, sensitization describes an increased response to a substance following repeated exposure, which can lead to heightened sensitivity to the drug's effects. Finally, the concepts of addiction and dependence, characterized by intense cravings, recurrent use, physical dependence, and withdrawal symptoms, underscore the significant implications of substance abuse and the necessity for effective treatment strategies in clinical psychology