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Biological Psychology

Chapter 2: Synapses

Introduction

• Definition of Synapses: Neurons communicate by transmitting chemicals at junctions called "synapses."

• Origin of Term: Coined by Charles Scott Sherrington in 1906 to describe the specialized gap between neurons.

• Study Method: Investigated neuron communication by studying reflexes, which are automatic muscular responses to stimuli, in a process known as a reflex arc.

Sherrington's Evidence for Synaptic Delay

• Observed Synaptic Delay: An impulse traveling through a synapse in the spinal cord is slower than one traveling a similar distance along an uninterrupted axon.

• Conduction Speed:

  • Speed along an axon: approximately $40 ext{ m/s}$.

  • Speed through a reflex arc: slower, sometimes $15 ext{ m/s}$ or less, indicating that the delay occurs at the synapse.

Excitatory Postsynaptic Potential (EPSP)

• Definitions:

  • Presynaptic Neuron: The neuron that delivers the synaptic transmission.

  • Postsynaptic Neuron: The neuron that receives the message.

  • EPSP: A graded depolarization that decays over time and space.

• Cumulative Effects: The cumulative effect of EPSPs is vital for both temporal and spatial summation.

Temporal Summation

• Observations by Sherrington:

  • Repeated stimuli over a short period produce a stronger response, leading to the concept of temporal summation.

  • The cumulative effect of repeated stimuli can produce a nerve impulse when a single stimulus is too weak.

• Operational Definition: Temporal summation is the summation of EPSPs or IPSPs due to repeated stimulation by one neuron.

Spatial Summation

• Observations by Sherrington:

  • Noted that several small stimuli in close proximity could produce a reflex when a single stimulus could not, leading to the idea of spatial summation.

• Cumulative Input: Synaptic input from several locations can accumulate to trigger a nerve impulse.

  • Each neuron receives numerous incoming axons that frequently produce synchronized responses, emphasizing the importance of spatial summation to brain function.

  • Interplay with Temporal Summation: Temporal and spatial summation generally occur together, with the order of axon activation affecting the results.

Electrical Potential Across Membrane (in Millivolts)

• Recordings from a Postsynaptic Neuron During Synaptic Activation:

  1. EPSP recorded leading to depolarization.

  2. Temporal summation: Several impulses from one neuron over time.

  3. Combining EPSPs can exceed threshold and produce action potentials.

  4. Simultaneous EPSPs: Combining spatially can also exceed threshold.

  5. Inhibitory Postsynaptic Potential (IPSP) is observed.

  6. Resting potential and action potentials travel along the axon.

Inhibitory Synapses

• Observations by Sherrington: Noted that when a dog’s leg was pinched, that leg retracted while the other legs extended, suggesting a role for interneurons.

• Inference: An interneuron in the spinal cord transmits an excitatory message to the flexor muscles of one leg while sending an inhibitory message to the other three legs.

Inhibitory Postsynaptic Potential (IPSP)

• Definition: IPSP is the temporary hyperpolarization of a membrane.

• Mechanism: Occurs when synaptic input selectively opens gates for positively charged potassium ions to leave or negatively charged chloride ions to enter the cell, functioning as an “active brake” to suppress excitation.

Sherrington's Inference of Inhibitory Synapses

• Diagrammatic Representation:

  • Sensory and Motor neurons: Shows how excitatory and inhibitory synapses operate in reflexes.

  • Interconnectedness: Body reflexes rely on both excitatory input and inhibition through interneurons to balance reactions.

A Possible Wiring Diagram for Synapses

• Diagram: Presents a basic wiring diagram for an "A or B" response with neurons through axons and dendrites in synapse coordination.

Knowledge Check

• Differences Between Temporal and Spatial Summation:

  • Temporal summation: Summation over time.

  • Spatial summation: Summation over space.

Nerves Send Messages by Releasing Chemicals

• Research by Otto Loewi: First to demonstrate that synaptic communication is chemical.

• Experiment: Stimulating one nerve in a frog’s heart slowed it, while stimulation of another increased the heartbeat, indicating that chemical interactions, not electrical ones, are responsible.

• Vagus Nerve Experiment: Transferred fluid from one heart to another demonstrated chemical transmission (decrease in heartbeat).

Transmission at a Synapse

1. Synthesis: Smaller neurotransmitters such as acetylcholine are synthesized.

2. Release Trigger: An action potential causes calcium to enter the presynaptic terminal, releasing neurotransmitters.

3. Diffusion and Binding: Released molecules diffuse across the synaptic cleft, attach to receptors, altering postsynaptic neuron activity.

4. Separation: Neurotransmitter molecules separate from receptors.

5. Reuptake: Neurotransmitters may be taken back into the presynaptic neuron for recycling or diffuse away.

6. Retrograde Signaling: Some postsynaptic cells release retrograde transmitters that slow further neurotransmitter release from presynaptic cells.

Types of Neurotransmitters

• Categories:

  • Amino Acids: Glutamate, GABA, glycine, aspartate.

  • Modified Amino Acids: Acetylcholine.

  • Monoamines: Include indoleamines like serotonin and catecholamines like dopamine, norepinephrine, epinephrine.

  • Neuropeptides: Chains of amino acids like endorphins, substance P.

  • Purines: ATP, adenosine.

  • Gases: NO (nitric oxide).

Synthesis of Transmitters

• Nutritional Sources: Neurons synthesize neurotransmitters from dietary substances.

• Examples:

  • Acetylcholine synthesized from choline (found in milk, eggs, nuts).

  • Tryptophan is a precursor for serotonin.

  • Catecholamines include epinephrine, norepinephrine, and dopamine (all contain a catechol group).

Anatomy of a Synapse

• Representation: Visual representation of synapse anatomy.

Effects on the Postsynaptic Cell

• Receptor Dependency: The effect of a neurotransmitter relies on its receptor on the postsynaptic cell.

• Ligand Definition: A ligand is a chemical that binds to a receptor.

Ionotropic Effects

• Function: When a neurotransmitter binds to a receptor, it immediately opens ion channels.

• Characteristics:

  • Effects occur quickly (often < 1 millisecond) and are short-lived.

  • Major neurotransmitters involved: Glutamate and GABA.

Metabotropic Effects and Second Messenger Systems

• Function: Occurs when neurotransmitters initiate slower, longer-lasting metabolic reactions.

• Neuromodulators: Chemicals that affect these receptors, including dopamine, norepinephrine, serotonin.

Metabotropic Mechanism

• Mechanistic Detail:

  1. Binding of neurotransmitter bends the receptor protein, leading to G protein activation.

  2. G protein activates a second messenger to alter metabolic pathways or open/close ion channels.

• Behaviors Involved: Taste, smell, pain, arousal, attention, hunger, thirst, emotion.

G-Proteins

• G-Protein Activation: Coupled to guanosine triphosphate (GTP); increases concentration of the second messenger for intracellular communication.

• Actions: May open/close ion channels or alter production of cellular proteins.

Drugs That Bind to Receptors

• Effects of Hallucinogens: Distort perception by resembling serotonin (e.g., LSD) and activating serotonin 5-HT2A receptors abnormally, leading to subjective effects.

• Opiates: Bind to specific receptors in the brain; endogenous morphines (endorphins) function similarly.

Inactivation and Reuptake of Neurotransmitters

• Reuptake Process: Neurotransmitters in the synapse are either inactivated or reabsorbed by the presynaptic neuron.

• Transporters: Membrane proteins that facilitate neurotransmitter reuptake.

Examples of Inactivation and Reuptake

• Serotonin: Taken back into the presynaptic terminal.

• Acetylcholine: Broken down by acetylcholinesterase into acetate and choline.

• Neurotransmitter Breakdown: Enzymes inactivate any remaining neurotransmitter not reabsorbed.

Stimulant Drugs

• Amphetamine and Cocaine: Stimulate dopamine synapses by increasing dopamine release from the presynaptic terminal.

• Methylphenidate (Ritalin): Blocks dopamine reuptake but at a gradual, controlled rate. Commonly prescribed for ADHD.

Negative Feedback from the Postsynaptic Cell

• Mechanisms of Negative Feedback:

  • Autoreceptors: Receptors that monitor released transmitter and inhibit further synthesis/release.

  • Postsynaptic Neuron Response: Chemicals released by stimulated postsynaptic neurons travel back to inhibit presynaptic release.

Cannabinoids

• Active Chemicals in Marijuana: Bind to anandamide or 2-AG receptors on presynaptic neurons or GABA, inhibiting neurotransmitter release.

• Effects: Decreased excitatory and inhibitory messages across neurons, typically resulting in reduced anxiety.

Electrical Synapses

• Special-purpose Synapses: Operate electrically, allowing faster communication than chemical synapses.

• Gap Junction: Direct contact between membranes of two neurons, causing depolarization in both, leading them to act as a single entity.

Hormones

• Definition: Chemicals secreted by glands or cells, transported to organs through the blood where they modify activity.

• Origin: Produced by endocrine glands, crucial for triggering long-lasting physiological changes.

Selective List of Hormones

The Pituitary Gland and the Hypothalamus

• Structure: Attached to the hypothalamus, consisting of two distinct parts.

  • Anterior Pituitary: Composed of glandular tissue, controlled via releasing/inhibiting hormones from the hypothalamus.

  • Posterior Pituitary: Composed of neural tissue; produces oxytocin and vasopressin, released in response to neural signals.

Pituitary Hormones

• Hormonal Flow: Describes arterial flow and relationships between anterior and posterior pituitary hormones affecting various bodily functions.

Negative Feedback in the Control of Thyroid Hormones

• Feedback Mechanism:

  • Hypothalamic Regulation: TSH-releasing hormone from hypothalamus stimulates anterior pituitary, which in turn influences thyroid hormone production by the thyroid gland.

  • Effects: Thyroid hormones provide excitatory effects on metabolism while inhibiting further production.

Discussion

• Group Discussion Prompt:

  • Examine the implication that LSD affects serotonin receptors while dopamine axons' destruction impairs amphetamines and cocaine.

  • Discuss underlying neurobiological mechanisms and implications with your group.

Study Questions

1. How did Charles Sherrington use behavioral observations to infer major synaptic properties?

2. How do EPSPs and IPSPs contribute to temporal and spatial summation?

3. Why is inhibition essential in the functioning of the nervous system?

4. What is the sequence at a synapse, from neurotransmitter synthesis to receptor stimulation and transmitter disposition?

5. What distinguishes ionotropic and metabotropic receptors, including functional mechanisms?

6. How do some drugs influence behavior through synaptic interactions?

7. What are common hormones and their effects on bodily functions or behaviors?