Neurons + Synaptic transmission

Question 1

Overview of Neurons

Answer 1

Neurons: Specialized cells in the nervous system that process and transmit electrical and chemical signals.
Three Types of Neurons:
* 1. Sensory Neurons: Carry signals from sensory receptors (e.g., skin) to the Central Nervous System (CNS).
* 2. Motor Neurons:Carry signals from the CNS to effectors like muscles and glands, enabling movement or secretion.
* 3. Relay Neurons: Found in the CNS, connecting sensory and motor neurons. They process information and form neural circuits.

Question 2

Structure of a Neuron

Answer 2

• Key Components:
  1. Cell Body (Soma): Contains the nucleus and genetic material.
  2. Dendrites: Branch-like structures that receive incoming signals from other neurons.
  3. Axon: The long fiber that transmits electrical signals from the cell body to the axon terminals.
  4. Myelin Sheath: A fatty layer that insulates the axon and speeds up electrical transmission. Formed by Schwann cells.
  5. Nodes of Ranvier: Gaps between myelin segments where action potentials “jump,” speeding up transmission.
  6. Axon Terminals: End of the axon, where the neuron communicates with the next neuron via synapses.
• Synapse: The junction between two neurons or a neuron and an effector cell.

Question 3

Resting State and Action Potential

Answer 3

• Resting Potential:
  o Neuron is at rest and polarized (more negative inside than outside).
  o Inside the neuron: High concentration of potassium (K+) ions.
  o Outside the neuron: High concentration of sodium (Na+) ions.
  o Charge Difference: -70mV (millivolts).
• Action Potential:
  1. Stimulation: Causes sodium channels to open, and Na+ ions flood into the neuron, depolarizing it.
  2. Threshold of Excitation: When the charge reaches -55mV, the neuron fires an action potential.
  3. Repolarization: Potassium ions exit, restoring the negative charge inside the neuron.
  4. Refractory Period: After firing, the neuron temporarily cannot fire again until it returns to resting potential.

Question 4

Synaptic Transmission - Definition and Process

Answer 4

• Definition: The process by which neurons communicate across a synapse via chemical signals (neurotransmitters).
• Steps of Synaptic Transmission:
  1. Action Potential: Reaches the axon terminal of the presynaptic neuron.
  2. Vesicle Fusion: Action potential triggers the release of neurotransmitter-filled vesicles into the synaptic cleft.
  3. Neurotransmitter Release: Neurotransmitters (e.g., dopamine, serotonin) cross the synaptic cleft and bind to receptors on the postsynaptic neuron.
  4. Post-Synaptic Potential: Binding of neurotransmitters to receptors causes either excitation (more likely to fire an action potential) or inhibition (less likely to fire an action potential).

Question 5

Excitatory vs Inhibitory Neurotransmitters

Answer 5

• Excitatory Neurotransmitters: Increase the likelihood of the postsynaptic neuron firing an action potential.
  o Example: Glutamate (the brain’s main excitatory neurotransmitter).
• Inhibitory Neurotransmitters: Decrease the likelihood of the postsynaptic neuron firing.
  o Example: GABA (the brain’s main inhibitory neurotransmitter).
• Summation: The combined effect of excitatory and inhibitory signals determines whether the postsynaptic neuron will fire an action potential.

Question 6

Types of Neurotransmitters and Their Functions

Answer 6

• Acetylcholine (ACh): Involved in muscle contraction, memory, and learning.
• Dopamine: Linked to movement, emotion, and reward pathways.
• Serotonin: Regulates mood, sleep, and appetite. Low levels are associated with depression.
• Noradrenaline (Norepinephrine): Associated with the fight-or-flight response and alertness.
• GABA: The main inhibitory neurotransmitter, it reduces neural excitability and calms the brain.

Question 7

Reuptake and Enzyme Breakdown

Answer 7

• After neurotransmitters bind to receptors, they are either:
  1. Reabsorbed by the presynaptic neuron (reuptake).
  2. Broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine).
• Drug Implications:
  o SSRIs (Selective Serotonin Reuptake Inhibitors): Used to treat depression by blocking serotonin reuptake, increasing serotonin levels in the synapse.
  o MAOIs (Monoamine Oxidase Inhibitors): Prevent the breakdown of neurotransmitters like serotonin and dopamine, increasing their levels.

Question 8

Evaluation of Synaptic Transmission

Answer 8

• Strengths:
  1. Empirical Evidence: Supported by research on neurotransmitters (e.g., dopamine’s role in Parkinson’s disease).
  2. Biological Basis: Provides insight into how mental disorders (e.g., depression, anxiety) are linked to neurotransmitter imbalances.
• Limitations:
  1. Reductionist: Focuses purely on biological factors and overlooks psychological or environmental influences on behaviour.
  2. Generalisation: Much research is based on animals, raising concerns about the generalisability of findings to humans.

Question 9

Key Exam Points for Neurons and Synaptic Transmission

Answer 9

• Key Terms: Ensure understanding of key terms (e.g., action potential, depolarization, synapse, neurotransmitters).
• Mechanisms: Be able to explain how neurons communicate (action potentials, neurotransmitter release, and receptor binding).
• Evaluation: Critically assess strengths and limitations, considering both biological evidence and reductionism.
• Neurotransmitter Impact: Explain how neurotransmitter imbalances can affect behaviour and contribute to mental disorders (e.g., depression, schizophrenia).