Neurons and Synapses Study Notes

Neurons and Synapses Study Notes

Introduction

• This study guide covers Chapter 3, focusing on the structure and function of neurons and synapses as described by OXFORD UNIVERSITY PRESS (2018).

Understanding Neurons

Neuron Doctrine

• Established by Ramon y Cajal, asserts that the brain is composed of many small, discrete cells.

• Approximately 100 billion neurons are present in the human brain.

• Like all other cells, neurons have:

  • Membrane

  • Nucleus

  • Specialized organelles

Structure of Neurons

Key Components of a Neuron

1. Dendrites:

   • Branching projections that collect information from other neurons.

   • Integrate thousands of tiny chemical signals.

   • Variety in shape.

2. Soma (Cell Body):

   • Central command center of the neuron containing the nucleus.

   • Integrates information received from dendrites.

3. Axon:

   • A long, slender extension responsible for conducting neural signals across distances.

   • Often described as a cable that rapidly conducts signals.

4. Axon Terminals:

   • Small swellings at the end of the axon that release signals to affect other neurons.

   • Involved in the release of neurotransmitters at synapses.

Neuron Types

Based on Function

• Sensory Neurons: Transmit signals from sensory receptors to the central nervous system.

• Motor Neurons: Send commands from the central nervous system to muscles and glands.

• Interneurons: Can be classified into two:

  • Projection Interneurons: Have long projections to other regions.

  • Local Interneurons: Stay within a region.

Based on Shape

• Multipolar Neurons:

  • Examples include pyramidal cells (in cerebral cortex) and Purkinje cells (in cerebellum).

  • Function: Carries commands to muscles and glands.

• Bipolar Neurons:

  • Found in sensory systems such as retina and cochlea.

• Unipolar Neurons:

  • Typically found near spinal cord; function in transmitting touch, temperature, and pain information.

Glial Cells (Glia)

• Essential in supporting neurons through various roles:

  • Speeding up neuronal signaling

  • Regulating extracellular chemicals

  • Enabling modification of neuron connections

• Types of glial cells by function and location:

  1. Astrocytes:

  • Provide structural and nutritional support.

  • Involved in blood-brain barrier function.

  1. Ependymal Cells:

  • Line the ventricles and move cerebrospinal fluid.

  1. Oligodendrocytes:

  • Myelinate axons in the CNS.

  1. Schwann Cells:

  • Myelinate axons in the PNS.

  1. Microglia:

  • Conduct debris cleanup and synapse removal.

Blood-Brain Barrier

• A semipermeable barrier between blood and brain formed by endothelial cells in capillaries.

• Area Postrema: A region in the medulla with a weaker blood-brain barrier that can detect poisons and trigger vomiting.

Axons and Dendrites

• Dendrites receive signals from other neurons through specialized structures called dendritic spines.

• Axons transmit signals away from the neuron.

• Key structures include:

  • Axon Hillock: Initiates action potentials.

  • Myelin Sheath: Provides insulation for electrical signals.

  • Nodes of Ranvier: Gaps in the myelin sheath facilitate saltatory conduction, where action potentials jump between nodes to increase speed.

Neural Communication

Resting Potential

• The voltage difference across the resting membrane is approximately $-70 mV$ (where the extracellular environment is assigned $0$).

• This is essential for maintaining the neuron's stable state before receiving signals.

Action Potentials

• An action potential is an electrical signal crucial for neural communication, characterized by a sequence of ionic changes across the neural membrane.

• Ionic Composition:

  • Intracellular fluid has a different concentration of sodium (Na+), potassium (K+), and chloride (Cl−) ions compared to extracellular fluid, affecting the neuron's electrical signaling capabilities.

• Generating Action Potentials:

  • Influenced by the relative concentrations of ions and the characteristics of ion channels in the membrane.

Propagation of Action Potentials

• The action potential reproduces itself down the length of the axon.

• Myelination Influence:

  • In unmyelinated axons, action potentials are reproduced at each segment, while in myelinated axons they propagate through nodes of Ranvier via saltatory conduction, enhancing conduction velocity.

Neurotransmitter Release at Synapses

• An action potential reaching the axon terminal triggers neurotransmitter release through exocytosis.

• The process is facilitated by the influx of calcium ions (Ca++).

• Axo-Axonic Synapses: Modulatory effects on neurotransmitter release by the target axon are observed here.

Axoplasmic Transport

• An active process that propels substances along axonal microtubules.

• Anterograde Transport: Movement from cell body toward axon terminals.

• Retrograde Transport: Movement from axon terminals back to the cell body.

Neurotransmitters

Types of Neurotransmitters

1. Small-Molecular-Weight Neurotransmitters:

   • Synthesis occurs locally in the axon terminal (examples include monoamines and amino acids).

2. Large-Molecular-Weight Neurotransmitters:

   • Typically require synthesis in the cell body (examples include neuropeptides).

Drug Interactions and Synaptic Function

• Agonists: Enhance the activity of a neurotransmitter.

• Antagonists: Reduce the activity of a neurotransmitter.

Synaptic Actions

• Reuptake: A process where neurotransmitters are reabsorbed back into the presynaptic neuron (e.g., SSRIs inhibit serotonin reuptake).

• Enzymatic Degradation: The breakdown of neurotransmitters by enzymes such as MAOs and AchE.

• Neuronal Excitation: Neurotransmitters can cause excitatory postsynaptic potentials (EPSPs) by opening channels that allow positively charged ions (e.g., $Na^+$) to enter, thus increasing the likelihood of neuronal firing.

Postsynaptic Potentials

Example Questions for Understanding

1. If a neuron has a resting membrane potential of $-70 mV$ and undergoes hyperpolarization to $-73 mV$, the size of the IPSP is:

   • 3 mV.

2. If a neurotransmitter binds to a receptor changing the membrane potential from $-70$ to $-73$, it undergoes:

   • an IPSP.

3. If the potential changes from $-70$ to $-67$, it undergoes:

   • an EPSP.

4. For a neuron’s membrane potential at $-70 mV$ with a threshold of $-60 mV$, how much depolarization is required?

   • 10 mV.

5. If the membrane potential is at $-73 mV$ and the threshold is $-60$, how much depolarization is necessary?

   • 13 mV (More hyperpolarized membranes require greater depolarization to reach firing threshold).