Study Guide for Chapter 3: Structure and Function of Neurons and Synapses

Study Guide: Structure and Function of Neurons and Synapses

Chapter Overview

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

Understanding Neurons

Neuron Doctrine

• Definition: Established by Ramon y Cajal, asserts that the brain is made up of many small, discrete individual cells.

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

• Cellular Composition: Neurons, like all other cells, have:

  • Membrane

  • Nucleus

  • Specialized organelles

Structure of Neurons

Neural Membrane

• Functions as a boundary between the cell and the external environment, separating intracellular fluid/cytoplasm from extracellular fluid surrounding the neuron.

• Uses different chemical compositions of the fluids to facilitate separation.

  • Phospholipid Membrane: Contains protein structures controlling permeability.

  • Ion Movement: Channels allow specific ions to move in or out.

Types of Ion Structures in Neurons

1. Ion Channels

   • Function: Allow ions to move passively without using energy.

   • Voltage Dependent Channels: Open and close in response to electrical status of the membrane.

2. Ion Pumps

   • Function: Requires energy to move ions across the membrane.

   • Sodium-Potassium Pumps: Move three sodium ions out while taking in two potassium ions, consuming 20-40% of the brain's energy.

   • Calcium Pumps: Specifically pump calcium ions out of the neuron.

Cytoskeleton

• Provides structural support and maintains the shape of the neuron.

• Composition: Made of three types of filaments:

  • Microtubules (largest, hollow tubes ~25 nm):

  • Move materials within the cell (e.g., vesicles).

  • Involved in anterograde (away from soma) and retrograde (towards soma) transport.

  • Tau Protein: Stabilizes microtubules; elevated levels associated with Alzheimer's disease.

  • Intermediate Filaments (about 10 nm):

  • Most common fiber, provide structural support.

  • Microfilaments (3-5 nm):

  • Participate in changing the shape and length of structures during development.

Key Components of a Neuron

1. Nucleus:

   • Central structure containing the nucleolus, where ribosomes are synthesized.

   • Ribosomes create proteins alone or with the endoplasmic reticulum (ER).

   • Endoplasmic Reticulum: Rough ER produces proteins; smooth ER processes proteins which are then sent to the Golgi apparatus.

   • Mitochondria: Utilize oxygen and transform sugars into ATP, the main energy source for neurons.

2. Dendrites:

   • Branching extensions that receive information from other neurons.

   • Integrate thousands of chemical signals.

   • Varied shapes with specialized gated ion channels that act as receptor sites for chemicals released from other neurons.

   • Dendritic Spines: Can change shape based on synaptic activity, contributing to learning and memory; abnormality is associated with autism, schizophrenia, and Alzheimer's.

3. Soma (Cell Body):

   • Acts as the neuron’s command center, integrating incoming signals and containing organelles.

4. Axon:

   • A single, long extension responsible for conducting neural impulses over distances.

   • Described as a cable with a diameter of less than 1 µm to about 25 µm.

   • Types of axons:

     • Local Circuit Neurons: Short axons communicate with nearby cells.

     • Projection Neurons: Long axons project to distant areas.

5. Axon Terminals:

   • Swellings at the end of axon collaterals that release signals to other neurons.

   • Involved in neurotransmitter release at synapses.

   • Axonal Varicosities: Intervals along unmyelinated segments of an axon containing mitochondria and synaptic vesicles.

Neuron Types-Based on Function

1. Sensory Neurons:

   • Transmit signals from sensory receptors to the central nervous system (CNS).

2. Motor Neurons:

   • Send commands from the CNS to muscles and glands.

3. Interneurons:

   • Classified into:

     • Projection Interneurons: Have long projections to other regions.

     • Local Interneurons: Stay within a region.

Neuron Types-Based on Shape

1. Multipolar Neurons:

   • The most common type in the vertebrate nervous system; serve as motor neurons with many branches extending from the soma.

   • Examples: Pyramidal cells (cerebral cortex), Purkinje cells (cerebellum).

2. Bipolar Neurons:

   • Feature two branches: one axon and one dendrite.

   • Found in sensory systems like the retina; includes Von Economo Neurons (VENs), specialized bipolar cells.

3. Unipolar Neurons:

   • A single branch from the cell body, found in sensory systems involved in touch, temperature, and pain, typically near the spinal cord.

Functional Variations in Neurons

• Sensory Neurons: Translate various external and internal stimuli into signals.

• Motor Neurons: Transmit commands directly to muscles and glands.

• Interneurons: The majority of neurons, acting as bridges between sensory and motor functions.

Glial Cells (Glia)

• Total of approximately 360 billion glial cells constitute 80-90% of cells in the CNS, also found in the peripheral nervous system.

• Functions: Support neurons by speeding up signaling, regulating chemicals, and modifying connections, help clean debirs and prune synaptic cnxs

• Types of Glial Cells:

  • Macroglia: Large varieties that develop from ectoderm.

  • Astrocytes: Most common in the CNS, maintain homeostasis, provide support, clean debris, and contribute to the blood-brain barrier.

  • Microglia: Small cells originating from mesoderm responsible for debris cleanup. They act as the brain's immune system but can damage surrounding cells if activated uncontrollably.

  • Ependymal Cells: Line ventricles and the spinal canal, moving cerebrospinal fluid and regulating stem cells.

  • Oligodendrocytes: Myelinate axons in the CNS, supporting structural stability.

  • Schwann Cells: Myelinate axons in the PNS, with one cell myelinating a single segment of an axon.

Blood-Brain Barrier

• A semipermeable barrier formed by endothelial cells in capillaries that protects the brain from toxins in the blood.

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

Axons and Dendrites

• Dendrites: Receive signals via specialized structures called dendritic spines, which can change shape according to synaptic activity; abnormalities may lead to neurological disorders.

• Axons: Transmit signals; key structures include:

  • Axon Hillock: Initiates action potentials, lacks myelin coverage.

  • Myelin Sheath: Insulates electrical signals.

  • Nodes of Ranvier: Gaps in the myelin facilitating saltatory conduction, where action potentials jump between nodes for faster signal propagation.

Neural Communication

Resting Potential

• Definition: The neural membrane has a resting potential of approximately -70 mV due to the concentration of potassium ions inside the cell.

• This resting state is maintained to prepare for incoming signals.

Action Potentials

• Definition: An action potential is an electrical signal essential for neural communication, generated in the presynaptic neuron.

• Sequence of ionic changes occurs across the neural membrane to propagate signals from the axon terminal to the postsynaptic neuron.

  • Ionic Composition: High sodium and chloride outside and high potassium inside, with the intercellular fluid being more negative than the exterior.

Generating Action Potentials

• Action potentials result from the movement of ions across the membrane, creating a difference in charge.

• Diffusion: The tendency for molecules to distribute evenly, leading to movement from high to low concentration.

• Electrostatic Pressure: Causes attraction of opposite charges and repulsion of like charges, influencing ion movement across membranes.

Propagation of Action Potentials

• Action potentials reproduce down the axon, influenced by myelination:

  • In unmyelinated axons, action potentials are reproduced at each segment.

  • In myelinated axons, they propagate via saltatory conduction, jumping from node to node, increasing conduction velocity.

Neurotransmitter Release at Synapses

Types of Synapses

1. Gap Junctions: Allow cells to communicate through shared cytoplasm, contributing to synchronization in brain functions and responding to hormone release.

2. Chemical Synapses: Wider gaps that utilize neurotransmitters to relay signals, allowing both excitatory and inhibitory interactions, impacting subsequent neuronal firing.

Neurotransmitter Release Mechanism

• An action potential reaching an axon terminal triggers neurotransmitter release via exocytosis:

  • Synaptic vesicles merge with the membrane to release contents into the synaptic gap.

  • Kiss-and-Run Mechanism: Vesicles partially empty their contents before retreating.

• Auto Receptors: Feedback mechanisms for neurotransmitter release and synthesis adjustments.

• Receptors:

  • Ionotropic: Directly influence ion channels upon binding neurotransmitters.

  • Metabotropic: Initiate longer-term changes through G proteins and second messengers.

Neurotransmitter Deactivation

• Neurotransmitters can be deactivated through:

  • Diffusion away from the synapse.

  • Enzymatic degradation (e.g., MAOs, AchE).

  • Reuptake process where pre-synaptic membranes recapture neurotransmitters.

Postsynaptic Potentials

• EPSPs (Excitatory Postsynaptic Potentials): Result from depolarization and increase likelihood of action potential firing; produced by open ion channels allowing positive ions in.

• IPSPs (Inhibitory Postsynaptic Potentials): Result in hyperpolarization and decrease likelihood of action potential firing; open channels allowing chloride or potassium ions movement.

Summation of Inputs

• Action potentials are produced when excitatory inputs exceed inhibitory inputs (spatial summation).

• Temporal summation: EPSPs and IPSPs may build up over time at active synapses, influencing action potential generation.

Axo-Axonic Synapses

• These synapses modulate neurotransmitter release; if a presynaptic neuron increases release, it is called presynaptic facilitation; if it decreases release, it is presynaptic inhibition.

Axoplasmic Transport

• An active process propelling substances along axonal microtubules.

• Anterograde Transport: From cell body toward axon terminals.

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

Types and Examples of Neurotransmitters

1. Small-Molecular-Weight Neurotransmitters:

   • Synthesized locally at axon terminals (e.g., monoamines, amino acids).

2. Large-Molecular-Weight Neurotransmitters:

   • Require synthesis in the cell body (e.g., neuropeptides).

Drug Interactions and Synaptic Function

• Agonists: Enhance neurotransmitter activity.

• Antagonists: Reduce neurotransmitter activity.

Synaptic Actions

Reuptake

• The process of reabsorbing neurotransmitters back into the presynaptic neuron (e.g., SSRIs inhibit serotonin reuptake).

Enzymatic Degradation

• Breakdown of neurotransmitters occurs through enzymatic activity (e.g., MAOs, AchE).

Neuronal Excitation

• Neurotransmitters can elicit EPSPs by facilitating the entry of positively charged ions (e.g., $Na^+$), increasing the chance of neuronal firing.

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).