The Building Blocks of the Brain: Neurons

The Building Blocks of the Brain: Neurons

Beginning to Understand the Nervous System Structure

  • The brain is the most complex 'machine' known.

  • It consists of approximately 85-100 billion neurons.

  • Contains between 125 trillion to 1 quadrillion synapses.

  • There are even more glial cells compared to neurons.

  • A starting point in understanding the nervous system is the neuron, which is the single unit of the nervous system.

  • Illustration by Ramón y Cajal displays different types of neurons.

Lecture Objectives

  • At the end of this section, you should be able to:

    1. Define what a neuron is, including its 4 major structural regions.

    2. Identify the role and location of dendrites, soma, axon, and axon terminals.

    3. Briefly define what a synapse is and mention its main components.

    4. List major nervous systems and how they are connected.

    5. Describe the 4 types of glial cells and their functions.

The Neuron

  • Neurons form very complex circuits and connections that integrate information to distribute it to other regions of the brain.

  • The human brain contains roughly 85-100 billion neurons, making the neuron the basic unit of the nervous system.

Neurons are Diverse in Shape

  • The shape and structure of neurons reveal indications of their functions:

    • Multipolar Neurons: Have multiple projections off the cell body.

    • Bipolar Neurons: Have two extensions, one on each side of the cell body.

    • Unipolar Neurons: Have a single extension that branches into two directions.

  • Neuronal shape and function are intrinsically interlinked, indicating the specialization necessary for different roles in the nervous system.

The Structural Components of a Neuron

  • The structure of a neuron is composed of 4 main divisions:

    1. The Input Zone: Composed of dendrites that process information from the environment or from other cells.

    2. The Integration Zone: Known as the soma or cell body, where the decision to produce a neural signal is made.

    3. The Conduction Zone: This occurs along the axon, where the information is electrically transmitted over distances.

    4. The Output Zone: Finished in specialized swellings called axon terminals or terminal boutons, where information is transferred to other cells.

Dendrites Receive Information at the Input Zone

  • Dendrites are extensions of the neuronal membrane at the input zone that receive information from other neurons through synapses.

  • They form smaller swellings known as dendritic spines, which provide additional spaces to form more synapses.

  • Reference: Shao, L.-X., et al. (2021) - Psilocybin induces rapid and persistent growth of dendritic spines in frontal cortex.

Information is Integrated at the Soma

  • The soma is the cell body of the neuron and contains all common cellular machinery.

  • It integrates the information received to determine whether to send a signal of its own.

  • The neuron’s electrical stimulus is initiated in the soma.

  • Reference: Zhao, X., van Praag, H. (2020) - Standardized quantification of adult neurogenesis.

Review Checkpoint

  1. What are the 4 major structural components of a neuron?

  2. Which of the 4 components includes the axon?

  3. TRUE or FALSE: Neurons can contain both Endoplasmic Reticulum and Mitochondria.

  4. What is ONE of the functions of dendritic spines?

The Axon Transmits the Information

  • The axon, also known as the nerve fiber, is a single membrane extension from the soma that carries the neuron’s electrical signal away from it.

  • An axon may form axon collaterals which branch off from the main body.

  • The direction of the signal transmitted along the axon is unidirectional.

Information is Sent Through Synapses at the Axon Terminals

  • The axon terminals are bulbous structures at the end of axons (or axon collaterals) forming the output zone where synapses occur.

  • A synapse is the cellular junction where information transmits from a presynaptic neuron to a postsynaptic neuron:

    1. Presynaptic Membrane: The membrane of the axon terminal.

    2. Synaptic Cleft: The gap between the two neurons.

    3. Postsynaptic Membrane: The membrane of the dendrite or cell body of the receiving neuron.

Neurons Create Networks

  • Individual neurons create circuits that develop into networks and systems within the nervous system.

Networks Create Systems

  • These neuronal networks can be categorized into two main systems:

    • Central Nervous System (CNS)

    • Peripheral Nervous System (PNS)

Central Nervous System: CNS

  • Composed of:

    • Brain

    • Spinal Cord

Peripheral Nervous System: PNS

  • Composed of:

    • Autonomic Nervous System

    • Somatic Nervous System

    • Includes sensory and motor pathways; divided into:

    • Sympathetic

    • Parasympathetic

Sympathetic vs. Parasympathetic Nervous Systems

  • Sympathetic Nervous System:

    • Responsible for physiological arousal and preparing the body for action, engaging in the 'Fight or Flight' response.

    • Features shorter axons that innervate sympathetic ganglia.

  • Parasympathetic Nervous System:

    • Responsible for returning the body to a baseline arousal level, preparing the body for recovery and repair, engaging in 'REST and DIGEST'.

    • Features longer axons that project to ganglia near target organs.

Sympathetic vs Parasympathetic Processes

  • Sympathetic Division Effects:

    • Dilates pupils

    • Inhibits salivation

    • Accelerates heartbeat

    • Increases glucose release

    • Inhibits digestion

    • Various effects on blood vessels and other organs.

  • Parasympathetic Division Effects:

    • Constricts pupils

    • Stimulates salivation

    • Slows heartbeat

    • Stimulates digestion

    • Various organs receive distinctive intra-organ responses.

Cells of the Nervous System: Glial Cells

  • Glial Cells serve critical support functions for neurons and help maintain cellular homeostasis in the nervous system in several ways:

    1. Astrocytes: Maintain the blood-brain barrier, provision nutrients to neurons and support synaptic function.

    2. Microglia: Function as immune cells of the brain, providing defense against pathogens.

    3. Oligodendrocytes: Provide myelin sheath to CNS neurons.

    4. Schwann Cells: Provide myelin sheath to PNS neurons.

Glial Cells Assist with Neuronal Networks

  • Oligodendrocytes and microglia interact to modify myelin during CNS development.

  • Reference: Hughes, A. N. & Appel, B. (2020) - Microglia modify developmental myelination.

Goals for Understanding Nervous System

  • Define and utilize critical components of the Scientific Method:

    • Identify independent variable (IV), dependent variable (DV), control group, and experimental group.

  • Apply knowledge on experimental methods that lead to causal inferences.

  • Evaluate the staining methods used to identify neurons.

  • Identify and label** parts of neurons** accurately.

  • List major nervous systems and understand their interconnections.

  • Analyze how different cell types and structures support neuron function effectively.