Introduction to Neural Cells

Definition of Neural Cells

• Neural or neuro- refers to the nervous system.
• Neural cells are integral to the nervous system and are involved in various functions:
  • Consciousness
  • Social interactions
  • Cognition
  • Emotion
  • Movement
  • Sensory perception
  • Regulation of body functions (e.g., circulation, respiration, digestion)

Two Main Categories of Neural Cells

1. Neurons
   • Traditionally called nerve cells.
   • Derived from the Greek word for nerve.
2. Glia
   • Also known as neuroglia or glial cells.
   • Derived from the Greek word for glue, as they were initially thought to only glue neurons together.

Structure of the Nervous System

• Central Nervous System (CNS)
  • Comprises the brain and spinal cord.
• Peripheral Nervous System (PNS)
  • Composed mostly of nerves extending throughout the body.
  • Includes nerves that extend into arms, legs, and other body parts.

Neural Cells in CNS and PNS

• Neurons and glial cells are distributed differently between CNS and PNS.
• Nerves in PNS contain:
  • Neurons
  • Glial cells
  • Other non-neural cells
• Neurons can be found in both CNS and PNS.
• Types of glial cells are specific to either CNS or PNS.

Origin of Neural Cells

• Most neural cells originate from:
  • Neural stem cells
  • Neural crest cells
• Derived from the ectoderm of the embryo.
• Neurons and glia in CNS typically come from neural stem cells.
• Neurons and glia in PNS typically come from neural crest cells.

Structure of Neural Cells

• Common Features:
  • Soma (Cell Body)
    • Contains nucleus and organelles.
  • Processes
    • Extensions from the soma which vary in:
      • Number
      • Length
      • Thickness
      • Degree of branching
    • Terminal structures and functions vary.

Functions

• Neurons: Process and transmit information.
• Glia: Support neurons in various ways.

Types of Neural Cells

• Numerous structural and functional types exist.
• Nervous system comprises billions of neurons forming trillions of connections.
• More glia than neurons.

Common Types of Glial Cells (to be covered in detail later)

• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes
• Schwann cells

Less Common Glial Cells

• Satellite cells
• Olfactory ensheathing cells

Note: Future videos will provide more detail on each type of glial cell.

Overview of Neuron Structure

Basic Components of a Neuron

• Soma: Also known as the cell body.
• Neurites: Processes extending from the soma, divided into:
  • Dendrites: Short, branched, often covered in spines to increase surface area.
  • Axon: Usually long and unbranched until the end; connects to the soma at the axon hillock.

Axon Details

• Axon Hillock: The initial segment where the axon leaves the soma.
• Axon Initial Segment/Trigger Zone: The first part of the axon.
• Axon Terminals: Branches at the end of the axon.
• Transport Systems: Essential for moving substances between soma and axon terminals.
• Myelin Sheath: Insulative sheath around large axons, interrupted by Nodes of Ranvier.

Synapses and Target Cells

• Synapse: The junction where an axon terminal is close to the target cell.
• Target Cells: Can be neurons, muscle cells, gland cells, or capillaries (for hormone secretion).

Structural Types of Neurons

Neural Development

• Neural Stem Cells: Can become any neural cell in CNS.
• Neuroblasts: Differentiate from neural stem cells; migrate and extend axons.
• Growth Cone: Structure at axon tips that follow cues to reach target cells.
• Neural Crest Cells: Equivalent peripheral nervous system cells.

Types of Neurons

1. Unipolar Neurons

   • One process (axon) during development.

2. Bipolar Neurons

   • One axon and one dendrite.

3. Multipolar Neurons

   • One axon and multiple dendrites.
   • Most common in adult humans.

4. Pseudounipolar Neurons

   • One short process from soma splits into two axons:
     • Peripheral Axon: Acts like dendrites.
     • Central Axon: Extends towards CNS.
   • Has a distinct structure from unipolar neurons.

Conclusion

• Different types of neurons have distinct structures and functions.
• Future videos will delve into functions of dendrites and axons.

Overview of Neuron Function

Key Concepts

• Neurons process and transmit information.
• Resting Membrane Potential: Stable electrical charge difference across the cell membrane. Negative inside, positive outside.
• Excitability: Neurons respond to inputs thanks to their resting potential.

Neuron Input

• Neurons receive excitatory or inhibitory input.
• Inputs usually come in through dendrites, sometimes soma or axon.
• Graded Potentials: Changes to the membrane potential, small and brief, proportional to input.
• Summation: Adding of excitatory and inhibitory graded potentials at the trigger zone (axon initial segment).

Trigger Zone

• Summation determines if the information fires down the axon.
• Analogous to a gun trigger.

Action Potentials

• Large, brief membrane potential changes traveling the axon's length.
• Conducted faster along larger axons and those with a myelin sheath.
• Consistent size and duration for each neuron.

Transmission to Target Cell

• Action potential reaches axon terminals, crosses gap to target cell.
• Neurotransmitters released, binding to receptors, changing target cell behavior.
• Neurotransmitter removal resets synapse.

Functional Types of Neurons

• Afferent Neurons: Bring information into the CNS (sensory neurons).
• Efferent Neurons: Carry information away from the CNS.
  • Motor Neurons: Control skeletal muscle (somatomotor neurons).
  • Autonomic Neurons: Control smooth muscle, cardiac muscle, glands (visceromotor neurons).
• Interneurons: Connect neurons, forming complex pathways in the CNS.

Neuron Categorization

• CNS (Central Nervous System)
• PNS (Peripheral Nervous System)

Summary

• Neurons function similarly to how a gun operates:
  • Loading (resting potential)
  • Triggering (summation at trigger zone)
  • Firing (action potential transmission).

Lecture on Astrocytes

Introduction to Astrocytes

• Origin: Astrocytes come from the Greek words for "star cell."
• Classification: They are glial cells in the central nervous system, derived from neural stem cells.
• Structure:
  • Soma: Variable in number and branches of processes.
  • Processes: Highly branched, resemble stars under a microscope.
  • End-feet: Special structures at the end of processes.

Functions of Astrocytes

Astrocytes perform numerous vital functions in the central nervous system:

Structural Support

• Scaffold Formation: Occupy a large space in the CNS, forming the majority of its structure.

Response to Injury

• Glial Scar Formation: Respond to CNS injury by proliferating, migrating to injury sites, hypertrophying their processes to wall off the injury.
  • Names for the Process: Gliosis, astrogliosis, astrocytosis, or reactive astrocytosis.
  • Function of Glial Scar: Provides structural support by shoring up the injury area.

Homeostasis

• Interstitial Fluid Balance: Monitors and maintains optimal ion concentrations, especially potassium, for neuronal function.
• Energy Support: Releases lactate, aiding neurons when their continuous oxygen and glucose supply is disrupted.

Blood-Brain Barrier Contribution

• Barrier Function: Works with blood vessel components to prevent large molecules from entering the CNS from the bloodstream.
• Role of End-feet: Plastered over blood vessels, contributing to the barrier.

Synaptic Function

• Synapse Clearance: Helps clear neurotransmitters at synapses to reset them for future neuronal communication.
• Prevention of Constant Activation: By clearing neurotransmitters, synapses are not left in a constantly activated state.

Influence on Neurons and Glia

• Substance Exchange: Influence other cells through the exchange of various substances.

Conclusion

• Astrocytes are crucial workhorses in the CNS, with diverse and critical roles in maintaining CNS function.

Microglia and Their Role in the Central Nervous System

Introduction to Microglia

• Definition: Microglia are glial cells in the central nervous system (CNS).
• Origin of Name: Derived from Greek words meaning "small glue."
• Size: Smaller than other glial cells, hence the name. Other glial cells are collectively referred to as macroglia.
• Embryological Origin: Likely derived from circulating monocytes from the bone marrow, originating from the mesoderm.

Morphology of Microglia

• Resting Microglia:

  • Small soma with long, highly branched processes.
  • Function: Sample interstitial fluid and monitor for trouble.

• Active Microglia:

  • Larger, amoeba-like shape.
  • Function: Respond to detected trouble, such as inflammation.

Functions of Microglia

Inflammatory Response

• Resting Microglia Activation:

  • Detects inflammation from tissue injury or infection (bacteria, virus).
  • Becomes active microglia, resembling macrophages.

• Active Microglia Actions:

  • Migrate to areas of inflammation.
  • Search for foreign cells or dead/damaged cells.

Cytotoxic Secretion

• Purpose: Kill foreign cells (e.g., bacteria) using cytotoxic substances like reactive oxygen species (ROS).
• Process: Secretion of cytotoxic factors to eliminate threats.

Phagocytosis

• Definition: Process of eating or engulfing debris.
• Target: Foreign cell debris (e.g., bacteria) or dead/damaged CNS cells.
• Process: Ingesting debris and breaking it down inside the cell.

Antigen Presentation

• Function: Present debris pieces on the surface for immune system cells, such as lymphocytes, to recognize.
• Antigen: Any molecule recognized by immune cells.
• Effect: Enhances and specifies inflammation response.

Interaction with Other Cells

• Communication: Exchange substances with neurons, other glial cells, and immune cells.
• Role: Major contributors to CNS inflammation and immune response.

Conclusion

• Microglia are essential for maintaining CNS health by monitoring, responding to inflammation, and interacting with other cells. They play a critical role in inflammation and immunity within the CNS.

Lecture on Ependymal Cells

Overview

• Ependymal cells are a type of glial cell in the central nervous system (CNS).
• They line spaces in the brain and spinal cord that are filled with cerebral spinal fluid (CSF).

Anatomy Context

• The brain and spinal cord contain interconnected spaces filled with CSF.
• These spaces are lined by a tissue called the ependyma, composed of ependymal cells.
• Greek origin: 'ependyma' means covering.

Origin and Structure

• Ependymal Cells
  • Derived from neural stem cells.
  • Form a simple, cuboidal epithelium (one layer of cube-shaped cells).
  • Function as a lining for cavities in the CNS.

Orientation

• Facing CSF
  • Surface facing CSF has microvilli to increase surface area.
  • Also contains cilia, which help move CSF.
• Facing Interstitial Fluid (IF)
  • Interstitial fluid is the fluid between cells in the CNS.

Functions

1. Barrier Formation

   • Ependymal cells create a barrier between CSF and interstitial fluid.
   • Barrier is relatively leaky compared to the blood-brain barrier.
   • This leakiness allows for medical sampling of CSF to understand CNS conditions.

2. Secretion of CSF

   • Ependymal cells participate in the secretion of CSF.
   • Specialized ependymal cells and capillaries form structures called tufts.
   • Fluid is secreted across ependymal cells to produce CSF.

Clinical Relevance

• The ability to sample CSF due to the leaky barrier provides valuable insights into CNS health and disease.

Oligodendrocytes Overview

Definition and Origin

• Oligodendrocytes are a type of glial cell in the central nervous system.
• They originate from neural stem cells.
• The name derives from Greek words meaning "cells with a few branches."

Structure and Role

• Neural Interaction: Oligodendrocytes extend multiple processes to wrap around the axons of neurons.
  • Each oligodendrocyte can form up to several dozen processes.
  • These processes contribute to the creation of the myelin sheath on axons.
• Myelin Sheath:
  • Composed mostly of lipids, akin to fat, providing a fatty sheath around axons.
  • Myelin sheath segments can be from different oligodendrocytes on a neuron's axon.
  • The sheath acts like rubber insulation on a wire, enhancing speed and efficiency of information transmission along axons.
• Connection Maintenance:
  • The myelin sheath remains connected to the oligodendrocyte soma via its processes.

Functionality

• Myelination:
  • Each process of an oligodendrocyte forms one segment of myelin on an axon.
  • A single oligodendrocyte can myelinate multiple axons.
• Cell Interaction:
  • Oligodendrocytes influence and exchange substances with neurons and other glial cells.

Variability and Mystery

• Some oligodendrocytes do not form myelin and are variably shaped in certain central nervous system regions.
• The function of these non-myelinating oligodendrocytes is not fully understood.

Schwann Cells

Overview

• Glia of the peripheral nervous system.
• Derived from neural crest cells.
• Named after the person who described them.

Types of Schwann Cells

Nonmyelinating Schwann Cells

• Structure: Generally shapeless with small troughs on the surface.
• Function: Provide support to small diameter axons by allowing them to sit in the troughs.
• Characteristics:
  • Associated with small diameter axons of neurons.
  • Do not form a myelin sheath.

Myelinating Schwann Cells

• Structure: Produce a myelin sheath around axons.
• Function:
  • Create a myelin sheath similar to that in the central nervous system.
  • Myelin sheath is interrupted by nodes of Ranvier.
  • Each Schwann cell myelinates only one segment of one axon.

Comparison with Central Nervous System

• Myelination:
  • In the central nervous system, oligodendrocytes myelinate multiple neurons.
  • In the peripheral nervous system, Schwann cells myelinate only one axon segment.
• Structure of Myelin Sheath:
  • Similar in function and structure to central nervous system myelin.

Detailed Structure of Myelin Sheath

• Composition: Myelin is rich in lipid.
• Arrangement: Cell membrane of Schwann cell wraps tightly around axon like a roll of tape.
• Cell Structure: Most of the Schwann cell membrane forms the myelin sheath with a small portion containing the nucleus and cytoplasm (like a soma) on the outside.

Additional Functions

• Schwann cells and neurons influence each other through the exchange of various substances.