35.1 Neurons and Glial Cells - Biology 2e _ OpenStax

Learning Objectives

• Functions of Neurons: Understand the structural components of neurons and their functions.

• Types of Neurons: Identify and describe the four main types of neurons.

• Glial Cell Functions: Compare the various functions of different types of glial cells.

Nervous System Overview

• Nervous systems vary in structure and complexity across the animal kingdom.

  • Simpler Organisms: Sea sponges lack a true nervous system.

  • Nerve Nets: Cnidarians (e.g., jellyfish) have decentralized nerve networks.

  • Bundled Nerves: Echinoderms like sea stars show more development with bundled nerves.

  • Central and Peripheral Nervous Systems: Flatworms have both a CNS (small brain & nerve cords) and PNS (system of nerves).

  • Insect Nervous System: More complex and decentralized with a brain, ventral nerve cord, and ganglia.

  • Cephalopods: Octopi possess highly organized neurons and complex brains similar to vertebrates.

Vertebrate Nervous System

• Vertebrates possess more complex, centralized, and specialized nervous systems than invertebrates.

  • CNS: Composed of the brain and spinal cord.

  • PNS: Comprises peripheral nerves extending into the body.

  • Structural Differences: Invertebrate nerve cords are usually ventral, while vertebrate spinal cords are dorsal.

Glial Cells

• Role of Glial Cells: Supportive functions in the nervous system, assisting neurons similarly to how workers maintain electric wiring.

• Diversity in Nervous Systems: Numerous types of neurons and glial cells exist, each fulfilling unique roles.

Neuron Structure and Function

• Average Neurons: Common laboratory fly (Drosophila melanogaster) has about 100,000 neurons; humans have around 86 billion.

• Neuronal Communication: Neurons communicate through signaling, foundational for behaviors such as reflexes and more complex actions.

Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and key organelles (endoplasmic reticulum, Golgi apparatus, etc.).

• Dendrites: Tree-like structures that receive signals from other neurons; increase surface area through dendritic spines.

• Axon: Tube-like structure that transmits signals to axon terminals, which connect to other neurons or target cells.

  • Myelin Sheath: Insulating layer produced by glial cells that speeds signal conduction, crucial for long axons.

  • Nodes of Ranvier: Gaps in the myelin sheath where action potentials are recharged as they propagate along the axon.

  • Synaptic Connections: Dendrites can receive signals from numerous neurons, exemplified by Purkinje cells which may connect with ~200,000 neurons.

Neuron Types

Neuron Classification

• Unipolar Neurons: One structure extends from the soma, found in some invertebrates (not in vertebrates).

• Bipolar Neurons: One axon, one dendrite; examples include retinal bipolar cells.

• Multipolar Neurons: Most common; have one axon and multiple dendrites; found in CNS (e.g., Purkinje cells).

• Pseudounipolar Neurons: Single process that branches into two; primarily sensory neurons.

Neurogenesis

• New Neurons in Adults: Researchers have discovered neurogenesis occurs in adults—first observed in songbirds.

• Hippocampus Role: Daily neuron generation is vital for learning and memory; correlates positively with rat learning tasks.

• Influencers of Neurogenesis: Factors like physical exercise and antidepressants can enhance neurogenesis, while stress may inhibit it.

Glial Cell Functions

• Astrocytes: Provide nutrients, regulate extracellular ion balance, form the blood-brain barrier, and modulate synapse activity.

• Oligodendrocytes: Create myelin sheaths for multiple CNS axons.

• Microglia: Scavenge pathogens and dead neurons, responding to nerve activity.

• Ependymal Cells: Line ventricles; involved in cerebrospinal fluid production.

• Schwann Cells (PNS): Provide myelin sheaths for individual axons.

• Satellite Cells (PNS): Offer nutrients and structural support to neurons.

Learning Objectives

• Functions of Neurons: Understand the structural components of neurons and their diverse functions, including signaling, communication, and integration of information necessary for detecting stimuli and coordinating responses.

• Types of Neurons: Identify and describe the four main types of neurons—unipolar, bipolar, multipolar, and pseudounipolar—emphasizing their structural differences and specific roles in both sensory and motor functions throughout different nervous systems.

• Glial Cell Functions: Compare the various functions of different types of glial cells, including astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells, and satellite cells, focusing on their supportive roles and how they maintain homeostasis in the nervous system.

Nervous System Overview

• Nervous systems vary widely in structure and complexity across the animal kingdom, reflecting their evolutionary adaptations.

• Simpler Organisms: Sea sponges are notable for lacking a true nervous system, relying on simple cell-to-cell communication.

• Nerve Nets: Cnidarians, such as jellyfish and sea anemones, possess decentralized nerve networks that allow for basic reflexive movements and interactions with their environment.

• Bundled Nerves: Echinoderms like sea stars show a more developed system with bundled nerves that facilitate locomotion and sensory perception.

• Central and Peripheral Nervous Systems: Flatworms represent a significant evolutionary step, possessing both a Central Nervous System (CNS) with a small brain and nerve cords, and a Peripheral Nervous System (PNS) consisting of various nerves branching throughout their body.

• Insect Nervous System: More complex than earlier examples, insects feature a brain, ventral nerve cord, and segmented ganglia, allowing for coordinated movements and behaviors.

• Cephalopods: Octopuses demonstrate highly organized neurons and complex brains, which provide advanced cognitive abilities, similar in complexity to some vertebrates.

Vertebrate Nervous System

• Vertebrates possess a more complex, centralized, and specialized nervous system compared to invertebrates, enabling advanced functions.

• CNS: Composed of the brain and spinal cord, responsible for processing information and generating responses.

• PNS: Comprises peripheral nerves extending into the body, facilitating communication between the CNS and limbs or organs.

• Structural Differences: Invertebrate nerve cords are typically ventral, while vertebrate spinal cords are dorsal, a reflection of their different evolutionary paths and functional requirements.

Glial Cells

• Role of Glial Cells: Glial cells perform numerous supportive functions in the nervous system, akin to how workers maintain electric wiring, ensuring optimal environment for neuronal function.

• Diversity in Nervous Systems: Numerous types of neurons and glial cells exist, each fulfilling unique roles essential for maintaining homeostasis, neural repair, and communication within the nervous system.

Neuron Structure and Function

• Average Neurons: The common laboratory fly (Drosophila melanogaster) has approximately 100,000 neurons, while humans have around 86 billion, illustrating the diversity and complexity of neuronal structures.

• Neuronal Communication: Neurons communicate through intricate signaling pathways, which are fundamental for behaviors ranging from simple reflexes to complex cognitive tasks.

• Parts of a Neuron:

  • Cell Body (Soma): Contains the nucleus and essential organelles (e.g., endoplasmic reticulum, Golgi apparatus) tasked with producing neurotransmitters and maintaining cellular activities.

  • Dendrites: Tree-like branches that receive signals from other neurons; increase surface area through dendritic spines, enhancing synaptic connectivity.

  • Axon: A long, tube-like structure that transmits signals away from the cell body to axon terminals, which connect to other neurons or target cells.

  • Myelin Sheath: An insulating layer produced by glial cells that speeds up electrical signal conduction, particularly crucial for long axons.

  • Nodes of Ranvier: Periodic gaps in the myelin sheath facilitate rapid signal transmission through saltatory conduction, allowing action potentials to jump from node to node.

  • Synaptic Connections: Dendrites can receive inputs from numerous neurons; for example, Purkinje cells in the cerebellum can connect with approximately 200,000 neurons, highlighting complex integration in neural circuits.

Neuron Types

Neuron Classification

• Unipolar Neurons: Possess one structure extending from the soma; primarily found in some invertebrates (not in vertebrates) and affiliated with sensory functions.

• Bipolar Neurons: Feature one axon and one dendrite; examples include retinal bipolar cells, which play critical roles in visual processing.

• Multipolar Neurons: The most common form, characterized by one axon and multiple dendrites; notably found in the CNS, such as cerebellar Purkinje cells that facilitate motor control.

• Pseudounipolar Neurons: Have a single process branching into two; primarily sensory neurons that transmit sensory information to the CNS.

Neurogenesis

• New Neurons in Adults: Recent research indicates that neurogenesis, the process of generating new neurons, occurs in adults, with significant discoveries first noted in songbirds.

• Hippocampus Role: Daily neuron generation in the hippocampus is essential for learning and memory, correlating positively with performance in learning tasks in animals like rats.

• Influencers of Neurogenesis: Factors such as physical exercise, enriched environments, and certain antidepressants can enhance neurogenesis, while chronic stress may inhibit the generation of new neurons.

Glial Cell Functions

• Astrocytes: Provide essential nutrients, regulate extracellular ion balance, form the blood-brain barrier, and modulate synaptic activity to maintain a stable environment for neurons.

• Oligodendrocytes: Responsible for creating myelin sheaths for multiple CNS axons, facilitating rapid electrical signal conduction.

• Microglia: Act as the immune defense in the nervous system, scavenging pathogens, dead neurons, and debris, and responding effectively to nerve injury and activity.

• Ependymal Cells: Line the brain's ventricles and are involved in the production and circulation of cerebrospinal fluid, playing a role in cushioning and nutrient distribution.

• Schwann Cells (PNS): Analogous to oligodendrocytes in the CNS, Schwann cells provide myelin sheaths for individual axons in the peripheral nervous system, crucial for their regeneration.

• Satellite Cells (PNS): Offer support and nutrients to neurons in ganglia, ensuring their health and functionality.

Here are the key terms used in the chapter along with their definitions:

1. Neurons: Specialized cells that transmit nerve impulses within the nervous system.

2. Glial Cells: Supportive cells in the nervous system that assist neurons in various functions, such as maintaining homeostasis and providing structural support.

3. Central Nervous System (CNS): The part of the nervous system that consists of the brain and spinal cord, responsible for processing information and generating responses.

4. Peripheral Nervous System (PNS): The nervous system component that consists of peripheral nerves and ganglia extending into the body, facilitating communication between the CNS and the rest of the body.

5. Unipolar Neurons: Neurons that have a single structure extending from the soma, primarily found in some invertebrates, typically associated with sensory functions.

6. Bipolar Neurons: Neurons with one axon and one dendrite, often found in the sensory pathways, such as retinal bipolar cells in the eye.

7. Multipolar Neurons: The most common type of neuron with one axon and multiple dendrites, crucial for integrating information; found in the CNS.

8. Pseudounipolar Neurons: Neurons that have a single process that branches into two, primarily functioning as sensory neurons that relay information to the CNS.

9. Astrocytes: A type of glial cell that provides nutrients, regulates ion balance, forms the blood-brain barrier, and modulates synaptic activity in the CNS.

10. Oligodendrocytes: Glial cells that create myelin sheaths for multiple axons in the CNS, facilitating faster electrical signal conduction.

11. Microglia: Immune defense cells in the nervous system that scavenge pathogens and debris, responding to injury and maintaining health in neural tissue.

12. Ependymal Cells: Glial cells that line the ventricles of the brain and produce cerebrospinal fluid (CSF).

13. Schwann Cells: Glial cells in the PNS that provide myelin sheaths for individual axons, playing a key role in their regeneration.

14. Satellite Cells: Glial cells in the PNS that provide structural support and nutrients to neuron cell bodies in ganglia.

15. Neurogenesis: The process through which new neurons are generated, particularly significant in certain brain regions such as the hippocampus.

16. Dendrites: Branch-like structures on neurons that receive signals from other neurons and increase the surface area for synaptic connections.

17. Axon: A long, tube-like structure that carries electrical impulses away from the cell body to other neurons or target cells.

18. Myelin Sheath: An insulating layer surrounding the axon, produced by glial cells, that accelerates the conduction of nerve impulses.

19. Nodes of Ranvier: Periodic gaps in the myelin sheath that facilitate rapid conduction of action potentials through saltatory conduction.

20. Synaptic Connections: Junctions where neurons communicate with each other, allowing signal transmission across synapses.

21. Hippocampus: A region of the brain important for learning and memory, known for its ability to generate new neurons (neurogenesis).

22. Nerve Nets: Simple neural structures found in organisms like cnidarians, allowing basic reflexive movements without a centralized brain.

23. Bundled Nerves: More developed neural structures found in echinoderms, facilitating locomotion and sensory perception.

24. Cephalopods: A class of mollusks (e.g., octopuses) known for their advanced nervous systems and cognitive abilities comparable to vertebrates.

25. Invertebrates: Animals without a backbone, often having simpler nervous systems compared to vertebrates.

26. Vertebrates: Animals with a backbone, typically possessing more complex and centralized nervous systems.

27. Laboratory Fly (Drosophila melanogaster): A model organism in neuroscience studies, known for having about 100,000 neurons, used to investigate neural function.

28. Neuronal Communication: The process by which neurons transmit signals to each other, fundamental for behaviors and bodily functions.

29. Cell Body (Soma): The part of a neuron that contains the nucleus and organelles, responsible for maintaining the neuron's health and function.

30. Cerebrospinal Fluid: A clear fluid that cushions the brain and spinal cord, produced by ependymal cells.

31. Neural Circuits: Complex networks of interconnected neurons that process and transmit information.

32. Ion Balance: The proper concentration of ions in the extracellular fluid that is essential for neuronal signaling and function.

33. Sensory Neurons: Neurons that transmit sensory information from receptors to the CNS.

34. Motor Functions: Actions or behaviors that result from the activation of skeletal muscles, facilitated by motor neurons.

35. Learning and Memory: Processes that involve the encoding, storage, and retrieval of information, significantly influenced by neurogenesis in the brain.

36. Reflexes: Automatic responses to stimuli, mediated by neural pathways that can involve both sensory and motor neurons.

37. Cognitive Abilities: Higher-level brain functions, such as thinking, reasoning, and problem-solving, often associated with complex neural networks.