Neurons and neuroglia are the building blocks of the nervous system. These specialized cells work together to process information, control bodily functions, and shape our thoughts and behaviors. Understanding their structure and function is key to grasping how the nervous system operates.

This section dives into the intricate world of neurons and supporting glial cells. We'll explore their unique features, different types, and roles in maintaining a healthy nervous system. Get ready to uncover the fascinating cellular machinery that powers our neural networks!

Neuron Structure and Function

Components of a Neuron

• Neurons are the basic functional units of the nervous system responsible for receiving, processing, and transmitting information through electrical and chemical signals
• The soma, or cell body, contains the nucleus and organelles necessary for cellular functions
  • Involved in protein synthesis (ribosomes, rough endoplasmic reticulum)
  • Responsible for energy production (mitochondria)
• Dendrites are branched extensions of the soma that receive signals from other neurons or sensory receptors
  • Covered in dendritic spines, which are small protrusions that increase the surface area for receiving signals
• The axon is a long, thin fiber that conducts electrical impulses away from the soma to other neurons, muscles, or glands
  • Can range in length from a few millimeters to over a meter (sciatic nerve)

Axon and Myelin Sheath

• The axon hillock is the junction between the soma and the axon, where action potentials are generated
  • High concentration of voltage-gated sodium channels allows for the initiation of action potentials
• The axon terminal is the endpoint of the axon, containing synaptic vesicles that release neurotransmitters into the synaptic cleft to communicate with the next neuron or target cell
  • Neurotransmitters can have excitatory or inhibitory effects on the postsynaptic cell
• The myelin sheath insulates the axon to increase the speed of electrical impulse propagation
  • Formed by Schwann cells in the peripheral nervous system (PNS)
  • Formed by oligodendrocytes in the central nervous system (CNS)
• Nodes of Ranvier are gaps in the myelin sheath that allow for the regeneration of action potentials, enabling saltatory conduction
  • Saltatory conduction allows for faster signal transmission compared to continuous conduction in unmyelinated axons

Neuron Types and Functions

Morphological Classification

• Unipolar neurons have a single process that divides into two branches, with one branch forming the axon and the other forming the dendrites
  • Primarily found in invertebrates and are rare in vertebrates
• Bipolar neurons have two processes extending from the soma, one forming the dendrite and the other forming the axon
  • Found in sensory systems (retina, olfactory epithelium)
• Multipolar neurons have multiple dendrites and a single axon extending from the soma
  • Most common type of neuron in the vertebrate nervous system
  • Involved in complex processing and integration of information

Functional Classification

• Sensory neurons, or afferent neurons, transmit information from sensory receptors to the central nervous system
  • Typically pseudounipolar, with the soma located in the dorsal root ganglia or cranial nerve ganglia
  • Examples include neurons that detect touch, pain, temperature, and proprioception
• Motor neurons, or efferent neurons, transmit signals from the central nervous system to muscles or glands to initiate a response
  • Multipolar neurons with the soma located in the spinal cord or brainstem
  • Examples include upper motor neurons in the primary motor cortex and lower motor neurons in the spinal cord
• Interneurons are neurons that form connections between other neurons within the central nervous system
  • Involved in processing, integrating, and modulating information
  • Examples include local circuit neurons in the cortex and Renshaw cells in the spinal cord

Neuroglia in the Nervous System

Glial Cells in the Central Nervous System

• Astrocytes are star-shaped glial cells that provide structural support, maintain the blood-brain barrier, regulate neurotransmitter levels, and participate in synaptic transmission and plasticity
  • Form the glial limitans, a barrier between the CNS and the surrounding tissue
  • Take up excess neurotransmitters (glutamate) from the synaptic cleft to prevent excitotoxicity
• Oligodendrocytes are glial cells in the central nervous system that form the myelin sheath around axons, enabling faster and more efficient signal transmission
  • A single oligodendrocyte can myelinate multiple axons
• Microglia are the immune cells of the central nervous system, monitoring for pathogens, damaged cells, and debris
  • Can phagocytose harmful substances and secrete cytokines to regulate immune responses
  • Activated in response to injury or inflammation
• Ependymal cells line the ventricles of the brain and the central canal of the spinal cord
  • Create the blood-cerebrospinal fluid barrier and facilitate the circulation of cerebrospinal fluid
  • Possess cilia that help circulate cerebrospinal fluid

Glial Cells in the Peripheral Nervous System

• Schwann cells are glial cells in the peripheral nervous system that form the myelin sheath around axons and aid in axon regeneration after injury
  • A single Schwann cell myelinates a single axon segment
  • Produce neurotrophic factors that support axon growth and survival
• Satellite cells surround the cell bodies of neurons in sensory, sympathetic, and parasympathetic ganglia
  • Provide structural and metabolic support to the neurons
  • Regulate the microenvironment around the neuron cell bodies

Neurogenesis and Brain Development

Embryonic Neurogenesis

• Neurogenesis is the process by which new neurons are generated from neural stem cells and progenitor cells
• During embryonic development, neurogenesis is widespread and crucial for the formation of the nervous system
  • Neural stem cells differentiate into neurons and glia
  • Neuronal migration and differentiation are guided by chemical and physical cues
• Radial glia are present during embryonic development and serve as scaffolds for the migration of neurons to their final destinations
  • Can also differentiate into neurons and other glial cells
  • Play a role in the formation of cortical layers and the organization of the developing brain

Adult Neurogenesis and Plasticity

• In the adult brain, neurogenesis is limited to specific regions
  • Subventricular zone of the lateral ventricles (olfactory bulb interneurons)
  • Subgranular zone of the hippocampal dentate gyrus (granule cells)
• Adult neurogenesis plays a role in learning, memory, and mood regulation
  • Newly generated neurons integrate into existing neural circuits
  • Enhances the brain's ability to adapt and store new information
• Factors such as exercise, environmental enrichment, and certain antidepressants can stimulate adult neurogenesis
  • Stress, aging, and certain disorders can impair adult neurogenesis
• Neurogenesis is an important mechanism of brain plasticity, allowing the brain to adapt and reorganize in response to new experiences, learning, and injury
  • May contribute to the brain's ability to recover from stroke or traumatic brain injury
• Understanding the mechanisms and regulation of neurogenesis may lead to the development of therapies for neurodegenerative disorders (Alzheimer's, Parkinson's)
  • Stimulating endogenous neurogenesis or transplanting neural stem cells are potential therapeutic strategies