8.1 Neurons and Neuroglia

5 min read•august 28, 2024

Neurons and neuroglia are the building blocks of our nervous system. Neurons transmit information through electrical and chemical signals, while neuroglia provide support and protection. Together, they form the intricate network that controls our body's functions and behaviors.

Understanding how neurons communicate and adapt is crucial for grasping the nervous system's complexity. From myelinated axons speeding up signals to neuroplasticity allowing our brains to change, these processes shape our ability to learn, remember, and respond to our environment.

Neuron Structure and Function

Neuron Components and Their Roles

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  • Neurons are the primary functional units of the nervous system responsible for receiving, processing, and transmitting information through electrical and chemical signals
  • The structure of a typical neuron consists of a cell body (soma), dendrites, and an axon
    • The cell body contains the nucleus and other organelles essential for cellular function and protein synthesis
    • Dendrites are branched extensions that receive signals from other neurons (Purkinje cells in the cerebellum)
    • The axon is a long, thin projection that carries electrical signals away from the cell body to other neurons or effector cells (motor neurons innervating skeletal muscle)

Neuroglia: Supporting Cells of the Nervous System

  • Neuroglia, also known as glial cells, are non-neuronal cells that provide support, protection, and maintenance for neurons in the nervous system
  • Types of neuroglia include astrocytes, oligodendrocytes, microglia, and ependymal cells, each with specific functions
    • Astrocytes regulate the extracellular environment, provide structural support, and contribute to the blood-brain barrier (star-shaped cells in the central nervous system)
    • Oligodendrocytes produce myelin sheaths that insulate axons in the central nervous system, enhancing signal transmission speed (cells with multiple processes extending to axons)
    • Microglia are the immune cells of the nervous system responsible for defending against pathogens and clearing cellular debris (small, highly mobile cells in the brain and spinal cord)
    • Ependymal cells line the ventricles of the brain and the central canal of the spinal cord producing cerebrospinal fluid (cuboidal or columnar cells with cilia)

Neuronal Communication

Electrical and Chemical Signaling in Neurons

  • Neuronal communication occurs through a combination of electrical and chemical signals, allowing information to be transmitted between neurons and throughout the nervous system
  • The process of neuronal communication begins with an electrical signal called an action potential generated when a neuron's membrane potential reaches a threshold level
  • Action potentials are all-or-none events that propagate along the axon resulting in the release of neurotransmitters at the synapse (rapid depolarization followed by repolarization of the axon membrane)

Synaptic Transmission and Neurotransmitters

  • Synapses are specialized junctions between neurons where chemical communication occurs consisting of a presynaptic neuron, a synaptic cleft, and a postsynaptic neuron
  • Neurotransmitters are chemical messengers released from the presynaptic neuron into the synaptic cleft which then bind to specific receptors on the postsynaptic neuron (glutamate, GABA, dopamine)
    • The binding of neurotransmitters to receptors can result in either excitatory or inhibitory postsynaptic potentials, depending on the type of neurotransmitter and receptor involved
    • Excitatory postsynaptic potentials (EPSPs) increase the likelihood of the postsynaptic neuron generating an action potential, while inhibitory postsynaptic potentials (IPSPs) decrease this likelihood (glutamate-mediated EPSPs, GABA-mediated IPSPs)
  • After neurotransmitters have exerted their effects, they are either degraded by enzymes in the synaptic cleft or taken back up into the presynaptic neuron through a process called reuptake (acetylcholinesterase breaking down acetylcholine, serotonin reuptake by presynaptic transporters)

Myelinated vs Unmyelinated Neurons

Myelin Sheath and Saltatory Conduction

  • Myelinated neurons have axons that are wrapped in a lipid-rich insulating substance called myelin produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system
  • Myelin sheaths are arranged in segments along the axon with gaps between the segments called nodes of Ranvier
  • The presence of myelin insulation allows for faster and more efficient signal transmission along the axon as action potentials "jump" from one node of Ranvier to the next, a process known as saltatory conduction (myelinated motor neurons in the spinal cord)

Differences in Signal Transmission and Implications

  • Unmyelinated neurons lack the myelin insulation around their axons resulting in slower signal transmission compared to myelinated neurons
  • In unmyelinated neurons, action potentials must propagate continuously along the axon membrane leading to a slower conduction velocity (C fibers in the peripheral nervous system)
  • The difference in signal transmission speed between myelinated and unmyelinated neurons has implications for the timing and coordination of neural activity in various parts of the nervous system (rapid reflexes vs. slow pain sensation)
  • Disorders that affect myelin, such as multiple sclerosis, can lead to impaired signal transmission and neurological dysfunction (demyelination of axons in the central nervous system)

Neuroplasticity and Learning

Synaptic Plasticity and Memory Formation

  • Neuroplasticity refers to the brain's ability to change and adapt its structure and function in response to experience, learning, and environmental stimuli
  • Neuroplasticity occurs at multiple levels including changes in synaptic strength, the formation of new synapses, and the reorganization of neural networks
  • Synaptic plasticity is a key mechanism underlying learning and memory as it allows for the strengthening or weakening of connections between neurons based on their activity patterns
    • Long-term potentiation (LTP) is a form of synaptic plasticity that results in a long-lasting increase in synaptic strength, thought to be a cellular basis for learning and memory (hippocampal LTP during spatial learning tasks)
    • Long-term depression (LTD) is another form of synaptic plasticity that results in a long-lasting decrease in synaptic strength which may be important for refining neural circuits and removing unnecessary connections (cerebellar LTD during motor learning)

Experience-Dependent Plasticity and Adult Neurogenesis

  • Neurogenesis, the formation of new neurons, is another aspect of neuroplasticity that occurs in specific regions of the adult brain such as the hippocampus which is crucial for learning and memory
  • Experience-dependent plasticity refers to changes in neural structure and function that occur as a result of an individual's experiences and interactions with the environment (increased cortical representation of frequently used fingers in musicians)
  • Neuroplasticity is not limited to early development but continues throughout life allowing the brain to adapt to new challenges, acquire new skills, and recover from injury or disease (cortical reorganization following stroke or limb amputation)
  • Understanding neuroplasticity has important implications for education, rehabilitation, and the treatment of neurological disorders as it suggests that the brain has the capacity to change and improve with targeted interventions and training (cognitive training in older adults, constraint-induced movement therapy for stroke patients)
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