Neurons

Neurons & Neuroglia

Dendrite: A short, branched extension of a neuron that receives signals and conducts impulses towards the cell body.
Cell body (soma): Contains the nucleus and organelles, serving as the metabolic center of the neuron where most of the cell's material and energy activities take place, and integrates incoming signals.
Myelin sheath: A fatty layer that surrounds the axon, composed of oligodendrocytes in the CNS and Schwann cells in the PNS; it accelerates electrical signal transmission through a process called saltatory conduction.
Schwann cell: A type of glial cell in the PNS that produces the myelin sheath and aids in repairing damaged neurons.
Axon: A long, slender projection that conducts electrical impulses away from the cell body to other neurons or muscles.
Terminal: The end of an axon, which releases neurotransmitters into the synapse, facilitating communication between neurons.
Node of Ranvier: Regularly spaced gaps in the myelin sheath along the axon that facilitate rapid signal transmission by allowing ions to flow in and out of the neuron.
Nucleus: A membrane-bound organelle that contains the neuron’s genetic material and regulates cellular activities.

Neurons exhibit significant variation in size, shape, and functionality, which allows for the diverse roles played by different neuron types in the nervous system.

Commonly referred to as the soma, this structure contains cytoplasm and embedded nucleus, supporting the cell’s metabolic functions.
One or more neurites (processes) extend from it to facilitate communication with other neurons, allowing for the complex interconnectivity within neural circuits.

Dendrites are characterized by a high degree of branching and are covered in dendritic spines, which enhance the surface area and increase the capacity for synaptic connections with other neurons.

Axons vary significantly in length and diameter; some can extend over a meter in length (as in motor neurons extending to the limbs), while others are much shorter.
They originate from the axon hillock, a specialized region of the cell body known for integrating signals and triggering action potentials.

A single axon can give rise to many terminal branches, allowing it to communicate with several other neurons simultaneously.
The distal ends of these branches, known as axon terminals or synaptic boutons, contain vesicles filled with neurotransmitters.

Communication at the synapse is unidirectional; signals travel from the presynaptic neuron (axon terminal) to the postsynaptic neuron (dendrites or cell body).
This intricate process involves neurotransmitter release, binding to receptors on the postsynaptic membrane, and subsequent signaling cascades.
Detailed mechanisms of synaptic transmission and types of neurotransmitters, along with their effects, are topics in further lectures.

Multipolar Neuron: Characterized by multiple dendritic branches and a single axon; most common in the CNS, including motor neurons.
Bipolar Neuron: Features one dendrite and one axon; often found in sensory pathways such as the retina.
Unipolar Neuron: Contains a single process that bifurcates, functioning as both a dendrite and axon; commonly found in sensory neurons of the PNS, facilitating the transmission of sensory information.

Glial cells outnumber neurons in the brain and are crucial for providing support, protection, and nourishment to the nervous system.

Ionic Balance Maintenance: Glial cells help regulate extracellular ion concentrations, which is vital for neuronal health and function.
Synaptic Modulation: They modulate synaptic activity by regulating the uptake and recycling of neurotransmitters, which can impact synaptic strength and plasticity.
Neural Development Support: Glial cells provide structural scaffolding for developing neurons during embryonic growth and neurodevelopment.
Injury Response: Some glial cells can aid in the recovery from neural injuries; however, excessive glial response can impede recovery and contribute to neuroinflammatory conditions.

Astrocytes: Star-shaped cells that maintain the blood-brain barrier and provide metabolic support, regulating the environment surrounding neurons.
Oligodendrocytes: Responsible for the formation of myelin sheaths around axons in the CNS, aiding in efficient signal transmission.
Microglia: Immune cells in the CNS that act as macrophages, clearing debris and modulating inflammatory responses, crucial for maintaining neuroprotective environments.

Schwann Cells: Equivalent to oligodendrocytes in the PNS; they produce myelin and play a major role in the regeneration of damaged peripheral nerves.
Satellite Cells: Specialized glial cells in peripheral ganglia that support neuronal cell bodies, providing nutrients and regulating the environment similar to astrocytes in the CNS.

Microglial Cell: Immune function and debris clearance in the CNS.
Myelin Sheath: Insulates axons to improve electrical conduction speed.
Blood Vessel: Supplies oxygen and nutrients to neural tissues.
Oligodendrocyte: Myelin-producing cells in the CNS; essential for rapid signal transmission.
Gap Junction: Specialized connections that allow for direct communication between neighboring neurons or glial cells.
Neuron (Interconnected with astrocytes at synapses): Facilitates complex communication in neural networks.

CNS (Central Nervous System): Comprised of the brain and spinal cord, featuring oligodendrocytes and distinct myelin sheaths that expedite nerve signal conduction.
PNS (Peripheral Nervous System): Encompasses all neural structures outside the CNS, including Schwann cells, myelin sheaths, and Nodes of Ranvier, important for mobilizing and transmitting signals to and from the periphery of the body.