Neuron Structure and Function

Neuron Structure and Function

Components of a Neuron

• The soma (cell body) contains the nucleus and organelles, maintaining the cell's health and supporting neuronal function.
• Dendrites are branched projections extending from the soma that receive chemical signals (neurotransmitters) and convert them into electrical impulses toward the soma.
• The axon is a slender projection carrying electrical signals away from the soma to other neurons or muscle cells.
• Teledendria are terminal branches facilitating communication, releasing neurotransmitters to transmit signals.
• The bouton (terminal button) is the axon's end, containing neurotransmitters in vesicles that fuse with the membrane to release them into the synaptic cleft upon an action potential.
• The synapse is the gap between the bouton and dendrites, where signal transfer occurs via neurotransmitter release.

Cellular Activity

• Neurons send and process information, integrating excitatory and inhibitory signals to influence bodily functions and behaviors.
• Neuroglia (glial cells) support neurons by maintaining homeostasis, forming myelin, and providing protection.

Myelination

• Myelination begins towards the end of pregnancy's third trimester and continues to about age 4, crucial for rapid signal transmission.
• The myelin sheath allows electrical impulses to conduct more quickly and efficiently along the axon which facilitates the rapid jumping of action potentials

Classification of Neurons

Based on Shape

• Multipolar neurons have numerous dendrites extending from the soma, common in the brain and spinal cord.
• Bipolar neurons feature one axon and one dendrite, often in sensory organs like the retina.
• Unipolar neurons possess one process splitting into two branches, typically sensory neurons in peripheral nerves.

Based on Axonal Length

• Golgi Type I neurons have long axons for transmitting signals over great distances.
• Golgi Type II neurons feature short axons, primarily involved in local processing in specific brain or spinal cord regions.

Communication Between Neurons

Resting Potential

• The resting membrane potential of a neuron is around -70 mV, maintained by the sodium-potassium pump transporting 3 Na+ ions out and 2 K+ ions into the cell.

Initiating an Action Potential

• When a neuron receive enough excitatory neurotransmitters, sodium channels open, allowing Na+ ions to cause a depolarization.
• If the membrane potential reaches about -55mV, an action potential is triggered.

Propagation of Action Potential

• Once generated at the axon hillock, the AP travels along the axon as a wave of depolarization.
• Voltage-gated sodium channels open in succession, allowing for rapid conduction of the electrical signal toward the axon terminals.

Repolarization

• After the peak of the action potential, potassium channels open, allowing K+ ions to flow out.
• This results in repolarization and restoration of the resting membrane potential.

Mechanism of Nerve Impulses

• Action potentials are facilitated by the coordinated opening and closing of ion channels.
• The influx of ions generates electrical currents, resulting in voltage changes across the neuronal membrane which propagate the nerve impulse.

Summary of Neuronal Communication Steps

• The intracellular and extracellular environments of neurons create positive and negative charges.
• Neurons maintain resting potential (no net charge, ready to fire)
• Dendrites receive excitatory or inhibitory signals, leading to graded potentials.
• The axon hillock integrates these signals to decide on starting an action potential
• The AP travels from the axon hillock down the axon to the bouton.
• The arrival of the AP at the bouton prompts calcium channels to open, allowing Ca2+ ions to enter.
• Calcium influx triggers the fusion of vesicles with the bouton membrane, releasing neurotransmitters into the synaptic cleft.
• Released neurotransmitters diffuse across the synapse and bind to receptors on the postsynaptic neuron, leading to either excitation or inhibition of that neuron.