Neurons and Their Anatomy

Neurons

• Neurons are specialized cells for receiving, conducting, and transmitting electrochemical signals.
• There are approximately 80-90 billion neurons in the adult human brain.
• Glial cells support neurons and contribute to information processing.

Neuron Anatomy

• Dendrites: Receive information from other neurons at synapses.
• Cell body (soma): Contains the nucleus and integrates incoming signals.
• Axon: A long, thin tube that carries electrical signals away from the cell body.
• Myelin sheath: Insulating layer around the axon, increasing signal speed.
• Nodes of Ranvier: Gaps in the myelin sheath where the action potential is regenerated.
• Axon terminals (terminal buttons): Release neurotransmitters to communicate with other cells.

Neuron Classification

• Multipolar: Many dendrites, one axon (most common).
• Unipolar: One process extending from the cell body, branching into input and output zones.
• Bipolar: One dendrite and one axon.
• Multipolar interneuron: Short axon or no axon.

Synapses

• Synapse: The junction between two neurons.
• Presynaptic membrane: Axon terminal of the sending neuron.
• Postsynaptic membrane: Dendrite or cell body of the receiving neuron.
• Synaptic cleft: Gap between the presynaptic and postsynaptic membranes.
• Synaptic vesicles: Contain neurotransmitters.
• Neurotransmitters: Chemical messengers.

Synapse Function

• Neurotransmitters are released into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic membrane.
• Binding of neurotransmitters causes changes in the postsynaptic membrane potential.

Neurons Classifications (by Function)

• Sensory neurons: Gather information from the environment.
• Motor neurons: Control muscle contractions.
• Interneurons: Connect circuits within the CNS.

Additional Notes

• Neural plasticity: Synaptic connections change constantly.
• Axoplasmic transport: Movement of substances along the axon.
• Anterograde: Cell body to axon terminals.
• Retrograde: Axon terminals to cell body.
• Glial cells: Astrocytes, microglia, oligodendrocytes (CNS) and Schwann cells (PNS)

Action Potentials

• Resting potential: Membrane potential of a neuron at rest (-70 mV).
• Polarization: Neuron is polarized at resting.
• Diffusion: Molecules move from high to low concentration.
• Electrostatic pressure: Opposing charges attract, like charges repel.
• Threshold of excitation: The minimum level of depolarization to trigger an action potential.
• Depolarization: Membrane potential becomes less negative (Na+ influx).
• Repolarization: Membrane potential returns to resting potential (K+ efflux).
• Refractory period: Time when a neuron cannot fire another action potential.
• All-or-none principle: Action potentials are always the same size.
• Rate law: Intensity of a stimulus is reflected by the rate of action potentials.
• Saltatory conduction: Action potential jumps between nodes of Ranvier.

Neurotransmitter Release

• Docking: Vesicles are positioned near the membrane.
• Release zone: Portion of the presynaptic membrane where neurotransmitters are released.
• Calcium channels: Opening allows calcium to enter and trigger neurotransmitter release.
• Fusion pore: Formed when vesicles fuse with the presynaptic membrane.
• Neurotransmitter types: Small vs. large molecule neurotransmitters. Small molecule neurotransmitters contain neurotransmitter molecules and are recycled into the nerve terminal via a kiss and run process, other neurotransmitters can be released through merge and recycle process.
• Kiss and run: Vesicles release then reabsorb neurotransmitters.
• Merge and recycle: Vesicles merge, then reform
• Bulk endocytosis: Recycling of vesicle membrane.

Synaptic Integration

• Spatial summation: Multiple synapses at different locations add their effects.
• Temporal summation: Multiple synapses that occur shortly in succession add their effects.
• Inhibitory postsynaptic potentials (IPSPs): Hyperpolarizes the membrane.
• Excitatory postsynaptic potentials (EPSPs): Depolarizes the membrane.

Synaptic Termination

• Reuptake: Neurotransmitters are taken back into the presynaptic neuron.
• Enzymatic deactivation: Neurotransmitters are broken down by enzymes.

Synapse Types

• Axodendritic: Axon to dendrite.
• Axosomatic: Axon to cell body.
• Axoaxonic: Axon to axon.
• Dendrodendritic: Dendrite to dendrite.

Additional Glials

• Astrocytes: Largest glial cell, involved in nutrient exchange, synapse formation and pruning, blood-brain-barrier control.
• Microglia: Smaller glial cells, involved in immune response and clearing debris.
• Oligodendrocytes: Produce myelin in the CNS.
• Schwann cells: Produce myelin in the PNS.