Neuron Structure, Function and Membrane Potential OSMOSIS - EASY

Neuron Structure and Function

• Neurons are the fundamental cells of the nervous system.
• They consist of three main components: dendrites, soma (cell body), and axon.
• Dendrites are branching extensions that receive signals from other neurons through neurotransmitters.
• The soma contains the neuron's organelles, including the nucleus.
• The axon is a long, slender projection intermittently wrapped in myelin (fatty substance).
• Myelin insulation facilitates rapid transmission of electrical signals down the axon.
• Neurons communicate with each other via neurotransmitters, which act as chemical signals.
• Within a neuron, signals are propagated using action potentials, which are electrical signals.
• Electrical signal races down the axon up to 100 meters per second.

Resting Membrane Potential

• A neuron's resting membrane potential is the electrical charge difference across its membrane when it's not actively signaling.
• It arises from uneven distribution of ions inside vs outside the cell.
• There are higher concentrations of Na+, Cl-, and Ca2+ ions outside the cell.
• There are higher concentrations of K+ and negatively charged anions (A-) inside the cell.
• This ion distribution results in a net negative charge inside the cell, approximately -65 millivolts.

Ligand-Gated Ion Channels and Postsynaptic Potentials

• Neurotransmitters bind to receptors on dendrites, opening ligand-gated ion channels.
• Ligand-gated channels respond to neurotransmitters (ligands).
• Opening of these channels allows specific ions to flow into or out of the cell, altering its charge.
• Influx of positive charge (e.g., Na+ entering) causes depolarization, making the cell less negative.
• An excitatory postsynaptic potential (EPSP) occurs when there is a net influx of positive charge, making the cell more likely to fire an action potential.
• Influx of negative charge (e.g., Cl- entering) causes repolarization or hyperpolarization, making the cell more negative and less likely to fire.
• An inhibitory postsynaptic potential (IPSP) occurs when there is a net influx of negative charge.
• A single EPSP or IPSP creates only a small change on the resting membrane potential,
• Collective effect of multiple EPSPs across dendrites can push membrane potential to threshold value.

Action Potential Generation

• If the combined effect of EPSPs reaches a threshold (typically around -55mV), it triggers an action potential.
• Voltage-gated Na+ channels open at the axon hillock in response to the threshold voltage.
• The opening of voltage-gated Na+ channels causes a rapid influx of Na+ ions into the cell.
• This influx leads to further depolarization and the opening of nearby voltage-gated Na+ channels, initiating a chain reaction down the axon.
• During the action potential, the inside of the cell becomes positively charged (up to +40mV).

Repolarization and Refractory Periods

• Depolarization ends when Na+ channels stop allowing Na+ to flow into the cells- a process known as inactivation.
• Voltage-gated K+ channels open slowly after Na+ channels, allowing K+ to flow out of the cell.
• Potassium flowing out of the cell down its own electrochemical gradient- removing some positive charge and blunting the effect of the sodium depolarization.
• The combined efforts of sodium-potassium pump and the extended opening of the potassium channels results in hyperpolarization of the neuron.
• Hyperpolarization is where the neuron becomes more negative relative to the resting potential.
• The sodium channels enter their initial closed state.
• The neuron enters an absolute refractory period when sodium channels are inactivated and cannot be reopened.
• This absolute refractory period prevents action potentials from happening to close together.
• Keeps the action potential moving in one direction.
• It then enters a relative refractory period when sodium channels are closed but can be activated with a strong stimulus.
• The neuron eventually returns to its resting membrane potential once all ion channels close.

Myelination and Saltatory Conduction

• Myelin is a fatty substance produced by glial cells (Schwann cells or oligodendrocytes) that insulates axons.
• Myelinated areas lack voltage-gated ion channels, preventing ion flow across the membrane.
• Ions can only flow through the spots between the myelin, called nodes of Ranvier.
• Instead of propagating via channels, the charge essentially jumps from node to node.
• Sodium ions rushing in bumps other positive sodium ions already inside the cell, which bumps another one, and so on until it reaches the next node.
• The action potential "jumps" from one node to the next in a process called saltatory conduction.
• Saltatory conduction greatly increases the speed of action potential propagation.