Neurons: Structure, Function, and Action Potentials

Neurons: Structure, Function, and Action Potentials

Major Characteristics of Neurons

Neurons are specialized cells responsible for transmitting information throughout the nervous system. They possess several defining characteristics that enable their function:

• Excitability: Ability to respond to stimuli and convert them into nerve impulses.

• Conductivity: Ability to transmit electrical signals along their membrane.

• Secretion: Release of neurotransmitters to communicate with other cells.

• Longevity: Most neurons can live and function for a lifetime.

• Amitotic: Most neurons do not divide after maturation.

Anatomic Structure of a Neuron

Neurons have a distinct structure that supports their function in signal transmission:

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Branched extensions that receive signals from other neurons.

• Axon: Long projection that conducts impulses away from the cell body.

• Axon Hillock: Region where action potentials are initiated.

• Axon Terminals (Synaptic Boutons): Release neurotransmitters to communicate with target cells.

Glia and Types of Glial Cells

Glia (or neuroglia) are non-neuronal cells that support and protect neurons. Types include:

• Astrocytes: Maintain the blood-brain barrier and provide nutrients.

• Oligodendrocytes: Form myelin sheaths in the central nervous system (CNS).

• Schwann Cells: Form myelin sheaths in the peripheral nervous system (PNS).

• Microglia: Act as immune cells in the CNS.

• Ependymal Cells: Line ventricles and produce cerebrospinal fluid.

Ions Involved in Neuronal Signaling

Neuronal signaling depends on the movement of specific ions across the membrane:

• Sodium (Na+): Moves through voltage-gated sodium channels.

• Potassium (K+): Moves through voltage-gated potassium channels and leak channels.

• Chloride (Cl-): Moves through chloride channels.

• Calcium (Ca2+): Enters through voltage-gated calcium channels, especially at synaptic terminals.

Sodium-Potassium Pump

The sodium-potassium pump (Na+/K+-ATPase) is essential for maintaining ion gradients:

• Actively transports 3 Na+ ions out and 2 K+ ions into the neuron per ATP hydrolyzed.

• Restores resting ion concentrations after action potentials.

Equation:

$3\mathrm{Na}^+_{\text{in}} + 2\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\mathrm{Na}^+_{\text{out}} + 2\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P}_i$

Membrane Potential and Resting Membrane Potential

Membrane potential is the voltage difference across a cell's plasma membrane, produced by unequal distribution of ions. The resting membrane potential is the steady-state voltage (typically -70 mV in neurons) when the cell is not transmitting signals, created by ion gradients and selective permeability.

Key Terms in Neuronal Signaling

• Resting Membrane Potential: The baseline electrical charge difference across the membrane.

• Depolarization: Membrane potential becomes less negative (more positive).

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Threshold: The critical level to which the membrane potential must be depolarized to initiate an action potential.

• Refractory Period: Time after an action potential when a neuron cannot fire another action potential (absolute and relative phases).

• Unidirectional Propagation: Action potentials travel in one direction along the axon.

• Myelinated: Axons covered with myelin sheaths, increasing conduction speed.

• All-or-None: Principle that an action potential either occurs fully or not at all, once threshold is reached.

Action Potential: Development and Recovery

Action potentials are rapid changes in membrane potential that propagate along the axon. Steps include:

1. Resting State: All voltage-gated channels closed; resting membrane potential maintained.

2. Depolarization: Na+ channels open, Na+ enters, membrane potential rises.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits, membrane potential falls.

4. Hyperpolarization: K+ channels remain open, membrane potential becomes more negative than resting.

5. Return to Resting State: K+ channels close, Na+/K+ pump restores ion gradients.

Action potentials occur at the axon hillock and propagate along the axon.

Refractory Period and Action Potential Propagation

The refractory period ensures one-way propagation of action potentials and limits their frequency:

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

All-or-None Law of Action Potentials

The all-or-none law states that if a stimulus reaches threshold, an action potential will fire with uniform magnitude; sub-threshold stimuli do not trigger action potentials.

Adaptations for Increased Conduction Speed

Organisms have evolved two main adaptations to increase action potential conduction speed:

• Increased Axon Diameter: Reduces resistance to ion flow, speeding conduction.

• Myelination: Myelin sheaths insulate axons, enabling saltatory conduction between nodes of Ranvier.

Role of Myelination

Myelination increases conduction speed by allowing action potentials to 'jump' between nodes of Ranvier (saltatory conduction), rather than propagating continuously along the axon.