Neuron Function and Signal Transmission

Neuron Structure

  • Four Main Parts of a Neuron:

    • Dendrites:

    • Function: Receive information

    • Cell Body:

    • Function: Processes and integrates received information

    • Axon:

    • Function: Carries information over long distances within the neuron

    • Axon Terminal:

    • Function: Transmits information to the subsequent cell in the neural pathway

  • Nerves:

    • Definition: A bundle of axons traveling together

    • Characteristic: Nerves can be very long to accommodate long-distance information transmission

Neuronal Signal Transmission

  • Dendrites:

    • Role: Receive incoming signals

    • Neuron Decision: Based on signal strength, the neuron decides if the signal will be transmitted

  • Action Potential:

    • Definition: The phenomenon when a neuron fires, transmitting a signal along the axon

  • Ionic Movement:

    • Importance: Signal transmission relies heavily on the movement of ions (charged particles)

    • Ions Involved: Sodium (Na+), Potassium (K+), and Chloride (Cl-)

Resting Neuron Characteristics

  • Resting State of Neuron:

    • Definition: A neuron is at rest when it is not sending a signal

    • Ionic Concentration:

    • Higher concentration of Sodium ions outside the cell

    • Higher concentration of Potassium ions inside the cell

    • Chemical Gradient:

    • Created by the unequal distribution of ions across the cell membrane

    • Electrical Gradient:

    • Created due to the difference in charge, with more positively charged ions outside than inside

  • Electrochemical Gradient:

    • Definition: The combined effect of the chemical and electrical gradients

    • Membrane Potential:

    • Resting potential of a typical neuron: approximately -70 millivolts

    • Indicates the inside of the cell is 70 millivolts less positive than the outside

    • Equilibrium:

    • Electrochemical equilibrium occurs when concentration and electrical gradients are equal and opposite

Ion Movement and Channels

  • Ion Channels:

    • Function: Facilitate ion movement across the membrane

    • Types of Ion Channels:

    • Voltage-Gated Channels: Open at certain membrane potentials

    • Ligand-Gated Channels: Open upon binding of a specific molecule

    • Mechanically-Gated Channels: Open in response to physical changes (length, pressure)

    • Selectively Permeable: Most channels allow only specific ions to pass

    • Types of Channels:

    • Voltage-gated Sodium Channels

    • Voltage-gated Potassium Channels

Graded and Action Potentials

  • Graded Potential:

    • Characteristics:

    • Varies in size (can be positive or negative)

    • Transient (short-lived)

    • Not typically caused by voltage-gated channels

  • Restoration of Resting Potential:

    • Process: Involves the Sodium-Potassium Pump

    • Mechanism:

    • The pump uses ATP hydrolysis to actively transport ions against their gradients

    • Transports 3 Sodium ions out and 2 Potassium ions into the cell

    • Energy Consumption:

    • This process accounts for 20% to 40% of the brain's total energy use

Action Potential Generation

  • Threshold Voltage:

    • Definition: The required membrane potential to trigger an action potential, typically -55 millivolts

  • Voltage-Gated Sodium Channels:

    • States:

    • Closed: At rest

    • Open: Upon reaching threshold, allowing Na+ influx

    • Inactivated: After overshoot, preventing further Na+ entry

  • Membrane Changes:

    • Depolarization:

    • Entry of Na+ causes the membrane potential to increase, moving toward 0 millivolts

    • Overshoot:

    • Membrane potential can exceed 0, reaching up to +30 millivolts

    • Repolarization:

    • Opening of voltage-gated potassium channels (K+) allows K+ to exit, reversing depolarization

    • Hyperpolarization:

    • Membrane potential temporarily becomes more negative than -70 millivolts due to slow closing of K+ channels

Refractory Periods

  • Absolute Refractory Period:

    • Definition: Time during which a neuron cannot fire another action potential regardless of stimulus strength

    • Function: Prevents rapid consecutive action potentials and retrograde signal transmission

  • Relative Refractory Period:

    • Definition: Follows absolute refractory period; neuron can fire again but requires a stronger stimulus

    • Reason: The neuron is still hyperpolarized due to ongoing K+ efflux

Action Potential Characteristics

  • All-or-Nothing Principle:

    • Definition: The amplitude of action potentials remains constant; they do not vary in magnitude with stimulus strength

  • Frequency of Action Potentials:

    • The number of action potentials per second can increase with stronger stimuli (e.g., intense pain) and decrease with weaker stimuli (e.g., gentle breeze)

Conduction Velocity

  • Factors Influencing Speed:

    • Presence of Myelin Sheaths:

    • Function: Increase transmission speed via saltatory conduction

    • Mechanism: Action potentials “jump” between nodes of Ranvier where the axon is not covered by myelin

  • Types of Myelination:

    • Peripheral Nervous System: Myelin formed by Schwann cells

    • Central Nervous System: Myelin formed by oligodendrocytes

Summary

  • Neurons at rest maintain a resting potential.

  • A small stimulus leads to a graded potential.

  • A sufficient stimulus that exceeds the threshold generates an action potential, leading to neuronal firing.