MD

kolbintro6e_lectureslides_ch04

INTRODUCTION TO NEURONAL ELECTRICAL SIGNALING

  • The process by which neurons use electrical signals to transmit information is fundamental to brain function and behavior.

BEHAVIORAL RESPONSE TO STIMULI

  • Nerves detect sensory stimuli and inform the brain.

  • The brain processes stimuli to decide responses.

  • Muscles are then commanded to execute behavioral responses.

THE BASICS: ELECTRICITY AND ELECTRICAL STIMULATION

Electrical Potential

  • Electrical potential refers to the ability to do work utilizing stored electrical energy.

Understanding Electricity

  • Electricity is the flow of electrons from areas of higher charge to lower charge.

  • Negative Pole: Source of electrons (higher charge).

  • Positive Pole: Receives electrons (lower charge).

STUDYING ELECTRICAL ACTIVITY IN ANIMAL TISSUE

  • Electrical Stimulation:

    • Electrical currents (2 to 10 millivolts) stimulate tissues without damaging them.

    • Current leaves a stimulator through an electrode, stimulating the tissue before returning via a reference electrode.

  • Electrical Recording:

    • Voltage differences measured between electrodes indicate electrical activity in the tissue.

EARLY CLUES LINKING ELECTRICITY AND NEURONAL ACTIVITY

Historical Discoveries

  • Galvani's Studies: Discovered electrical current could elicit muscle twitching.

  • Fritsch and Hitzig: Electrical stimulation of the neocortex prompted limb movements.

  • Bartholow (1874): Documented the effects of direct brain stimulation in humans.

ELECTRICAL RECORDING STUDIES

Early Measurements

  • Caton: First to measure brain electrical currents; developed the electroencephalogram (EEG).

  • von Helmholtz: Estimated the flow of information in nerves as slower than electricity, highlighting that ions create a charge wave rather than travel themselves.

TOOLS FOR MEASURING NEURON'S ELECTRICAL ACTIVITY

Measurement Devices

  • Voltmeters: Measure electrical potential differences.

  • Oscilloscopes: Record changes in voltage on an axon.

  • Microelectrodes: Deliver current to a single neuron or measure its activity.

ION MOVEMENTS AND ELECTRICAL CHARGES

Ion Types

  • Cations: Positively charged ions (e.g., Na+, K+).

  • Anions: Negatively charged ions (e.g., Cl−).

Ion Movement Principles

  • Diffusion: Movement from high to low concentration.

  • Voltage Gradient: Charges moving from high to low voltage areas.

MOVING TO EQUILIBRIUM

  • Equilibrium achieved when ion concentrations balance via diffusion and voltage gradients in models of cell membranes.

RESTING POTENTIAL

Understanding Resting Potential

  • The negative electrical charge across the cell membrane at rest is approximately -70 mV.

  • Ion distributions maintain this potential, with higher concentrations of Na+ outside and K+ inside the neuron.

MAINTAINING RESTING POTENTIAL

  • Membrane Properties: Impermeable to large proteins; sodium channels typically closed, while K+ channels are gated allowing some movement.

  • Na+-K+ Pumps: Move Na+ out and K+ into the cell, crucial for maintaining ion differentials.

GRADED POTENTIALS

Characteristics

  • Changes in membrane voltage due to ion concentration shifts.

  • Hyperpolarization: Becomes more negative, usually from inhibitory Cl– influx.

  • Depolarization: Becomes less negative, typically due to Na+ influx.

ACTION POTENTIAL

Mechanism

  • Action potential is a rapid reversal of membrane polarity, triggered when voltage reaches threshold (-50mV).

  • Involves Na+ and K+ voltage-activated channels.

REFRACTORY PERIODS

Types

  • Absolute Refractory Period: No new action potential can occur during repolarization.

  • Relative Refractory Period: A stronger stimulus may elicit a response during later phases.

NERVE IMPULSES

  • Action potentials propagate along axons due to refractory periods, ensuring one-way travel of the impulse.

SALTATORY CONDUCTION

Importance of Myelin

  • Myelin speeds up impulses; nodes of Ranvier facilitate rapid signal transmission along the axon.

  • Impairment of myelin (e.g., in Multiple Sclerosis) disrupts neural signaling.

NEURONAL INTEGRATION OF INFORMATION

Input Management

  • Neurons can integrate numerous inputs from thousands of connections through dendritic spines.

  • EPSPs and IPSPs affect whether a neuron will fire an action potential based on their cumulative effects.

EPSPs AND IPSPs

Differences

  • EPSP: Brief depolarization making neurons more likely to fire.

  • IPSP: Brief hyperpolarization making firing less likely.

SUMMATION OF INPUTS

Temporal and Spatial Summation

  • Temporal: Summation of signals occurring in close time; Spatial: Summation of signals from nearby regions.

VOLTAGE-ACTIVATED CHANNELS

Integration and Initiation

  • Action potentials are initiated at the axon hillock, where many voltage-activated channels are located.

  • Dendrites typically do not generate action potentials but may in specific neurons.

SENSORY STIMULI AND ACTION POTENTIALS

Mechanisms

  • Sensory neurons have ion channels that start generating nerve impulses upon stimulation.

  • Example: Stretch-activated channels activated by hair displacement in touch neurons.

NERVE IMPULSES AND MOVEMENT

Connection to Muscles

  • Spinal motor neurons transmit impulses that lead to muscle contractions via neurotransmitter acetylcholine.

  • Contraction occurs through combined influx of Na+ and K+ ions.

TRIGGERING ACTION POTENTIALS

  • Initiation of action potentials is based on the integration of various synaptic inputs at the axon hillock.

CONCLUSION

  • Understanding how neurons process electrical signals is essential for grasping the broader mechanisms of brain activity and behavioral responses.