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.