Chapter 11: Bioelectricity and Fundamentals of the Nervous System

Bioelectricity: Basic Principles of Electricity

Relationship Between Current, Voltage, and Resistance

Understanding bioelectricity requires knowledge of fundamental electrical concepts as they apply to neurons:

• Voltage: The measure of charge difference between two points, representing potential energy. In neurons, this is the difference in charge across the cell membrane.

• Current: The flow of electrical charge, which in biological systems is the movement of ions across membranes. Current depends on both voltage and resistance.

• Resistance: The hindrance to charge flow. Biological membranes act as insulators (high resistance), while ion channels act as conductors (low resistance).

• Equation:

$I = \frac{V}{R}$

• Insulators: Materials with high resistance (e.g., cell membrane).

• Conductors: Materials with low resistance (e.g., open ion channels).

• In the body: Currents are generated by the movement of ions (Na+, K+, etc.) across the neuronal membrane.

Membrane Ion Channels

Types of Ion Channels

Ion channels are proteins that allow ions to cross the cell membrane, crucial for electrical signaling:

• Leak Channels: Always open, allow passive ion movement.

• Gated Channels:

  • Chemically Gated: Open in response to neurotransmitter binding.

  • Voltage-Gated: Open in response to changes in membrane potential.

  • Mechanically Gated: Open in response to physical deformation (stretch or pressure).

• Electrochemical Gradient: The combined effect of concentration gradient and electrical forces driving ion movement.

Resting Membrane Potential (RMP)

Definition and Electrochemical Basis

The resting membrane potential is the voltage difference across the neuronal membrane when the cell is not transmitting signals:

• Typical Value: Approximately –70 mV (inside negative relative to outside).

• Causes:

  • Ion Concentration Differences: More Na+ outside, more K+ inside.

  • Membrane Permeability: Membrane is more permeable to K+ than Na+.

  • Na+/K+ Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining gradients.

Example: The Na+/K+ pump is essential for maintaining the RMP and for recovery after action potentials.

Changes in Membrane Potential

Depolarization and Hyperpolarization

• Depolarization: The inside of the cell becomes less negative, increasing the likelihood of generating an impulse.

• Hyperpolarization: The inside becomes more negative, decreasing the likelihood of generating an impulse.

Graded Potentials

Graded potentials are local changes in membrane potential:

• Occur over short distances and have variable strength.

• Decay with distance from the stimulus.

• Can trigger action potentials if they reach threshold.

Action Potentials

Action potentials are rapid, long-distance electrical signals:

• Do not decay with distance.

• Occur only on axons.

Action Potential: Generation and Propagation

Steps of Action Potential

1. Resting State: All voltage-gated Na+ and K+ channels are closed; only leak channels are open.

2. Depolarization: Voltage-gated Na+ channels open, Na+ enters, making the membrane positive.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits, restoring negativity.

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

• Threshold: Must be reached for action potential to occur; follows an all-or-none principle.

• Propagation: Action potential triggers adjacent areas, moving like dominoes along the axon.

• Stimulus Intensity: Determined by frequency of action potentials, not their size.

Refractory Periods

Absolute vs. Relative Refractory Periods

• Absolute Refractory Period: No new action potential can be generated; Na+ channels are open or inactive.

• Relative Refractory Period: A stronger stimulus may trigger an action potential; membrane is repolarizing.

Conduction Velocity

Factors Affecting Speed

• Larger Axon Diameter: Faster conduction due to lower resistance.

• Myelination: Increases speed via saltatory conduction (action potential jumps between nodes of Ranvier).

Saltatory vs. Continuous Conduction

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps from node to node, increasing speed.

• Continuous Conduction: Occurs in unmyelinated axons; action potential moves along the entire membrane, slower.

Synapses

Definition and Types

• Synapse: Junction where neurons communicate.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

Electrical vs. Chemical Synapses

Type	Structure	Transmission	Location
Electrical Synapse	Direct ion flow via gap junctions	Fast, bidirectional	Rare; brain regions, embryos, cardiac muscle
Chemical Synapse	Neurotransmitter release across synaptic cleft	Slower, unidirectional	Most common; CNS and PNS

Chemical Synapse Events

1. Action potential arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters.

3. Synaptic vesicles release neurotransmitter into synaptic cleft.

4. Neurotransmitter binds to postsynaptic receptor, causing graded potential.

5. Signal ends by reuptake, enzymatic breakdown, or diffusion away.

Postsynaptic Potentials

Excitatory vs. Inhibitory Postsynaptic Potentials

• EPSP (Excitatory Postsynaptic Potential): Depolarizes postsynaptic membrane, increasing chance of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes postsynaptic membrane, decreasing chance of action potential.

Summation

• Temporal Summation: Rapid signals in succession from one presynaptic neuron.

• Spatial Summation: Multiple inputs from different presynaptic neurons at the same time.

Synaptic Plasticity

• Potentiation: Stronger responses with repeated stimulation.

• Inhibition: Reduced neurotransmitter release or response.

Neurotransmitters

Definition and Classification

Neurotransmitters are chemical messengers released by neurons to transmit signals across synapses.

Class	Examples	Main Function
Acetylcholine (ACh)	ACh	Muscle contraction
Biogenic amines	Dopamine, Serotonin, Norepinephrine, Histamine	Emotion, mood, arousal
Amino acids	GABA, Glutamate	Inhibition, excitation
Peptides	Endorphins, Enkephalins	Pain modulation
Purines	ATP	Energy signaling
Gases/Lipids	NO, CO	Modulation
Endocannabinoids	Natural opiates	Pain, appetite

Neurotransmitter Effects

• Excitatory vs. Inhibitory: Effect depends on the receptor type.

• Direct: Open ion channels for fast responses.

• Indirect: Use second messengers for slower, longer-lasting effects.

Neurotransmitter Receptors

• Channel-linked Receptors: Fast, ligand-gated ion channels.

• G-protein Coupled Receptors: Slow, modulatory, use second messengers.

Example: Dopamine acts as both an excitatory and inhibitory neurotransmitter depending on the receptor subtype.

Additional info: Synaptic events can be modified by drugs, disease, or learning, affecting neurotransmitter release, receptor sensitivity, or signal termination.