Electrical Activity in Neurons and Synaptic Transmission: Cell Biology Study Notes

Introduction to the Nervous System

Overview

The nervous system is a complex network responsible for transmitting electrical and chemical signals throughout the body, enabling rapid communication and coordination of physiological processes. This section introduces the foundational concepts of neuronal function, focusing on the cellular mechanisms underlying electrical activity and synaptic transmission.

Electrical Activity in Neurons

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuronal membrane when the cell is not actively transmitting a signal. Typically, neurons have a resting membrane potential of -70 mV.

• Definition: The voltage inside the cell relative to the outside.

• Mechanisms:

  • Large negatively charged molecules inside the cell.

  • Na+/K+ pumps: Move 3 Na+ out and 2 K+ in, creating a net negative charge inside.

• Gradients:

  • Electrical gradient: More negative inside.

  • Concentration gradient: [K+] higher inside, [Na+] higher outside.

Altering Membrane Potential

Neurons and muscle cells can change their membrane potentials, a property known as excitability. Changes in membrane potential are caused by alterations in the permeability to specific ions, primarily through ion channels.

• Electrochemical gradient: Combination of concentration gradient and attraction to opposite charges.

• Ion currents: Flow of ions through channels, generating electrical signals.

Types of Electrical Activity

• Graded potentials: Local changes in membrane voltage, typically occurring in dendrites and cell bodies.

• Action potentials: Rapid, large changes in membrane voltage that propagate along axons.

Graded Potentials

Generation and Examples

• Depolarization (EPSP):

  • Ligand-gated channels permeable to Na+ open, allowing Na+ influx.

  • Membrane potential becomes less negative.

  • EPSPs (excitatory postsynaptic potentials) can bring the neuron closer to threshold for action potential generation.

• Hyperpolarization (IPSP):

  • Ligand-gated channels permeable to Cl- open, allowing Cl- influx.

  • Membrane potential becomes more negative.

  • IPSPs (inhibitory postsynaptic potentials) move the neuron further from threshold.

Characteristics Table

Characteristic	Graded Potential	Action Potential
Part of neuron	Dendrites and cell body	Axon hillock, down the axon
Channels	Ligand gated	Voltage gated
Direction Volt.	Pos. or Neg.	Positive and then negative
Amount Volt.	Small change	Large
Magnitude	Varies	All or none
Duration	1 msec to few sec.	1-2 msec
Change in intensity	Local, decreases with distance	No loss along the axon

Action Potentials

Generation of an Action Potential

Action potentials are generated when the sum of EPSPs and IPSPs depolarizes the neuron to the threshold (about -55 mV).

• All-or-None Law:

  • Once threshold is reached, an action potential occurs.

  • Amplitude and duration are not affected by stimulus size.

• Ion Channels:

  • Na+ voltage-gated channels open at threshold, allowing Na+ influx.

  • K+ channels (leakage and voltage-gated) allow K+ efflux, repolarizing the membrane.

Phases of Action Potential

• Depolarization: Na+ channels open, Na+ enters.

• Repolarization: K+ channels open, K+ exits.

• After-hyperpolarization: Membrane potential overshoots, reaching about -85 mV before returning to resting potential via Na+/K+ pumps.

Refractory Periods

• Absolute refractory period: Na+ channels are inactive; no new action potential can be generated.

• Relative refractory period: K+ channels are still open; a very strong stimulus can generate another action potential.

Coding for Stimulus Intensity

• Frequency modulation: Stronger stimuli increase the frequency of action potentials.

• Recruitment: More neurons are activated by stronger stimuli.

• Temporal coding: Placement and timing of action potentials are critical for information processing.

Action Potential Propagation

Cable Properties of Neurons

• Neurons conduct charges through their cytoplasm, but this is inefficient due to high internal resistance and ion leakage.

• Action potentials must propagate along the membrane rather than relying solely on cable properties.

Propagation in Unmyelinated Neurons

• Action potentials are generated at every patch of membrane along the axon.

• Conduction is slow because each action potential is an individual event.

Propagation in Myelinated Neurons

• Myelin: Insulates the axon, increasing conduction speed.

• Nodes of Ranvier: Gaps in myelin where voltage-gated Na+ and K+ channels are concentrated.

• Saltatory conduction: Action potentials "leap" from node to node, greatly increasing speed.

Conduction Speed

• Increased by larger diameter of the neuron and by myelination.

• Example speeds:

  • Thin, unmyelinated: 1.0 m/sec

  • Thick, myelinated: 100 m/sec

Synaptic Transmission

Introduction to the Synapse

• A synapse is the functional connection between a neuron and the cell it signals.

• In the CNS, the second cell is another neuron.

• In the PNS, the second cell may be a neuron, muscle, or gland (neuromuscular junction).

Electrical Synapses

• Cells are joined by gap junctions, allowing ions and molecules to pass directly from one cell to another.

• Connexin proteins form channels spanning both membranes.

• Important for synchronizing neural activity (e.g., in the hippocampus).

Chemical Synapses

• Most common type of synapse.

• Presynaptic neuron releases neurotransmitters stored in synaptic vesicles into the synaptic cleft.

• Neurotransmitters bind to receptors on the postsynaptic neuron, causing graded potentials.

Otto Loewi and Chemical Synapses

• Otto Loewi demonstrated that synaptic transmission is chemical, not just electrical, at the neuromuscular junction (vagus nerve and heart).

• His work confirmed the existence of chemical synapses and earned him the Nobel Prize in 1936.

Summary Table: Graded vs. Action Potentials

Feature	Graded Potential	Action Potential
Location	Dendrites/cell body	Axon hillock/axon
Channels	Ligand-gated	Voltage-gated
Amplitude	Variable	All-or-none
Duration	Variable	1-2 ms
Propagation	Decreases with distance	No loss along axon

Key Equations

• Nernst Equation: Used to calculate equilibrium potential for an ion: $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Resting Membrane Potential (Goldman-Hodgkin-Katz Equation): $V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Example: The rapid transmission of nerve impulses in myelinated axons allows for complex behaviors and fast reflexes in vertebrates.

Additional info: These notes expand on the original slides by providing definitions, mechanisms, and context for each concept, as well as key equations and tables for comparison.