LIN350C 2 Graded and Action Potentials

Assistant Professor, Department of Linguistics, The University of Texas at Austin
Course: SLH/LIN 350(C): LANGUAGE AND THE BRAIN
Topics: Graded and Action Potentials

The Neuron Doctrine: A fundamental principle in neuroscience.
Theories proposed by two scientists: - Ramon y Cajal: Proposed that neurons communicate by contact (neural communication does not involve continuity between neurites).
- Camillo Golgi: Suggested that neurites of different cells fuse together to form a continuous network (akin to a vascular system).
This debate laid the foundational understanding of neuron structure and function.

Golgi Stain: A staining technique that allows visualization of a limited number of cells in their entirety.
Nissl Stain: Stains rough endoplasmic reticulum, highlighting cell bodies and proximal dendrites.
Theoretical Support: The Golgi stain supports Golgi’s theory of neuronal continuity.

Resting Potential: The state of a neuron when it is not firing, typically around -70 mV.
Graded Potential: Variations in membrane potential that can lead to action potentials if they reach a certain threshold.
Action Potential: A rapid rise and subsequent fall in voltage (a 'spike') across a cellular membrane.

The Salty Banana Model: A metaphor used to describe neuronal ion concentrations.
Inside the neuron: High concentration of potassium (K+).
Outside the neuron: High concentration of sodium (Na+).
Ion distribution creates an electrochemical gradient which is essential for action potentials.

The membrane potential of a cell at rest is approximately -70 mV.
Important for maintaining homeostasis within neurons before channels open to initiate graded or action potentials.

Leak Channels: Allow passive movement of ions (K+, Na+) across the membrane, contributing to resting potential.
Na+/K+ Pump: Actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell, maintaining concentration gradients essential for action potentials.

Events occurring at the synapse:
- Synaptic Vesicles: Store neurotransmitters. - Voltage-Gated Ca2+ Channels: Open upon action potential arrival, allowing calcium ions into the terminal, triggering neurotransmitter release.
- Chemically-Gated Channels: Respond to specific neurotransmitters leading to graded potential changes in the postsynaptic neuron.
Synaptic cleft is the space between axon terminal and dendrites where neurotransmitters are released, allowing for depolarization or hyperpolarization of the postsynaptic membrane.

Membrane Potential Changes: - When neuron depolarizes and reaches -50 mV, voltage-gated Na+ channels open, resulting in rapid influx of sodium ions, causing depolarization to +30 mV.

Following peak depolarization, voltage-gated K+ channels open when membrane potential reaches +30 mV.
K+ exits the neuron, repolarizing the membrane back toward -70 mV.

After repolarization, the membrane potential may temporarily dip below resting potential due to prolonged K+ channel opening (undershoot phase).

Action potentials propagate along the axon due to sequential opening of voltage-gated Na+ channels in response to depolarization, allowing a wave-like movement of the signal.

1865: The action potential was first recorded by physiologist Julius Bernstein using frog nerve’s electrodes.
Referred to as the “negative variation” prior to the identification of Golgi stains and the sodium-potassium pump in later years (identified in 1957).

All-or-Nothing Principle: Proposed by Warren McCulloch and Walter Pitts in the 1940s, stating that action potentials occur fully or not at all.
This principle has influenced modern computational models by John von Neumann.

Subsequent studies with various animals (e.g., bullfrogs, eels, mice, squids) affirmed the all-or-nothing nature of neural spikes.
The squid model was pivotal in developing a mathematical model of neuron behavior by Hodgkin and Huxley, enhancing understanding beyond McCulloch-Pitts' initial ideas.