Action Potential

Overview of Action Potentials

  • Definition: An action potential is a neural impulse, which is an electrical signal that travels down the axon of a neuron, facilitating the communication of information.

  • Importance of Action Potentials: Without action potentials, neural communication cannot occur. They are essential for sending messages within the nervous system.

Types of Neurons

  • Classification: The three main types of neurons identified include sensory neurons, interneurons, and motor neurons.

    • Sensory Neurons (Afferent Neurons):

    • Function: Transmit information from sensory receptors to the brain.

    • Example: When touching a hot surface, sensory neurons relay the pain signal from skin receptors to the brain.

    • Interneurons:

    • Location: Found exclusively in the brain and spinal cord.

    • Function: Process and relay information between sensory and motor neurons, effectively integrating sensory input and motor output.

    • Motor Neurons (Efferent Neurons):

    • Function: Send commands from the brain to the body's muscles and glands.

    • Example: Motor neurons signal the hand to withdraw away from the hot surface.

Neural Communication Process

  • Stages of Neural Communication:

    • Dendrites: Receive information from other neurons.

    • Soma (Cell Body): Contains the nucleus and fuels the neuron.

    • Axon: Carries the message away from the soma.

    • Axon Terminal: Sends neurotransmitters across the synapse to communicate with other neurons.

  • Synapse:

    • Neurons do not touch; there is a synaptic gap between them.

    • An area the size of a pinhead can contain about 30,000 neurons.

Action Potential Overview

  • Generate: An action potential is generated by a change in the neuron's electrical charge, primarily involving the movement of sodium (Na+) and potassium (K+) ions across the axon's membrane.

Neuron at Rest

  • The resting state of a neuron is characterized by:

    • Polarized State:

    • Charge: -70 millivolts (mV)

    • Description: Inside is negative compared to the outside due to the distribution of ions.

    • Semi-permeable membrane allows selective passage of ions.

Initiation of Action Potential

  • Signal Reception:

    • When a signal is received, if it is excitatory, sodium channels open, and Na+ rushes into the neuron, causing depolarization.

    • Threshold Voltage: An action potential is initiated when the membrane potential reaches -55 mV.

  • All-or-None Principle:

    • The action potential either occurs fully, or it does not occur at all, analogous to firing a gun.

Phases of Action Potential

  1. Depolarization:

    • Na+ influx raises the internal charge to approximately +40 mV.

    • Initial brief change in electrical charge occurs due to sodium rushing in.

  2. Repolarization:

    • When the action potential peaks, Na+ channels close, and K+ channels open, allowing K+ to flow out, restoring a more negative charge.

  3. Hyperpolarization:

    • The charge temporarily dips below -70 mV (about -90 mV), leading to a refractory period where another action potential cannot be initiated.

Refractory Period

  • Definition: The time during which the neuron cannot fire again until it returns to the resting state (back to -70 mV).

  • Analogy: Comparable to a toilet after flushing—it cannot flush again until it is refilled.

Ion Movement and Pumps

  • Sodium-Potassium Pump:

    • Function: Maintains resting potential by moving 3 Na+ out for every 2 K+ brought into the neuron.

    • This action ensures that the internal environment remains more negative than the external environment.

Chemical Transmission

  • Neurotransmitter Release:

    • Occurs at the axon terminal when an action potential arrives, causing vesicles to release neurotransmitters into the synapse.

  • Post-Synaptic Effects:

    • The neurotransmitters can:

    1. Bind to receptors on the next neuron's dendrites (ideal scenario).

    2. Be destroyed or washed away in the synapse.

    3. Go through a reuptake process, where they are reabsorbed by the presynaptic neuron.

  • Lock and Key Model:

    • Receptors are specific; a neurotransmitter must be the correct type to bind effectively, just like a key fits a specific lock.

Drug Interaction with Neurotransmission

  • Mechanisms of Drug Action:

    • Antagonists: Block neurotransmitter receptors (e.g., painkillers blocking substance P).

    • Agonists: Mimic neurotransmitters to enhance signal transmission (e.g., heroin mimicking endorphins).

    • Reuptake Inhibitors: Prevent reabsorption of neurotransmitters, increasing their availability (e.g., cocaine blocking dopamine reuptake).

Summary of Key Concepts

  • Neurons: Composed of dendrites, soma, axon, and axon terminal.

  • Action Potential: Key to neural impulse transmission characterized by a change in electrical charge due to Na+ and K+ movements across membranes.

  • Neurotransmission: Involves chemical signaling across a synapse, influenced by various drugs through antagonism, agonism, or altering reuptake processes.