Part C: The Physiology of a Neuron

Structure and Function of the Human Body ASE109 Lecture: The Nervous System (I) Part C: The Physiology of a Neuron

Topics

  • Nerve Impulses

  • The Resting Potential

  • The Action Potential

The Physiology of a Neuron: The Nerve Impulse

  • Definition of Nerve Impulses

    • Nerve impulses are electrochemical changes that convey information within the nervous system.

    • Neurons transmit these nerve impulses.

    • The resting potential of a neuron allows for the generation of these impulses. - Nerve impulses are electrochemical changes that convey information within the nervous system, acting as signals for communication between neurons and other cells. These impulses are crucial for rapid information transmission, allowing for reflexes, sensory perception, and complex neural processing. - Neurons are specialized cells responsible for transmitting these nerve impulses. They consist of a cell body, dendrites that receive signals from other neurons, and an axon that carries the impulse away from the cell body. The axon is often encased in a myelin sheath, which facilitates faster signal transmission through a process called saltatory conduction. - The resting potential of a neuron is the result of a difference in ion concentrations across the plasma membrane, primarily involving sodium (Na+) and potassium (K+) ions. Maintaining this resting potential is essential for generating nerve impulses, as it sets the stage for action potentials, which are the actual nerve impulses. - Neurons operate under the all-or-nothing principle, meaning that once a threshold is reached (typically around -55 mV), a full action potential is triggered and propagates along the neuron without diminishing in strength. This process is vital for effective communication in the nervous system and underpins all neural activity.

The Physiology of a Neuron: The Resting Potential

  • Definition of Resting Potential

    • The resting potential is the potential energy of a neuron when it is at rest.

    • It is analogous to the potential energy stored in a battery, resulting from the separation of positive and negative charges across a membrane.

  • Mechanism of Resting Potential

    • The potential energy can be converted to electrical energy to perform work, similar to how a battery powers devices like a mobile phone or a flashlight.

    • The resting potential exists because the plasma membrane of the neuron is polarized, exhibiting:

    • Higher positive charge outside the cell

    • Higher negative charge inside the cell

    • Measured in millivolts (mV), typically around -70 mV.

The Resting Potential: Measurement

  • Measurement of Resting Potential

    • The resting potential can be measured in volts.

    • For example, a typical nerve cell has a resting potential of approximately -0.070 volts or -70 millivolts (mV), designated with a negative sign.

    • Key Points:

    • Sodium ions (Na+) are predominantly found outside the plasma membrane.

    • The plasma membrane is selectively permeable, allowing potassium ions (K+) to diffuse out but restricting sodium ions.

    • This results in a negative internal charge due to large negatively charged proteins and other molecules trapped within the cell.

Maintaining the Resting Potential

  • Importance of Maintaining Resting Potential

    • Neurons must maintain their resting potential akin to a rechargeable battery to send out electrical signals.

  • Mechanism

    • Neurons actively transport sodium ions (Na+) out of the cell while bringing potassium ions (K+) back into the cytoplasm.

    • The sodium-potassium pump is a protein carrier in the plasma membrane that performs this function.

    • This action resets the neuron to its resting potential following an action potential.

The Physiology of a Neuron: The Action Potential

  • Definition of Action Potential

    • Action potentials refer to the conduction of nerve impulses and are described as the movement of an electrical signal along the axon.

The Action Potential: How it Occurs

  • Stimulus Requirement

    • A stimulus must activate the neuron, with sufficient strength to reach a specific threshold for an action potential to be generated.

    • Threshold Voltage: The minimum voltage required is approximately -55 mV.

    • All-or-Nothing Principle:

    • Once the threshold is reached, an action potential occurs completely.

    • If the threshold is not reached, no action potential occurs.

Generation of an Action Potential

  • Initial Conditions

    • Measurement begins at -55 mV, indicating the threshold potential has been reached.

    • Sodium channels located in the plasma membrane open in response to the threshold stimulus, allowing sodium ions (Na+) to rapidly influx into the cell.

Generation of an Action Potential: Depolarization

  • Definition

    • Depolarization occurs when the influx of positively charged sodium ions causes the inside of the axon to become more positive compared to the outside.

  • Process

    • Sodium channels open along the axon, permitting sodium ions to flow inside.

    • This change shifts the internal charge from negative to positive.

    • Typical measurements during this phase peak between +30 mV and +35 mV.

Generation of an Action Potential: Repolarization

  • Definition

    • Immediately following depolarization, repolarization occurs when potassium (K+) channels open, allowing potassium ions to exit the cell.

  • Process

    • This outflux of potassium ions restores the negative internal charge, resulting in a voltage drop, often below -70 mV. Following depolarization, repolarization occurs when potassium (K+) channels open, allowing potassium ions to exit the cell. This outflux of potassium ions is initiated by the change in membrane potential reaching a certain level, which leads to the opening of these channels. As potassium ions exit, they carry positive charge out of the cell, which restores the negative internal charge, resulting in a voltage drop that often goes below -70 mV.

Restoring the Resting Potential

  • Post-Repolarization Action

    • After repolarization, potassium levels drop below -70 mV, and the resting potential needs to be restored.

    • The sodium-potassium pump plays a crucial role:

    • It moves potassium ions back into the cell and sodium ions outwards, restoring the original resting membrane potential.

The Physiology of a Neuron: Graph of An Action Potential

  • Duration of Action Potential

    • The entire process from depolarization to repolarization is rapid, typically requiring only 3 to 4 milliseconds to complete.

  • Visual Representation

    • Researchers plot the voltage changes over time to visualize these rapid fluctuations in voltage across the axonal membrane, typically presented in graphs illustrating action potentials.

Generation of an Action Potential: Summary

  • Sequence of Events

    • When a stimulus raises the resting potential to the threshold, the following occurs:

    1. Depolarization: Sodium ions enter the axon, raising the internal voltage from -70 mV to approximately +35 mV.

    2. Repolarization: Following the peak, potassium ions exits the axon, restoring internal negativity and briefly overshooting below resting potential.

    3. Restoration: The sodium-potassium pump reinstates the resting potential after the action potential is concluded.