Nerve Impulses

Nerve Impulses and Reflexes

Cell Membrane Potential Steps

  • Resting Potential: -70 mV

  • Threshold Potential / Summation: -55 mV

  • Action Potential: Increases to +30 mV

Resting Potential

  • Mechanisms:

    • Leak channels allow K+ (potassium) to exit and Na+ (sodium) to enter.

    • More K+ channels result in a net loss of positive charge inside the cell, creating a resting membrane potential of -70 mV.

    • Sodium-Potassium pump actively transports 3 Na+ out for every 2 K+ in, maintaining the concentration gradient.

Threshold and Summation

  • Excitatory Postsynaptic Potential (EPSP): Brings neuron closer to firing.

  • Inhibitory Postsynaptic Potential (IPSP): Brings neuron closer to resting potential.

  • Together, EPSP/IPSP affect the action hillock (gatekeeper) determining if the action potential (AP) is initiated when reaching -55 mV.

Action Potential

  • Process (Three Steps):

    1. Depolarization

    2. Repolarization

    3. Hyperpolarization

1st Step: Depolarization

  • When threshold (-55mV) is met, voltage-gated Na+ channels open, allowing Na+ influx.

  • Na+ channels close at +30 mV, stopping further Na+ entry.

2nd Step: Repolarization

  • At +30 mV, voltage-gated K+ channels open, allowing K+ to exit the neuron, returning the inside to a more negative potential.

3rd Step: Hyperpolarization

  • K+ channels close at -70 mV; however, the delayed closing causes excess K+ to leave.

  • Importance:

    1. Prevents immediate re-firing of the neuron.

    2. Ensures action potential travels in one direction, preventing backpropagation.

Return of Na+/K+ to Correct Locations

  • The sodium-potassium pump restores Na+ to the outside and K+ to the inside of the cell.

Action Potential Summary

  • Phases:

    1. Resting State: -70 mV

    2. Threshold Initiation: -55 mV

    3. Peak Action Potential: +30 mV

    4. Repolarization: Returns towards -70 mV

    5. Hyperpolarization: Brief potential drop due to K+ flow out.

Nerve Impulse Speed

  • Speed of impulse is proportional to axon diameter: larger diameter = faster impulse.

  • Myelinated vs. Unmyelinated Axons:

    • Continuous conduction in unmyelinated axons (voltage channels along the entire length).

    • Saltatory conduction in myelinated axons (impulses jump between nodes of Ranvier).

Nerve Pathway of a Stimulus

  • Pathway: Dendrite → Cell Body → Axon → Synapse → Dendrite of next neuron.

How Neurons Communicate

  • Neurotransmitter release at synapse facilitates communication between neurons.

  • Receptors on dendrites receive neurotransmitters, leading to either excitatory or inhibitory responses.

Types of Neurotransmitters

  • Excitatory: Increase membrane permeability, encouraging threshold achievement by increasing Na+ permeability.

  • Inhibitory: Decrease permeability, reducing likelihood of threshold achievement by increasing Cl- permeability.

Examples of Neurotransmitters

  • Acetylcholine: Stimulates muscle contraction.

  • Dopamine: Influences pleasure, motivation, and motor control in the brain.

  • Serotonin: Affects mood, sleepiness, and appetite regulation.

  • Endorphins: Involved in stress response and pain management, enhancing mood.

  • Commonly released together during activities like exercising (dopamine, serotonin, endorphins).

Answer Key

  • Neurotransmitters: Dopamine, Acetylcholine, Serotonin, Endorphins