HSCI Health Science 3020: Human Physiology - Neuronal Integration Part 3 Study Notes

HSCI Health Science 3020: Human Physiology - Neuronal Integration Part 3

I. Introduction to Neuronal Integration

  • Focus on temporal and spatial summation within the context of neuronal integration.

  • Recap on excitatory (EPSP) and inhibitory postsynaptic potentials (IPSPs).

II. Excitatory and Inhibitory Postsynaptic Potentials

  1. Definitions:

    • Excitatory Postsynaptic Potential (EPSP):

      • A change in the membrane potential that makes it more likely to fire an action potential.

      • Caused by the release of excitatory neurotransmitters from the presynaptic terminal binding to receptors on the postsynaptic terminal, bringing the threshold potential closer to action potential threshold.

    • Inhibitory Postsynaptic Potential (IPSP):

      • A change in membrane potential that makes it less likely to fire an action potential.

      • Caused by the binding of inhibitory neurotransmitters, pushing the potential further from the threshold.

  2. Significance:

    • Neurons don't operate as isolated entities; multiple synaptic terminals influence the postsynaptic terminal.

    • The axon hillock acts as a decision-making center, determining whether the neuron fires based on the integrative input of EPSPs and IPSPs.

III. Summation of Potentials

  1. Summation Concept:

    • Summation refers to the accumulation of EPSPs and IPSPs over time and space at the postsynaptic terminal to determine whether an action potential reaches threshold.

    • Membrane Events: It involves depolarizing (EPSP) and hyperpolarizing (IPSP) stimuli acting upon a neuron.

  2. Temporal Summation:

    • Occurs when successive EPSPs or IPSPs stimulate the same synaptic location within a short timeframe, allowing the potentials to overlap and add together.

    • For example, if stimuli are given within milliseconds (such as 1 to 6 ms apart), they have the potential to summate.

    • Important Note: An action potential itself lasts about one millisecond. Graded potentials may last longer; thus, the timing of stimuli is crucial for the effect of summation.

  3. Spatial Summation:

    • Refers to the summation of inputs from multiple presynaptic terminals affecting one postsynaptic neuron.

    • If multiple input potentials happen simultaneously at different locations on the membrane, they can summate and potentially reach threshold.

    • Example: If two small excitatory potentials occur at the same time but are spatially separated, they may not reach threshold if they are not close enough to each other.

IV. Action Potentials and Their Properties

  1. All-or-None Principle:

    • Once a neuron fires an action potential, it is always of the same magnitude and does not vary in size with increased stimulus strength.

    • The threshold potential must be reached for an action potential to occur, regardless of the magnitude of stimulus beyond that threshold.

    • Diagram: A graph displays stimulus strength versus membrane potential—only reaching a specific threshold (e.g., 5 mV) results in an action potential regardless of exceeding stimulus strength (e.g., 6–8 mV).

V. Neurotransmitter Dynamics and Feedback Systems

  1. Glutamatergic neurons:

    • Neurons that release glutamate are referred to as glutamatergic neurons, which are often excitatory in nature.

    • When a glutamatergic neuron is activated, action potentials travel along the axon, leading to calcium channel opening and subsequent glutamate release into the synaptic cleft.

  2. Interplay of Excitatory and Inhibitory Neurons:

    • Glutamate can activate inhibitory neurons (e.g., GABAergic neurons), demonstrating a feedback loop where excitatory signals can dampen further excitatory release.

    • The release of GABA from inhibitory neurons reduces future glutamate release, showcasing negative feedback in neuronal networks.

VI. Regulation of Neuronal Firing

  1. Frequency of Neuronal Firing:

    • Firing frequency can vary, represented as signals in Hertz (Hz) from 1 Hz (one action potential per second) to 50 Hz (50 action potentials per second).

    • Increased firing rates (temporal summation) or increased neuron participation (spatial summation) can enhance neuron activity.

  2. Feedback Mechanisms:

    • Mechanisms exist to fine-tune neurotransmitter release via presynaptic receptors responding to incoming neurotransmitters, ensuring that excessive activity is mitigated through inhibitory feedback.

    • Example: A presynaptic membrane can have glutamate receptors that sense glutamate release, adjusting subsequent neurotransmitter levels automatically.

VII. Conclusion

  • Summation mechanisms, both temporal and spatial, are critical for understanding how neurons integrate signals to determine action potential firing. Feedback mechanisms further regulate neurotransmitter release and neuron excitability.

  • Students are encouraged to review materials before class discussions for deeper understanding.