Nerves – Graded Potentials and Synapses

Graded Potentials

  • Graded potentials are changes in membrane potential that decrease in strength as they spread out from the point of origin.

    • Reason for decrease in strength: Current leak out of the cell.

Integration and Coding of Impulses

  • Integration occurs in the neuron cell body, where multiple graded potentials may converge and affect action potential generation at the axon hillock.

  • Subthreshold Graded Potential

    • Definition: A graded potential that, although it starts above threshold, decreases in strength and reaches the trigger zone below threshold.

    • Implication: Does not initiate an action potential.

  • Suprathreshold Graded Potential

    • Definition: A graded potential that remains above threshold by the time it reaches the trigger zone due to a stronger stimulus.

    • Implication: Initiates an action potential.

Synaptic Transmission

  • Synaptic transmission is the process by which a signal is transferred from one neuron to another.

    • Chemical synapse structure:

    • Presynaptic neuron releases neurotransmitter into the synaptic cleft.

    • Postsynaptic cell has specific receptors for the neurotransmitter.

    • Synaptic cleft: The space between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic neuron.

Synaptic Transmission Process

  1. An action potential reaches the axon terminal of the presynaptic neuron.

  2. Voltage-gated Ca2+ channels open, allowing Ca2+ to enter the neuron, which triggers synaptic vesicles to release neurotransmitters.

  3. The neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane, leading to a postsynaptic response.

  • Termination of Neurotransmitter Action:

    • Neurotransmitter can be broken down by enzymes in the synaptic cleft.

    • Neurotransmitters can be taken back into cells or diffuse away from the synapse.

Types of Synaptic Responses

  • Ionotropic Receptors:

    • Fast responses mediated by ion channels that create rapid, short-acting synaptic potentials.

  • Metabotropic Receptors:

    • Slow responses mediated by G protein-coupled receptors that create long-lasting effects.

Postsynaptic Responses

  • Excitatory Postsynaptic Potentials (EPSPs):

    • Result in depolarization of the postsynaptic membrane, making it more likely to trigger an action potential.

  • Inhibitory Postsynaptic Potentials (IPSPs):

    • Result in hyperpolarization of the postsynaptic membrane, making it less likely to trigger an action potential.

Integration of Synaptic Signaling

  1. Spatial Summation:

    • Definition: Occurs when graded potentials are initiated simultaneously from multiple synapse locations that are close together.

    • Implication: Can reach threshold to generate an action potential if the summed effects are strong enough.

  2. Temporal Summation:

    • Definition: Occurs when graded potentials occur in close succession over time.

    • Implication: If the graded potentials are frequent enough, they may sum up to trigger an action potential.

Action Potentials

  • Characteristics:

    • Action potentials are all-or-nothing events; they either occur at full amplitude or not at all.

    • Only occur at the axon hillock after reaching the threshold.

  • Comparison of Graded and Action Potentials:

    • Graded potentials can be summated, while action potentials cannot.

    • Graded potentials occur due to the opening of ligand-gated or mechanically-gated channels, while action potentials occur due to voltage-gated channels for Na+ and K+.

Communication in the Nervous System

  • Electrical Synapses:

    • Formed by gap junctions, they are rapid and allow direct electrical communication between neurons.

The Autonomic Nervous System (ANS)

  • Components:

    • Divided into sympathetic and parasympathetic divisions.

    • Sympathetic: Activates the fight-or-flight response.

    • Parasympathetic: Rest-and-digest functions.

Neurotransmitters in ANS

  • Acetylcholine:

    • Acts at muscarinic receptors.

  • Norepinephrine and Epinephrine:

    • Act at adrenergic receptors (α and β receptors - β1 responds to both NE and E, β2 responds to E).

Neurotoxins and Drugs

  • Neurotoxins can disrupt synaptic transmission by affecting the release of neurotransmitters or the function of postsynaptic receptors.

  • Examples include botulinum toxin, which inhibits vesicle docking, and reuptake inhibitors used in antidepressants.

Summary of Key Concepts

  • The role of neuron cell bodies includes integration of signals from dendrites and influencing the firing of action potentials.

  • Connection between graded potentials and action potential generation.

  • Synaptic transmission process and mechanisms of neurotransmitter termination.

  • Different types of synaptic responses and their implications for neural communication.

  • Integration and summation of synaptic signals are crucial for neuron firing.

  • Relationship and antagonistic roles of sympathetic and parasympathetic nervous systems.