Impulse Transmission II

Overview

This study guide covers the mechanisms of impulse transmission in neurons, focusing on postsynaptic potentials, the distinction between graded and action potentials, factors affecting conduction velocity, and the roles of neurotransmitters and their receptors. Understanding these concepts is essential for comprehending how the nervous system processes and transmits information.

Postsynaptic Potentials

Definition and Types

Postsynaptic potentials are changes in membrane potential in the postsynaptic neuron following neurotransmitter binding. These can be either excitatory or inhibitory, depending on the type of ion channels activated.

• Excitatory Postsynaptic Potential (EPSP): A depolarizing event that brings the membrane potential closer to the threshold for firing an action potential.

  • Caused by activation of specific ion channels, typically allowing more Na+ influx than K+ efflux.

  • Results in depolarization (membrane potential becomes less negative).

• Inhibitory Postsynaptic Potential (IPSP): A hyperpolarizing event that moves the membrane potential further from the threshold.

  • Caused by activation of channels that allow Cl- influx or K+ efflux.

  • Results in hyperpolarization (membrane potential becomes more negative).

Example: Glutamate typically produces EPSPs, while GABA produces IPSPs in the central nervous system.

Synaptic Integration: Summation

Summation Mechanisms

Neurons integrate multiple synaptic inputs through summation, which determines whether an action potential will be generated.

• Single EPSP: Cannot induce an action potential (AP) alone.

• Summation: Multiple EPSPs can combine to reach threshold and trigger an AP. IPSPs can also summate, counteracting EPSPs.

• Excitatory and Inhibitory Inputs: Most neurons receive both types from thousands of other neurons.

• Action Potential Initiation: Occurs only if the sum of EPSPs brings the membrane potential to threshold.

Types of Summation:

• Temporal Summation: Multiple signals from a single synapse in rapid succession.

• Spatial Summation: Simultaneous signals from multiple synapses at different locations.

• Spatial Summation of Opposing Stimuli: EPSPs and IPSPs can cancel each other out if they occur at the same time and location.

Example: If two EPSPs occur close together in time or space, their effects add up, increasing the likelihood of reaching threshold.

Graded Potentials vs. Action Potentials

Comparison Table

The following table summarizes the key differences between graded potentials and action potentials:

Additional info: Graded potentials are typically found in dendrites and cell bodies, while action potentials occur along axons.

Conduction Velocity

Factors Affecting Speed of Impulse Transmission

The speed at which action potentials travel along axons (conduction velocity) is influenced by several factors:

• Axon Diameter: Larger diameter axons conduct impulses faster due to lower resistance to ion flow.

• Myelination: Myelin sheaths insulate axons, preventing charge leakage and allowing action potentials to "jump" between nodes of Ranvier (saltatory conduction), greatly increasing speed.

• Example: Myelinated axons can conduct impulses up to 30 times faster than unmyelinated axons.

Clinical Relevance: In diseases like Multiple Sclerosis (MS), the immune system attacks myelin in the CNS, leading to slowed or blocked nerve conduction, resulting in symptoms such as muscle weakness, vision and speech disturbances, and loss of coordination.

Neurotransmitters and Receptors

Types of Neurotransmitter Receptors

Neurotransmitter receptors are classified based on their structure and mechanism of action:

• Ionotropic Receptors: Ligand-gated ion channels that open directly in response to neurotransmitter binding, allowing specific ions to flow across the membrane.

  • Effects are immediate and brief.

  • Example: Nicotinic acetylcholine receptors (Na+ influx, excitatory).

• Metabotropic Receptors: G-protein-coupled receptors that do not contain ion channels but activate second messenger systems (e.g., cyclic AMP, cyclic GMP, Ca2+).

  • Can open or close ion channels indirectly, activate kinases, or influence gene expression.

  • Effects are slower, more complex, and longer-lasting.

  • Example: Muscarinic acetylcholine receptors (can be excitatory or inhibitory).

Agonists are drugs that mimic neurotransmitter action, while antagonists block receptor activity.

Major Neurotransmitters and Their Receptors

• Acetylcholine (ACh):

  • Nicotinic Receptors: Ionotropic, excitatory, found at neuromuscular junctions and in the autonomic nervous system.

  • Muscarinic Receptors: Metabotropic, can be excitatory or inhibitory, found in the heart, smooth muscle, glands, and CNS.

• Catecholamines: Includes epinephrine, norepinephrine, and dopamine.

  • Primarily act through metabotropic (G-protein-coupled) receptors.

  • Involved in functions such as the fight-or-flight response, pleasure, and motor control.

  • Beta-blockers are antagonists that reduce heart rate and prevent arrhythmias.

• Amino Acid Neurotransmitters:

  • Glutamate: Main excitatory neurotransmitter in the CNS; most receptors are ionotropic (depolarizing), but some are metabotropic (hyperpolarizing).

  • GABA (gamma-aminobutyric acid): Main inhibitory neurotransmitter; receptors can be ionotropic (Cl- channels) or metabotropic.

Additional info: The balance between excitatory and inhibitory neurotransmission is crucial for normal nervous system function and is a target for many pharmacological agents.