Lecture_15__Impulse_II

Lecture Overview

• Lecture #15: Impulse Transmission II

• Topics: Postsynaptic Potentials, Graded Potentials vs. Action Potentials, Conduction Velocity, Neurotransmitters and Receptors

Postsynaptic Potentials

• Neurotransmitter Activation of Ion Channels

  • Can activate different ion channels in the postsynaptic membrane.

• Excitatory Postsynaptic Potential (EPSP)

  • Caused by:

    • Activation of Na+ channels (depolarization)

    • Nonspecific ion channel activation (more Na+ than K+, depolarization)

    • Ca2+ channel activation (depolarization)

• Inhibitory Postsynaptic Potential (IPSP)

  • Caused by:

    • K+ channel activation (hyperpolarizing)

    • Cl- channel activation (hyperpolarizing)

• Membrane Potential Levels

  • +30 mV (peak), -55 mV (threshold), -70 mV (resting)

Synaptic Integration: Summation

• EPSP and IPSP Effects

  • A single EPSP cannot induce an Action Potential (AP).

  • EPSPs can summate, influencing the postsynaptic neuron.

  • IPSPs can also summate; most neurons receive both inputs.

• Action Potentials

  • Started only if EPSPs dominate, bringing membrane potential to threshold.

Types of Graded Potential Summation

• Temporal Summation

  • Multiple EPSPs occur in rapid succession at the same synapse.

• Spatial Summation

  • Multiple EPSPs from different synapses add together.

• Cancellation of Potentials

  • Changes in membrane potential can cancel each other out (e.g., overlapping EPSPs and IPSPs).

Graded Potentials vs. Action Potentials

• Graded Potentials

  • Activated by ligand-gated channels.

  • Varies with initiating event, decreases with distance.

  • Duration varies, both depolarizing and hyperpolarizing effects.

  • Can be summed (no threshold or refractory period).

• Action Potentials

  • Activated by voltage-gated channels.

  • All-or-none response; amplitude remains constant along axon.

  • Only depolarizing effects, with a defined threshold and refractory period.

Conduction Velocity

• Factors Influencing Velocity

  • Axon Diameter: Larger diameter = faster conduction (less resistance).

    • Example: Squid giant axon.

  • Myelination: Myelin insulates and prevents charge leakage.

    • Action potentials in myelinated neurons propagate about 30 times faster than in unmyelinated neurons.

    • Saltatory Conduction: Charge jumps across myelinated regions via Nodes of Ranvier.

Importance of Myelin Sheaths: Multiple Sclerosis (MS)

• Overview of MS:

  • Autoimmune disease affecting primarily young adults.

  • Immune system attacks myelin in the CNS, leading to inflammation and scarring (scleroses).

  • Impulse conduction slows and can cease, leading to symptoms such as visual disturbances, weakness, and loss of muscular control.

Types of Neurotransmitter Receptors

• Ionotropic Receptors:

  • Contain an ion channel; neurotransmitter binding opens the pore, resulting in either depolarization (Na+ or Ca2+ influx) or hyperpolarization (Cl- influx or K+ efflux).

  • Responses are immediate and brief.

• Metabotropic Receptors:

  • Transmitter binds to G-protein-coupled receptors; indirect effects through second messengers (e.g., cyclic AMP).

  • Responses are complex, slow, and often prolonged.

• Drug Interactions:

  • Agonists: Mimic neurotransmitters; Antagonists: Inhibit responses.

Acetylcholine Receptors (Cholinergic)

• Nicotinic Receptors:

  • Ionotropic, excitatory; involved in somatic nervous system and some CNS pathways.

• Muscarinic Receptors:

  • Metabotropic, excitatory or inhibitory; parasympathetic effects (e.g., heart, glands).

Catecholamines

• Compounds include epinephrine, norepinephrine, and dopamine.

• All are metabotropic receptors, acting as G-protein coupled receptors, influencing sympathetic and parasympathetic responses.

  • Examples of Actions:

    • Heart rate modulation.

    • Respiratory and digestive system impacts.

Motor Neurons and Neurotransmitters in Systems

• Somatic Nervous System: Single neuron from CNS to effector organs, using ACh.

• Autonomic Nervous System: Two-neuron chain (preganglionic and postganglionic), using ACh (parasympathetic) or NE (sympathetic).

• Effect: Stimulatory or inhibitory, depending on neurotransmitter and receptor types.

Amino Acid Neurotransmitters

• Excitatory Neurotransmitter: Glutamate

  • Majority ionotropic (depolarization), some metabotropic (hyperpolarization).

• Inhibitory Neurotransmitters: GABA (gamma-Aminobutyric acid) and Glycine.

  • GABA receptors can be ionotropic (Cl-) or metabotropic (hyperpolarizing).

  • Mixing alcohol or barbiturates with benzodiazepines should be avoided, as they can enhance GABA effects and potentially cause severe outcomes.