Anatomy Lecture Pt.2

Overview of Neuronal Impulse Transmission

  • Neuronal impulse transmission occurs via a series of electrical and chemical processes, primarily involving the movement of ions and neurotransmitters across synapses between neurons.

Neuron Structure and Function

  • Presynaptic Neuron: The neuron before the synapse that releases neurotransmitters.

  • Synaptic Knob: The end of the presynaptic neuron containing neurotransmitter-filled vesicles.

  • Postsynaptic Neuron: The neuron receiving signals from the presynaptic neuron.

Depolarization at the Synaptic Knob

  • When depolarization reaches the synaptic knob:

    • Calcium Channels Open: Voltage change opens calcium channels in the presynaptic membrane.

    • Calcium Influx: Calcium ions ( ext{Ca}^{2+}) enter the neuron, triggering a series of events.

    • Calcium's Role: Essential for the exocytosis process of neurotransmitter release.

      • The influx of calcium causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis.

Neurotransmitter Binding and Graded Potential Generation

  • Neurotransmitter Release: Once released, neurotransmitters travel across the synaptic cleft to bind with receptors on the postsynaptic neuron’s dendrites.

  • Graded Potential: The binding of neurotransmitters generates a graded potential in the postsynaptic neuron

    • Definition: A graded potential is a transient change in the membrane potential that varies in amplitude (strength) and can either be excitatory or inhibitory.

    • Sodium Channels Open: Excitatory neurotransmitters open sodium channels, allowing sodium ions ( ext{Na}^+) to flow in, leading to depolarization of the postsynaptic neuron.

Action Potential Generation

  • If the graded potential reaches a threshold ( ext{-55 mV}), it triggers an action potential.

  • Action Potential Definition: A rapid, all-or-nothing depolarization that travels down the axon, maintaining its amplitude due to sequential opening of sodium channels.

    • Role of Axon Hillock: The site where the sum of graded potentials occurs and where the action potential is initiated if the threshold is reached.

Neurotransmitters and Their Effects

  • Types of Neurotransmitters:

    • Excitatory Neurotransmitters: Cause depolarization, increase likelihood of generating action potentials.

    • Example: Acetylcholine (ACh) is released at neuromuscular junctions, opening sodium channels in muscle fibers to initiate contraction by releasing calcium from the sarcoplasmic reticulum.

    • Inhibitory Neurotransmitters: Cause hyperpolarization, decrease likelihood of action potentials.

    • Can act by opening potassium channels ( ext{K}^+) or chloride channels ( ext{Cl}^-).

    • Hyperpolarization Definition: The membrane potential becomes more negative than the resting potential due to efflux of potassium ions or influx