Neurobiology III

Action potential refractory periods

  • Absolute refractory period

    • Unable to generate another action potential, regardless of the strength of the stimulus.

    • This is due to the inactivation of voltage-gated sodium channels.

    • starts with the initiation of the AP until the end of the Na+ channel inactivation

  • Relative refractory period

    • immediately following the absolute refractory period during which a stronger-than-normal stimulus is required to generate an action potential.

    • the membrane potential is hyperpolarized and it takes more depolarization to reach the threshold for firing another action potential.

    • the voltage-gated potassium channels are still open, causing hyperpolarization of the neuron.

    • Na+ channels are ready to depolarize but K+ are still open

Voltage gated Sodium channel during absolute refractory period

Propagation of an action potential

  • When curent flows into the neuron, it flows passively in both directions.

  • When Na+ flows into the neuron, positive current flows passively in both directions but APs are conducted in one direction

  • inactivation gate of voltage gated Na+ channels ensure unidirectional APs

  • Propagation of an action potential along an axon

Stimulus strength is encoded by AP frequency

  • Stronger the stimulus, the higher frequency of action potentials

  • Strong sustained depolarization is necessary for high frequency APs

Transmission of signals between neurons

  • Neurons transmit their signals at electrical or chemical synapses

  • Synaptic vesicles release neurotransmitter by exocytosis

  • Voltage gated Ca2+ channels open in response to AP-induced depolarization

    • increased intracellular Ca2+ facilitates neurotransmitter release

Neurotransmitter

  • synthesized in neuron

  • released at pre synapse following depolarization

  • bind and cause effect at post synapse

  • lot of mitochondria in the neuron to make neurotransmitters

Ionotropic and metabatropic receptors

  • Ionotropic

    • directly open ion channels upon binding with a neurotransmitter. They mediate fast synaptic transmission in the nervous system.

    • When a neurotransmitter binds to an ionotropic receptor, it causes a conformational change that allows ions, such as sodium (Na+), potassium (K+), or calcium (Ca2+), to flow through the channel. , resulting in the generation of an electrical signal.

    • Examples of ionotropic receptors include nicotinic acetylcholine receptors and NMDA receptors.

  • Metabatropic receptors

    • activate intracellular signaling pathways through G-proteins. They have a slower response time but can produce longer-lasting effects.

    • Metabotropic receptors play a role in various physiological processes, including neurotransmission, sensory perception, and regulation of mood and behavior.

    • Examples include GABA-B receptors, dopamine receptors, and serotonin receptors.