Synapses

Resting Potential

  • The resting potential is the electrical potential across the membrane of a neuron when not transmitting an impulse.

  • Resting potential value: Around -70mV.

  • Mechanism:

    • Sodium-potassium pumps actively transport 3 Na⁺ ions out of the neuron and 2 K⁺ ions into the neuron.

    • The membrane is more permeable to K⁺ ions than Na⁺ ions, causing a negative charge inside relative to the outside.

4. Action Potential

  • The action potential is a brief change in electrical charge across the neuron membrane when a nerve impulse is transmitted.

  • Steps in Action Potential:

    1. Depolarization: If the membrane potential reaches the threshold (-55mV), voltage-gated Na⁺ channels open, and Na⁺ ions flood into the neuron, making the inside positive (+40mV).

    2. Repolarization: Na⁺ channels close and K⁺ channels open, allowing K⁺ ions to leave the cell, restoring the negative charge inside.

    3. Hyperpolarization: K⁺ channels close slowly, causing an overshoot where the inside becomes more negative than the resting potential.

    4. Restoration: The sodium-potassium pump restores the resting potential.

5. Propagation of Action Potentials

  • Continuous Conduction: In unmyelinated neurons, action potentials move along the axon in a wave-like manner.

  • Saltatory Conduction: In myelinated neurons, action potentials "jump" from one Node of Ranvier to the next, speeding up transmission.

6. Synaptic Transmission

  • Synapse: The junction between two neurons or between a neuron and an effector cell.

  • Chemical Synapses:

    • When an action potential reaches the axon terminal, voltage-gated Ca²⁺ channels open.

    • Ca²⁺ ions enter the terminal and trigger the release of neurotransmitters (e.g., acetylcholine) from vesicles into the synaptic cleft.

    • The neurotransmitters bind to receptors on the post-synaptic membrane, causing ion channels to open and generate a new action potential (or inhibit one, depending on the type of synapse).

  • Enzyme Breakdown or Reuptake: Neurotransmitters are either broken down by enzymes (e.g., acetylcholinesterase) or reabsorbed by the presynaptic neuron.

7. Refractory Period

  • Absolute Refractory Period: The neuron cannot fire another action potential during this time, no matter the stimulus strength (due to inactivated Na⁺ channels).

  • Relative Refractory Period: A stronger-than-normal stimulus is required to initiate another action potential.

8. Role of Ions in Nervous Transmission

  • Sodium (Na⁺): Key in depolarization.

  • Potassium (K⁺): Key in repolarization.

  • Calcium (Ca²⁺): Triggers neurotransmitter release at synapses.

9. Factors Affecting Speed of Transmission

  • Axon Diameter: Larger diameter fibers conduct impulses faster due to less resistance.

  • Temperature: Higher temperatures increase the rate of diffusion of ions, speeding up transmission.

  • Myelination: Myelinated fibers transmit impulses faster than unmyelinated ones due to saltatory conduction.

10. Neurotransmitters

  • Excitatory Neurotransmitters: Increase the likelihood of an action potential (e.g., acetylcholine, glutamate).

  • Inhibitory Neurotransmitters: Decrease the likelihood of an action potential (e.g., GABA, glycine).