Action potentials

Neurons consist of several parts, including dendrites and synapses.
Dendrites are primarily responsible for signal reception, significantly contributing to the receptive part of the neuron along with the cell body.
Synapses primarily facilitate communication between neurons.

Neurotransmitters play a crucial role in signal transmission between neurons.
They bind to specific receptors located on the postsynaptic neuron's membrane, similar to the action in muscle cells.
These receptors function as chemically gated ion channels, remaining closed until stimulated.

When neurotransmitters bind, ion channels open, allowing charged particles (ions) to move across the membrane based on concentration gradients.
- Types of ions involved:
- Sodium (Na⁺)
- Potassium (K⁺)
- Chloride (Cl⁻)
This ion movement alters the membrane's electrical potential, changing from the resting potential of approximately -70mV.

Graded potentials are changes in membrane potential that vary in magnitude.
- They can occur in small increments (e.g., from -70 to -68 mV).
Action potentials, by contrast, are rapid, uniform changes in electrical potential that reach approximately +30 mV.
Graded Potentials
- Variable in change magnitude (e.g., 2-8 mV).
- Occur in the dendrites and cell body, dissipating quickly due to their localized nature.

Initiated by releasing neurotransmitters that bind to receptors on postsynaptic neurons causing:
- Opening of chemically gated cation channels, primarily allowing Na⁺ to enter the cell, and K⁺ to exit.
- Net influx of positive charges (more Na⁺ entering than K⁺ leaving), making the inside of the cell less negative (e.g., from -70 to -68 mV).
- Brings the neuron closer to the action potential threshold, thus “exciting” the neuron.

Triggered when neurotransmitters bind to different receptors, leading to:
- Opening of chemically gated channels that allow K⁺ to exit or Cl⁻ to enter, resulting in a more negative membrane potential (e.g., from -70 to -72 mV).
- Moves the cell farther from the action potential threshold, thus inhibiting neuronal firing.

Neurons typically receive inputs from multiple presynaptic neurons, leading to a cumulative effect:
- Integration of multiple EPSPs and IPSPs occur at the axon hillock, where the decision is made whether to fire an action potential.

If the summation of inputs results in reaching the threshold of approximately -55 mV, an action potential is generated:
- This follows an All-or-Nothing Law:
- Action potentials occur fully or not at all; sub-threshold stimuli do not produce action potential.
Action Potential Characteristics:
- Rapid depolarization (due to influx of Na⁺) from threshold to approximately +30 mV followed by repolarization (outflow of K⁺).
- Sequential activation of voltage-gated ion channels along the axon results in a propagating electrical signal.

The axon is responsible for transmitting electrical impulses away from the cell body toward other neurons:
- Action potentials propagate by sequentially opening voltage-gated sodium and potassium channels, resulting in a wave of depolarization along the axon.
- The axon membrane can regenerate action potentials through the sequential opening/closing of ion channels across its length.

The refractory period is divided into two phases:
- Absolute Refractory Period: No action potential can occur regardless of stimulus strength.
- Relative Refractory Period: Action potential can occur only if the stimulus is strong enough to surpass the threshold, beyond what is usually needed.

Overall, the neuron functions as a network, with various postsynaptic potentials influencing its activity, culminating in the generation and propagation of action potentials, which are essential for neuronal communication and signaling characteristics throughout the nervous system.