Nerve Conduction and Action Potentials

Nerve Conduction and Synapses

• Nerve cells generate electrical signals using ion flow across their membranes.
• Neurons create a resting membrane potential, which is a negative potential.
• Action potentials temporarily remove the negative resting potential, making the transmembrane potential positive.
• The resting membrane potential is not zero.
• Action potentials propagate along axons; these are the fundamental signals for information transmission in the nervous system.
• These signals arise from ion fluxes across nerve cell membranes, which are selectively permeable and have uneven ion distributions.

Action Potential

• Action potentials involve a rapid depolarization (upstroke) followed by repolarization of the membrane potential.
• Opening Na+ channels causes depolarization (sodium influx).
• Opening K+ channels causes repolarization (potassium efflux).
• The action potential has a threshold.
• Failed action potential initiations can occur if the stimulus does not reach the threshold.
• The "all-or-none" response means that once an action potential begins, it will complete its course or not occur at all.
• Depolarization lessens the membrane potential, hyperpolarization increases it.
• Overshoot is when the membrane potential becomes positive inside the cell.

Velocity of Action Potential

• Action potential speed depends on fiber diameter (larger is faster) and myelination (myelinated nerves are faster).
• Myelin allows for "saltatory conduction," where the action potential jumps between Nodes of Ranvier, making it faster than continuous conduction.
• Myelinated neurons are faster than unmyelinated ones.

Synapses

• Synapses are junctions connecting neurons or between a neuron and another cell type, like muscle.
• Most synapses are chemical, using neurotransmitters to relay signals.
• Some synapses are electrical, using direct connections.
• At chemical synapses, neurotransmitters are released at the presynaptic terminal, cross the synaptic gap, and bind to receptors on the postsynaptic neuron.
• Neurotransmitters can be excitatory or inhibitory; their effect depends on the type of receptor.

Different Types of Synapses

• Chemical synapses release neurotransmitters, which either cause depolarization (excitation) or hyperpolarization (inhibition) in the postsynaptic neuron.
• The influx of Ca2+ in the axon terminal triggers neurotransmitter release.
• Voltage-gated Ca2+ channels open when an action potential reaches the axon terminal.
• Neurotransmitters diffuse across the synaptic cleft to bind to receptors on the postsynaptic cell membrane, which can be excitatory (Na+ influx) or inhibitory (Cl- influx).
• Glutamate is a common excitatory neurotransmitter.
• GABA is a common inhibitory neurotransmitter.

Functions of the Synapse

• Summation of EPSPs (excitatory postsynaptic potentials) and IPSPs (inhibitory postsynaptic potentials) determines whether an action potential occurs in the postsynaptic neuron.
• If the sum of EPSPs is large enough to reach the threshold, an action potential occurs.
• The summed inhibitory potentials push the membrane away from the firing threshold.

Multiple Sclerosis

• Multiple sclerosis is an autoimmune disease affecting the myelin sheath.
• It leads to damaged myelin, forming scar tissue, which slows the conduction of action potentials.