Electrical Signals in the Nervous System

Electrical Signals in the Nervous System

• Cells produce electrical signals called action potentials, which are responsible for transmitting information from one part of the body to another.
• The membrane potential is a result of ionic concentration differences across the plasma membrane and the permeability of the membrane.
• The Na+/K+ pump and membrane permeability are responsible for the ion concentrations across the membrane.
• The inside of a cell has a high concentration of K+ and proteins, while the outside of a cell has a high concentration of Na+ and Cl-.
• The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron when it is not transmitting signals.
• The resting membrane potential exists in an unstimulated (resting) cell due to the permeability characteristics of the membrane and differences in ion concentrations on each side of the membrane.
• The membrane is more permeable to K+ due to leak channels, resulting in a higher concentration of K+ outside the cell, creating a negative charge inside the cell.
• The voltage-gated Na+ channels activate at threshold potential, allowing Na+ to move into the cell, causing depolarization.
• The voltage-gated Na+ channels close and K+ channels open during repolarization, causing K+ to move out of the cell, making the inside of the cell more negative, restoring the resting membrane potential.
• The refractory period refers to the time when a neuron is less sensitive to further stimulation after an action potential.
• Absolute Refractory Period: Neuron is completely insensitive to another stimulus.
• Relative Refractory Period: A stronger-than-threshold stimulus can initiate another action potential.
• The number of action potentials produced per unit of time is called action potential frequency.
• A subthreshold stimulus does not cause an action potential, while a threshold stimulus does.
• Maximal Stimulus: Produces the highest frequency of action potentials.
• Submaximal Stimulus: Stimuli between the threshold and maximal stimulus.
• Supramaximal Stimulus: Stimuli stronger than the maximal stimulus, but they cannot produce a greater frequency of action potentials.
• Action potentials propagate down an axon in a one-way direction due to the refractory period.
• Continuous Conduction: Action potential in one site triggers the next site.
• Saltatory conduction: Action potentials jump from node to node in myelinated axons.
• Myelinated axons conduct action potentials faster than unmyelinated axons.
• Myelinated axons have thicker myelin sheaths, which increases the conduction speed.
• Larger-diameter axons conduct action potentials faster than smaller-diameter axons because they have a greater surface area and more voltage-gated Na+ channels.
• Type A fibers: Large-diameter, myelinated, conduct at 15-120 m/s.
• Type B fibers: Medium-diameter, lightly myelinated, conduct at 3-15 m/s.
• Type C fibers: Small-diameter, unmyelinated, conduct at 2 m/s or less.

The Synapse

• A synapse is a junction between two cells where action potentials in one cell cause action potentials in another.
• Presynaptic Cell: Transmits signals toward the synapse.
• Postsynaptic Cell: Target cell receiving the signal.
• Electrical Synapse: Cells connected by gap junctions allowing graded current to flow between adjacent cells.
• Chemical Synapse: Uses neurotransmitters to transmit information between neurons.
• Chemical synapses have three main components: a presynaptic terminal, a synaptic cleft, and a postsynaptic membrane.
• Neurotransmitters are released from synaptic vesicles in the presynaptic terminal by action potentials.
• Neurotransmitters bind to receptors on the postsynaptic membrane, leading to the opening of ligand-gated ion channels.

Receptor Molecules in Synapses

• Neurotransmitters bind to specific receptors, only influencing cells with receptors for that neurotransmitter.
• Certain neurotransmitters can be excitatory in some cells and inhibitory in others.
• Neurotransmitters can also attach to presynaptic terminals and modulate their own release.

Neurotransmitter Removal

• Neurotransmitters are removed from the synapse by different mechanisms:
  • Acetylcholine (ACh): Acetylcholinesterase breaks down ACh into acetic acid and choline, which is recycled in the presynaptic neuron.
  • Norepinephrine: Recycled in the presynaptic neuron or diffuses away from the synapse.
  • Monoamine oxidase (MAO): Enzyme that breaks down norepinephrine.
  • Absorbed into circulation: Broken down in the liver.

Neurotransmitters and Neuromodulators

• Chemical messengers secreted by neurons, with some neurons secreting more than one type.
• Major chemical classes of neurotransmitters:
  • Acetylcholine (ACh): Best understood; composed of acetic acid and choline.
  • Biogenic amines: Catecholamines and indoleamines.
  • Amino acids: Examples include glycine and glutamate.
  • Purines: Nitrogen-containing compounds derived from nucleic acids; examples include adenosine and ATP.
  • Neuropeptides: Short chains of amino acids.
  • Gases and Lipids: Examples include nitric oxide (gas), carbon monoxide (gas), and endocannabanoids (lipid-derived).

Responses at the Postsynaptic Cells: Excitatory and Inhibitory Postsynaptic Potentials

• Excitatory postsynaptic potential (EPSP): Depolarization occurs, which is a stimulatory response that might reach threshold, producing an action potential and a cellular response.
• Inhibitory postsynaptic potential (IPSP): Hyperpolarization occurs, which is an inhibitory response, decreasing the likelihood of an action potential by moving the membrane potential further away from the threshold.

Neuromodulation

• Neuromodulators: Influence the likelihood of an action potential being produced in the postsynaptic cell.
• Axoaxonic synapses: The axon of one neuron synapses with the presynaptic terminal (axon) of another neuron. This is common in the central nervous system.
• Presynaptic inhibition: Reduces the amount of neurotransmitter released from the presynaptic terminal.
• Presynaptic facilitation: Increases the amount of neurotransmitter released from the presynaptic terminal.

Spatial and Temporal Summation

• A single postsynaptic potential is often not enough to reach threshold.
• Spatial summation: Two or more presynaptic neurons stimulate a postsynaptic neuron simultaneously, their graded potentials summate at the trigger zone to produce a graded potential that exceeds threshold, generating an action potential.
• Temporal summation: Two or more action potentials arrive at the presynaptic membrane in close succession, their graded potentials summate at the trigger zone to reach threshold and produce an action potential.

Explore the fundamentals of electrical signals within the nervous system in this quiz. Learn about action potentials, membrane potentials, and the role of ion concentrations and permeability. Test your understanding of how these electrical signals contribute to neural communication.