Nervous System: Electrical Signaling and Synaptic Transmission

Resting Membrane Potential (RMP)

The resting membrane potential (RMP) is the voltage difference across the cell membrane of a neuron when it is not actively transmitting a signal. This potential is essential for the function of excitable cells such as neurons and muscle cells.

• RMP is generated by the unequal distribution of ions (mainly Na+ and K+) across the cell membrane.

• Na+ is more concentrated outside the cell, while K+ is more concentrated inside.

• Negatively charged anions (phosphates, proteins, nucleic acids) are more abundant inside the cell, making the interior more negative.

• The cell membrane is polarized due to this unequal distribution.

Maintaining Resting Membrane Potential

Leakage of Na+ and K+ through their respective channels can disrupt the RMP. The sodium-potassium pump (Na+/K+ ATPase) is crucial for maintaining this potential.

• The pump actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP.

• This maintains the concentration gradients and the negative charge inside the cell.

• Formula for the pump's action:

Nerve Signaling: Ion Channels

Nerve signaling depends on the opening and closing of ion channels in the plasma membrane. There are two primary types:

• Ligand-gated channels: Open in response to binding of a specific chemical (neurotransmitter).

• Voltage-gated channels: Open in response to changes in membrane potential.

Action Potential: Generation and Phases

An action potential is a rapid change in membrane potential that propagates along the axon. It consists of four main phases:

1. Resting State: All voltage-gated Na+ and K+ channels are closed; only leakage channels are open.

2. Depolarization: Voltage-gated Na+ channels open, Na+ rushes into the cell, making the inside more positive. Positive feedback causes more Na+ channels to open. Threshold is typically -55 to -50 mV.

3. Repolarization: Na+ channels inactivate, voltage-gated K+ channels open, K+ exits the cell, restoring negative membrane potential.

4. Hyperpolarization: Some K+ channels remain open, causing the membrane potential to become more negative than the resting state. Na+ channels reset.

Conducting a Nerve Signal

The action potential propagates along the axon in a domino effect, creating a nerve signal. After an action potential, the membrane enters a refractory period during which it cannot be stimulated again.

• Continuous conduction: Occurs in unmyelinated axons; action potential is generated at every point along the axon.

• Saltatory conduction: Occurs in myelinated axons; action potential jumps from node to node (Nodes of Ranvier), increasing speed and efficiency.

Clinical Homeostatic Imbalance: Multiple Sclerosis (MS)

Multiple sclerosis (MS) is an autoimmune disease that destroys myelin sheaths in the CNS, impairing nerve signal conduction.

• Symptoms include visual disturbances, weakness, loss of muscular control, speech difficulties, and incontinence.

• Treatment involves drugs that modify immune system activity; high vitamin D levels may reduce risk.

Transmitting a Nerve Signal to the Next Cell

Neurons communicate with other cells via synapses. There are two main types:

• Electrical synapses: Less common, joined by gap junctions, rapid communication, most abundant in embryonic tissue.

• Chemical synapses: Most common, involve neurotransmitter release.

Chemical Synapse Transmission

• When an action potential reaches the axon terminal, synaptic vesicles release neurotransmitters into the synaptic cleft.

• The presynaptic neuron releases neurotransmitters, which bind to receptors on the postsynaptic neuron.

• Neurotransmitters are broken down by enzymes or reabsorbed by the presynaptic neuron.

• Examples of neurotransmitters: acetylcholine (ACh), norepinephrine, serotonin, dopamine.

Clinical Homeostatic Imbalance: Parkinson's Disease

Parkinson's disease is characterized by progressive loss of motor control, typically beginning in a person's 50s or 60s. It is caused by degeneration of dopamine-releasing neurons.

• Dopamine acts as an inhibitory neurotransmitter, preventing excessive activity.

• Loss of dopamine neurons leads to involuntary muscle contractions and symptoms such as tremors, rigidity, and shuffling gait.

Summary Table: Ion Distribution and Membrane Potential

Key Formula:

Example: The sodium-potassium pump maintains the RMP by actively transporting ions against their concentration gradients, ensuring the neuron is ready to fire an action potential when stimulated.

Additional info: The refractory period ensures unidirectional propagation of the action potential and prevents overlapping signals.