Cellular Membrane Transport and Nerve Signal Transmission

Active Transport Across Cell Membranes

Active transport enables certain types of proteins within the cell membrane to move substances. This process consumes energy, typically provided by ATP molecules, allowing transport against a concentration gradient.

The Sodium-Potassium Pump

The sodium-potassium pump is a transmembrane protein that actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. This counter-gradient transport exhibits ATPase activity, meaning it breaks down ATP to power the movement. For every three Na+ ions pumped out, two K+ ions are pumped in, consuming one ATP molecule. This action generates potential differences across the membrane, contributing to the membrane potential. This potential is crucial for regulating substances entering and leaving the cell.

Other examples of active transport include cotransport mechanisms, where the movement of one substance down its gradient powers the movement of another against its gradient (though the initial gradient is established by active transport). The calcium pump and proton pump are also vital examples of active transport systems.

Understanding the Nerve Impulse

Neurons receive signals and transmit nerve impulses, which are a consequence of rapid changes in the neuron's membrane potential. A nerve impulse typically originates in the dendrites, travels through the cell body, and propagates along the axon like an electrical wave. The established membrane potential is fundamental for this impulse transmission.

Phases of a Nerve Impulse

1. Resting Potential

At rest, the neuron maintains a resting potential, primarily due to the action of the Na+-K+ pump. This pump actively transports more potassium ions into the cell and sodium ions out, establishing a negative charge inside relative to the outside. The typical resting potential is approximately -70 millivolts (mV), meaning the inside of the cell is 70mV more negative than the outside.

2. Action Potential (Depolarization)

When a neuron receives a sufficiently strong stimulus, it triggers an action potential. This involves the rapid opening of voltage-gated sodium channels, allowing a massive influx of sodium ions into the cell. This influx causes the inside of the membrane to become very positive, reaching approximately +40 mV. This rapid change from negative to positive is known as depolarization of the membrane.

3. Impulse Propagation and Repolarization

Immediately after depolarization, voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This outflow causes the membrane to return to its negative resting potential, a process called repolarization. Following repolarization, the sodium-potassium pump works to restore the original ion concentrations across the membrane.

The influx of sodium ions during depolarization in one area of the neuron causes the adjacent membrane region to depolarize as well, opening new sodium channels and propagating the impulse. This self-propagating wave ensures the impulse travels along the entire neuron. Nerve impulse conduction speed can reach up to 120 meters per second (m/s) in unmyelinated axons. In myelinated axons, the impulse "jumps" between the Nodes of Ranvier (spaces between myelin sheaths), a process called saltatory conduction, increasing the speed to up to 200 m/s. This is because the action potential only occurs at these unmyelinated gaps.

Synaptic Transmission: Connecting Neurons

A synapse is the specialized junction where one neuron communicates with another neuron, or with an effector cell (like a muscle or gland). This connection involves a small gap called the synaptic cleft, located between the axon terminal of the presynaptic neuron and the dendrite, cell body, or axon of the postsynaptic neuron.

The transmission of a nerve impulse across a synapse is primarily mediated by chemical substances called neurotransmitters. When an action potential reaches the axon terminal of the presynaptic neuron, it triggers the opening of voltage-gated calcium channels, leading to an influx of calcium ions. This calcium influx causes synaptic vesicles, which contain neurotransmitters, to fuse with the presynaptic membrane and release their contents into the synaptic cleft.

These neurotransmitters then diffuse across the cleft and bind to specific receptors on the postsynaptic neuron's membrane. This binding can produce two main effects:

• Depolarization (Excitatory Postsynaptic Potential - EPSP): If the neurotransmitter causes the opening of sodium channels, it leads to depolarization of the postsynaptic membrane. If this depolarization reaches the threshold, it can initiate a new nerve impulse in the postsynaptic neuron. These are typically carried out by excitatory neurotransmitters.
• Hyperpolarization (Inhibitory Postsynaptic Potential - IPSP): Alternatively, the neurotransmitter might cause the membrane to become even more negative (hyperpolarization), making it harder for a new action potential to be generated. This inhibitory effect is often achieved by opening chloride or potassium channels.