Describe the changes in membrane potential and ion movement during the generation of an action potential in a neuron.

At rest, a neuron maintains a resting membrane potential of about -70 mV, with a higher concentration of sodium ions ($\text{Na}^{+}$) outside the cell and potassium ions ($\text{K}^{+}$) inside the cell. The cell membrane is more permeable to $\text{K}^{+}$ than $\text{Na}^{+}$, and the sodium-potassium pump actively transports 3 $\text{Na}^{+}$ out for every 2 $\text{K}^{+}$ in, contributing to this negative charge inside.

When a stimulus reaches the neuron and causes the membrane potential to reach the threshold potential (around -55 mV), voltage-gated $\text{Na}^{+}$ channels open rapidly. This allows a large influx of positively charged $\text{Na}^{+}$ ions into the cell, causing the interior of the neuron to become less negative and then positive, a process called depolarization. The membrane potential quickly rises to about +30 mV.

As the membrane potential reaches its peak positive value, the voltage-gated $\text{Na}^{+}$ channels inactivate, stopping the influx of $\text{Na}^{+}$. Simultaneously, voltage-gated $\text{K}^{+}$ channels open, allowing $\text{K}^{+}$ ions to flow out of the cell. This efflux of positive charge causes the membrane potential to return to a negative state, a process known as repolarization.

The voltage-gated $\text{K}^{+}$ channels are slow to close, leading to an overshoot where too many $\text{K}^{+}$ ions leave the cell. This makes the membrane potential temporarily more negative than the resting potential, a phase called hyperpolarization. Eventually, the $\text{K}^{+}$ channels close, and the sodium-potassium pump works to restore the ion concentrations and the resting membrane potential.

Information processing begins at the sensor, which is a specialized receptor cell or organ (like the eye for vision or skin for touch) that detects specific stimuli from the environment. These sensors convert the physical or chemical stimuli into electrical signals, known as sensory input. For example, light hitting the retina or pressure on the skin generates these initial signals.

Once generated, the electrical signals, or action potentials, are transmitted along the sensory neurons towards the central nervous system (CNS). This transmission involves the rapid movement of ions across the neuron's membrane, as described in Question 7, propagating the signal from the dendrites, through the cell body, and down the axon.

When the action potential reaches the end of a neuron's axon, it arrives at a synapse. Here, the electrical signal is converted into a chemical signal. Neurotransmitters are released from the presynaptic neuron into the synaptic cleft, binding to receptors on the postsynaptic neuron. This binding either excites or inhibits the postsynaptic neuron, allowing the signal to be relayed across the gap to the next neuron.

The signals from multiple sensory neurons converge in the central nervous system (brain and spinal cord), specifically in areas like the thalamus and cerebral cortex. This is the integration phase, where the brain processes, interprets, and makes sense of the incoming sensory information. It involves complex neural networks that analyze the signals, compare them with stored memories, and formulate an appropriate response.

After integration, the brain generates a motor output signal, which is an electrical command sent from the CNS. This command travels along motor neurons from the brain or spinal cord towards the peripheral nervous system. The motor output dictates the appropriate action or response based on the integrated information.

The motor output signals ultimately reach effectors, which are muscles or glands. For example, if the integrated information indicates a need to move, the motor neurons will stimulate specific muscles to contract, resulting in a physical action (e.g., moving a limb). If it's a glandular response, the gland will secrete hormones or other substances. This completes the information processing cycle, leading to an immediate behavior or contributing to memory formation.