Question

How are electrical signals transmitted in the human brain and nervous system?

Answer 1

Electrical signals are crucial for communication within the human brain and nervous system. These signals, also known as nerve impulses or action potentials, are responsible for transmitting information between neurons (nerve cells) and coordinating various physiological processes. Let's break down how electrical signals are transmitted in the human brain and nervous system:

Neurons: Neurons are specialized cells that form the basic building blocks of the nervous system. They consist of three main parts: the cell body (soma), dendrites, and an axon.

Resting Membrane Potential: When a neuron is at rest, there is a difference in electrical charge between the inside and outside of the cell. This difference is known as the resting membrane potential. The inside of the neuron is negatively charged compared to the outside, due in part to the presence of ions such as sodium (Na+), potassium (K+), chloride (Cl-), and negatively charged proteins.

Action Potential Generation:
a. Threshold: Neurons receive input from other neurons through their dendrites. If the combined input reaches a certain threshold level, typically around -55 to -50 millivolts (mV), it triggers an action potential.
b. Depolarization: When the threshold is reached, voltage-gated sodium channels in the neuron's membrane open. This allows sodium ions to rush into the cell, causing a rapid depolarization of the membrane. The membrane potential becomes positive.
c. Repolarization: After depolarization, the voltage-gated sodium channels close, and voltage-gated potassium channels open. Potassium ions move out of the cell, repolarizing the membrane and restoring the negative charge.
d. Hyperpolarization: In some cases, the membrane potential briefly becomes more negative than the resting potential. This is known as hyperpolarization and is caused by the continued movement of potassium ions.
e. Sodium-Potassium Pump: After an action potential, the sodium-potassium pump actively transports sodium ions out of the cell and potassium ions back in, restoring the resting membrane potential.

Action Potential Propagation: Once an action potential is generated at the initial segment of the axon (axon hillock), it travels down the axon in a self-propagating manner. This is achieved through a process called saltatory conduction in myelinated axons, where the action potential "jumps" from one node of Ranvier to the next, significantly increasing the speed of signal transmission.

Synaptic Transmission: When an action potential reaches the axon terminals, it triggers the release of neurotransmitters into the synapse—the small gap between the axon terminal of one neuron and the dendrite of another neuron or a target cell (such as a muscle or gland). Neurotransmitters bind to receptors on the receiving neuron's membrane, leading to the generation of postsynaptic potentials.

Postsynaptic Potentials: Neurotransmitters can elicit either excitatory postsynaptic potentials (EPSPs), which make the postsynaptic neuron more likely to generate an action potential, or inhibitory postsynaptic potentials (IPSPs), which make the postsynaptic neuron less likely to generate an action potential. The summation of these potentials at the axon hillock determines whether an action potential will be generated.

In summary, electrical signals in the human brain and nervous system are transmitted through a complex interplay of ion movements across neuronal membranes, the generation and propagation of action potentials, and the release and reception of neurotransmitters at synapses. This intricate process enables the communication and coordination of various functions throughout the body.