3.1 Neuronal structure and function

The nervous system's building blocks are neurons, specialized cells that transmit signals. These cells have a soma, dendrites for receiving signals, and an axon for sending them. Understanding their structure is key to grasping how our nervous system functions.

Neurons communicate through action potentials, electrical signals that travel along axons. These signals are generated by changes in ion concentrations across the cell membrane. Myelin sheaths and specialized ion channels help speed up signal transmission, allowing for efficient neural communication.

Neuron Structure

Components of a Neuron

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• Neurons are the basic functional units of the nervous system that transmit electrical and chemical signals
• Consist of a cell body (soma) which contains the nucleus and other organelles necessary for cellular function
• Have specialized projections called dendrites that receive signals from other neurons and transmit them towards the soma
• Possess a single long projection called an axon that transmits signals away from the soma to other neurons or effector cells (muscles, glands)

Axon Structure and Function

• Axons are long, thin, cable-like projections that extend from the soma and transmit electrical signals to other neurons or effector cells
• Axons are surrounded by a lipid-rich insulating layer called the myelin sheath which is produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system
• The myelin sheath is interrupted at regular intervals by gaps called nodes of Ranvier which allow for faster signal transmission via saltatory conduction (jumping from node to node)
• Axons terminate at synapses where neurotransmitters are released to communicate with other neurons or effector cells (neuromuscular junctions)

Neuronal Signaling

Action Potential Generation and Propagation

• Neurons transmit signals through changes in their membrane potential called action potentials
• At rest, neurons maintain a negative membrane potential (around -70 mV) due to the unequal distribution of ions (primarily K+ and Na+) across the membrane
• When a neuron receives a sufficiently strong depolarizing stimulus, voltage-gated Na+ channels open allowing Na+ to flow into the cell, further depolarizing the membrane
• If the membrane potential reaches a threshold value (around -55 mV), an action potential is generated and propagates down the axon
• After the peak of the action potential, voltage-gated K+ channels open allowing K+ to flow out of the cell, repolarizing the membrane back to its resting potential

Ion Channels and Saltatory Conduction

• Ion channels are proteins embedded in the neuronal membrane that allow specific ions to flow in or out of the cell
• Voltage-gated ion channels (Na+ and K+) are essential for the generation and propagation of action potentials
• Ligand-gated ion channels (such as those activated by neurotransmitters) are important for synaptic transmission
• In myelinated axons, action potentials jump from one node of Ranvier to the next in a process called saltatory conduction
• Saltatory conduction allows for faster signal transmission compared to continuous conduction in unmyelinated axons because the action potential is regenerated only at the nodes of Ranvier (less energy and time required)