The Body's Command Center
The nervous system is the body's primary communication network. It is composed of two main parts:
Central Nervous System (CNS): Consists of the brain and the spinal cord. It acts as the integration and command center.
Peripheral Nervous System (PNS): Consists of all the nerves that branch out from the CNS to the rest of the body. It carries messages to and from the CNS.
The Neuron: The Basic Unit
The fundamental unit of the nervous system is the neuron, a specialized cell designed to transmit information.
Dendrites: Branch-like extensions that receive signals from other neurons.
Cell Body (Soma): Contains the nucleus and other organelles; integrates incoming signals.
Axon: A long, slender projection that transmits signals away from the cell body to other neurons, muscles, or glands.
Myelin Sheath: A fatty insulating layer that covers the axon of many neurons, produced by Schwann cells (in the PNS) or oligodendrocytes (in the CNS). It dramatically increases the speed of signal transmission.
The Action Potential: The Electrical Signal
An action potential is a rapid, temporary change in the electrical potential across the axon membrane. It is an 'all-or-none' event.
1.Resting State: The neuron is 'polarized'. The inside of the axon is negatively charged relative to the outside, due to the action of the sodium-potassium pump.
2.Depolarization: A stimulus causes sodium channels to open. Positively charged sodium ions (Na⁺) rush into the cell, making the inside of the membrane positive.
3.Repolarization: The sodium channels close, and potassium channels open. Positively charged potassium ions (K⁺) rush out of the cell, restoring the negative charge inside.
4.Propagation: This wave of depolarization and repolarization travels down the axon. In myelinated axons, the action potential 'jumps' between gaps in the myelin called Nodes of Ranvier, a process called saltatory conduction, which is much faster.
The Synapse: Chemical Communication
A synapse is the junction where a signal is transmitted from one neuron to another.
When an action potential reaches the end of an axon (the axon terminal), it triggers the release of chemical messengers called neurotransmitters into the synaptic cleft (the space between neurons).
These neurotransmitters diffuse across the cleft and bind to receptors on the dendrites of the post-synaptic neuron.
This binding can either excite the next neuron (making an action potential more likely) or inhibit it (making it less likely).
Common neurotransmitters include acetylcholine (muscle contraction), dopamine (reward and motivation), and serotonin (mood regulation).