nervous system neurons

The Nervous System comprises two main divisions: the Central Nervous System (CNS), containing the brain and spinal cord, and the Peripheral Nervous System (PNS), encompassing all nerves outside the CNS. The PNS subdivides into the somatic nervous system (voluntary control of skeletal muscles) and autonomic nervous system (involuntary control of organs, glands). The autonomic further splits into sympathetic (fight-or-flight responses) and parasympathetic (rest-and-digest functions).

Neurons are specialized cells transmitting electrical and chemical signals. Three types exist: sensory neurons (carry information from receptors to CNS), motor neurons (transmit commands from CNS to effectors), and relay neurons/interneurons (connect neurons within CNS). Each neuron contains key structures: dendrites receive signals, the cell body/soma contains the nucleus, the axon conducts electrical impulses, myelin sheath insulates the axon (produced by Schwann cells), nodes of Ranvier are gaps enabling saltatory conduction, and axon terminals release neurotransmitters at synapses.

Action potentials are electrical impulses traveling along axons following the all-or-nothing principle. At rest, neurons maintain a resting potential of approximately -70mV through sodium-potassium pumps (3 Na⁺ out, 2 K⁺ in). When stimulation exceeds threshold (~-55mV), voltage-gated sodium channels open causing depolarization to +40mV, followed by repolarization as potassium channels open, then hyperpolarization before returning to resting state.

Mnemonic: "Sensory Ascends, Motor Descends" (SAMD) - sensory neurons carry information UP to brain, motor neurons carry commands DOWN to muscles.

Understanding the nervous system becomes clearer through everyday analogies. Think of the CNS as a command centre like air traffic control coordinating all operations, while the PNS resembles communication cables connecting the control tower to aircraft (your body parts).

Real-world sympathetic activation: When encountering a potential threat—perhaps walking alone at night and hearing footsteps—your sympathetic nervous system activates instantaneously. Pupils dilate (enhanced vision), heart rate accelerates (increased oxygen delivery), breathing deepens, and digestion halts (energy diverted to muscles). This fight-or-flight response prepared our ancestors to escape predators. Conversely, after eating Sunday lunch, your parasympathetic system dominates: heart rate slows, digestion activates, pupils constrict—the body focuses on conservation and restoration.

Neurons function like electrical circuits: Imagine a line of dominoes (action potential propagation). Once the first domino falls past a certain angle (threshold), all subsequent dominoes inevitably fall—this illustrates the all-or-nothing principle. The myelin sheath acts like insulation on electrical wires, preventing signal loss and dramatically increasing transmission speed through saltatory conduction (signal "jumps" between nodes of Ranvier).

Clinical application: Multiple Sclerosis (MS) demonstrates neuronal function's importance. MS destroys myelin sheaths, causing signals to slow or fail entirely. Patients experience coordination problems, muscle weakness, and sensory issues—direct evidence of myelin's crucial role in rapid neural communication. Understanding this helps psychologists appreciate how biological damage creates psychological and behavioral changes, exemplifying the biological approach's core assumption that mind and body are interconnected.