Chapter 19: Introduction to Nerves & the Nervous System

The fundamental unit is the neuron, comprising a cell body (soma), short receiving extensions (dendrites), and a long transmitting process (axon). Nerves primarily communicate using electrical action potentials, which are sudden, temporary reversals in membrane charge. This process is regulated by the sodium–potassium pump, which maintains a polarized resting state (negative charge inside). When stimulated, depolarization occurs as sodium channels open, allowing positive sodium ions to rush in, reversing the electrical charge. Repolarization immediately restores the resting membrane potential. The speed of transmission is enhanced in myelinated nerves by Schwann cells, which facilitate saltatory or leaping conduction, where the impulse skips between exposed areas called the nodes of Ranvier. Maintenance of these action potentials requires sufficient energy (oxygen and glucose) and the correct balance of electrolytes like sodium and potassium. When the electrical impulse reaches the axon terminal, communication transitions to a chemical process at the nerve synapse, which includes the presynaptic nerve, the synaptic cleft, and the postsynaptic effector cell. Neurotransmitters, such as acetylcholine, catecholamines (norepinephrine, epinephrine, dopamine), GABA, and serotonin, are released into the cleft and interact with specific receptor sites to either stimulate or inhibit the effector cell. To prepare the nerve for subsequent stimulation, neurotransmitters are rapidly inactivated either through reuptake by the presynaptic nerve or breakdown by specific enzymes. Protection of the CNS is provided by bone, meninges, and the blood–brain barrier, which presents challenges for drug penetration. The brain’s protective blood supply is ensured by the circle of Willis. Functionally, the brain is organized into the hindbrain (pons, medulla) controlling vital functions, the midbrain containing the reticular activating system (RAS) for arousal and sleep cycles, and the forebrain, which includes the cerebral cortex (thinking and learning), the hypothalamus (major sensor for water balance, temperature, and endocrine control), and the limbic system, which manages the expression of emotions. Learning begins as a reverberating electrical circuit, or engram (short-term memory), and relies on sufficient oxygen, glucose, and sleep to become a permanent memory. Pharmacological interventions frequently target the nervous system by altering synaptic activity, such as blocking neurotransmitter reuptake or enzyme breakdown, underscoring the necessity of understanding this complex physiology.