Editing Anatomy and Physiology (section)

== Nervous System ==

=== Organization ===

* '''Central nervous system (CNS)''': brain and spinal cord.
* '''Peripheral nervous system (PNS)''': cranial and spinal nerves, divided into the somatic (voluntary) and autonomic divisions.
* '''Autonomic nervous system''': the '''sympathetic''' division produces the fight-or-flight response (increased heart rate and contractility, bronchodilation, pupil dilation, decreased gastrointestinal motility, glycogenolysis), and the '''parasympathetic''' division produces rest-and-digest effects (decreased heart rate, bronchoconstriction, pupil constriction, increased gastrointestinal motility). The vagus nerve carries most parasympathetic output.

=== The Neuron and Impulse Transmission ===

A neuron has dendrites (receive), a cell body, and an axon (transmit). Myelin, produced by Schwann cells in the periphery and oligodendrocytes in the CNS, speeds conduction by saltatory transmission. Demyelination, as in multiple sclerosis and Guillain-Barre syndrome, slows or blocks conduction.

Impulse transmission depends on sodium influx (depolarization) and potassium efflux (repolarization), with the sodium-potassium pump restoring the gradient. Neurotransmitters cross the synapse: acetylcholine, dopamine, serotonin, norepinephrine, gamma-aminobutyric acid, and glutamate.

=== Brain Regions ===

* '''Frontal lobe''': voluntary movement, judgment, personality, expressive speech (Broca area).
* '''Parietal lobe''': sensory interpretation, spatial orientation.
* '''Temporal lobe''': hearing, memory, receptive speech (Wernicke area).
* '''Occipital lobe''': vision.
* '''Cerebellum''': balance, coordination, fine motor control. Cerebellar injury produces ataxia and a positive Romberg sign.
* '''Brainstem''' (midbrain, pons, medulla): the medulla contains the cardiac, vasomotor, and respiratory centers. Brainstem compression is lethal.
* '''Hypothalamus''': temperature regulation, thirst, hunger, and control of the pituitary.
* '''Thalamus''': sensory relay station.

=== Cranial Nerves ===

There are 12 pairs. Those most often assessed in nursing practice include II (optic, vision), III (oculomotor, pupil constriction and most eye movement), V (trigeminal, facial sensation and chewing), VII (facial, facial expression and taste on the anterior tongue), VIII (vestibulocochlear, hearing and balance), IX and X (glossopharyngeal and vagus, gag and swallow), and XII (hypoglossal, tongue movement).

'''Nursing application:''' Cranial nerves IX, X, and XII govern swallowing. Assessing gag, cough, and voice quality before giving anything by mouth after a stroke is an aspiration-prevention measure grounded directly in this anatomy.

=== Intracranial Pressure ===

The '''Monro-Kellie doctrine''' states that the skull is a fixed container holding brain tissue, blood, and cerebrospinal fluid. An increase in any one component must be offset by a decrease in another or intracranial pressure rises. Normal intracranial pressure in an adult is approximately 5 to 15 mmHg.

Early signs of increasing intracranial pressure: a change in level of consciousness (the earliest and most sensitive sign), restlessness, headache, and pupillary changes. Late signs: '''Cushing triad''' of widening pulse pressure with rising systolic pressure, bradycardia, and irregular respirations, along with posturing and fixed dilated pupils.

'''Nursing application:''' Elevate the head of the bed about 30 degrees with the head midline to promote venous drainage, avoid hip flexion, avoid neck flexion or rotation, cluster care to limit stimulation, and prevent Valsalva maneuvers such as straining, coughing, and vomiting. Suction only briefly and as needed. Hypotonic fluids are avoided because they worsen cerebral edema.

=== Spinal Cord ===

The spinal cord transmits motor commands down and sensory information up, and mediates reflexes. Injury above T6 risks '''autonomic dysreflexia''', an exaggerated sympathetic response to a stimulus below the level of injury (most often a distended bladder or impacted bowel) causing severe hypertension, pounding headache, bradycardia, flushing above the injury, and pallor below it. The nursing response is to sit the patient upright first, then find and remove the stimulus.

Injury at or above C3 to C5 threatens diaphragm innervation by the phrenic nerve, which is why the memory aid "C3, 4, 5 keeps the diaphragm alive" matters clinically.