5.2 Higher Brain Functions, Sleep, the Autonomic Nervous System, Cerebral Blood Flow & the Special Senses

Why this area matters

The second half of the neurophysiology TOS list covers the cerebral cortex and intellectual functions, learning and memory, states of brain activity, the autonomic nervous system, cerebral blood flow, CSF and brain metabolism, and the special senses. These topics come back in Medicine (stroke, dementia), Pharmacology (autonomic drugs), Surgery (head injury, ENT and ophthalmology) and Pediatrics (hearing and vision screening).

Cortical functions and language

The left hemisphere is dominant for language in more than 90% of right-handed people and most left-handed people.

Learning, memory and states of brain activity

• Declarative memory (facts and events) needs the hippocampus and medial temporal lobe to consolidate new memories. Bilateral hippocampal damage causes anterograde amnesia. Procedural memory depends on the basal ganglia and cerebellum.
• Long-term potentiation, triggered through NMDA glutamate receptors, is the cellular basis of learning.
• Wernicke-Korsakoff syndrome results from thiamine deficiency affecting the mammillary bodies and medial thalamus. It causes amnesia with confabulation.
• Sleep stages:

The suprachiasmatic nucleus of the hypothalamus sets the circadian rhythm and drives melatonin release from the pineal gland at night.

The autonomic nervous system

Organophosphate poisoning (insecticides, a common rural poisoning in the Philippines) floods both muscarinic and nicotinic synapses with acetylcholine. Muscarinic signs are salivation, lacrimation, urination, diarrhea, bronchorrhea, bradycardia and miosis; nicotinic signs are fasciculations and weakness. Atropine reverses the muscarinic effects, and pralidoxime reactivates the enzyme if given early.

Cerebral blood flow, CSF and brain metabolism

• The brain receives about 750 mL/min, about 15% of cardiac output, or about 50 mL per 100 g per minute. It uses about 20% of resting oxygen consumption.
• It depends almost entirely on glucose; ketones are used in prolonged fasting. Loss of flow causes unconsciousness within seconds and irreversible damage within minutes.
• Flow is autoregulated across a mean arterial pressure of about 60 to 150 mmHg. Chronic hypertension shifts this range to higher pressures.
• CO2 is the strongest local regulator: hypercapnia dilates cerebral vessels, and hypocapnia constricts them. Brief hyperventilation can therefore lower intracranial pressure as a temporary rescue measure.
• Cerebral perfusion pressure = mean arterial pressure - intracranial pressure. Under the Monro-Kellie doctrine, the skull's fixed volume means a mass forces out CSF and venous blood before pressure rises steeply.
• CSF:
  • It is made by the choroid plexus at about 500 mL per day; about 150 mL is present at any time.
  • It flows from the lateral ventricles through the foramina of Monro to the third ventricle, then through the cerebral aqueduct to the fourth ventricle, then out through the foramina of Luschka and Magendie into the subarachnoid space.
  • It is absorbed through arachnoid granulations into the dural venous sinuses.
  • Blocked flow causes non-communicating hydrocephalus; impaired absorption causes communicating hydrocephalus.

Vision

• Refraction:
  • Myopia focuses images in front of the retina and is corrected with concave lenses.
  • Hyperopia focuses images behind the retina and is corrected with convex lenses.
  • Astigmatism comes from unequal corneal curvature.
  • Presbyopia is the age-related loss of accommodation.
• Photoreceptors:
  • Rods serve dim-light vision, use rhodopsin (vitamin A), and lose function in vitamin A deficiency (night blindness).
  • Cones serve color and acuity and are concentrated in the fovea. Red-green color deficiency is X-linked and affects roughly 8% of males.
  • In phototransduction, light activates transducin, phosphodiesterase lowers cGMP, sodium channels close, and the photoreceptor hyperpolarizes.
• Pupillary light reflex: the optic nerve (CN II) is the afferent limb. The pretectal nuclei project bilaterally to the Edinger-Westphal nuclei, and CN III is the efferent limb. That is why light in one eye constricts both pupils, and why a relative afferent pupillary defect points to an optic nerve lesion.

Hearing and balance

• The middle-ear ossicles and the tympanic membrane amplify pressure about 20-fold, matching the impedance of air to fluid. The stapedius (CN VII) and tensor tympani (CN V3) damp loud sounds.
• The cochlea is tonotopic: the base detects high frequencies and the apex low frequencies. Noise-induced and age-related loss affect high tones first.
• Hair cells transduce sound. Auditory pathways project to both sides above the cochlear nuclei, so a unilateral cortical lesion does not cause deafness.
• Tuning-fork tests (512 Hz): in conductive loss, Weber lateralizes to the affected ear and Rinne shows bone conduction greater than air. In sensorineural loss, Weber lateralizes to the normal ear and Rinne is positive (air conduction greater than bone) in both ears.
• Vestibular system: semicircular canals detect angular acceleration, and the utricle and saccule (otoliths) detect linear acceleration and gravity. Vestibulo-ocular reflexes stabilize gaze.

Taste and smell

• Taste (sweet, salty, sour, bitter, umami) is carried by CN VII (chorda tympani, anterior two-thirds of the tongue), CN IX (posterior third) and CN X (epiglottis) to the nucleus of the solitary tract.
• Smell (CN I) is the only sense that reaches the cortex (piriform and entorhinal) without first relaying in the thalamus. Anosmia follows cribriform plate fractures, viral infections including COVID-19, and Kallmann syndrome (anosmia with hypogonadotropic hypogonadism).