Neurophysiology & Sensory Systems — Comprehensive Study Notes Summary & Study Notes

🧠 Myelin and Schwann Cells

Schwann cells wrap around the axon of a peripheral nerve fiber, spiraling to form many layers of their plasma membrane. Most of the cytoplasm is squeezed out, leaving a compact, fatty myelin sheath that electrically insulates the axon. Gaps between adjacent Schwann cells, the nodes of Ranvier, permit saltatory conduction—action potentials jump node-to-node, increasing conduction velocity.

🧩 Neuroglia and Neurotransmitter Control

Astrocytes regulate the chemical environment around neurons, including uptake and buffering of glutamate. By controlling extracellular glutamate levels they protect motor neurons from excitotoxic injury, which is relevant to diseases like amyotrophic lateral sclerosis (ALS).

🦠 Peripheral Neuropathy and Receptors

Peripheral neuropathy affects peripheral nerves (not central tracts), causing sensory, motor, or autonomic deficits. Diseases like leprosy (Hansen’s disease) damage peripheral cutaneous receptors—including mechanoreceptors, nociceptors, and thermoreceptors—leading to loss of pain and temperature sensation, unnoticed injury, infection, and deformity.

⚡ Membrane Permeability and Local Anesthetics

Local anesthetics reduce sodium permeability of the neuronal plasma membrane. Blocking Na+ entry prevents depolarization and therefore action potential generation and propagation, producing loss of sensation (no pain signal transmitted).

🧭 Major Brain Structures & Functions

• Hypothalamus: regulates temperature, water balance, and most autonomic functions.
• Optic chiasm: site where medial optic nerve fibers cross.
• Corpora quadrigemina (midbrain): reflex centers for vision and hearing.
• Cerebellum: posture, coordination, and complex motor planning.
• Thalamus: major relay for sensory information to the cortex.
• Medulla oblongata: autonomic centers for BP, HR, respiration, plus cough/sneeze/swallow centers.
• Corpus callosum: major fiber tract connecting cerebral hemispheres.
• Mammillary bodies: olfactory relay stations.
• Cerebral aqueduct: connects third and fourth ventricles.
• Midbrain: location of cerebral peduncles; part of brainstem.

🎛 Cranial Nerves — Overview and Clinical Tests

Mnemonic: On Occasion Our Trusty Truck Acts Funny—Very Good Vehicle Anyhow. Know nerve number, primary function, and simple bedside tests:

• I (olfactory): sensory, smell (e.g., coffee).
• II (optic): sensory, visual acuity, visual fields, pupillary light reflex.
• III (oculomotor): motor, eye movements and pupil constriction.
• IV (trochlear): motor, downward/inward gaze.
• V (trigeminal): both, facial sensation and jaw clench.
• VI (abducens): motor, lateral gaze.
• VII (facial): both, facial expression, anterior 2/3 taste.
• VIII (vestibulocochlear): sensory, hearing and balance.
• IX (glossopharyngeal): both, gag reflex and posterior 1/3 taste.
• X (vagus): both, voice quality, ‘‘say ah’’ for palate elevation.
• XI (accessory): motor, shoulder shrug and head turn.
• XII (hypoglossal): motor, tongue protrusion and deviation.

Common clinical links: Bell’s palsy = CN VII involvement; chewing = CN V; listening/vertigo = CN VIII; salivation/taste = CN VII and IX; eye rolling uses CN III, IV, VI together.

↔️ Brain Lesions, Lateralization, and Consciousness

A lesion in the right internal capsule producing left-sided paralysis reflects that corticospinal fibers have crossed (decussated), so contralateral motor loss occurs when fibers are damaged above the decussation.

Trauma to the brainstem is more dangerous than focal frontal lobe injury because the brainstem houses vital autonomic centers controlling respiration, heart rate, and blood pressure; damage can cause coma or death.

Loss of signal transmission to the cerebral cortex with intact cortex and brainstem often implicates damage to relay/ascending systems—particularly the thalamus or the reticular activating system (RAS)—which interrupt cortical arousal and awareness (as in unresponsive wakefulness syndrome).

🧱 Major Nerve Plexuses & Key Nerves

• Cervical plexus (C1–C4): supplies neck and diaphragm; phrenic nerve is essential for breathing.
• Brachial plexus (C5–T1): supplies shoulder, arm, hand; important motor/sensory nerves include median, radial, and ulnar.
• Lumbar plexus (L1–L4): hip flexion and knee extension; supplies anterior thigh and medial leg.
• Sacral plexus (L4–S4): hip extension, knee flexion, most foot movements; contains the sciatic nerve.

Injuries: wrist drop results from radial nerve impairment (loss of wrist extensor function).

🦠 Viral Latency: Varicella-Zoster (Shingles)

After chickenpox, varicella-zoster virus can lie dormant in dorsal root ganglia at multiple spinal/cranial levels. It can reactivate later as shingles; reactivation more than once is possible because the virus persists lifelong and immunity can wane.

🔁 Reflex Arc — Components

A spinal reflex arc has five components: receptor, sensory (afferent) neuron, integration center (spinal interneurons), motor (efferent) neuron, and effector (muscle or gland). Reflex testing helps localize injuries.

🦵 Common Reflexes and Their Clinical Use

• Patellar (knee-jerk): tests L2–L4; contraction of quadriceps femoris.
• Calcaneal (ankle-jerk/Achilles): tests S1–S2; stretches gastrocnemius/soleus.
• Crossed-extensor reflex: withdrawal from pain with contralateral limb extension to maintain balance.
• Plantar reflex: normal adults show plantarflexion; Babinski sign (dorsiflexion of big toe) suggests corticospinal (pyramidal) tract damage.
• Corneal reflex: sensory via CN V₁ (ophthalmic branch), motor via CN VII—blinking protects the eye.
• Gag reflex: sensory CN IX, motor CN X—protects airway.
• Ciliospinal reflex: painful stimulation of neck/face (C8–T2) produces ipsilateral pupillary dilation via sympathetic pathways.
• Salivary reflex: taste/olfactory input via CN VII (anterior 2/3) and CN IX (posterior 1/3) activates parasympathetic salivatory nuclei to stimulate salivary glands.

Reflex organs: micturition and defecation depend on stretch receptors and coordinated sacral spinal outputs; carotid sinus baroreceptors modulate BP via brainstem cardiovascular centers.

⚙️ Abnormal Reflex Function & Pathology

Hyporeflexia (weak but present reflexes) can result from damage to skeletal muscle (reduced contraction), sensory (afferent) neurons (diminished input), or motor (efferent) neurons (reduced output).

Tetanus toxin blocks inhibitory interneuronal neurotransmission in the spinal cord, removing reciprocal inhibition—agonist and antagonist muscles contract simultaneously, producing rigid, uncontrolled contractions (e.g., lockjaw).

👋 Cutaneous Sensation Tests & Adaptation

Clinical sensory tests include two-point threshold (spatial tactile resolution), tactile localization, and tests of receptor adaptation for touch and temperature. Receptors reduce firing to a constant stimulus over time, allowing the CNS to focus on new or changing stimuli. Referred pain shows how visceral pathology (e.g., myocardial ischemia) can be perceived as somatic pain (left shoulder/arm), which is clinically significant.

👁 Visual System: Field Lesions, Macula, and Near Vision

• Lesions of the optic tract produce contralateral homonymous hemianopia (loss of the same visual field halves in both eyes).
• Macular degeneration preferentially damages the macula lutea—the area of greatest visual acuity—so loss here severely impairs central, high-resolution vision while peripheral vision may remain.
• The near triad for near vision consists of lens accommodation, pupillary constriction, and eye convergence. Strabismus (misalignment) primarily affects convergence and involves extrinsic (extraocular) muscles such as the medial rectus.

Clinical eye tests: demonstrating the blind spot, measuring near point of accommodation, assessing visual acuity, testing for astigmatism and color blindness, and checking reflex activity of intrinsic and extrinsic eye muscles. Binocular vision and the primary visual cortex contribute to depth perception.

🔊 Hearing & Equilibrium: Labyrinth, Tests, and Disorders

• Acute labyrinthitis: inflammation of membranous labyrinth structures (cochlea, vestibule, semicircular canals, endolymphatic system) causing vertigo, imbalance, nausea, hearing changes, and sometimes abnormal eye movements (nystagmus).

Hearing tests:

• Weber test: distinguishes conductive vs sensorineural hearing loss by lateralization of a tuning fork.
• Rinne test: compares bone conduction to air conduction to help classify hearing loss.

Equilibrium tests:

• Romberg test: assesses balance with eyes closed.
• Barany (caloric) test and other vestibular tests evaluate inner ear function.
• Nystagmus is involuntary rapid eye movement seen in vestibular dysfunction.
• Balance testing helps detect disorders of the inner ear or vestibular pathways contributing to vertigo and dizziness.

These notes summarize essential principles for peripheral nerves, reflexes, cranial nerves, sensory testing, vision, hearing, and balance—key foundations for clinical localization and understanding common neurological disorders.