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Strabismus
Strabismus (squint) is misalignment of the eyes, affecting 2-5% of people. Types include esotropia, exotropia, hypertropia; treated with glasses, prisms, exercises, or eye muscle surgery, sometimes with adjustable sutures.
Straight leg raise
The straight leg raise, also called the Lasègue sign, lifts a patient's extended leg to detect lumbosacral nerve root irritation, most often evaluated in low back pain with sciatica.
Stratified squamous epithelium
Stratified squamous epithelium, keratinized or non-keratinized, is a multilayered protective epithelium with flattened surface cells, lining skin, esophagus, vagina, cornea, and oral mucosa, renewed by basal cell mitosis.
Stretch reflex
Stretch (myotatic, deep tendon) reflex: involuntary muscle contraction on rapid stretch, via monosynaptic Ia afferent–alpha motor neuron spinal connections; basis of knee jerk, ~20 ms upper-limb latency, delayed 14 ms in older adults.
Striatal cholinergic-GABAergic neuron
A striatal cholinergic-GABAergic neuron, or CGIN, is a large aspiny striatal interneuron that releases both acetylcholine and GABA, making up about half of the striatum's cholinergic interneurons.
Striated Muscle Contraction
Striated muscle contraction: force generation in skeletal and cardiac muscle via actin-myosin ATP hydrolysis in sarcomeres, triggered by calcium binding troponin, with DHPR-RyR1 triads in skeletal and CICR in cardiac muscle.
Striated muscle tissue
Striated muscle tissue: skeletal and cardiac muscle with sarcomere-based actin-myosin banding, sliding-filament contraction, DHPR versus CICR calcium coupling, fiber types, and diseases such as DMD and rhabdomyolysis.
Stroke volume
Stroke volume is the volume of blood ejected by a ventricle with each contraction, typically 60 to 100 mL per beat in healthy adults at rest.
Stromal cell of thymus
Thymic stromal cells are the non-lymphocyte support cells of the thymus, including thymic epithelial cells, fibroblasts, endothelial cells, and pericytes, which build the microenvironment for T cell development, selection, tolerance, and age-related involution.
Structural heart interventions in adults
Structural heart interventions in adults are catheter-based procedures that repair or replace heart valves, close septal defects, and occlude the left atrial appendage without open surgery, a subspecialty grouped from older pediatric techniques.
Sturge–Weber syndrome
Sturge–Weber syndrome, also called encephalotrigeminal angiomatosis, is a rare congenital neurocutaneous disorder featuring a facial port-wine birthmark, leptomeningeal angioma, and glaucoma, caused by a GNAQ mutation.
Subacute combined degeneration of spinal cord
Subacute combined degeneration (SACD, SCD) is vitamin B12 deficiency myelopathy of the dorsal and lateral spinal cord columns, causing sensory loss and spasticity; increasingly caused by nitrous oxide inactivation of B12.
Subclavian artery
Subclavian artery: paired artery of the upper thorax and neck; left arises from the aortic arch, right from the brachiocephalic trunk; three scalene-defined parts; becomes the axillary artery at the first rib.
Subclavian steal syndrome
Subclavian steal syndrome: symptoms from reversed vertebral artery flow caused by proximal subclavian artery stenosis, also called coronary subclavian steal after LIMA grafting; mostly left-sided, atherosclerotic, usually benign, treated conservatively with stenting or bypass for selected symptomatic cases.
Subcutaneous implantable cardioverter-defibrillator
Subcutaneous implantable cardioverter-defibrillator (S-ICD), EMBLEM S-ICD: a defibrillator with a single lead entirely under the skin, no transvenous lead. PRAETORIAN showed noninferiority to transvenous ICDs; PRAETORIAN-XL showed 42% fewer major complications at 8 years; no antitachycardia pacing.
Subcutaneous tissue
Subcutaneous tissue (tela subcutanea, hypodermis, superficial fascia) is the loose connective and fatty layer beneath the dermis; it insulates, cushions, stores energy, and is the target of subcutaneous injections.
Subdural hygroma
Subdural hygroma (SDHy, SDG): a subdural collection of pure CSF from traumatic arachnoid tearing or effusion in intracranial hypotension, dehydration, or atrophy; coined by Dandy 1934; may convert to chronic subdural hematoma.
Subluxation
Subluxation: in medicine, a partial joint dislocation; in chiropractic, a putative spinal lesion central to the profession since DD Palmer's 1895 founding, whose detection and clinical significance remain scientifically contested.
Submandibular gland
Submandibular gland: paired major salivary gland below the mandible, second largest after the parotid; drains via Wharton's duct, makes ~70% of resting saliva, and accounts for over 80% of salivary stones.
Suboccipital nerve
The suboccipital nerve is the dorsal ramus of the C1 spinal nerve, a mainly motor nerve supplying the four suboccipital muscles and semispinalis capitis within the suboccipital triangle alongside the vertebral artery.
Subscapularis muscle
Subscapularis (musculus subscapularis): largest rotator cuff muscle, filling the scapula's subscapular fossa, inserting on the humerus' lesser tubercle; the cuff's only internal rotator, innervated by upper and lower subscapular nerves (C5-C6), prone to underdiagnosed tears.
Substance P
Substance P (SP) is an 11-amino acid tachykinin neuropeptide, discovered 1931 by von Euler and Gaddum, sequenced 1971, signaling mainly through the NK1 receptor; its antagonists treat chemotherapy-induced nausea.
Substantia gelatinosa of Rolando
Substantia gelatinosa of Rolando (SGR), or substantia gelatinosa, is the translucent lamina II of the spinal dorsal horn, described by Rolando in 1824, that receives C and Aδ fiber input and modulates nociception via gate control circuitry.
Substantia nigra
The substantia nigra, Latin for "black substance", is a nucleus of the midbrain that supplies dopamine to the basal ganglia and helps modulate motor movement and reward.
Sudden cardiac death of athletes
Sudden cardiac death of athletes: unexpected cardiac death during sport, the leading medical cause of athlete death. Incidence 0.24-6.8 per 100,000 person-years; causes differ by age; screening and AED-based prevention debated.
Summation (neurophysiology)
Summation (neurophysiology): the combining of postsynaptic potentials in a neuron, via temporal summation from one input over time and spatial summation from multiple inputs, read out at the axon hillock; dendrites add linear, sublinear, and supralinear modes.
Superficial temporal artery
Superficial temporal artery (arteria temporalis superficialis): terminal branch of the external carotid artery supplying the face and scalp, dividing into frontal and parietal branches; biopsied for giant cell arteritis and used for STA-MCA bypass.
Superior canal dehiscence syndrome
Superior canal dehiscence syndrome (SCDS), a third-mobile-window inner ear disorder first described by Minor et al. in 1998, causing sound- and pressure-induced vertigo, autophony, and pulsatile tinnitus, diagnosed by Bárány criteria and repaired surgically.
Superior colliculus
Superior colliculus (optic tectum in non-mammals): paired layered midbrain structure integrating visual, auditory, and somatosensory maps to generate saccades, orient head and eyes, detect looming threats, and support attention and categorization.
Superior laryngeal nerve
Superior laryngeal nerve (nervus laryngeus superior), a vagus nerve branch dividing into an internal sensory branch to the supraglottic larynx and an external motor branch to the cricothyroid muscle; injury causes pitch-range loss and dysphagia.