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SLAP tear
SLAP tear (superior labrum anterior to posterior tear), also called SLAP lesion: an injury to the top of the shoulder's glenoid labrum at the biceps tendon anchor, caused by trauma or overhead sports, diagnosed by MRI, treated first with physical therapy.
Sleep
Sleep is a naturally recurring, reversible state of reduced responsiveness in which the brain cycles through REM and non-REM phases, regulated by circadian rhythms and sleep pressure.
Sleep cycle
Sleep cycle: the repeating NREM (stages 1-3) then REM sequence during sleep, about 80-100 minutes per cycle, four to six per night, deep sleep early and REM later; discovered 1953 by Aserinsky and Kleitman.
Sleep inertia
Sleep inertia: temporary grogginess, disorientation, and impaired performance immediately after waking, lasting 30-60 minutes typically; worst during circadian low and after deep sleep; managed with short naps, caffeine, and light.
Sleep paralysis
Sleep paralysis: temporary inability to move or speak while falling asleep or waking, caused by REM atonia persisting into wakefulness; affects about 8% of people, often with intruder or incubus hallucinations, and interpreted culturally as Pandafeche, kanashibari, or jinn attacks.
Sleep Stages
Sleep stages: the AASM's five human sleep states W, N1, N2, N3, and REM (stage R), scored from 30-second polysomnography epochs by EEG, EOG, and EMG patterns; N2 has spindles and K-complexes, N3 delta slow waves, REM atonia.
Slice preparation
Slice preparation: cutting 300–400 µm living brain sections for acute electrophysiology or long-term organotypic culture, from Harrison's 1907 explant to Stoppini's 1991 membrane-interface method, with ice-cold ACSF cutting and 6–12 hour acute viability.
Slow-wave sleep
Slow-wave sleep (SWS, deep sleep, stage N3) is the deepest NREM sleep stage, defined by high-amplitude 0.5–2 Hz delta waves on EEG, homeostatically regulated, central to memory consolidation, and reduced with aging.
Small intestine
Small intestine: the 3–5 m duodenum–jejunum–ileum tube between stomach and colon; performs 90–95% of nutrient absorption, absorbs 7–8 of 9 daily liters of water, and shows five molecular regions.
Smegma
Smegma: a natural buildup of sebum, dead skin cells, and moisture under the foreskin or around the clitoral hood; harmless, not an STI, and not carcinogenic despite a long medical myth.
Smell
Smell, or olfaction, is the chemical sense by which gaseous odorant molecules are detected and identified, mediated by the olfactory nerve and closely coupled to taste in flavor perception.
Smooth muscle
Smooth muscle: involuntary, non-striated muscle of hollow organs and vessels; single-unit and multi-unit types; myosin light chain phosphorylation via Ca2+-calmodulin-MLCK; latch state sustains tone cheaply; VSMC plasticity drives vascular disease.
Smooth Muscle Contraction
Smooth muscle contraction: calcium-calmodulin activation of myosin light-chain kinase phosphorylates myosin regulatory light chains, driving actin-myosin sliding anchored at dense bodies and focal adhesions; also caldesmon pathway, Rho kinase calcium sensitization, latch-bridges.
Smooth pursuit
Smooth pursuit: continuous eye movement tracking a slowly moving object to hold its image on the fovea; requires a moving target, has 100–125 ms latency, and involves MT/MST, FEF, cerebellum, and vestibular nucleus pathways.
Sneezing
Sneezing (sternutation): a protective airway reflex expelling nasal irritants via trigeminal nerve and brainstem NMB pathway; caused by allergies, infections, light; up to 40,000 droplets per sneeze.
Social neuroscience
Social neuroscience: interdisciplinary field studying neural, hormonal, cellular, and genetic mechanisms of social behavior; term coined early 1990s (Cacioppo and Berntson); distinct from the journal Social Neuroscience (2006).
Society for Cardiovascular Magnetic Resonance
Society for Cardiovascular Magnetic Resonance (SCMR), international professional society for cardiovascular magnetic resonance imaging; publishes JCMR, training levels I-III, guidelines, annual scientific sessions; headquartered East Dundee, Illinois.
Society for Neuroscience
The Society for Neuroscience (SfN) is an international professional society for brain researchers, founded in Washington, D.C. in 1969, that runs a large annual meeting.
Society for Neuroscience Oral History Project
The Society for Neuroscience Oral History Project is an archival program of video interviews and written autobiographies of senior neuroscientists, conceived by Larry Squire during his 1993 to 1994 SfN presidency.
Society of Thoracic Surgeons
The Society of Thoracic Surgeons (STS), founded 1964, is the world's largest cardiothoracic surgery organization with 8,000+ members in 102 countries, known for the STS National Database, risk-adjusted star ratings, and The Annals of Thoracic Surgery.
Soft palate
Soft palate (velum, palatum molle): mobile muscular posterior third of the palate ending in the uvula, with five muscles innervated by the vagus via the pharyngeal plexus except tensor veli palatini (V3); central to swallowing, speech, and sleep apnea.
Sole (foot)
The sole (planta pedis) is the bottom weight-bearing surface of the foot, with three arches, the plantar aponeurosis, 14 intrinsic muscles, and thick hairless skin; clinically central to diabetic foot ulcers and plantar pressure measurement.
Soleus muscle
Soleus muscle (musculus soleus): broad flat deep calf muscle of the triceps surae, originating from tibia and fibula, inserting via the Achilles tendon; slow-twitch postural plantar flexor, venous pump, and common site of calf strain and DVT.
Solitary nucleus
Solitary nucleus (nucleus tractus solitarii, NTS): dorsomedial medulla gray matter receiving visceral and taste afferents via cranial nerves VII, IX, X; first relay for baroreflex, respiration, and gut reflexes, using glutamatergic vagal input.
Somatic nervous system
Somatic nervous system (SoNS): PNS division for voluntary movement and sensory input via skeletal muscles; 12 cranial and 31 spinal nerve pairs, single-neuron efferent path releasing acetylcholine, distinct from the two-neuron autonomic system.
Somatomotor neuron
Somatomotor neuron (somatic motor neuron, CL:0005024) is a cholinergic, excitatory motor neuron innervating skeletal muscle, subdivided into alpha, beta, and gamma types, and distinct from visceromotor autonomic neurons.
Somatosensory system
Somatosensory system: the nervous system's network for touch, temperature, pain, and proprioception, carried by dorsal column-medial lemniscal and spinothalamic pathways to the somatosensory cortex.
Sorsby fundus dystrophy
Sorsby fundus dystrophy (Sorsby macular dystrophy, SFD) is a rare autosomal dominant retinal disease caused by TIMP3 mutations, causing macular atrophy and choroidal neovascularization with central vision loss from the fourth to fifth decade; anti-VEGF therapy helps if given early.
Sound localization
Sound localization: identifying a sound's direction and distance using interaural time and level differences plus pinna spectral (HRTF) cues; duplex theory, precedence effect, hearing-device limits, and animal comparisons.
Spasm
Spasm: sudden involuntary contraction of one or more muscles (also called muscle cramp, charley horse). Distinct from dystonia and spasticity; quinine reduces cramps but is restricted for toxicity, magnesium shows no benefit in older adults.