2,701 articles
Rod cell
Rod cell: retinal photoreceptor mediating scotopic (night) vision via rhodopsin; single-photon sensitivity, outer segment of 1000–2000 disks, absent from fovea, saturates in bright light, unlike cones.
Romano–Ward syndrome
Romano–Ward syndrome is the most common, autosomal dominant form of congenital long QT syndrome, in which the heart recharges slowly between beats, risking fainting and sudden death without deafness.
Romberg's test
Romberg's test: bedside balance test, feet together, eyes open then closed; positive when balance is lost with eyes closed, indicating dorsal column proprioceptive loss; variants include sharpened tandem Romberg and CTSIB.
Rosaï-Dorfman disease
Rosai-Dorfman-Destombes disease (RDD), also called sinus histiocytosis with massive lymphadenopathy, is a rare non-Langerhans cell histiocytosis causing massive cervical lymphadenopathy, S100+/CD68+/CD1a− histiocytes with emperipolesis, and MAPK mutations in some cases.
Rotator cuff
Rotator cuff: the four-muscle group (supraspinatus, infraspinatus, subscapularis, teres minor) stabilizing the shoulder joint. Covers anatomy, tears, impingement, diagnosis, and surgery-versus-therapy evidence.
S-antigen
S-antigen (retinal S-antigen, 48-kDa protein, arrestin-1, gene Sag) is a retinal photoreceptor protein that binds light-activated phosphorylated rhodopsin to shut off phototransduction, first identified in 1977 as a uveitis-inducing autoantigen.
Saccade
Saccade: rapid ballistic eye movement (up to ~500 deg/s, under 100 ms) shifting gaze between targets onto the fovea; also called saccadic eye movement. Controlled by brainstem burst neurons, superior colliculus, and frontal eye field; kinematics follow the main sequence.
Sacral and coccygeal spinal nerves
The sacral and coccygeal spinal nerves are five pairs of sacral nerves (S1–S5) and one coccygeal nerve, the lowest spinal nerves, supplying the pelvis and lower limb.
Sacral plexus
Sacral plexus (plexus sacralis): pelvic nerve network formed by the lumbosacral trunk (L4, L5) and S1–S4, anterior to piriformis; gives the sciatic, gluteal, posterior femoral cutaneous, perforating cutaneous, and pudendal nerves and the S2–S3 parasympathetic outflow.
Sacroiliac joint
Sacroiliac joint (SIJ): bilateral sacrum-ilium articulation, the body's largest axial joint, transferring spinal load to the legs; moves under 3 degrees and 2 mm, and contributes to 15-30% of low back pain.
Sacrum
Sacrum (os sacrum): triangular bone of five fused vertebrae at the base of the spine, transferring trunk weight to the pelvis via the auricular surfaces; 58-60 ossification centers, four foramina pairs, common sacralization and coccygeal fusion variations.
Sagittal plane
Sagittal plane: vertical antero-posterior anatomical plane dividing the body into right and left parts; median (midsagittal) plane bisects it equally, parasagittal planes unequally; flexion and extension occur in it.
Salience (neuroscience)
Salience (neuroscience): the prominence of stimuli that captures attention, computed by the salience network (anterior insula, dorsal ACC). Covers saliency/priority maps, the DMN/CEN switching hypothesis, incentive salience, aberrant salience in psychosis, and 2024 depression findings.
Saliency map
A saliency map is a two-dimensional map showing which locations in a visual scene stand out or, in machine learning, which inputs influence a model's prediction.
Saline (medicine)
Saline (medicine): 0.9% sodium chloride injection, also called normal saline, an intravenous crystalloid with 154 mEq/L each of sodium and chloride; introduced in the 1832 cholera epidemic, causes hyperchloremic acidosis in large volumes, increasingly replaced by balanced crystalloids.
Salivation
Salivation, the discharge of saliva from the salivary glands, keeps the mouth moist and aids digestion; healthy adults secrete about 0.3-0.4 mL/min unstimulated and 1-3 mL/min stimulated, under autonomic control.
Salk Institute for Biological Studies
Salk Institute for Biological Studies: independent nonprofit biological research institute founded 1960 by Jonas Salk in La Jolla, California; Louis Kahn-designed landmark campus, NCI-designated basic cancer center, discoveries in phosphorylation, neurohormones, and neurogenesis.
Sanpaku (三白)
Sanpaku (三白, "three whites") is a Japanese face-reading term for eyes with sclera visible on three sides of the iris; yin (below) and yang (above). Western medicine calls it scleral show, usually a normal eyelid-position feature, sometimes from thyroid eye disease.
Sarcomere
Sarcomere: the repeating contractile unit of striated muscle, delimited by Z-discs, about 2 µm long, built from actin, myosin, troponin, tropomyosin, and titin; force peaks at 2.0–2.2 µm and sarcomeric gene mutations drive cardiomyopathies.
SARM1
SARM1 (sterile alpha and TIR motif containing 1), also called SARM or SAMD2, is a human NAD+-cleaving enzyme on chromosome 17q11.2 that executes programmed axon degeneration after injury and is a neuroprotective drug target.
Sartorius muscle
Sartorius (musculus sartorius), the tailor's muscle, is the longest muscle in the human body, about 60 cm, running from the anterior superior iliac spine to the pes anserinus on the medial tibia, innervated by the femoral nerve L2-L3, with segmental type IV blood supply and flap and tendon-transfer surgical uses.
Scalene muscles
Scalene muscles (scalenus anterior, medius, posterior): three paired lateral neck muscles from cervical vertebrae to ribs 1-2; flex the neck, elevate ribs as accessory respiratory muscles, and bound the interscalene triangle carrying the brachial plexus and subclavian artery.
Scalp
Scalp: the five-layer soft tissue covering the skull (skin, connective tissue, galea aponeurotica, loose areolar tissue, pericranium, mnemonic SCALP), supplied by five paired arteries and trigeminal plus cervical nerves; site of hair growth, dandruff, and alopecia.
Scandinavian Simvastatin Survival Study
Scandinavian Simvastatin Survival Study (4S), 1994 Lancet trial of simvastatin in 4,444 Nordic coronary heart disease patients; first statin trial to show 30% lower all-cause mortality versus placebo over 5.4 years.
Scaphoid bone
The scaphoid bone, also called the hand navicular or os scaphoideum, is a boat-shaped carpal bone on the thumb side of the wrist and the most commonly fractured carpal bone.
Scapula
Scapula (shoulder blade): flat triangular pectoral-girdle bone on the upper back; glenoid cavity holds the humeral head, spine ends in the acromion, coracoid anchors muscles, over 10 muscles attach.
Scapuloperoneal spinal muscular atrophy
Scapuloperoneal spinal muscular atrophy (SPSMA, neurogenic scapuloperoneal amyotrophy, New England type) is a rare autosomal dominant TRPV4-related motor neuron disease causing scapular and peroneal muscle atrophy, laryngeal palsy, and distal weakness.
Schwann cell
Schwann cell: PNS glial cell (Theodor Schwann, 1839) forming myelin sheaths of peripheral axons, one internode per cell, derived from neural crest; transdifferentiates into repair cells after injury and is used in nerve repair and spinal cord injury trials.
Schwannoma
Schwannoma, also called neurilemmoma, is a usually benign nerve sheath tumor arising from Schwann cells, the most common peripheral nerve tumor in adults, often on the vestibular nerve.
Sciatic nerve
Sciatic nerve: largest human nerve, from L4–S3 roots, exiting the pelvis below piriformis and dividing into tibial and common peroneal nerves near the knee; supplies posterior thigh and most leg and foot motor and sensory function.