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Spatial audio
Spatial audio is a family of recording and playback techniques that let listeners perceive sound sources in three-dimensional space, most commonly on headphones using head-related transfer functions.
Sperm Motility
Sperm motility, the movement characteristics of spermatozoa in fresh semen, is graded by WHO as progressive, non-progressive, or immotile, with a 30% progressive reference limit, measured manually or by CASA.
Sphenoid bone
Sphenoid bone (os sphenoidale): unpaired wedge-shaped midline skull-base bone with body, greater and lesser wings, pterygoid processes; sella turcica, sphenoidal sinus, foramina ovale, rotundum, spinosum, optic canal.
Sphincter
A sphincter is a ring-shaped muscle that relaxes or tightens to open and close passages in the body, controlling the flow of food, urine, and stool.
Spike-timing-dependent plasticity
Spike-timing-dependent plasticity (STDP), a Hebbian synaptic learning rule: pre-before-post spiking within roughly 10–100 ms windows drives LTP, reverse order drives LTD; found at over 20 synapse types from insects to humans.
Spinal cord
Spinal cord: the CNS cylinder of gray and white matter with 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), ascending sensory and descending motor tracts, reflex function, and regeneration failure after injury.
Spinal cord abscess
Spinal cord abscess: pus collection in or around the spinal cord, usually bacterial (staphylococcus) and often a complication of epidural abscess; rare, treated with surgery and IV antibiotics.
Spinal cord arteriovenous malformation, nidus type
Spinal cord arteriovenous malformation, nidus type (SCAVM, ORPHA:715284): a rare glomerular spinal AVM with an intervening vessel network between artery and vein. Presents younger, with hemorrhage; complete obliteration 27% versus 72% for fistulous type.
Spinal cord injury
Spinal cord injury (SCI): damage to the spinal cord from trauma or disease, causing complete or incomplete loss of movement, sensation, and autonomic function below the injury level; about 15.4 million people affected globally in 2021.
Spinal cord segments and vertebral levels
Spinal cord segments and vertebral levels do not align: the adult cord ends at L1–L2, so its 31 nerve segments sit higher than the vertebrae of the same name.
Spinal cord stimulator
Spinal cord stimulator (SCS): implanted epidural leads plus pulse generator treating chronic neuropathic pain refractory to medical management. Rooted in 1965 gate control theory; first implant 1967, Medtronic system 1968. Complications include infection and lead migration.
Spinal cord traction injury
A spinal cord traction injury is damage to the spinal cord caused by stretching rather than compression, occurring during spinal deformity surgery, delivery, or chronic tethered cord syndrome.
Spinal grey matter
Spinal gray matter is the central, butterfly- or H-shaped region of the spinal cord containing neuronal cell bodies, organized into ten Rexed laminae and surrounded by white matter.
Spinal muscular atrophy
Spinal muscular atrophy, historically called Werdnig-Hoffmann or Kugelberg-Welander disease, is a hereditary disease in which loss of spinal motor neurons causes progressive muscle weakness, treatable with drugs approved since 2016.
Spinal muscular atrophy with respiratory distress type 1
Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a rare genetic motor neuron disease that causes diaphragmatic paralysis and muscle weakness, typically in infants aged 6 weeks to 6 months.
Spinal nerve
Spinal nerve: one of 31 paired mixed nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) formed by ventral and dorsal roots, exiting via intervertebral foramina as part of the peripheral nervous system.
Spinal stenosis
Spinal stenosis, narrowing of the spinal canal or nerve root spaces pressing on the spinal cord and nerves; most common in the lumbar and cervical spine, caused mainly by aging, with leg symptoms eased by flexion.
Spinocerebellar ataxia
Spinocerebellar ataxia (SCA): group of over 40 rare autosomal dominant neurodegenerative disorders causing cerebellar degeneration and progressive ataxia; SCA3 most common; no FDA-approved SCA drug.
Spinocerebellar ataxia type 19/22
Spinocerebellar ataxia type 19/22 (SCA19/22, ORPHA:98772) is a rare autosomal dominant cerebellar ataxia caused by KCND3 mutations encoding the Kv4.3 potassium channel, with slowly progressive ataxia, cognitive impairment, myoclonus, and tremor.
Spinocerebellar ataxia type 31
Spinocerebellar ataxia type 31 (SCA31) is an autosomal dominant neurodegenerative disease causing late-onset, slowly progressive cerebellar ataxia. Caused by a TGGAA repeat insertion on chromosome 16, it is Japan's third most common inherited ataxia.
Spinocerebellar ataxia type 6
Spinocerebellar ataxia type 6 (SCA6), an autosomal dominant ataxia from CAG repeat expansion in CACNA1A (Cav2.1 calcium channel), causing late-onset, slowly progressive pure cerebellar ataxia with nystagmus; slowest-progressing common SCA.
Spinothalamic tract
The spinothalamic tract, or spinal lemniscus, is a nerve fiber bundle that conveys sensations of pain, temperature, and crude touch from the spinal cord to the thalamus.
Spiral ganglion
The spiral ganglion is a group of sensory neurons in the cochlea of the inner ear that relays sound information from hair cells to the brainstem.
Spleen
Spleen: largest secondary lymphoid organ in adult humans, in the left upper quadrant; filters blood, recycles iron, stores 25-30% of RBCs, and mounts immune responses; human spleen lacks a rodent-style marginal zone.
Spleen filtration of nanoparticles
Spleen filtration of nanoparticles is the process by which the spleen traps or destroys intravenously injected nanoparticles; particles above 200 nm are captured, so designs stay near 100 nm or exploit the filter for immunotherapy.
Spleen imaging
Spleen imaging uses ultrasound, CT, MRI, and nuclear scintigraphy to measure the spleen and assess enlargement or focal abnormality, with ultrasound the most validated first-line test.
Spleen transplantation
Spleen transplantation (splenic autotransplantation) reimplants a patient's own splenic fragments, usually in the greater omentum, after splenectomy to regenerate splenic tissue and reduce overwhelming post-splenectomy infection risk; protection against OPSI remains unconfirmed.
Splenectomy
Splenectomy is surgical removal of the spleen, done open, laparoscopic, or robotically for trauma, hematologic disorders, and splenomegaly; asplenic patients need lifelong pneumococcal, meningococcal, and Hib vaccination and antibiotic prophylaxis.
Splenic cyst
A splenic cyst is a fluid-filled cavity in the spleen, classified as a true epithelial-lined cyst, a pseudocyst, or a parasitic hydatid cyst; about 75% are pseudocysts, most following abdominal trauma.
Splenic infarction
Splenic infarction: death of spleen tissue from occlusion of the splenic artery or its branches. Also called splenic infarct. Causes include cardioembolism, hematologic disease, malignancy, sepsis; diagnosed by contrast-enhanced CT; usually managed conservatively.