1,240 articles
Recurrent laryngeal nerve
Recurrent laryngeal nerve (RLN, nervus laryngeus recurrens): vagus nerve branch supplying the larynx's intrinsic muscles; right loops under the subclavian artery, left under the aortic arch; injury is a key thyroidectomy complication.
Red eye
Red eye: redness from inflammation of the eye's anterior segment; also called conjunctival hyperaemia. Most cases are benign conjunctivitis, blepharitis, or subconjunctival haemorrhage, but pain, photophobia, or reduced vision signal sight-threatening disease needing urgent ophthalmology referral.
Reelin
Reelin (RELN gene) is a large extracellular matrix glycoprotein controlling neuronal migration and cortical layering via ApoER2/VLDLR-Dab1 signaling; absent in reeler mice, lost in human lissencephaly, and protective COLBOS variants delay Alzheimer's disease.
Reflex
A reflex is an automatic, involuntary response to a stimulus mediated by a fixed neural pathway, the reflex arc, ranging from the knee jerk to protective withdrawal.
Reflex arc
A reflex arc, or reflex circuit, is the neural pathway that produces an automatic, involuntary response to a stimulus, integrating it in the spinal cord or brainstem without conscious involvement.
Refractory period (physiology)
Refractory period (physiology): post-action-potential interval in neurons and muscle, with absolute phase (Na+ channel inactivation, no second spike) and relative phase (elevated K+ conductance, stronger stimulus needed); limits firing rate and ensures unidirectional conduction.
Renshaw cell
Renshaw cell: spinal inhibitory interneuron (named for Birdsey Renshaw, 1941) in the ventral horn's lamina VII/IX; excited cholinergically by α-motoneuron collaterals, it inhibits motoneurons glycinergically, mediating recurrent inhibition.
Respiratory center
Respiratory center: the brainstem neuronal network (medullary pre-Bötzinger complex, Bötzinger complex, RTN/pFRG, pontine respiratory group) that generates and modulates the automatic breathing rhythm, shaped by CO2/pH chemoreceptor and lung mechanoreceptor feedback.
Resting potential
Resting potential (resting membrane potential, RMP): the inside-negative voltage across a cell membrane at rest, about -70 mV in neurons, set mainly by K+ leak permeability and maintained by the Na+/K+-ATPase, described by Nernst and GHK equations.
Reticular dystrophy of the retinal pigment epithelium
Reticular dystrophy of the retinal pigment epithelium (dystrophia reticularis laminae pigmentosae retinae, Sjögren 1950) is a rare inherited pattern macular dystrophy with a bilateral fishnet RPE pigment pattern, adult onset, good visual prognosis, PRPH2 linkage.
Reticular formation
Reticular formation (formatio reticularis): brainstem neuron network from midbrain to medulla regulating arousal, consciousness, sleep-wake cycles, and autonomic functions; source of the ascending reticular activating system (RAS) identified by Moruzzi and Magoun in 1949.
Reticular pseudodrusen
Reticular pseudodrusen, or subretinal drusenoid deposits, are yellowish subretinal lesions above the retinal pigment epithelium in age-related macular degeneration, strongly predicting progression to geographic atrophy and detectable only with multimodal imaging.
Retina
The retina is the light-sensitive neural tissue lining the back of the eye, containing rods and cones plus bipolar, horizontal, amacrine, and ganglion cells; the fovea is rod-free with peak cone density, and OCT imaging and AAV gene therapy are central clinical advances.
Retina bipolar cell
Retinal bipolar cells are retinal interneurons relaying photoreceptor input to ganglion cells; about 12 primate and 13+ mouse types, split into ON (mGluR6) and OFF pathways.
Retina horizontal cell
Retina horizontal cells are inhibitory interneurons in the inner nuclear layer that feed back to photoreceptors and forward to bipolar cells, creating center-surround receptive fields; primates have H1 and H2 types, mice one, zebrafish four.
Retinal
Retinal (11-cis-retinal), the aldehyde of vitamin A, is the universal light-absorbing chromophore of rhodopsin and cone pigments; photoisomerized to all-trans and regenerated by the RPE65 visual cycle and a Müller-cell cone cycle.
Retinal artery occlusions
Retinal artery occlusions (CRAO, BRAO, ophthalmic artery occlusion) are embolic blockages of retinal arteries causing sudden painless vision loss; treated as acute stroke with urgent stroke-center referral, though no proven vision-restoring therapy exists.
Retinal cell
Retinal cell: any of the neuron and glia types of the retina, including rods, cones, horizontal, bipolar, amacrine, and ganglion cells; over 130 cell types, with rods outnumbering cones 20-30 to 1 in humans.
Retinal detachment with retinal break
Retinal detachment with retinal break (rhegmatogenous retinal detachment, RRD): retina separates from the retinal pigment epithelium because a tear lets vitreous fluid pass under it. Treated by pneumatic retinopexy, scleral buckling, or vitrectomy; modern reattachment success exceeds 90%.
Retinal gene therapy using lentiviral vectors
Retinal gene therapy using lentiviral vectors: EIAV- and HIV-1-derived integrating retroviral vectors carrying genes up to ~8 kb, targeting RPE and photoreceptors via subretinal injection; used in Stargardt (SAR422459) and Usher 1B (UshStat) trials, distinct from non-integrating AAV.
Retinal haemorrhage
Retinal haemorrhage (retinal hemorrhage) is bleeding into the retina, classified by layer (flame, dot-and-blot, preretinal, subretinal, vitreous). Causes include diabetes, hypertension, vein occlusion, trauma, and abusive head trauma in infants; diagnosis uses OCT and angiography.
Retinal pigment epithelium
Retinal pigment epithelium (RPE): pigmented monolayer behind the retina supporting photoreceptors via phagocytosis, the visual cycle, and the outer blood-retinal barrier; its degeneration drives AMD, and stem cell-derived RPE transplants are in early trials.
Retinal vascular occlusions
Retinal vascular occlusions (ICD-11 9B74) are blockages of retinal arteries or veins, mainly RVO (CRVO, BRVO) and RAO. Prevalence 0.77% ages 30–89; anti-VEGF is first-line for RVO macular edema; acute RAO requires stroke-center referral.
Retinex
Retinex is a theory of human color vision and a family of image-enhancement algorithms, proposed by Edwin H. Land and John J. McCann in 1971.
Retinitis
Retinitis: inflammation primarily of the retina, usually infectious and necrotizing (CMV, HSV/VZV, Toxoplasma, syphilis). CMV retinitis affects AIDS patients with CD4 <50 cells/µL; HAART cut incidence over 80%.
Retinitis pigmentosa
Retinitis pigmentosa (RP), hereditary retinal dystrophy, is a group of inherited retinal diseases causing progressive photoreceptor degeneration, night blindness, tunnel vision, and legal blindness; over 130 genes involved, no cure, vitamin A and emerging gene therapies studied.
Retinitis punctata albescens
Retinitis punctata albescens (RPA, OMIM 136880) is a rare progressive inherited retinal dystrophy with white fundus dots and childhood night blindness, usually caused by RLBP1 mutations and related to retinitis pigmentosa.
Retinoblast
Retinoblast: multipotent embryonic retinal progenitor cell of the optic cup that generates all retinal neurons and Müller glia; distinct from retinoblastoma, the Rb-gene childhood tumor.
Retinoblastoma
Retinoblastoma is a childhood cancer of the retina, caused by RB1 tumor suppressor inactivation (Knudson two-hit hypothesis) or rarely MYCN amplification. Key sign is leukocoria; survival ranges from 99.5% in high-income to 57.3% in low-income countries.
Retinopathy
Retinopathy is a general term for disease-related damage to the retina, the light-sensitive tissue at the back of the eye, most often from diabetes or high blood pressure.