Human structure and function

Inferior mesenteric vein

The inferior mesenteric vein, or vena mesenterica inferior, is the vein draining the left colon and upper rectum into the portal system, most often through the splenic vein.

Inferior phrenic arteries (arteriae phrenicae inferiores): paired arteries supplying the diaphragm's inferior surface and adrenal glands, with highly variable origins from the aorta or celiac trunk; key extrahepatic collateral in hepatocellular carcinoma.

Inferior vena cava

Inferior vena cava (IVC), vena cava inferior: the largest vein below the diaphragm, formed by the common iliac veins, carrying lower-body blood to the heart; site of IVC filters (PREPIC, PREPIC2) and ultrasound fluid assessment.

Inferior vena cava filter

Inferior vena cava filter, a metal device placed in the IVC to trap clots and prevent pulmonary embolism when anticoagulation fails or is contraindicated; reduces PE but raises DVT risk without mortality benefit.

Inflammatory diseases of the retina

Inflammatory diseases of the retina (ICD-11 9B72, 網膜の炎症性疾患): retinal inflammation category covering viral retinitis (CMV, HIV) and unspecified forms; retinal vasculitis coded separately at 9B78.12.

Infraspinatus muscle

Infraspinatus (Musculus infraspinatus), a rotator cuff muscle of the scapula's infraspinous fossa inserting on the humerus greater tubercle; external rotation and shoulder stabilization, suprascapular nerve C5-C6, prone to tendinopathy and tears.

Inguinal lymph nodes

Inguinal lymph nodes (nodi lymphoidei inguinales), groin lymph nodes in superficial and deep groups, drain the legs, genitals, perineum, and lower abdominal wall into the external iliac nodes; key sites for infection signs and cancer staging.

Inherited retinal dystrophies

Inherited retinal dystrophies (IRDs), genetic retinal degenerations including retinitis pigmentosa, Stargardt disease, and Leber congenital amaurosis, affect about 1 in 3,000 people; over 350 genes are known, and Luxturna was the first approved gene therapy.

Oct 15, 2026

Inhibin

Inhibin is a gonadal glycoprotein hormone of the TGF-β superfamily, secreted by Sertoli and granulosa cells, that suppresses FSH secretion; it is used in prenatal screening for Down syndrome and as a marker of ovarian reserve.

Inhibitory interneuron

Inhibitory interneurons are local-circuit neurons, mostly GABAergic (glycinergic in spinal cord), about 20% of cortical neurons, subdivided into PV, SOM, and 5HT3aR classes that stabilize networks, generate oscillations, and prevent epilepsy.

Inhibitory postsynaptic potential

Inhibitory postsynaptic potential (IPSP): hyperpolarizing or shunting postsynaptic response at GABA/glycine synapses, discovered by Eccles in 1952 in cat spinal motoneurons, driven by chloride and potassium conductances.

Oct 22, 2026

Injectable bioelectronic mesh

Injectable bioelectronic mesh is a flexible, biocompatible electronic mesh injected through a syringe needle that unfurls inside soft tissue to record from or stimulate many neurons.

Injury of abdominal aorta

Injury of abdominal aorta (ICD-11 NB90.0): traumatic damage to the abdominal aorta from blunt or penetrating trauma. Rare in blunt trauma (<1%), graded 1-4, mostly managed nonoperatively or endovascularly, mortality about 29%.

Injury of axillary nerve

Injury of axillary nerve: damage to the C5-C6 posterior cord branch supplying deltoid and teres minor, usually from anterior shoulder dislocation or proximal humeral fracture; mostly non-operative recovery, surgery if no recovery by 3-6 months.

Injury of cauda equina

Injury of cauda equina (ICD-11 NB72): damage to the lumbosacral nerve roots below the conus medullaris, causing lower motor neuron deficits, saddle sensory loss, and bladder, bowel, and sexual dysfunction; traumatic and compressive causes, treated by urgent decompression.

Injury of facial nerve

Injury of facial nerve: damage to cranial nerve VII causing facial weakness or paralysis, from trauma (3% of facial palsy cases), surgery, infection, or Bell palsy; graded by House-Brackmann scale.

Injury of heart

Injury of heart (ICD-11 NB31): traumatic damage to the heart from blunt or penetrating chest trauma, ranging from contusion to chamber rupture; mortality 11.3% in US hospital data, higher for penetrating injury.

Injury of lens

Injury of lens is trauma to the crystalline lens of the eye, including traumatic cataract, subluxation, and dislocation, and a leading cause of vision impairment after ocular trauma.

Injury of oculomotor nerve

Injury of oculomotor nerve (ICD-11 NA04.2) is damage to cranial nerve III, causing oculomotor nerve palsy with down-and-out eye deviation, ptosis, diplopia, and sometimes pupil dilation; causes include microvascular disease, aneurysm, tumor, and head trauma.

Injury of spleen

Injury of spleen (ICD-11 NB91.0), traumatic damage to the spleen, mostly blunt trauma (over 94%); graded AAST I-V, managed nonoperatively when stable, with splenectomy reserved for instability or peritonitis.

Injury of the auricle

Injury of the auricle (ICD-11 NA0B): trauma to the external ear's cartilaginous pinna, including hematoma, laceration, avulsion, and fracture; key risks are cauliflower ear deformity, chondritis, and the need for prompt drainage or specialist repair.

Injury of thoracic aorta

Injury of thoracic aorta (ICD-11 NB30.0), traumatic aortic wall damage, mainly blunt thoracic aortic injury from motor vehicle deceleration at the aortic isthmus; graded I–IV, diagnosed by CTA, treated with TEVAR or non-operative management.

Injury of vagus nerve

Injury of the vagus nerve (CN X), usually surgical, causing vocal fold paralysis with hoarseness and dysphagia; prognosis assessed by laryngeal electromyography, most recovery within 6 months.

Inner ear

Inner ear (labyrinth): cochlea for hearing and vestibular system for balance, in the temporal bone's bony labyrinth with perilymph and membranous labyrinth with endolymph; hair cells transduce sound and head motion via tip-linked MET channels.

Inspiratory Capacity

Inspiratory capacity (IC) is the maximum air volume inhaled from normal end expiration (FRC) to total lung capacity, equal to tidal volume plus inspiratory reserve volume; a spirometry measure used to gauge lung hyperinflation in COPD and exercise testing.

Inspiratory Reserve Volume

Inspiratory reserve volume (IRV) is the extra air inhaled beyond a normal tidal breath, up to total lung capacity; part of vital capacity, measurable by spirometry, and reduced in dynamic hyperinflation during exercise and COPD.

Instinct

Instinct: unlearned, inherited, species-typical behavior, also called innate behavior. Traced from Darwin and James through McDougall, the 1920s critiques (Dunlap, Kuo), ethology's fixed action patterns, and Lehrman's critique.

Insular cortex

Insular cortex (insula, Island of Reil): cortical region hidden in the lateral sulcus, first described by Reil in 1809; hub for interoception, emotion, salience network, cognitive control, and social cognition.

Insulin Secretion

Insulin secretion is glucose-regulated release of insulin from pancreatic beta cells via KATP-dependent triggering and KATP-independent amplifying pathways, producing biphasic secretion modulated by incretins and paracrine signals.