Purely imaginary nonzero eigenvalues make a real planar linearization a centre equilibrium. They do not control the nonlinear radial drift. For example,has the same linearization as a uniform rotation. In polar coordinates, however, $\dot{r}=-r^3$ and $\dot{\theta }=1$. Every sufficiently small nonzero orbit spirals towards the origin, so the origin is an asymptotically stable weak focus, and no nonzero orbit is periodic. Reversing the signs of the cubic terms gives a repelling weak focus. Removing them gives a centre. Thus a nonlinear centre equilibrium requires a further argument, such as a suitable first integral; the imaginary pair alone is insufficient.

Ammonification converts nitrogen in organic compounds to Ammonia or ammonium during their breakdown. For example, microbial degradation of dead organisms can release the nitrogen that had been incorporated in amino acids.

Ammonification differs from nitrogen fixation, which begins with molecular nitrogen, and from nitrification, which oxidizes reduced inorganic nitrogen. Separating these transformations explains how decomposers reconnect living material to the nutrient pools used by primary producers.^[ref]

A historical nitrate-recognition reaction involving reduction by iron(II) and formation of a brown iron–nitrosyl species. Nitrite, reagent contamination and procedural conditions can interfere.

The linear system defining one step of Newton's method for $F(x)=0$ is the displayed first-order correction equation. A nonsingular Jacobian matrix gives a unique Newton correction. For an unconstrained stationary-point problem $F=\nabla f$, the system uses the Hessian matrix in place of $DF$.

An inhibitory receptor activated by glycine; blockade contributes to strychnine poisoning.

Many glycine receptors are ligand-gated chloride channels. Their effect on excitability depends on the chloride electrochemical gradient, so identifying an inhibitory transmitter does not eliminate the need to specify the ionic conditions. ^[ref]

A metabotropic receptor for acetylcholine.^[ref]

Destructive softening of brain tissue, often following cerebral infarction. Gross consistency does not establish its cause.

Destructive softening of spinal cord tissue following injury such as ischemia, compression or inflammation.

An examination boundary on the body below which a sensory modality changes, useful in localising a spinal cord lesion.

The pharyngeal plexus is a network of nerve branches supplying the pharynx. It includes motor, sensory and autonomic contributions. Its organization helps coordinate a shared passage used in swallowing and vocal activity.

The cardiac plexus is a network of autonomic and sensory pathways associated with the heart. Sympathetic and vagal contributions influence cardiac function through different mechanisms. Sharing a plexus does not make every fiber’s effect identical.

The pulmonary plexus is a network of autonomic and sensory pathways associated with the roots of the lungs and airways. Vagal and sympathetic contributions connect central regulation with local airway and vascular targets.

The oesophageal plexus is a neural network around the oesophagus, receiving vagal and sympathetic contributions. It links distributed autonomic pathways to the wall of the digestive passage. A network’s position should be distinguished from the origins of its fibers.

The lumbar plexus is a network of upper lumbar spinal pathways supplying parts of the abdominal wall and lower limb. Its branches include the femoral nerve and obturator nerve. Shared spinal origins and separate peripheral destinations help explain overlapping injury patterns.

Interruption or loss of a structure's nerve supply. Its physiological consequences can include direct motor or secretory changes and indirect effects through altered perfusion.

The femoral nerve arises from the lumbar plexus and supplies anterior-thigh motor pathways and sensory routes to part of the lower limb. Its relation to quadriceps femoris links it particularly to knee extension.

The tibial nerve is a major branch of the sciatic nerve carrying fibers toward posterior-leg and plantar structures. Its motor and sensory components have different targets. A lesion’s level determines which downstream branches are affected.

The common fibular nerve is a major branch of the sciatic nerve that supplies pathways to the lateral and anterior leg and part of the foot. Its course near the fibular neck gives nerve location practical importance when relating injury to motor and sensory changes.

A cheese-like pattern of necrosis, often associated with tuberculous granulomas but not uniquely diagnostic of tuberculosis. Architecture, microbiology and the surrounding viable tissue help establish the cause.

A clinical and pathological description of substantial tissue death, commonly associated with impaired blood supply and sometimes infection. Dry and wet forms describe different tissue conditions rather than a single pathogen.