The reflex that links an unconditional stimulus and an unconditional response, e.g. a series of neurons that links the presence of food in the mouth to secretion by the salivary glands.

The response evoked by an unconditional stimulus. An example is salivation triggered by the presence of food in the mouth.

A stimulus that evokes a response even in the absence of a history of conditioning, e.g. food in the mouth with regard to salivation.

One of the four nucleotide bases that occur in RNA. Uracil is not present in DNA (thymine (T) occurs in its place).

The number of covalent single bonds that an atom can make. For example, from the formula of methane, CH₄, a carbon atom can be seen to have a valency of 4.

(neural) A nodule on an axonal branch which contributes to the postsynaptic influence of the neuron by releasing large quantities of transmitter. This type of synapse is characteristic of neurons projecting to the striatum from the substantia nigra.

A tube passing into a vein through which a drug can be passed into the body.

Molecular aggregates of lipids and proteins. VLDL are formed in the liver. They contain much less protein than lipid.

The probability that a fertilized egg will go on to develop into an individual that can reproduce.

Parts of the central nervous system characterised by their relatively whitish appearance. The appearance is due to the presence of a large percentage of myelinated axons and the myelin contributes to the whitish appearance.