cavity

DEFINITION:

1. A hollow place or space, or a potential space, within the body or in one of its organs; it may be normal or pathological.

organ

DEFINITION:

A somewhat independent part of the body that performs a special function or functions.

effector organ - An organ that produces an effect, e.g., contraction or secretion, in response to nerve stimulation.

apical membrane

DEFINITION:

See under membrane.

apex

DEFINITION:

1. A general term used in anatomical nomenclature to designate the superior aspect of a body, organ, or part, or the pointed extremity of a conical structure such as the heart or lung; called also tip.

2. The point of greatest activity, or the point of greatest response to any type of stimulation, such as electrical stimulation of a muscle.

basolateral membrane

DEFINITION:

See under membrane.

base

DEFINITION:

1. The main ingredient of a compound.

2. In chemistry, the nonacid part of a salt; a substance that combines with acids to form salts; a substance that dissociates to give hydroxide ions in aqueous solutions; a substance whose molecule or ion can combine with a proton (hydrogen ion); a substance capable of donating a pair of electrons (to an acid) for the formation of a coordinate covalent bond.

3. In genetics, a nucleotide, particularly one in a nucleic acid sequence.

4. Indicating the bottom of an organ or other object.

5. Foundation.

6. A side or face on which a geometrical figure stands; also, the length of a base.

7. The point of beginning an act or operation.

8. A place on which a force depends for supplies.

9. A number (as 5 in 5⁷) that is raised to a power; especially, a number that when raised to a power equal to the logarithm of a number yields the number itself (the logarithm of 100 to ~ 10 is 2 since 10² = 100).

10. The number of units in a given digit's place of a number system that is required to give the numeral 1 in the next higher place (the decimal system uses a ~ of 10); also, such a system using an indicated base (convert from ~ 10 to ~ 2).

11. Any of the four stations at the corners of a baseball diamond.

12. To form or serve as a base for.

13. Establish.

14. Of inferior quality; debased, alloyed.

15. Contemptible, ignoble.

16. Menial, degrading.

17. Of little value.

plasma membrane

DEFINITION:

The structure enveloping a cell, enclosing the cytoplasm, and forming a selective permeability barrier; it consists of lipids, proteins, and some carbohydrates, the lipids thought to form a bilayer in which integral proteins are embedded to varying degrees.

A cell

DEFINITION:

1. Alpha cell (definition 1).
2. Acidophil (definition 2).
3. Amacrine cell.

cytoplasm

DEFINITION:

The protoplasm of a cell exclusive of that of the nucleus; it consists of a continuous aqueous solution and the organelles and inclusions suspended in it, and is the site of most of the chemical activities of the cell.

selective

DEFINITION:

Having a high degree of selectivity.

permeability

DEFINITION:

The state of being penetrable, esp. having pores or openings that permit liquids or gases to pass through.

proteins

DEFINITION:

Any of a group of complex organic compounds which contain carbon, hydrogen, oxygen, nitrogen, and usually sulfur, the characteristic element being nitrogen, and which are widely distributed in plants and animals. Proteins, the principal constituents of the protoplasm of all cells, are of high molecular weight and consist essentially of combinations of a-amino acids in peptide linkages. Twenty different amino acids are commonly found in proteins, and each protein has a unique, genetically defined amino acid sequence which determines its specific shape and function. They serve as enzymes, structural elements, hormones, immunoglobulins, etc., and are involved in oxygen transport, muscle contraction, electron transport, and other activities throughout the body, and in photosynthesis.

binding protein - any of a number of plasma proteins (See below) that bind to hormones of low solubility (chiefly the thyroid and steroid hormones), thus providing a transport system for them; some are specific for particular hormones, while others bind to any sparingly soluble hormones. Called also carrier protein or transport protein (See below).

carrier proteins - A binding protein (See above).

plasma proteins - The hundreds of different proteins present in blood plasma, including carrier proteins (such as albumin, transferrin, and haptoglobin), fibrinogen and other coagulation factors, complement components, immunoglobulins, enzyme inhibitors, precursors of substances such as angiotensin and bradykinin, and many other types of proteins.

transport protein - binding protein (See above).

bilayer

DEFINITION:

A membrane consisting of two molecular layers, such as the cell membrane or the envelope of some viruses.

integral

DEFINITION:

1. Essential to completeness.

2. Formed as a unit with another part.

3. Composed of parts that make up a whole.

4. Entire.

proteins

DEFINITION:

Any of a group of complex organic compounds which contain carbon, hydrogen, oxygen, nitrogen, and usually sulfur, the characteristic element being nitrogen, and which are widely distributed in plants and animals. Proteins, the principal constituents of the protoplasm of all cells, are of high molecular weight and consist essentially of combinations of a-amino acids in peptide linkages. Twenty different amino acids are commonly found in proteins, and each protein has a unique, genetically defined amino acid sequence which determines its specific shape and function. They serve as enzymes, structural elements, hormones, immunoglobulins, etc., and are involved in oxygen transport, muscle contraction, electron transport, and other activities throughout the body, and in photosynthesis.

binding protein - any of a number of plasma proteins (See below) that bind to hormones of low solubility (chiefly the thyroid and steroid hormones), thus providing a transport system for them; some are specific for particular hormones, while others bind to any sparingly soluble hormones. Called also carrier protein or transport protein (See below).

carrier proteins - A binding protein (See above).

plasma proteins - The hundreds of different proteins present in blood plasma, including carrier proteins (such as albumin, transferrin, and haptoglobin), fibrinogen and other coagulation factors, complement components, immunoglobulins, enzyme inhibitors, precursors of substances such as angiotensin and bradykinin, and many other types of proteins.

transport protein - binding protein (See above).

postsynaptic membrane

DEFINITION:

See under membrane.

plasma membrane

DEFINITION:

The structure enveloping a cell, enclosing the cytoplasm, and forming a selective permeability barrier; it consists of lipids, proteins, and some carbohydrates, the lipids thought to form a bilayer in which integral proteins are embedded to varying degrees.

neuron

DEFINITION:

Any of the conducting cells of the nervous system. A typical neuron consists of a cell body, containing the nucleus and the surrounding cytoplasm (perikaryon); several short radiating processes (dendrites); and one long process (the axon), which terminates in twiglike branches (telodendrons) and may have branches (collaterals) projecting along its course. The axon together with its covering or sheath forms the nerve fiber. Called also nerve cell.

synapse

DEFINITION:

A junctional connection between two neurons (nerve cells) across which a signal can pass. A single neuron may form thousands of these connections with adjacent nerve cells.

A typical neuron has one long fiber (axon) that projects from its cell body; it splits into several smaller branches and twigs, each ending in a terminal that forms a synapse, usually close to the cell body of an adjacent neuron. At a synapse, the two neurons do not come directly into contact; their surface membranes are separated by a gap called the synaptic cleft. When an electrical signal passes along a neuronal axon and reaches a synapse, it cannot bridge the cleft directly; instead, it causes the release of a chemical, called a neurotransmitter. The chemical travels across the cleft and is received at the surface membrane of the next neuron, where it changes the electrical potential of the membrane.

The axonal membrane from which the neurotransmitter is released is called the presynaptic membrane; the neuronal membrane at which it is received is called the postsynaptic membrane.

A synapse may be excitatory or inhibitory:

excitatory synapse - When a neurotransmitter passes across an excitatory synapse, the effect is to excite the postsynaptic membrane, making it more likely that the receptor neuron will "fire" and propagate an electrical impulse in turn.

inhibitory synapses - Inhibitory synapses decrease the excitation of the next neuron.

Most drugs affecting the nervous system work through their effects on synapses. They may modify neurotransmitter release or may modify the effects of neurotransmitters on postsynaptic membranes.

neurotransmitters

DEFINITION:

Any of a group of substances that transmit nerve impulses across a synapse.

presynaptic membrane

DEFINITION:

See under membrane.

plasma membrane

DEFINITION:

The structure enveloping a cell, enclosing the cytoplasm, and forming a selective permeability barrier; it consists of lipids, proteins, and some carbohydrates, the lipids thought to form a bilayer in which integral proteins are embedded to varying degrees.

synapse

DEFINITION:

A junctional connection between two neurons (nerve cells) across which a signal can pass. A single neuron may form thousands of these connections with adjacent nerve cells.

A typical neuron has one long fiber (axon) that projects from its cell body; it splits into several smaller branches and twigs, each ending in a terminal that forms a synapse, usually close to the cell body of an adjacent neuron. At a synapse, the two neurons do not come directly into contact; their surface membranes are separated by a gap called the synaptic cleft. When an electrical signal passes along a neuronal axon and reaches a synapse, it cannot bridge the cleft directly; instead, it causes the release of a chemical, called a neurotransmitter. The chemical travels across the cleft and is received at the surface membrane of the next neuron, where it changes the electrical potential of the membrane.

The axonal membrane from which the neurotransmitter is released is called the presynaptic membrane; the neuronal membrane at which it is received is called the postsynaptic membrane.

A synapse may be excitatory or inhibitory:

excitatory synapse - When a neurotransmitter passes across an excitatory synapse, the effect is to excite the postsynaptic membrane, making it more likely that the receptor neuron will "fire" and propagate an electrical impulse in turn.

inhibitory synapses - Inhibitory synapses decrease the excitation of the next neuron.

Most drugs affecting the nervous system work through their effects on synapses. They may modify neurotransmitter release or may modify the effects of neurotransmitters on postsynaptic membranes.

neurotransmitters

DEFINITION:

Any of a group of substances that transmit nerve impulses across a synapse.

semipermeable membrane

DEFINITION:

See under membrane.

solvent

DEFINITION:

1. Dissolving; effecting a solution.
2. A liquid that dissolves or that is capable of dissolving; the component of a solution that is present in greater amount.

solute

DEFINITION:

A dissolved substance.