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Glossary of Physiology Terms – A
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A band
Definition:
The A band is the region of a striated muscle sarcomere that contains myosin thick filaments. In fact, the A band is the entire length of the thick filament of the sarcomere. Its length is approximately 1 μm. The center of the A band is located at the center of the sarcomere (M line).

Related glossary terms/phrases:
H zone
I band
M line
Z disk



Absolute refractory period
Definition:
The absolute refractory period refers to a period during the action potential. This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. The absolute refractory period starts immediately after the initiation of the action potential and lasts until after the peak of the action potential. Following this period, the relative refractory period begins.

Related glossary terms/phrases:
Relative refractory period

See also:
Neuronal Action Potential - Refractory Periods



Acetylcholine
Abbreviation:
ACh

Definition:
Acetylcholine (ACh) is a chemical neurotransmitter used by the central nervous system (CNS) as well as the peripheral nervous system (PNS). Acetylcholine is a classical neurotransmitter and, in fact, it was the first of the classic neurotransmitters to be discovered. It was discovered in 1914 by Henry Hallett Dale while conducting experiments on the heart.

Acetylcholine is the neurotransmitter used by the somatic division of the nervous system at the neuromuscular junction (where a somatic motor neuron makes synaptic contact with a skeletal muscle cell). Acetylcholine is also used extensively by both branches of the autonomic nervous system; sympathetic and parasympathetic. It is the primary neurotransmitter released in autonomic ganglia by preganglionic autonomic neurons. It is also the primary neurotransmitter released by parasympathetic postganglionic neurons. A few sympathetic postganglionic neurons also release acetylcholine. The diverse actions of acetylcholine are exerted via the activation of nicotinic and muscarinic ACh receptors.

Other resources:
Acetylcholine (Wikipedia)



Acetylcholinesterase
Definition:
An enzyme found in the synaptic cleft at cholinergic synapses. It degrades acetylcholine to choline and acetate and, thus, terminates the action of acetylcholine at the synapse. Neither choline nor acetate can bind to acetylcholine receptors (nicotinic or muscarinic).



ACh
Definition:
Acetylcholine

See:
Acetylcholine



Action potential
Definition:
The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. The neuronal action potential forms an important basis for information processing, propagation, and transmission. In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction. Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Related glossary terms/phrases:
Graded potential

See also:
Neuronal Action Potential



Adenohypophysis
Definition:
Anterior pituitary gland

Related glossary terms/phrases:
Hypophysis
Neurohypophysis



Adrenarche
Definition:
An early sexual maturation stage, occurring 1 or 2 years before the onset of puberty, during which the zona reticularis of the adrenal cortex releases increased levels of dehydroepiandrosterone (DHEA) and androstenedione. These adrenal androgens are ultimately responsible for the appearance of pubic and axillary hair, increased oil production by facial skin, acne, and distinct adult body odor.

Related glossary terms/phrases:
Menarche
Pubarche
Thelarche



Aerobic
Definition:
A process that requires oxygen (O2); for example, aerobic metabolism.



Afferent
Definition:
Leading toward a region or structure of interest.

In the nervous system, afferent fibers (i.e., neurons) transmit information from a peripheral receptor to the spinal cord or the brainstem. Afferent neurons are also referred to as sensory neurons.

In the kidneys, the afferent arteriole carries blood to the glomerular capillaries.

Related glossary terms/phrases:
Efferent



Agonist
Definition:
An agonist is a molecule that binds to a receptor and activates a physiological response similar to that induced by the naturally occurring physiological ligand of the receptor. Therefore, agonist binding to a receptor mimics the action of the natural ligand.

For example, acetylcholine (ACh) is the naturally occurring physiological ligand that activates nicotinic and muscarinic acetylcholine receptors. Nicotine is an agonist of nicotinic ACh receptors (nAChR), and muscarine is an agonist of muscarinic ACh receptors (mAChR).

Related glossary terms/phrases:
Antagonist



All-or-nothing
Definition:
All-or-nothing is usually used when describing the action potential. It refers to the well-known observation that an action potential always occurs in its full size (i.e., full magnitude of voltage change).

Many physiologists use all-or-nothing and all-or-none interchangeably.

See also:
Important Features of the Neuronal Action Potential



Amenorrhea
Definition:
Absence of menstruation in sexually mature female individuals.



Amiloride
Definition:
Amiloride inhibits epithelial Na+ channels (ENaC), and in doing so, it acts as a diuretic by inhibiting sodium reabsorption in the late distal convoluted tubules, connecting tubules, and collecting ducts in the kidneys.

Other resources:
Amiloride (Wikipedia)



Anion
Definition:
A negatively charged ion.

Related glossary terms/phrases:
Cation



Antagonist
Definition:
An antagonist is a molecule that binds to a receptor, however, it does not activate the physiological response induced by the naturally occurring physiological ligand of the receptor. Moreover, once bound to the receptor, an antagonist prevents the physiological ligand from activating the receptor. Therefore, antagonist binding to a receptor prevents or blocks the action of the natural ligand.

For example, acetylcholine (ACh) is the naturally occurring physiological ligand that activates nicotinic and muscarinic acetylcholine receptors. Tubocurarine and succinylcholine are antagonists of the nicotinic ACh receptor (nAChR), and atropine is an antagonist of the muscarinic ACh receptor (mAChR).

Related glossary terms/phrases:
Agonist



Anterograde
Definition:
Moving along an axon away from the neuronal cell body toward the axon terminal.

Related glossary terms/phrases:
Retrograde



Antiport
Definition:
Exchange

See:
Exchange



Anucleate
Definition:
Without a nucleus. In particular, it refers to a cell that lacks a nucleus. For example, prokaryotic cells are anucleate. In addition, red blood cells (erythrocytes) are anucleate.



Aorta
Definition:
The large blood vessel (artery) that receives the output of the left ventricle of the heart. The aorta is the first and largest (in diameter) artery of the systemic circulation.

Following ventricular contraction (systole), hydrostatic pressure within the ventricle exceeds the pressure in the aorta, which forces blood out of the left ventricle, through the aortic semilunar valve, and into the aorta. As the ventricle relaxes during diastole, the pressure within the left ventricle drops to a level below that in the aorta. At this point, the aortic semilunar valve closes, which prevents the backflow of blood from the aorta to the left ventricle.



Atropine
Definition:
Atropine is a blocker (i.e., inhibitor) of muscarinic acetylcholine receptors. Therefore, atropine blocks the action of postganglionic parasympathetic neurons.



Autoregulation
Definition:
The ability of a tissue or organ to regulate its own function without extrinsic neural or hormonal input.

For example, in the kidneys, autoregulation ensures relatively constant blood flow and pressure through the kidney tubules in order to maintain a fairly constant glomerular filtration rate (GFR).



Axon hillock
Definition:
The region of the neuron cell body from which the axon originates. The axon hillock is generally the site of action potential initiation. It is also referred to as the initial segment.









Posted: Sunday, October 17, 2010
Last updated: Thursday, February 11, 2016