Depolarization Health Dictionary

  1. Health Encyclopedia
  2. Depolarization

Depolarization: From 1 Different Sources

n. the sudden surge of charged particles across the membrane of a nerve cell or a muscle cell that accompanies a physicochemical change in the membrane and cancels out, or reverses, its resting potential to produce an *action potential. The passage of a *nerve impulse is a rapid wave of depolarization along the membrane of a nerve fibre.
Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

Neuron

The term used to describe a nerve cell. A typical neuron consists of a cell body, several branching projections called dendrites, and a filamentous projection called an axon (also known as a nerve fibre). An axon branches at its end to form terminals through which electrical signals are transmitted to target cells. Most axons are coated with a layered insulating myelin sheath, which speeds the transmission of the signals. The myelin sheath is punctuated along its length by gaps called nodes of Ranvier, which help this process. Because the myelin sheath is nonconductive, ion exchange (depolarization) only occurs at a node, and signals leap from node to node along the length of the axon.

The nervous system contains billions of neurons, of which there are 3 main types: sensory neurons, which carry signals from sense receptors into the central nervous system (CNS); motor neurons, which carry signals from the CNS to muscles or glands; and interneurons, which form all the complex electrical circuitry within the CNS itself.

When a neuron transmits (“fires”) an electrical impulse, a chemical called a neurotransmitter is released from the axon terminals at synapses (junctions with other neurons). This neurotransmitter may make a muscle cell contract, cause an endocrine gland to release a hormone, or affect an adjacent neuron.

Different stimuli excite different types of neurons to fire. Sensory neurons, for example, may be excited by physical stimuli, such as cold or pressure. The activity of most neurons is controlled by the effects of neurotransmitters released from adjacent neurons. Certain neurotransmitters generate a sudden change in the balance of electrical potential inside and outside the cell (an “action potential”), which occurs at one point on the cell’s membrane and flows at high speed along it. Others stabilize neuronal membranes, preventing an action potential. Thus, the firing pattern of a neuron depends on the balance of excitatory and inhibitory influences acting on it.

If the cell body of a neuron is damaged or degenerates, the cell dies and is never replaced. A baby starts life with the maximum number of neurons, which decreases continuously thereafter.... neuron

Action Potential

the change in voltage that occurs across the membrane of a nerve or muscle cell when a *nerve impulse is triggered. It is due to the passage of charged particles across the membrane (see depolarization) and is an observable manifestation of the passage of an impulse.... action potential

Facilitation

n. 1. (in neurology) the phenomenon that occurs when a neuron receives, through a number of different synapses, impulses that are not powerful enough individually to start an *action potential but whose combined activity brings about some *depolarization of the membrane. In this facilitated state any small additional depolarization will suffice to trigger off an impulse in the cell. 2. (in education and group therapy) the process of running, leading, or controlling a group discussion.... facilitation

Muscle Relaxant

an agent that reduces tension in voluntary muscles. Drugs such as *baclofen, *dantrolene, and *diazepam are used to relieve skeletal muscular spasms in various spastic conditions, parkinsonism, and tetanus. The drugs used to relax voluntary muscles during the administration of anaesthetics in surgical operations act by blocking the transmission of impulses at neuromuscular junctions. Nondepolarizing muscle relaxants, e.g. *atracurium besilate, cisatracurium, pancuronium, and rocuronium, bind to receptor sites normally occupied by acetylcholine; depolarizing muscle relaxants, e.g. *suxamethonium, mimic the action of acetylcholine but *depolarization is prolonged.... muscle relaxant

Nerve Impulse

the electrical activity in the membrane of a *neuron that – by its rapid spread from one region to the next – is the means by which information is transmitted within the nervous system along the axons of the neurons. The membrane of a resting nerve is charged (polarized) because of the different concentrations of ions inside and outside the cell. When a nerve impulse is triggered, a wave of *depolarization spreads, and ions flow across the membrane (see action potential). Until the nerve has undergone *repolarization no further nerve impulses can pass.... nerve impulse

Q–t Interval

the interval on an *electrocardiogram between the beginning of ventricular depolarization (*Q wave) and the beginning of *repolarization (the T wave). See also long QT syndrome.... q–t interval

Q Wave

the downward deflection on an *electrocardiogram that indicates the beginning of ventricular depolarization. An abnormally deep and wide Q wave is an indication of prior heart muscle damage due to heart attack.... q wave

R On T Phenomenon

(in *electrocardiography) the superimposition of an R wave (ventricular *depolarization) on the T wave (ventricular *repolarization) of the preceding heartbeat. It may trigger ventricular fibrillation. See QRS complex; Q–T interval; S–T segment.... r on t phenomenon

S–t Segment

the segment on an *electrocardiogram that represents the interval between the end of ventricular *depolarization (QRS complex) and the beginning of ventricular *repolarization (T wave). The S–T segment is usually depressed by *ischaemia of the heart muscle but raised in the initial phase of myocardial infarction. It is therefore very useful for diagnosis in patients presenting with chest pain.... s–t segment

Health encyclopedia icon Health Encyclopedia

Recent Searches