Signal reception

Receptors for neurotransmitters can be either channel-linked (ligand-gated) or non-channel-linked. Channel-linked receptor proteins are transmitter-gated ion channels and are invariably multipass, transmemebrane proteins. The action of neurotransmitters involving channel-linked receptors (such as the acetylcholine receptor, ACh-receptor) is fast, direct and specific, acting on a single post-synaptic cell. Non-channel-linked receptors, on the other hand, are involved in slow, spatially and temporally diffuse, complex and long-lasting processes and a single neurotransmitter can act on many cells. Non-channel-linked receptors can be either (i) catalytic or (ii) G-protein-linked. Catalytic receptor proteins are generally tyrosine-specific single-pass transmembrane proteins. They act, on activation by their ligands, directly as enzymes. G-protein-linked receptors exert and indirect action on plasma-membrane-bound enzymes or ion channels, mediated through GTP-binding regulatory protein (G-protein), activating or inactivating adenylate cyclase and in that process controlling the concentrations of intracellur messengers — Ca⁺⁺ and/or cyclic AMP. These intracellular messengers act to alter the functions of other target proteins in the cell. The generator potential of any sensory receptor is a graded potential change, and not an 'all-or-nothing' potential.

Receptors can also be classified broadly according to their functions in sensory cells — chemo-, electro-, mechano-, photo-, and sono-receptors etc., which are specialised neural elements to detect and translate special forms of external impulses. Sensory cells (organs) are specialised transducers, (like all neurons), converting stimuli of different kinds (muscle stretch, sound vibrations and light etc.) into electrical signals. The outer segments of sensory cells, where stimuli are received, are differently structured, according to stimulus characteristics. For example, in mechano-receptors of auditory cells, the tip of the outer segment contains a special group of microtubles with an integral membrane unit; in photoreceptors, the outer segment has an enlarged membrane surface to accommodate a sufficient number of photopigment molecules.

Define the following words

Spatially, temporally, to mediate, a ligand, auditory, to accommodate.

Complete the sentences

1. The action of neurotransmitters involving channel-linked receptors…

2. Non-channel-linked receptors, on the other hand,…

3. Non-channel-linked receptors can be either… or…

4. G-protein-linked receptors exert…

5. These intracellular messengers act…

6. Receptors can also be classified …

7. The outer segments of sensory cells, where stimuli are received,...

Put the following words and word combinations into the gaps. Some of them may be used more than once.

protrude / clusters / passed on / dendrites / sensory receptors / impulses / neuronal response / projections / transduced / nodes / transmission / stimuli / myelin sheath

The external world is perceived by organisms through ______________ received by them. These can be mechanical, thermal or chemical. They are ______________ by the ______________ of the organism into electrical signals and ______________ to the central nervous system. The internal communications within the organism are taken care of by the nervous system. This ______________ of information relating to the environment and the ability of multi-cellular organisms to respond appropriately to them is essential for survival.

The nervous system consists of neurons, which transmit information from one part of the organism to the other. This is known as the ______________. The nerve cells (neurons and Swann cells) have a cell body from which small projections known as ______________ and large ______________ called axons ______________. The ______________ carry ______________ towards the central cell body. The cell body is the thick region of the neuron containing the nucleus and most of the cytoplasm. The axons carry impulses away from the cell body. A fatty covering known as the ______________ covers most of the larger axons. The ______________ is interrupted somewhat periodically at sites called the ______________ of Ranvier. Several axons are grouped together in each nerve. Similarly nerve cell bodies form ______________ known as ganglia. Some axons are very long and can extend up to 3 or 4 metres.

Translate the text into Russian orally

Если сообщить волокну два последовательных стимула, разделенные некоторым интервалом времени, то поведение волокна зависит от этого интервала. Немедленно после генерации импульса данный участок волокна находится в абсолютном рефрактерном состоянии, т. е. не может быть возбужден вновь. Затем следует относительное рефрактерное состояние, в котором увеличено значение порогового потенциала. Продолжительность всего рефрактерного состояния варьирует от одной до нескольких миллисекунд.

В состоянии покоя аксоплазма заряжена отрицательно по отношению к внешней среде. Потенциал покоя равен примерно ~80 мВ. Пороговое возбуждение имеет потенциал, равный примерно — 50 мВ, а потенциал действия равен 40 мВ. Прохождение импульса определяется изменением состояния мембраны аксона при ее стимуляции. Как уже говорилось, в состоянии покоя клеточной мембраны в результате активного транспорта ионов концентрация ионов К+ в цитоплазме (аксоплазме) значительно превосходит их концентрацию во внешней среде, а для ионов Na+ ситуация противоположна.

При наложении на мембрану стимулирующего потенциала вначале увеличивается ее проницаемость для ионов Na+. Ионы Na+ входят в аксон, в результате чего внутренняя поверхность мембраны изменяет знак своего заряда с отрицательного на положительный. Иными словами, происходит деполяризация мембраны.