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| Transmission Of Messages |
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| The messages are transmitted in form of electrical impulses along the fibres of the neurons. There are three types of transmission of impulses - along the nerve fibre, from one nerve fibre to the other and from one nerve fibre to the muscle fibre or gland. |
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| The junction between the axon and the dendrites of the next neuron is called the synapse. The end of the axon is branched into many fibres that are devoid of the myelin sheath. At the synapse, the axon fibres and dendrites are not in direct contact. The space between them is called synaptic cleft. The axon fibres at their ends have swellings called the synaptic knobs. Within these knobs are synaptic vesicles having chemicals called neurotransmitters. The impulses that reach the ends of the axon fibres make the vesicles release these chemicals into the synaptic cleft. The chemicals reach the dendrites of the next neuron. This sets up a new impulse which is transmitted to the axon. The speed of transmission is about 120m/s. The two main neurotransmitters in vertebrates are acetylcholine and noradrenaline. |
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| Neural Synapse |
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| The axons of the motor nerve fibre end on a muscle fibre or a gland. The synapse formed at the junction of the motor nerve fibre and the muscle fibre is called the neuromuscular junction. |
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| At the neuromuscular junction also, the axon forms branches that are devoid of myelin sheath. At the region of the axon fibres, the muscle fibre has a specialised region called the motor end-plate. The cell membrane of the muscle fibre cell is called the sarcolemma. Sarcolemma at the neuromuscular junction shows depressions into which the synaptic knobs of the motor nerve fibre fit. The transmission of an impulse is by release of the neurotransmitters into the muscle fibre through the sarcolemma from the synaptic knob. |
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| The impulse is received by the dendrites and passed through the cell body to the axon. An impulse is an electrical disturbance. |
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| (a) |
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| (b) |
| A typical action potential in an axon (a) Potential distribution across the axonal membrane (b) Relationships between membrane potentials |
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| All along the nerve fibre, it is surrounded by a fluid called the extracellular fluid (ECF). There is a differential distribution of sodium and potassium ions across the membrane due to |
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| i) The poor permeability of the resting membrane for Na+ ions and higher permeability for the K+ ions. |
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| ii) The sodium pump of the membrane that carries out active transport of three Na+ ions for every two K+ ions across the membrane. |
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| Thus, the cytoplasm of the axon is more electronegative than the layer of ECF just outside the plasmalemma. |
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| This results in a potential called the resting potential (measuring about -80mV) and the membrane is said to be polarized. |
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| When the dendrite receives the neurotransmitters, they change the properties of the cell membrane which in turn changes the distribution of the ions across the membrane. The cytoplasm becomes more electropositive than the ECF. Thus the membrane at this point becomes depolarized. This result in a different potential called the action potential. |
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| The change in potential is then transmitted along the neuron to the ends of the axon. The transmission of the impulse in myelinated fibres is faster than that in non-myelinated fibres. This is because the manner of transmission of impulses in these two types of fibres differs in the following manner: |
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| Transmission along non-myelinated fibre |
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| (a) |
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| (b) |
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| Difference in the local circuits produced in (a) non-myelinated axon (continuous) (b) myelinated axon (saltatory) |
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| Once the impulse is generated at a particular point on the membrane, that point becomes depolarized. The cations diffuse from the electropositive depolarized region to the electronegative polarized region through the axoplasm (cytoplasm of the axon). Simultaneously, the cations in the ECF diffuse from the electropositive polarized region to the electronegative depolarized region through the ECF. Thus, the neighbouring region becomes depolarized. In this manner, the impulses travel all along the non-myelinated fibres. |
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| Transmission along myelinated fibres |
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| In myelinated fibres, the axons are covered by the myelin sheath all along except in regions called the Nodes of Ranvier. Thus the depolarization can take place only at the nodes. The diffusion of cations can also take place only at the nodal regions. So, the impulse that is generated at one node 'jumps' to the next node. This type of transmission is called the saltatory conduction. In this type of transmission, the impulses do not have to travel all along the length of the fibres and hence they are faster. |
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| The impulse then reaches the synapses and the neuromuscular junctions. Here, the impulse induces the release of the neurotransmitter. The release of the neurotransmitter at the synapse is diagrammatically shown- |
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| There are mainly two types of synapses. Electrical and Chemical depending upon the nature of transfer of information across the synapse |
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| a) In electrical synapses, the cells are separated by a gap, the synaptic cleft of only 0.2 nm and there are specialised for rapid signal transmission. |
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| b) Chemical synapses, the commonest type of synapse consist of a bulbous expansion of a nerve terminal, called synaptic knob. The cells are separated by a gap of 20nm. |
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| In this manner, the message is transmitted as a wave of impulse along the lengths of connecting neurons. |
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| The speed of conduction, as mentioned earlier, is faster in myelinated fibres. It can reach up to 120m/s in these fibres. However, the presence of the sheath slows down the conduction to, at times, 0.5m/s. The speed of conduction in the non-myelinated fibres also depends on the diameter of the fibres. Thinner the fibre, slower is the conduction. Thus, animals with long arms like the giant squids where the impulse has to travel fast over a longer distance have thick nerve fibres. This has increased the speed of conduction to about 100m/s in some cases. |
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