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Explain the following processes: 

(i) Polarisation of the membrane of a nerve fibre. 

(ii) Depolarisation of the membrane of a nerve fibre. 

(iii) Conduction of a nerve impulse along a nerve fibre. 

(iv) Transmission of a nerve impulse across a chemical synapse.

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(i) Polarisation of the membrane of a nerve fibre:

When a neuron is not conducting an impulse, i.e., resting, the axonal membrane is more permeable to K+ and nearly impermeable to Na+ . Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm contains high concentration of K+ and negatively charged proteins and low concentration of Na+ . In contrast, the fluid outside contains a low concentration of K+ , a high concentration of Na and thus forms a concentration gradient. These ionic gradients across the resting membrane are maintained by the sodium-potassium pump. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.

(ii) Depolarization of the membrane of a nerve fibre: 

(a) Stimulation of an axon immediately enhances manifold its membrane permeability to Na+ . As a result, Na+ ions diffuse across the membrane from the extracellular fluid (ECF) where their concentration is higher, to the interior of the fibre where the concentration is much lower. But the membrane permeability to K+ starts rising somewhat later only, so there is simultaneous rise in the outward diffusion of K+ from the cell interior having a higher K+ concentration. 

(b) These effects lower the overall cation concentration outside and enhance its concentration inside the membrane. 

(c) The membrane is, thus deposited, with its interior becoming electropositive to the exterior. 

(d) The depolarization spreads a local current. It induces nearby passive Na+ channels to open and so as to depolarize the nearby site. 

(e) Hence the initial depolarization passes outward over the membrane and spreads out in all directions from the site of stimulation.

(iii) Conduction of nerve impulse along a nerve fibre: 

(a) It is a property of nerve fibre to become excited by a stimulus and then conduct that stimulus for the required and appropriate response. 

(b) In conducting a stimulus, the nerve axon has to pass through resting phase to active phase and then the recovery phase. 

(iv) Transmission of a Nerve impulse across a chemical Synapse: 

(a) The physiological junction between two neurons across which nerve impulses can be transmitted is known as synapse. 

(b) Synapse occur between the knob like axon endings of one neuron and the dendrites of cell body of another. 

(c) At the junction of two neurons a narrow fluid filled space called synaptic cleft is present. 

(d) Knob like endings of one neuron form many membrane bound vesicles called synaptic vesicles. 

(e) Since they help in the transmission of nerve impulse, they are also called as neurotransmitters. 

(f) When a nerve impulse reaches the axon terminal the synaptic vesicles get stimulated and release their stored chemicals in the synaptic cleft. These chemicals then diffuse through these clefts to reach the membrane of the next neurons and stimulate the next neurons.

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