Explain the following processes:
$(a)$ Polarisation of the membrane of a nerve fibre
$(b)$ Depolarisation of the membrane of a nerve fibre
$(c)$ Conduction of a nerve impulse along a nerve fibre
$(d)$ Transmission of a nerve impulse across a chemical synapse

(N/A) During the resting state,the concentration of $K^+$ ions is higher inside the axoplasm,while the concentration of $Na^+$ ions is higher outside. Due to the selective permeability of the membrane,$K^+$ ions diffuse out faster than $Na^+$ ions move in. Consequently,the outer surface of the axonal membrane becomes positively charged,and the inner surface becomes negatively charged. This state is called polarisation.
$(b)$ When a stimulus is applied to a site on the polarised membrane,the permeability to $Na^+$ ions increases significantly. $Na^+$ ions rapidly influx into the axoplasm,causing the inner side of the membrane to become positively charged and the outer side to become negatively charged. This reversal of polarity is known as depolarisation.
$(c)$ Nerve impulses are conducted differently in myelinated and non-myelinated fibres. In myelinated fibres,the myelin sheath acts as an insulator,restricting ionic exchange to the nodes of Ranvier. Consequently,the action potential 'jumps' from one node to the next,a process called saltatory conduction,which is much faster. In non-myelinated fibres,the ionic exchange and depolarisation occur continuously along the entire length of the membrane.
$(d)$ $A$ chemical synapse is the junction between the axon terminal of one neuron and the dendrite of the next. When an action potential reaches the axon terminal,it triggers synaptic vesicles to fuse with the pre-synaptic membrane,releasing neurotransmitters (e.g.,acetylcholine) into the synaptic cleft. These neurotransmitters bind to specific receptors on the post-synaptic membrane,opening ion channels and generating a new action potential in the next neuron. The enzyme acetylcholinesterase then breaks down the neurotransmitter to terminate the signal and allow the membrane to repolarise.