question_answer 3)

Explain the following processes (a) Polarisation of the membrane of a nerve fibre (b) Depplarisation 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  

Answer:

Polarisation of the Membrane of a Nerve Fibre In resting state, the neuron membrane is polarized with outer surface positively charged and inner surface negatively charged. The resting membrane potential is about - 40 mV to - 90 mV with an average of -70 mV. (b) Depolarisation of the Membrane of a Nerve Fibre In a depolarized membrane, a stimulus causes the opening of voltage gated Na+ channels. There is a rapid inflow of Na+ ions which wipes out the local electrical potential difference. The threshold stimulus for opening of Na4' channel is generally -55 to - 60 mV (about 10 mV less than the resting potential. (c) Conduction of a Nerve Impulse along a Nerve Fibre A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may . not be separated by a gap called synaptic cleft. There are two types of synapses namely, electrical synapses and chemical synapses. At electrical synapses, the membranes of pre-and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. At a chemical synapse, the membranes of the pre and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemical called neurotransmitters are involved in the transmission of impulses of these synapes. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. (d) Transmission of a Nerve Impulse Across a Chemical Synapse A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the. membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft. At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a hew potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.