Explain the following processes:
(a) Polarisation of the membrane of a nerve fibre.
(b) Depolarisation of the membrane of a nerve fibre.
© Conduction of a nerve impulse along a nerve fibre.
(d) Transmission of a nerve impulse across a chemical synapse.
Polarization of the membrane of a nerve fibre :
(i) Nerve fibres becomes excited by a stimulus and conduct the stimulus for the required and appropriate response.
(ii) In conducting a stimulus it has to pass through from resting phase to the active phase and then recovery phase.
(iii) In resting phase, the membrane is 30 times more permeable to potassium ions (K+) than the sodium (Na+) ions. As a result of which inside the axon there is more K+ and likewise more Na+ on outside of the membrane. Inner side of polarized membrane is electro negative while the outer side is electropositive.
(b) Depolarization of the membrane of a nerve fibre:
(i) 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
(ii) These effects lower the overall cation concentration outside and enhance that inside the membrane
The membrane is, thus deposited, with its interior becoming electropositive to the exterior.
(iv) The depolarization spreads a local current. It induces nearby passive Na+ channels to open and so to depolarize the hearby site.
(v) Hence the initial depolarization passes outward over the membrane and spreads out in all directions from the site of stimulation.
© Conduction of nerve impulse along a nerve fibre:
(i) It is a property of nerve fibre to become excited by a stimulus and then conduct that stimulus for the required and appropriate response.
(ii) In conducting a stimulus, the nerve axon has to pass through resting phase to active phase and
then the recovery phase.
(I) Resting phase : In resting phase of the nerve fibre,the axon membrane is 30 times more permeable
to potassium (K+) ions than to sodium ions (Na+).
Thus sodium ions are outside. The membrane
shows a net positive (+) charge on the outside
and negative charge inside due to the presence of
organic radicals. It is called resting potential, which remains at a value of - 70 mV and the membrane is said to be polarized. The axon membrane remains
impermeable to them so long as the nerve is at rest,
(II) Active phase : When the nerve fibre is stimulated
electrically, thermally, chemically or mechanically,
a disturbance occurs at the point of stimulation. It
is called local excitatory state. The axon membrane
is said to be depolarized. It results a wave of excitation along the nerve fibre get spread which
is called nerve impulse. Thus during active phase of the nerve fibre, K+ diffuses out of the axon membrane and Na+ ions leaving it so that positive ,and negative charges on the outside and inside of
the axon membrane are reversed. The change in membrane potential is called action potential.
(III) Recovery’ phase : In this phase the reverse
phenomenon takes place i.e., the axon membrane is positively charged outside and negative charge
inside due to an active transport of Na+ outside and
K+ ions inside the axon.
Two factors that can increase the velocity of nerve
impulse are:
(a) Myelination and (b) The diameter of fibre.
(I V) Transmission of a Nerve impulse across a chemical Synapse:
(a) The physiological junction between two
neurons across which nerve impulses can be
transmitted.
(b) Synapse occur between the knob like axon
endings of one neuron and the dendrites of cell
body of another. .
© 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 and 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.