Page 6 - LN-NEURAL CONTROL & COORDINATION

Re-polarization
+
(i) The rise in the stimulus-induced permeability to Na is extremely short-lived.
+
It is quickly followed by a rise in permeability to K .
+
+
(ii) Within a fraction of a second, Na influx stops and K outflow begins until
the original resting state of ionic concentration is achieved. Thus, resting
potential is restored at the site of excitation, which is called repolarisation of
the membrane. This makes the fibre once more responsive to further
stimulation.
(iii) In fact until repolarisation occurs neuron cannot conduct another impulse.
The time taken for this restoration is called refractory period.
Note:
* When an impulse travels along a myelinated neuron, depolarisation occurs

only at the nodes of Ranvier. It leaps over the myelin sheath from one node to
the next. This process, is called saltatory conduction.
* This process accounts for the greater speed of an impulse travelling along a
myelinated neuron than along a non-myelinated one. It is upto 50 times faster
than the non-myelinated nerve fibre.
A nerve impulses is transmitted from one neuron to another through junctions
called synapses. It is formed by the membranes of a pre-synaptic neuron and a

post-synaptic neuron.
There are mainly two types of synapses
Electrical Synapses
(i) The membranes of pre and post-synaptic neurons are in very close proximity
(i.e., in continuity). The continuity is provided by the gap junction (small
protein tubular structures) between the two neurons.
(ii) In electrical synapse, there is minimal synaptic delay because of the direct