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Generation, conduction and transmission of nervous impulse

Neurons are excitable cells because their membranes are in a polarised state. Do you know why the membrane of a neuron is polarised? Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e. resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium tons (Na+). Similarly. the membrane Is impermeable to negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains high concentration
of K' and negatively charged proteins and low concentration of Na+. In contrast, the fluid outside the axon contains a low concentration of K+, a high concentration of Na+ and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell.

 As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface become negatively charged and therefore is polarised. The electrical potential difference across the testing plasma membrane is called as the resting potential(-70mv).

Some important terms
Stimulus:- a charge which can generate an action potential.
Threshold stimulus:- stimulus if minimum strength that can generate an action potential. After receiving threshold stimulus a nerve fibre will conduct an impulse.

Threshold stimulus - 55 to - 6o mv

Leak channel- Na+ and k+ move along with concentration gradient.
Resting membrane is more permeable for k+

The Mechanism of generation of nerve impulse and it's conduction
Depolarisation- opening of Na+ voltage gated channels.
Influx of Na+ inside- inside of the membrane become (+)ve and outside is (-)ve action potential.
Repolarization- voltage-gated k+ channel open efflux of K+ outside- inside become (-)ve and outside become (+)ve
Hyperpolarisation- inside become more (-)ve due to excess efflux of K+ represent by black curved line
Refractory period - In this period second stimulus can't generate impulse. This period for axon is 1/1000th of a second.

          In myelinated neurons
Saltatory conduction- impulse move one node to another node not all through axonal membrane and the characteristic of
vertebrates is faster mode of conduction, speed 120m/s and also economical.

Speed of conduction - depend on myelination - speed is greater for myelinated neurons
Diameter of axon - larger in diameter more in speed of conduction
Temperature- speed of conduction is more in warm blooded animal.

             Transmission of Impulse
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. Transmission of an impulse across
electrical synapses is very similar to impulse conduction along a single.
axon. Impulse transmission across an electrical synapse is always faster
than that across a chemical synapse. Electrical synapses are rare in our
System.
Chemical synapse- in this case impulse can be modulate due to chemical synapse but in case of electrical synapse this is not possible and this is more advance then electrical synapse.



   Transmission of nerve impulse at
              Synapse
- opening Ca++ gated channels on the membrane of synapse knob
- influx of Ca++
- exocytosis of neurotransmitters from pre-synaptic membrane.
- neurotransmitters bind to receptors on post- synaptic membrane
- excitatory neurotransmitters(  depolarisation of post-synaptic membrane)
- inhibitory(hyperpolarisation)

 Electrical synapse- in this case synaptic cleft(d) is approx 2nm and impulse jump directly from pre-synaptic to post-synaptic

Generation , conduction and transmission of nerve impulse

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