This nervous system controls internal environment or working of visceral organs. It gives response to internal receptors and is involuntary except micturition.
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| Formation of typtcal Spinal never |
ANS consists of motor nerve fibres only. On the basis of location of ganglion ANS is divided in two parts.
a) Sympathetic Nervous system : It is formed by 22 pairs of sympathetic ganglia linearly arranged on two sympathetic cords running on either on either side of the vertebral column. It is connected to CNS by spinal nerve fibres. This system works during stress, pain, anger, fear or emergency (Fight, Flight or Fright). It secretes Adrenaline or nor adrenaline as emergency hormones.
b) Parasympathetic nervous system : This system consist of nerve fibres running along some cranial nerves and sacral nerves. Parasympathetic ganglia present on the side of visceral organs like heart, lungs, stomach, kidney etc. and parasympathetic fibres, This system releases acetylcholine which acts as neurotransmitter as well as inhibitor of visceral organs. This system works during rest and brings about relaxation, comfort, pleasure etc.
Excitation and Conduction of Nerve Impulse :
What is a nerve impulse and how is it generated and propogated? Why the membrane of a neuron is polarised?
Neurous are excitable cells surrounded by extracellular fluid (ECF). Their membrane is selectively permeable permeable to different ions. In the resting state i.e. when the neuron is not conducting any impulse, it is in a polarised state. At this time the interior of axon is more electronegative (due to less cations) as compared to ECF which is electropositive (due to more cations). The major cations are potassium ions (K+) and sodium ions (Na+).
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| Conduction of nerve Impulse |
On application of a stimulus the membrane becomes freely permeable to cause the rapid influx of the Na+. This action reverses the polarity at that site i.e. the inner side becomes positively charged. The charge of polarity results in potential difference at the site and the axonal end. As a result local circuit is formed and current flows from dendron to axon. In the same manner sequential local circuits are formed along the length of the axon and the impulse is conducted along length of a nerve fibre. The stimulus induced permeability to Na+ is short lived. Once the impulse is conducted the membrane is repolarized and the fibre becomes responsive to further stimulation once again. Then the impulse is transmitted to next neuron.