Gray and white rami communicans of spinal nerves

Gray and white rami communicans 
Gray and white rami communicans are related with sympathetic system.

Both gray and white rami communicans are attached with ventral ramus of thoracic and lumbar spinal nerves. Only gray rami communicans are attached with ventral ramus of cervical, sacral and coccygeal spinal nerves.

Through white rami communicans pre-ganglionic sympathetic fibre leave the spinal nerves and enter into sympathetic ganglia . pre- ganglionic fibres are myelinated so it looks white. Through gray rami communicans post-ganglionic sympathetic fibre again enter into the spinal nerve , leave the sympathetic ganglia. Post ganglionic sympathetic fibres are non- myelinated so it looks gray.

Gray and white rami communicans are the lateral branch of sympathetic ganglia which connect ventral ramus of corresponding spinal nerve. White rami communicans has distal connection with ventral ramus of spinal nerve. Gray rami communicans has proximal connection with ventral ramus of spinal nerve. In thoracic and lumbar region pre-ganglionic fibres reach to sympathetic ganglia by using white rami communicans and corresponding ventral ramus of spinal nerve . in cervical and sacral region there are no corresponding white rami communicans of ventral ramus of spinal nerves . pre- ganglionic fibres enter into the corresponding sympathetic ganglia , some are ascend to cervical sympathetic ganglia and some fibres are descend to the sacral sympathetic ganglia. So cervical and sacral region lacks white rami communicans .

 pre-ganglionic sympathetic neurons are present in the lateral horn of gray matter of spinal cord ... pre-ganglionic fibres leave the spinal cord through the ventral root  of spinal nerve, spinal nerve trunk,.... through white rami communicans they enter into the sympathetic ganglion   

Post ganglionic neurons are present within the sympathetic ganglia ---- post -ganglionic fibres leave the sympathetic ganglia through gray rami communicans --- again enter into the ventral ramus of spinal nerve and distributed along the ventral and dorsal ramus of spinal nerve --- they innervate smooth muscle cells of vessel, sweat gland , arrector pili muscle(smooth muscle )  of the skin