Peripheral nervous system

Question 1

How is the PNS functionally divided?

Answer 1

The nervous system is divided in central nervous system and peripheral nervous system: the CNS is formed by the brain and the spinal cord, and bring sensory information to the brain, where they are processed and transformed in motor impulses; the PNS is subdivided in enteric NS, the one that control organ contraction, and the autonomic NS, which regulates visceral function. The ANS is subdivided in sympathetic and parasympathetic NS, based on the function these nerves have: the sympathetic is resposible for the fight/flight action, while the parasympathetic NS is resposnible for the rest and digest action.

Neurons are structurally different depending on the function they have: somatic neurons have the cell body in the CNS and a very long axon, or somatic motor fiber, in the PNS; the sympathetic neurons have the cell body in the CNS, a pre-ganglion fiber, a ganglion with which the neuron makes synapse with, and the organ to which it is connected to; the parasympathetic neurons are the opposite than sympathetic ones, having the cell body in the CNS, than they make the synapse with the ganglion and then a very long post-ganglion fiber connect the CNS to the organ.

Question 2

What are spinal nerves? What is their function? How do they develop?

Answer 2

There are 31 pairs of spinal nerves:

• 8 cervical pairs
• 12 thoracic pairs
• 5 lumbar pairs
• 5 sacral pairs
• 1 coccygeal pair

These spinal nerves control the whole body: sympathetic neurons, which are on the top and on the bottom of the spinal cord, are responsible for the motor output to body wall, thoracic cavity, limbs, head and neck; the motor neurons, start from the ventral part of the spinal cord and grow via the dorsal root ganglia; the sensory neurons run via dorsal root ganglia.

Spinal nerves are organised in dermatomes, segments, which is innervated by sensory fibers of one spinal nerve.

Question 3

How does PNS develop?

Answer 3

At week 5, neurulation start and the neural crest form, which will develop into the PNS.

1. Due to neuronal migration the sympathetic ganglia are formed as well as bilateral chain, as the second migration occur, the prevertebral ganglia forms, and as he third one occur, the organ plexi.
2. the somatic motorneurons bundle together, from the cervical part in a cranio-caudal direction.
3. somatic, autonomous and sensory neurons bundle together, forming the spinal nerves.
4. the sympathetic chain ganglia form.
5. growth of axons from sympathetic chain ganglia towards target organs.