09 Control and Coordination - part 09 - Peripheral Nervous System (PNS)

 

09 Control and Coordination - part 09 - Peripheral Nervous System (PNS)

Peripheral Nervous System (PNS) : 

• Connects the central nervous system to the different parts of the body having receptors and effectors. 
• Depending on the connection to the CNS, the peripheral nerves are classified into two main types -

1. Cranial nerves - connected to the brain.
2. Spinal nerves - connected to the spinal cord.
   
   
   
   
   
   

1. Cranial Nerves : 

• Develop from the brain, in all amniotes (reptiles, birds and mammals). 
• There are 12 pairs of cranial nerves. 
• Roman numbering I to XII is used to denote them. 
• Originate from or terminate into the brain.
• According to their function, these are classified as- 

1. sensory (I, II, VIII)
2. motor (III, IV, VI, XI, XII) and 
3. mixed (V, VII, IX, X) nerves.

2. Spinal Nerves : 

• Thirty one pairs of spinal nerves originate from the spinal cord. 
• They are mixed nerves.
• Provide two way communication between the spinal cord and part of the upper and lower limbs, neck and trunk.
  

Do you know ?

1. Of the 12 pairs of cranial nerves, only the X (vagus) passes into the body and innervates internal organs.
2. Vagus has the maximum number of branches and longest distribution.
3. V/trigeminal/Dentist’s nerve is the largest cranial nerve
4. VI/abducens is the smallest cranial neve.

Formation of a typical spinal nerve : 

• Mixed type.
• Have some nerve fibre as sensory and some motor. 
• Formed inside the neural canal of verterbral column by two roots - 

1. Posterior or dorsal sensory root and 
2. Anterior or ventral root. 

• Anterior root receives the sensory nerve from the dorsal root ganglion (cell bodies of sensory neurons are located in the ganglion), while the anterior/ventral root gives out the motor nerve. 
• The dorsal sensory and the ventral motor nerves together form the mixed spinal nerve. 
• It emerges out from both sides of the spinal cord through the intervertebral foramen. 
• As soon as it emerges out of vertebral column, it shows three branches viz-

1. Ramus dorsalis
2. Ramus ventralis
3. Ramus communicans

1. Ramus dorsalis : 

• From skin and to muscles of dorsal side

2. Ramus ventralis : 

• Largest of the three supplies the organs and muscles on lateral and anterior side

3. Ramus communicans : 

• Smallest of the three.
• Given out from 1st thoracic upto 3rd lumbar (L3) spinal nerve. 
• It joins the sympathetic ganglia.

Reflex Action : 

• Sudden, spontaneous automatic, involuntary response to stimulus.
• The response to stimulus is said to be involuntary as it is carried out without any conscious effort by the brain. 
• The path along which the action is carried out is called reflex arc.
• Human nervous system is divided into CNS, PNS, ANS. 
• PNS consist of network of nerves arising from or going to the CNS.
• Accordingly peripheral nerves are classified as

1. Afferent nerves
2. Efferent nerves

Afferent nerve fibres :

• Transmit sensory impulse from tissue or organ to the CNS.

Efferent nerve fibres :

• Transmit regulatory or motor impulses from the CNS to the various peripheral tissues and organ.
• However according to recent studies, the extent of PNS has been broadened to incorporate the ANS.
• According to this view, the PNS is divided into - 

1. Somatic nervous system and
2. Autonomic nervous system

• The somatic nerous system relays impulses from CNS to the skeletal or voluntary muscles of the body.