Module 3 : Physiology

Lecture 14 : Nervous System (Part-II)

 

  1. Spinal Cord

Spinal cord is continuation of medulla oblongata which comes out from foramen of magnum and continues in neural canal of vertebral column. It is also covered by duramater, arachnoid and piamater. Narrow space between duramater and vertebra is known as epidural space. The outer part of spinal cord is of white matter while inner part is of grey matter. The grey matter projects outside and forms the dorsal and ventral horn. Dorsal and ventral horn continues in a tube like structure known as root of dorsal and ventral horn. In the root of dorsal horn, ganglia are present known as dorsal root ganglia. Sensory neurons are found in the dorsal root ganglia whose axon extend and get embedded into grey matter of spinal cord. These sensory neurons make synapse with ventral root neuron. Motor neurons are found in ventral root whose cyton is found in ventral horn while its dendrons are embedded into grey matter of spinal cord. Both sensory and motor nerve fiber combined and comes out from intervertebral foramen and form spinal nerve.
Spinal cord acts as bridge between brain and organ of the body. It regulates and conducts the reflex action as well as it provides relay path for the impulses coming from brain.

Reflex Action
Reflex actions are involuntary actions, completed very quickly as compared to normal actions.
Reflex actions are of two types:

  1. Cranial reflex: These actions are completed by brain. These are slow actions e.g. watering of mouth to see good food.
  2. Spinal reflex: These actions are completed by spinal cord. These are fast actions e.g. withdrawal of arm at the time of pinching by any needle.

Peripheral Nervous System
All nerves arise from brain and spinal cord are included in peripheral nervous system. Nerves arises from brain is termed as cranial nerve whereas nerves arises from spinal cord is termed as spinal nerves. All reptiles, birds and mammals have 12 pairs of cranial nerve. Amphibians and fishes have only 10 pairs of cranial nerves. In human, I, II, and VIII cranial nerve are pure sensory in nature. III, IV, VI, XI and XII cranial nerve are motor nerve and rest others out of 12 cranial nerves are mixed type of nerves.