Class 12 Nervous System Spinal Cord NEET AIIMS

Spinal Cord

Spinal Cord

           45 cm. A long start at the lower end of the medulla sixth ends at the level of 1st lumbar vertebrae (L1).

           It is situated in the Neural canal of the vertebral column.

           It has a dorsal fissure and a ventral fissure.

           The central cavity is called a neurocoel, which originates from the metacoel and it is full of CSF.

           At the lower end, it tapers known as Conus Medullaris after that it becomes a non-nervous & thread like called filum terminate.

           White matter is external & divided into three columns or Funciculi i.e., ventral, dorsal and lateral. Funculi have bundles of nerve fibres which descend or ascend and are called nerve tract two types.

       1.   Ascending Tract  

              Sensory or different in nature. Conduct sensory impulses to the brain. Mainly located in dorsal funiculus.

       2.   Descending Tract

Motor or efferent in nature, Conduct motor impulses away from the brain.         

Mainly located in ventral and lateral Funiculus.


       It conducts sensory and motor impulses to and from the brain, It controls reflex action of the body.


           In Brain, Grey matter is external and white matter is internal.

           In Spinal Cord, Grey matter is internal and white matter is external.

           In Frog Cord, Grey matter is Rectangular.

           In mammals, it is Butterfly shaped.

           The premotor area in the frontal lobe is the highest centre for the muscle for the autonomic nervous system.

Autonomic nervous system

 This system controls and coordinates the involuntary activity of various organs.

    It controls such activity in which voluntary will power is not involved e.g. body secretion.

           Autonomic N.S. is not autonomic, but it is also controlled by the brain.

           Both sympathetic & parasympathetic N.S. are antagonist to each other, but neither is exclusively excitatory nor inhibitory.

           It consists of the sympathetic and parasympathetic nervous system.

Sympathetic Nervous System

           Also known as thoracolumbar because they are made up of a thoracolumbar chain of the ganglion.

           Sympathetic nerve release Adrenaline, therefore, called adrenergic nerve fibre.

           The sympathetic chain is formed of 21 lateral or chain ganglion – 3 cervical, 12 thoracics, 5 lumbar and sacral. In an emergency, both the sympathetic system and adrenal gland work together and this integral unit is called the sympathetic-adrenal system.     

           The sympathetic ganglia give rise to post-ganglionic nerve fibres to the visceral organs.

           There are three pre-vertebral ganglia in the abdominal region of viz. —celiac, superior mesenteric & inferior mesenteric ganglia.

           The pre-ganglionic nerve fibres are smaller and they secrete acetylcholine as a neurohormone, hence they are cholinergic.

Parasympathetic Nervous System

           It’s also known as Craniosacral outflow because Parasym] thetic information travel through cranial nerve – 3, 7, 9, 10 & sacral plexus.

           Parasympathetic nerve release Acetyl-choline so-called Cholinergic nerve fibre. Auerbach Plexus & Meissner Plexus – Plexus of parasympathetic neuron present on wall intestine, and they control peristalsis and secretion of the intestine.

           The parasympathetic pre-ganglionic fibre of III cranial nerve ends into a ciliary ganglion. The post-ganglion fibre from this ganglion innervate iris & ciliary body of the eye.

           The pre-ganglionic fibre of IX cranial nerve ends into an otic ganglion. It gives out post-ganglionic nerve fibres which innervate the buccal cavity and parotid salivary glands.

           The pre-ganglionic fibre of X cranial nerve innervates muscles of the alimentary canal, heart, larynx, lungs, BVs, liver, gall bladder, pancreases etc.

           The pre-ganglionic fibres of sacral spinal nerves innervate intestine, kidneys, urinary bladder, reproductive organs etc.

           The pre-ganglionic nerve fibres are much longer than the post-ganglionic nerve fibres.

           They secrete acetylcholine as a neurohormone.

Function of ANS

           The autonomic nervous system is involuntary so that the brain is not indulged.

           It co-ordinates various body organs.

           The autonomic nervous system can meet emergencies and combat stress.

           Autonomic nervous system by its two components sympathetic and parasympathetic can accelerate, deaccelerate inhibits, the activity of various organs.

           It helps in the coordination of the body.

          The response of various organs towards ANS is as follows:–

Reflex Action

           It is an involuntary, autonomic, immediate response towards a stimulus without the conscious will of an organism.

           It is the autonomic response of the body.

           Reflex action is comparatively more rapid than normal reactions.

           The reflex arc is the arrangement of neurons in the pathway that always passes through the central nervous system.

           The reflex arc is formed by receptor-spinal cord-effector (muscle or gland).

           The reflex arc may be monosynaptic (sensory and motor) or polysynaptic (sensory, association or internuncial or interneurons and motor neurons).

           Reflex actions are of two types: simple reflex and acquired reflexes.

Reflex Arc

           It is the path travelled by nerve impulse generated by sense organ in response to an external stimulus to reach the effector organ during a reflex action.

           The reflex arc consists of:–  

             Sense organ – Sensory neuron – Spinal cord – Motor neuron – Muscle of effector organ  (afferent)   (efferent)

           Reflex action is of two types : (a) Simple Reflex      (b) Conditioned Reflex

Simple Reflex

           In this way, no learning is required.   

           The simple reflex is also known as an unconditioned reflex. It is inborn, the unlearned reflex to a stimulus.

           The simple reflex is mostly protective in function.

           Examples of simple reflexes are :   

             ng, sneezing and yawning.

Conditioned Reflex

           A conditioned reflex is also known as an acquired reflex. It is not ibron but acquired and dependent on past experience training and learning.

           Demonstration of conditioned reflex was first made by Russian physiologist Ivan Petrovitch Pavlov (1846-1936) in the hungry dog.

           Pavlov rang the bell while feeding the dog, thus, associated the unconditioned response with additional stimulus.

           Examples of conditioned reflexes are learning of dancing, cycling, swimming, singing, driving, etc. These actions are under cerebral control during learning.

Electroencephalogram (EEG)

           The electrical activity of the brain and cerebral cortex can be recorded through instruments like electroencephalography or cathode-ray oscilloscope.

           The recording of the spontaneous electrical activity of the brain is electroencephalogram.

           The electrical tracing of the cerebral cortex is called EEG.

          Berger in 1929 was first to record EEG.            Electroencephalic record in normal human subjects consists of four different types of rhythmic waves namely Alpha, Beta, Theta and Delta waves.

(i)  Alpha wave : 10-12 cycles/sec. Alpha waves are most prominent when the brain is under quiet rest.

Alpha waves disappear when eyes remain open.

 Alpha waves disappear entirely during sleep.

       (ii) Beta waves : 15-60 cycles/sec. Beta waves appear during tension or during intense activities of CNS.  

       (iii) Theta waves: 5-8 cycles/sec. Theta waves are prominent in children between ages 2 to 5 years.

       (iv) Delta waves : 1-5 cycles/sec. Delta waves rarely found in normal adults during waking periods,  but are usually found during deep sleep.                Delta waves in EEG also indicate brain damage.