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 Laws Related to Sensory Physiology

A. Muller’s Doctrine of specific nerve energies:

No matter where along the nerve pathway one stimulates, the type of sensation will depend on which part of the brain is finally going to be stimulated.


Law of Projection:

No matter where along the nerve pathway one stimulates, the sensation will be felt at the site of the receptor. Eg PHANTOM LIMB


C.  Weber Fechner Law & Stevens’ power law

These two laws relate the sensation felt (S) to the intensity of the stimulus (I).

S = K log I – Weber-Fechner Law

S = KIa – Steven’s Power Law


Bell-Magendie Law

This law states that the dorsal root is sensory and ventral root is motor.


Labelled – line theory

All the sensations from the different parts of the body travel along specified paths.



  1. In the lateral spinothalamic tract, fibers from the lower parts of the body are placed laterally.
  2. In the posterior column, fibers from the lower parts of the body are placed medially.



Sensory Cortex

  • Present in Postcentral Gyrus area 3,1,2
  • Body is represented Vertical & upside down called as Sensory homunculus.
  • Not only is there detailed localization of the fibers from the various parts of the body in the postcentral gyrus, but also the size of the cortical receiving area for impulses from a particular part of the body is proportionate to the use of the part.
  • The cortical areas for sensation from the trunk and back are small, whereas very large areas are concerned with impulses from the hand and the parts of the mouth concerned with speech.



Representation in Cerebral cortex is : (AIIMS May 09)
A. Vertically  
B. Horizontally
C. Tangentially     
D. Tandem



A. Vertically




Lesion of Post Central Gyrus

  • Decreases the sensations, but do not entirely abolish them.

a. Fine touch & proprioception are almost totally lost.

b. Temp. sensibility less affected.

c. Pain sensibility only slightly affected.


NOTE: A certain degree of perception is possible in the absence of cortex.


Upon Recovery

a.  Pain sensibility returns first.

b.  Followed by temperature sense.

c.   & finally proprioception & fine touch.

Types of Inhibition:



This is because of the release of an inhibitory neurotrasmitter (NT) from neuron A; this produces an IPSP in neuron B.


Pre-synaptic Inhibition

‘A’ releases an excitatory NT, to excite B. But ‘C’ ends presynaptically as ‘A’ to decrease the release of the excitatory NT. Salient Features of Presynaptic Inhibition

a.     It is example of axo-axonic synapse

b.     It is stimulated by general anaesthetics

c.      It is inhibited by picrotoxin

d.     It is mostly due to GABA



Example: Basket / Stellate cell inhibiting the purkinje cell in the cerebellum


Feed-forward inhibition: ‘A’ stimulates ‘B’; but ‘B’ inhibits ‘C’


Feedback or Renshaw cell inhibition:
‘A’ stimulate the muscle. However, a collateral from A (‘B’) ends on an inhibitory interneuron (C) in the anterior horn of the spinal cord. This inhibitory interneuron is called a Renshaw cell.


Lateral or afferent or surround inhibition:
This is one of the cornerstones of sensory physiology. Stimulation of required neuron inhibits the adjacent neurons (by a collateral). This enhances contrast and sharpens the image.

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