What is referred pain

1. When the sensation of pain is experienced at the site other than the injured or diseased part, it is called referred pain. If it seems to spread from the local area to the distant site, it is called radiating pain . For example:
(i) Phantom limb pain .
(ii) Pain of stone in the gall bladder referred to the tip of the right shoulder.
(iii) Pain in testicles due to stone in the ureter.
(iv) During a heart attack, pain is often experienced at the tip of left shoulder and tends to spread in the inner aspect of left arm.

2. Pain is usually referred to a structure that developed from the same segment during the embryonic development. This principle is called the dermatomal rule. For example, the heart and the arm have the same segmental origin; similarly, the testicles and kidney developed from the same primitive urogenital ridge.

3. Theories of referred pain 
(i) Convergence theory
(a) The nerves from the visceral structures and the somatic structure to which pain is referred enter the CNS at the same level and converge on the same spinothalamic neurons.
(b) Since somatic pain is far more common than visceral pain, therefore, when the same afferent pathway is stimulated by signals that originate in visceral afferent nerves, the signal that reaches the somatosensory cortex is identical and is misinterpreted as having arisen within the somatic area.

(ii) Facilitation theory. The afferent impulses from visceral structures produce subliminal fringe effects that lower the excitability threshold of spinothalamic neurons which receive afferent fibers from somatic areas. Therefore, any slight activity in the pathways transmitting pain impulses from somatic regions, and which normally would die out within the spinal cord, is facilitated and thus reaches conscious levels.

properties of receptors

a. Specificity
Each type of receptor is highly specific for a particular stimulus for which it is designed and is non responsive to normal intensities of other type of stimuli (e.g) Rods and cones respond to normal intensities of light, but respond to only high intensity of touch Adequate stimulus: The stimulus which can easily stimulate a receptor is the adequate stimulus for that receptor (e.g) Light is the adequate stimulus for rods & cones. & Labelled Line Principle:

The specificity of nerve fibers for transmitting only one modality of sensation is called labeled line principle.

d. Doctrine of specific nerve energies:

Also called as Muller’s law. The sensation evoked by impulses generated in a receptor depends

in part upon the specific part of the brain they ultimately activate. Law of projection:

When a stimulus is applied anywhere in the pathway of a sensation, the sensation is projected to

the receptors. (c.g) Phantom limb & Phantom pain

Phantom Limb: The non existing limb in an amputated person gives the sensation of pain &

proprioception as if it is existing.

Phantom pain: The pain sensation from the non existing limb of an amputated person can be explained by law of projection ie., the stimulus applied anywhere in the pathway causes projection of sensation to receptors)

Mechanism Amputation formation of neuromas→→ discharge of impulses by pressure or Spontaneously sensation produced is projected to the place where the receptors were

presented L Adaptation:

Reduction in sensitivity of receptors in the presence of a constant stimulus Phasic receptors: Fast adapting receptors (e.g) receptors for smell & pacinian corpuscles.

Tonic receptors: Slow-adapting receptors (e.g) proprioceptors

Receptors that do not adapt at all Pain receptors (Nociceptors)

write short note on properties of receptors

PROPERTIES OF RECEPTORS

a. Specificity:
Each type of receptor is highly specific for a particular stimulus for which it is designed and is non responsive to normal intensities of other type of stimuli (e.g) Rods and cones respond to normal intensities of light, but respond to only high intensity of touch.
b. Adequate stimulus:
The stimulus which can easily stimulate a receptor is the adequate stimulus for that receptor.
(e.g) Light is the adequate stimulus for rods & cones.
c. Labelled Line Principle:
The specificity of nerve fibers for transmitting only one modality of sensation is called labeled
line principle.
d. Doctrine of specific nerve energies:
Also called as Muller’s law. The sensation evoked by impulses generated in a receptor depends
in part upon the specific part of the brain they ultimately activate.
e. Law of projection:
When a stimulus is applied anywhere in the pathway of a sensation, the sensation is projected to the receptors. (e.g) Phantom limb & Phantom pain
Phantom Limb: The non existing limb in an amputated person gives the sensation of pain & proprioception as if it is existing.
Phantom pain: The pain sensation from the non existing limb of an amputated person can be explained by law of projection ie., the stimulus applied anywhere in the pathway causes projection of sensation to receptors
 Mechanism
Amputation → formation of neuromas → discharge of impulses by pressure or Spontaneously→ sensation produced is projected to the place where the receptors were presented.
f. Adaptation:
Reduction in sensitivity of receptors in the presence of a constant stimulus
• Phasic receptors: Fast adapting receptors (e.g) receptors for smell & pacinian corpuscles.
•Tonic receptors: Slow – adapting receptors (e.g) proprioceptors
•Receptors that do not adapt at all – Pain receptors (Nociceptors)

g. Intensity discrimination:
Weber Fechner Law: The magnitude of sensation felt is proportionate to the log of intensity of stimulus
                                            R=KSA
(R = Magnitude of sensation felt, S=intensity of stimulus, K & A = constants)
Intensity discrimination depends upon
• Number of receptors stimulated (spatial summation)
• Frequency of action potential reaching the cortex (Temporal summation)