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L2, sensory receptors and sensory transduction - physiology

 sensory receptors and sensory transduction

 Lecture video



Data 

 Definition of receptors

Receptors are specialized structures present at the peripheral end of afferent neurons and are sensitive to changes in the environment

i.e. sensitive to stimuli.

Function

Receptors respond to different forms of stimuli (energy)., e.g. thermal, chemical electrical, etc., and transform it to electric energy or depolarization

i.e. they act as transducers


Mechanism 


Classification


Physiological classification -


a-Mechanoreceptor


Stimulated by mechanical stimuli, which cause deformation of the receptors


Examples

Stretch receptors present in skeletal muscle [muscle spindle] and wall of blood vessels

 Tension receptors present in tendons of muscles [Golgi tendon receptor

Touch and pressure receptors present in the skin and subcutaneous tissue

Joint receptors: detect position and movement sense

[ Auditory receptors [Organ of Corti

Vestibular receptors [Macula & Crista ampullaris]


b-Chemoreceptors

Responds to chemical stimuli, may be
 External chemoreceptors: detect exogenous chemical stimuli
 e.g
taste and smell receptors.
Internal chemoreceptors: detect endogenous chemical stimuli
e.g
- Oz and CO2 receptors: peripheral in carotid and aortic bodies and central in the medulla.
- Glucoreceptors and osmoreceptorsin hypothalamus.



C- Thermoreceptors: 

cold and hot receptors



d-Pain receptors [Nociceptors]:

Responds to injurious or noxious stimuli (tissue damage).


e-Photoreceptors [Light or Electromagnetic receptors]:

Responds to light waves as rods and cones inthe retina.



According to the site


a-Superficial or cutaneous receptors (Exteroceptors):

 present in skin and subcutaneous tissues

e.g.
 Free nerve endings for pain, temperature and crude touch.
 Hair follicle, Meissner's corpuscle, Pacinian's corpuscle, and Merkel's disc for touch  


b-Deep receptors (Proprioceptors):

 present in structure deep to skin,
e.g.
 Free nerve endings for pain.
 Pacinian's corpuscle in joint capsule for sense of movement.
Ruffini's endings and receptor similar to Golgi tendon organ [Spray type endings] present in joint capsule for sense of position.
Golgi tendon organ in the tendon for tension sense
Muscle spindle stretch receptor in the muscle


Visceral receptors: 

present in internal viscera,

eg.
 Free nerve endings for pain.
Stretch receptors in the wall of hollow viscera as urinary bladder and rectum.
Chemoreceptors, e.g. in carotid and aortic bodies.

properties

Specificity (Muller's Law of Specific Nervous Energy)

•- Each receptor is most sensitive to a particular stimulus called the adequate stimulus, e.g.. rod and cones are most sensitive to electromagnetic waves.
•- Each receptor when stimulated gives only one type of sensation called the modality [Type] of sensation specific to that receptor regardless the method of stimulation

Cold sensation


Adaptation

Means gradual decrease in response inspite of constant stimulation.


Classification of receptors according to the speed (rate) of adaptation


• a. Slowly adapting ( tonic or static ) receptor 

e.g. Pain , mechanoreceptors (muscle spindle, Golgi tendon receptor, and vacular baroreceptor


• B. Rapidly adapting [phasic] 

e.g. touch receptors as hair receptors Pacinian corpuscle or Meissner's corpuscles.


• c. Moderately adapting receptors

 e.g., temperature or pressure receptor

The rate of adaptation depends on the importance of sensation carried by the receptor , the more important the sensation (e.g.. pain) the less the rate o adaptation 



Mechanism of adaptation is not the same for all receptors, e.g
A- mechanoreceptors 
e.g.
 Pacinian corpuscle
Readjustment remodelingj in the structure of the receptor due to visco-elastic properties of its lamellae
2. Accommodation of the nerve fiber to the stimulus due to inactivation of Na" channels as a result of continuous current flow 

b. Photoreceptors (rods and cones)
adapt by changing the concentration of the light sensitive pigment inside it.


Excitability (Receptor potential, generator potential

•When receptor is stimulated, it responds by a state of partial depolarization called receptor potential or 

 generator potential.

Properties of generator potential


a. It is caused by increased permeability to
Na+
b. It is a graded response, i.e. when the stimulus is increased its amplitude and duration increases [Does not obey "all or non" law
c. It is not followed by absolute refractory period.
d. It can be summated.
e. Its duration is more than 5 m.sec, ie. longer than the duration of action potential.
f. When reach firing level of the afferent nerve it leads to repetitive action potential along the afferent nerve. The frequency of action potentials along the afferent nerve is directly proportional to
 logarithm the intensity of the stimulus [Weber- Fechner Law
i.e.
when the intensity of the stimulus increases 100 folds the frequency of impulses along the afferent increases only 2 folds and when the intensity of the stimulus increases 1000 folds the frequency of
impulses along the afferent increases only 3 folds.


Compression character of receptor

The receptors compress marked changes in stimulus intensity into smaller changes in frequency of action potential along the afferent nerve and hence actual sensation of changes in stimulus intensity.












































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