1. SENSORY SYSTEM – CLASSIFICATION OF SOMATIC SENSES, RECEPTORS AND SENSORY TRANSDUCTION
CLASS: I M.Sc.,
UNIT: 4
PREPARED BY: A. BENNO SUSAI

2. Sensory Systems All nerve impulses arrive to CNS as Action Potential
They reach different brain regions so different senses sensed
Intensity depends upon number of action potentials received.

3. Sensory Receptors
Input to the Nervous System is provided by sensory receptors that detect such sensory stimuli as Touch, Sound, Light, Pain, Cold and Warmth.

4. Types of Sensory Receptor and its Stimuli
Mechanoreceptor
Which detect mechanical compression of stretching of the receptor or of tissues adjacent to the receptor.
2. Thermoreceptor
Detect change in temperature.
Nociceptor ( Pain Receptor)
Detect damage occurring in the tissues due to physical or chemical damage

5. Electromagnetic receptors
Detect light on the retina of the eye.
Chemoreceptors
Detects taste in the mouth, Smell in the nose, Oxygen level in the arterial blood, Osmolality of the fluids, CO2 concentration.

6. Receptor types

7. 7

8. Sensory Transduction
Sensory cells respond to stimuli because they possess stimulus-gated ion channels in their membranes.
Sensory stimulus produces a change in the membrane potential.
receptor potential
greater the sensory stimulus, the greater the depolarization of the receptor potential and higher frequency of action potentials

9. Categories of Sensory Receptors
Sensory information is conveyed to the CNS and perceived in a four-step process.

10. Modality of Sensation Labeled Line:
All of the principal types of sensation ~ Touch, Sound, Light, Pain, Cold and Warmth are called Modality of Sensation.
Each( Any) receptor will sense the sensations by the point in the nervous system by which the fiber leads.
This specificity of nerve fibers for transmitting only one modality of sensations is called the labeled line principle.

11. Classification of sensory Receptors
S.No
Receptor Types
Receptors 1.
Mechanoreceptor
Skin Tactile sensibilities, Deep tissue sensibilities, Hearing, Equilibrium, Arterial Pressure. 2.
Thermoreceptor
Warm and cold 3.
Nociceptor
Pain 4.
Electromagnetic receptors
Vision 5.
Chemoreceptors
Taste, Smell, Arterial Oxygen

12. Events in Sensory Transduction

13. Action potential
If VM reaches threshold, Na+ channels open and Na+ influx ensues, depolarizing the cell and causing the VM to increase. This is the rising phase of an AP.
Eventually, the Na+ channel will have inactivated and the K+ channels will be open. Now, K+ effluxes and repolarization occurs. This is the falling phase.
K+ channels are slow to open and slow to close. This causes the VM to take a brief dip below resting VM. This dip is the undershoot and is an example of hyperpolarization.

16. 1
They have 2 gates.
At rest, one is closed (the activation gate) and the other is open (the inactivation gate).
Suprathreshold depolarization affects both of them. 2

17. 3 4 5

18. Sensory stimuli depolarize the cell resulting in a receptor Potential
1. Receptor releases
neurotransmitter on afferent nerve and
depolarizes nerve
2. Afferent nerve activity reaches
threshold and APs fire
3. APs propagate down nerve fiber to
the CNS first synapse
4. Neurotransmitter release results in
AP in post synaptic cell B.
Some sensory receptor cells a
neurons produce APs
Directly

20. Nerve Fiber
Classification

21. General Classification
While running the momentary position of the legs at each fraction of a second during running is faster.
Whereas the aching pain do not need to be transmitted rapidly.
Size of the nerve fiber determines the velocity of the conduction ie., 0.5 to 20 micrometer in dia. The range of velocity is m/sec.

22. The fibers are divided into types A and C, and the type A fibers are further subdivided into α,β,γ and δ fibers.
Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves.
Type C fibers constitute more than one half of the sensory fibers in most peripheral nerves as well as the postganglionic autonomic fibers.

23. There is an alternative classification used by sensory physiologists.
Group Ia
Group Ib
Group II
Group III
Group IV

24. REFERENCES
Arthur C. Guyton, 2005, Text Book of Medical Physiology, WB Saunders’s, USA.
Kathleen, J.W. Wilson and Anne Waugh Ross and Wilson Anatomy and Physiology in health and illness. (8th Edition). Churchchill Livingstone, New York.