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

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.

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.

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

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.

Receptor types

7

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

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

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.

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

Events in Sensory Transduction

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.

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

3 4 5

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

Nerve Fiber Classification

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 0.5-120 m/sec.

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.

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

REFERENCES Arthur C. Guyton, 2005, Text Book of Medical Physiology, WB Saunders’s, USA. Kathleen, J.W. Wilson and Anne Waugh. 1998. Ross and Wilson Anatomy and Physiology in health and illness. (8th Edition). Churchchill Livingstone, New York.