Sensory system

1. Sensory System

2. 2 General senses Receptors that are widely distributed throughout the body Skin, various organs and joints Special senses Specialized receptors confined to structures in the head Eyes, ears, nose and mouth

3. 3 Receptors, Sensation, and Perception Sensory receptors Specialized cells or multicellular structures that collect information from the environment Stimulate neurons to send impulses along sensory fibers to the brain Sensation A feeling that occurs when brain becomes aware of sensory impulse Perception A person’s view of the stimulus; the way the brain interprets the information

4. 4 Receptor Types Chemoreceptors Respond to changes in chemical concentrations Pain receptors (nociceptors) Respond to tissue damage Thermoreceptors Respond to changes in temperature Mechanoreceptors Respond to mechanical forces Photoreceptors Respond to light Osmoreceptors Respond to changes in solute concentration

5. Heat Light sound pressure chemical change Modalities

6. Receptors transform an external signal into a membrane potential Receptor Potential - separate receptor Generator Potential - a specialized ending of an afferent neuron

7. 7 Sensory Impulses Stimulation of receptor causes local change in its receptor potential A graded electrical current is generated that reflects intensity of stimulation If receptor is part of a neuron, the membrane potential may generate an action potential If receptor is not part of a neuron, the receptor potential must be transferred to a neuron to trigger an action potential Peripheral nerves transmit impulses to CNS where they are analyzed and interpreted in the brain

8. 8 Sensory Adaptation The a bility to diminish the extent of their depolarization despite sustained stimulus strength Types of receptors according to their speed of adaptation Tonic (Do not adapt or adapt slowly e.g. muscle stretch receptors and joint proprioceptors ) Phasic (Rapidly adapting receptors e.g. Tactile receptors)

9. Mechanism of adaptation

10. Nerve Fiber Classification General classification scheme A fibers: Myelinated Subtypes: a, b, g, d, some overlap in ranges Fastest conducting and largest diameter – a (120 m/sec, 20 m) B fibers: Slower myelinated C fibers: Un myelinated Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 m )

11. Nerve Fiber Classification Sensory nerve classification I, II, III fibers: Myelinated Subtypes: Ia , Ib Fastest conducting and largest diameter – Ia IV fibers: Unmyelinated Slower conducting than IIIs and smallest diameter

12. Summation Spatial Temporal Relaying of signals through neuronal pools Threshold--- Subthreshold stimuli Excitation or Facilitation Divergence and Convergence of signals passing through neuronal pool After Discharge Synaptic after discharge Reverberatory circuit Continuous signal output Rhythmical signal output

13. 13 General Senses Senses associated with skin, muscles, joints and viscera Three (3) groups: Exteroceptive senses ( exteroceptors ) Senses associated with body surface such as touch, pressure, temperature, and pain Visceroceptive senses ( interoceptors ) Senses associated with changes in the viscera such as blood pressure stretching blood vessels and ingestion of a meal Proprioceptive senses Senses associated with changes in muscles and tendons such as at joints

14. 14 Touch and Pressure Senses Free nerve endings Common in epithelial tissues Simplest receptors Sense itching Tactile ( Meissner’s ) corpuscles Abundant in hairless portions of skin and lips Detect fine touch; distinguish between two points on the skin Adapt rapidly Lamellated ( Pacinian ) corpuscles Common in deeper subcutaneous tissues, tendons and ligaments Detect heavy pressure and vibrations Expanded Tip receptors (Merkel’s discs---- Iggo dome receptors Initially strong but partially adapting Localize touch and determine texture Hair end organ Adapts readily Touch (movement of object on surface of body + initial contact) Ruffini’s endings Slow adaptation found in joint capsules

15. 15 Touch and Pressure Receptors Epidermis Dermis (a) (b) (c) Section of skin Free nerve endings Epithelial cells Sensory nerve fiber Epithelial cells Tactile (Meissner’s) corpuscle (touch receptor) Sensory nerve fiber Lamellated (Pacinian) corpuscle (pressure receptor) Connective tissue cells Sensory nerve fiber Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b, c: © Ed Reschke

16. 17 Summary of Receptors of the General Senses

17. Dorsal column Medial leminiscal system Fine touch Vibration Position Pressure Anterolateral System Pain Temperature Crude touch Tickle and itch Sexual sensations

18. Layers of somatosensory cortex Incoming sensory signals excite layer VI I & II Receive signals from lower centers of brain (Controls excitability of respective area stimulated) II &III send axons to related portions on opposide side of cortex through corpus callosum V & VI Send axons to deeper parts of brain V ( more distant areas) VI ( Thalamus)

19. Functions of somatosensory Area I Localize Determine Pressure against body Judge weights of objects Judge shapes or forms Judge texture

20. Two point discrimination Vibratory sense Position sense

21. Two point discrimination

22. Vibratory sense 30-800 cycles/sec ( Pacinian ) 2- 80 cycles/sec( Meissners ) Position sense Multiple types of receptors for joint angulation Spindle (mid ranges of motion) Pacinian , Ruffinis , Golgi tendon Extremes of joint angulation