1. بسم الله الرحمن الرحيم

2. RECEPTORS AND SENSORY PATHWAYS

3. LEARNING OBJECTIVES Classify the sensory receptors
Enumerate the receptors found in the skin for different sensory modalities
Explain the terms receptor potentials and sensory transduction
Describe the sensory pathways that mediates touch, proprioception and vibration sense
Describe the organization of sensory cortex

4. Peripheral Nervous System
Consists of nerve fibers that carry information between the CNS and other parts of the body
Afferent division
Detects, encodes, and transmits peripheral signals to the CNS (sends information from internal and external environment to CNS)
To maintain homeostasis
Visceral afferent
Incoming pathway for information from internal viscera (organs in body cavities)
Sensory afferent
Somatic (body sense) sensation
Sensation arising from body surface and proprioception
Special senses
Vision, hearing, taste, smell

5. Receptors Structures at peripheral endings of afferent neurons
Detect stimuli (change detectable by the body), exists in various forms e.g. heat, light, sound, pressure & chemical change
Each receptor is specialized to one type of stimulus ,its adequate stimulus e.g. eyes- light,
ears- sound, warmth- heat
Convert forms of energy into electrical signals (action potentials)
Process is called transduction

6. Types of Receptors Photoreceptors
Responsive to visible wavelengths of light
Mechanoreceptors
Sensitive to mechanical energy
Thermoreceptors
Sensitive to heat and cold
Osmoreceptors
Detect changes in concentration of solutes in body fluids and resultant changes in osmotic activity
Chemoreceptors
Sensitive to specific chemicals
Include receptors for smell and taste and receptors that detect O2 and CO2 concentrations in blood and chemical content of digestive tract
Nociceptors
Pain receptors that are sensitive to tissue damage or distortion of tissue

9. Tactile Receptors Free nerve endings: Detect pain, touch and pressure
found everywhere in the skin and viscera
Meissner’s corpuscles:
rapidly adapting (within a fraction of a second) and detect movement of light objects over skin, position sense ,dynamic fine touch (manipulation)
-found on nonhairy skin, fingertips and lips
Merkel’s discs:
respond rapidly at first and then slowly adapt, detect the “steady state”, position, static touch(shape, edges, texture)
-found on hairy as well a non hairy skin

10. Tactile Receptors Hair end organ
Adapts rapidly and detects movement over the body
Ruffini’s end organ
Slowly adapting and respond to continual deformation of the skin and joint rotation
Pacinian corpuscle
Very rapidly adapting and is stimulated only by rapid movement
detects vibration and other rapid changes in the skin

11. Receptors
May be:
-Specialized ending of an afferent neuron
-Separate cell closely associated with peripheral ending of a neuron
-Stimulus alters receptor’s permeability which leads to graded receptor potential
-Usually causes nonselective opening of all small ion channels
-This change in membrane permeability can lead to the influx of sodium ions. This produces receptor (generator) potentials.
-The magnitude of the receptor potential represents the intensity of the stimulus.
-A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS.

12. Conversion of Receptor and Generator Potentials into Action Potentials

14. Receptors May adapt slowly or rapidly to sustained stimulation
Types of receptors according to their speed of adaptation
Tonic receptors
Do not adapt at all or adapt slowly
Ex: Muscle stretch receptors, joint proprioceptors
CNS must continually get info. about degree of muscle length & joint position
So, these receptors must continue to generate APs to relay information to CNS
Phasic receptors
Rapidly adapting receptors
No longer responds to maintained stimulus
Important to signal a change in stimulus intensity, rather than relay status quo information
Tactile (touch) receptors in skin

16. Adaptation of receptors
When a continuous stimulus is applied, receptors respond rapidly at first, but response declines until all receptors stop firing.

17. Transmission of Receptor Information to the Brain
-The larger the nerve fiber diameter the faster the rate of transmission of the signal
-Velocity of transmission can be as fast as 120 m/sec or as slow as 0.5 m/sec
-Nerve fiber classification
type A - myelinated fibers of varying sizes, generally fast transmission speed
subdivided into a, b, d, g
type C - unmyelinated fibers, small with slow transmission speed

18. Classification of Somatic Sensations
Mechanoreceptive - stimulated by mechanical displacement
Tactile
touch
pressure
vibration
tickle and itch
Position or proprioceptive
static position
rate of change

19. Classification of Somatic Sensations
Thermoreceptive:
Detect heat and cold
Nociceptive:
Detect pain and are activated by any factor that damages tissue

20. Pathways for the Transmission of Sensory Information
Almost all sensory information enters the spinal cord through the dorsal roots of the spinal nerves
Two pathways for sensory information:
-Dorsal column-medial lemniscal system
- Anterolateral system

21. Dorsal Column System
Contains large myelinated nerve fibers for fast transmission ( m/sec).
High degree of spatial orientation maintained throughout the tract.
Transmits information rapidly and with a high degree of spatial fidelity (i.e., discrete types of mechanoreceptor information).
Touch, vibration, position, fine pressure

22. Anterolateral System
Smaller myelinated and unmyelinated fibers for slow transmission ( m/sec)
Low degree of spatial orientation
Transmits a broad spectrum of modalities
Pain, thermal sensations, crude touch and pressure, tickle and itch, sexual sensations.

23. Somatosensory Pathways
Pathways conveying conscious somatic sensation-
Consists of chains of neurons, or labeled lines, synaptically interconnected in particular sequence to accomplish processing of sensory information
First-order sensory neuron
Afferent neuron with its peripheral receptor that first detects stimulus
Synapses on Second-order neuron
Second-order sensory neuron
Either in spinal cord or medulla
Synapses with third-order neuron
Third-order sensory neuron
Located in thalamus
Terminate in somatosensory cortex

24. Somatic Sensory Cortex
Located in the postcentral gyrus
Highly organized distinct spatial orientation
Each side of the cortex receives information from the opposite side of the body
Unequal representation of the body
lips have greatest area of representation followed by the face and the thumb
trunk and lower body have the least area

26. Acuity Refers to discriminative ability
Influenced by receptive field size and lateral inhibition

27. RECEPTIVE FIELD -Receptor has its receptive field ie. It
responds to stimulus within a restricted region of skin
-Receptive field is inversely proportional to density of receptors
-Smaller is the receptive field, greater is acuity or discriminative ability
-More receptors on finger tips as compared to elbow

28. LATERAL INHIBITION

30. Perception
Conscious interpretation of external world derived from sensory input
from a pattern of nerve impulses delivered to brain
Why sensory input does not give true reality perception
Humans have receptors that detect only a limited number of existing energy forms
Information channels in our brains are not high-fidelity recorders
Cerebral cortex further manipulates the data

31. What Do You Perceive? Do you “see” a white square that is not really
there?
Variable perceptions from the same visual input.
Do you see two faces in profile, or a wineglass?

32. Uses For Perceived Information
Afferent input is essential for control of efferent output
-Processing of sensory input by reticular activating system in brain stem is critical for cortical arousal and consciousness
-Central processing of sensory information gives rise to our perceptions of the world around us
-Selected information delivered to CNS may be stored for further reference
-Sensory stimuli can have profound impact on our emotions

33. References
Human physiology by Lauralee Sherwood, seventh/eighth edition
Text book physiology by Guyton &Hall,12th edition
Text book of physiology by Linda .s contanzo,third edition