Sensory receptors

1. Sensory receptors By Dr Farah Amir Ali

2. Learning objectives: 1: Discuss the general design of nervous system: 2: Define synapse, its types (chemical and electrical) and properties along with anatomic consideration. 3: Explain the functions of synapses and transmitter substances. 4: Discuss the special characteristic of synaptic transmission. 5: Differentiate between spatial and temporal summation. 6: Define the following terms: a: Excitatory postsynaptic potentials b: Inhibitory postsynaptic potential. c: Presynaptic facilitation and inhibition d: Post synaptic inhibition e: : Convergence f: Divergence 7: Define neuropeptides with examples. 8: Explain the classification of sensory receptors. 9: Define labeled line principle. 10 Discuss receptor potential with example of pacinian corpuscle. 11- Distinguish between tonic and phasic receptors with examples. 12-Discuss the physiological classification of nerve fiber with its functions. 13- Describe instability and stability of neuronal circuits. 14- Explain synaptic fatigue as mean of stabilizing the nervous system

3. Sensations -- Action potentials that reach the brain via sensory neurons • Perceptions - the brain’s interpretation of sensations • Sensory reception - the detection of the energy of a stimulus by receptors in sensory cells • Sensory receptors specialized neurons or epithelial cells that respond to specific or generalized stimuli

4. A sensory receptor converts a stimulus into a signal which can trigger an action potential in a sensory neuron. • Accessory structures may modify receptor responses

5. General Receptors or Special Receptors • General receptors -- • throughout skin • Or body: muscle spindles and tendon organs Special Sense Receptors • Given location(s) where stimuli are detected • Taste buds, nasal cavity • Eyes and ears

6. Sensory Receptor Types Figure 10-1: Sensory receptors

7. Most sensory neurons have their cell bodies in spinal(dorsal root) ganglia

8. Sensory Receptors Functions of Sensory Receptors 1. Transduction 2. Amplification 3. Transmission 4. Integration

9. Conversion of stimulus energy into membrane potential of receptor cell Begins by a change in membrane permeability results in “graded” change in membrane potential “RECEPTOR POTENTIAL” Is graded: proportional to strength of stimulus can be due to change in ion permeability : 1. as gated ion channels respond to receptor molecule (a ligand binds to) 2. or due to actual stretching of membrane in response to pressure. Sensory Transduction

10. Sensory Receptors 2. Amplification - • strengthening of a stimulus too weak to be carried into nervous system • Direct - Complex organ, ear: sound waves magnified 20X • Part of transduction in eye: 100,000X of action potential in signal to brain from eye, vs. few photons of light energy trigger process

11. 3. Transmission • - conducting impulses to CNS • Stimulus does not turn on/off production of action potential • but controls the frequency with which they are generated… • can detect a change in stimulus intensity, • not just presence or absence of stimuli

12. Integration • -processing of information > begins immediately • integration via summation of graded potentials

13. Post synaptic potentials IPSP: Transient hyperpolarization of postsynaptic membrane potential caused by presynaptic release of neurotransmitter EPSP:Transient postsynaptic membrane depolarization by presynaptic release of neurotransmitter

14. Types of Sensory Receptors - type of energy transduced 1. Mechanoreceptors - mechanical energy: • pressure, touch, stretch, motion, sound 2. Nocioreceptors - pain (mechanical or heat energy) 3. Thermoreceptors - temperature: heat or cold 4. Chemoreceptors - general or specific chemical molecule binds to site on membrane receptor protein 5. Electromagnetic receptors - electromagnetic energy (elec. Mag. spectrum) light, electricity, magnetism Photoreceptors -- special kind of E.M.R. organized into “eyes”

15. I. Mechanoreceptors Skin tactile sensibilities (epidermis and dermis) Free nerve endings Expanded tip endings Merkel’s discs Ruffini’s endings Meissner’s corpuscles Krause’s corpuscles Hair end-organs Deep tissue sensibilities Ruffini’s endings Pacinian corpuscles muscle spindles Golgi tendon receptors Hearing Equilibrium Arterial pressure Classification of Sensory Receptors

16. II. Thermoreceptors Cold Cold receptors Warmth Warm receptors III. Nociceptors Pain Free nerve endings IV. Electromagnetic receptors Vision Rods Cones V. Chemoreceptors Taste Smell Arterial oxygen Osmolality Blood CO2 carotid bodies

17. Sensory coding • Converting a receptor stimulus to a recognizable sensation is termed sensory coding. All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity, and duration.

18. Modality • Humans have four basic classes of receptors based on their sensitivity to one predominant form of energy: mechanical, thermal, electromagnetic, or chemical. The particular form of energy to which a receptor is most sensitive is called its adequate stimulus.

19. The term sensory unit is applied to a single sensory axon and all its peripheral branches. These branches vary in number but may be numerous, especially in the cutaneous senses Lateral inhibition Two point discrimination Location

22. Intensity • The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. the greater intensity of stimulation also will recruit more receptors into the receptive field.

24. Duration • When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potentials in its sensory nerve declines over time. This phenomenon is known as adaptation or desensitization. • The degree to which adaptation occurs varies from one sense to another. Receptors can be classified into rapidly adapting (phasic) receptors and slowly adapting (tonic) receptors.

25. TONIC receptors (slowly adapting) Slow or no adaptation Continuous signal transmission for duration of stimulus Monitoring of parameters that must be continually evaluated, e.g.: chemoreceptors Phasic Receptors (Rapidly adapting) Rapid adaptation Cease firing if strength of a continuous stimulus remains constant Allow body to ignore constant unimportant information, e.g.Smell

27. Importance of rate receptors • Their predictive function • Eg joint receptors and semicircular canals

28. Labeled line Principle • Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation. • Yet despite the fact that we experience these different modalities of sensation, nerve fiber transmit only impulses. Therefore, how is it that different nerve fibers transmit different modalities of sensation?

29. Labeled line Principle • This specificity of nerve fibers for transmitting only one modality of sensation is called Labeled Line Principle

30. General classification of nerve fibers

33. The 18th and 19th C debate about the nature ofcommunication in the nervous system:Electrical or Chemical?? • After a presynaptic neuron is stimulated the delay is about 0.3 ms for the postsynaptic neuron to respond. This is too long for electric transmission. • If you stimulate the postsynaptic neuron , no response in the presynaptic one.

34. NEUROMODULATORS • Some chemicals are released by neurons have little or no effect on their own but can modify the effects of neurotransmitters • Neuromodulation is frequently produced by neuropeptides and by circulating steroids and steroids produced in the nervous system

35. Classification of neurotransmitters and neuromodulators • Small molecule transmitters • Neuropeptides

37. Neuropeptides are present in tissues at much lower concentrations In striking contrast, neuropeptides are initially synthesized in the cell soma, sequestered within the lumen of the secretory pathway and transported down the axon while undergoing cleavages and other processing events Neurotransmitters are present in tissues at much higher concentrations The supply of conventional neurotransmitters in small synaptic vesicles is replenished in nerve terminals by local synthesis, and many conventional neurotransmitters are recaptured after secretion Difference b/w small molecule neurotransmitters and neuropeptide Neuropeptides and small molecule transmitters can coexist and be co-released from same cell

38. Acetylcholine • Present both in the central and peripheral nervous systems (CNS &PNS) • Synthesized by the combination of AcetylCoA, which is a product of the Krebbs cycle in the mitochondria, and choline, which is obtained from food (egg yolk, legumes). • Distribution: Neuromuscular junction Autonomic nervous system Many synapses throughout brain Receptors: Nicotinic (ionotropic) Muscarinic (metabotropic) Clearance: Broken down enzymatically In the PNS, it is the transmitter of the neuromuscular junction between neurons and all types of muscle (cardiac/smooth/skeletal) and thus is responsible for muscle contraction.

40. Biosynthesis of Ach

41. Destruction of Ach and reuptake

42. Monoamines 1)Catecholamines: 􀁺 Dopamine 􀁺 Norepinephrine 􀁺 Epinephrine 2)Indolamines: 􀁺 Serotonin (=5-HT)

44. Dopamine 􀂾DA neurons primarily in midbrain 􀂾 Project to limbic system, basal ganglia, cortex 􀂾 Involved in movement, reward, stress response

46. Norepinephrine 􀂾Most NE neurons found in locus ceruleus in brainstem. Project widely to other regions 􀂾 Functions: Arousal, stress, mood, memory

49. Neuronal pools • The central nervous system is composed of thousands to millions of neuronal pools; some of these contain few neurons, while others have vast numbers. • For instance, the entire cerebral cortex could be considered to be a single large neuronal pool. • Other neuronal pools include the different basal ganglia and the specific nuclei in the thalamus, cerebellum, mesencephalon, pons, and medulla. Also, the entire dorsal gray matter of the spinal cord could be considered one long pool of neurons.

50. Relaying of Signals ThroughNeuronal Pools • Threshold and Subthreshold Stimuli—Excitation or Facilitation • the discharge zone of the incoming fiber, also called the excited zone or liminal zone. • To each side, the neurons are facilitated but not excited, and these areas are called the facilitated zone, also called the subthreshold zone or subliminal zone.