3 SENSORY RECEPTORS physiology slides.pptx

Sensory Receptors

1. PHYSIOLOGY OF SENSORY RECEPTORS

2. INTRODUCTION Sensory receptors are specialized cells that detect changes In the environment and convert these changes in to electrical signals that can be processed by the nervous system. These cells are located in specialized organs such as eyes, ears, nose, skin and mouth.

3. Types of sensory receptors and stimuli they detect There are 5 types of sensory receptors 1. mechanoreceptors - detect physical distortion of the receptor or cells adjacent to the receptor. Eg pressure, vibration, fine touch or gross touch 2. thermoreceptor – detect changes in temperature both cold and warmth receptors. 3. nociceptors- (pain receptor) – detect physical or chemical damage to tissues 4. photoreceptor/electromagnetic receptor- detect light on the retina of the eye.

4. 5. chemoreceptors – detect changes in chemicals, test in mouth, smell in nose PH, and ions.

5. Transduction of sensory stimulus into receptor potential. A receptor potential (generator potential) is a change in potential across the receptor membrane when the stimulus that excite the receptor pass through it. Different receptors can be excited in different ways to cause receptor potentials

6. Mechanism of receptor potential 1. mechanical deformation of the receptor which stretch the receptors and open ion channels. 2. Application of chemical to a membrane which open ion channels 3. Change of temperature of membrane which alter permeability of the membrane. 4. Effect of electrochemical radiation such as light which directly or indirectly Change the membrane characteristics and allow ions to flow through membrane channels

7. INTRODUCTION TO SYNAPSE

8. Synapse: Definition Is the structure in which nerve signals from one neuron is transmitted into other neuron or an effector tissue Is the structure through which neurons communicate with themselves or communicate with other body tissues (effector tissues such as muscles and glands ) The neuron that brings the nerve signals is called Pre-synaptic neuron/fibre The neuron that receives the nerve signal (in case of neuron-neuron transmission) is called post-synaptic neuron

9. Properties of the Synapse 1. Plasticity: Ability of the synapse to change its strength, number and size to enhance or decline its strength 2. Fatiguability: Continuous activation of post-synaptic membrane leads to reduced sensitivity of the target neuron/tissue to NT. 3. Synaptic delay: Due to dynamics of the transmission (electrical-chemical-electrical), there is a short delay about 0.5-1ms of transmission 4. Summation: The effect of NT at the post-synaptic membrane (generation of EPSPs or IPSPs) can be summed up to get the overall effect (generation of action potential or over inhibition)

10. Properties of the Synapse 5. Summation: The effect of NT at the post-synaptic membrane can be summed up to get the overall effect (generation of action potential or over inhibition) Temporal summation (sum of signals separated by time) Spatial summation (sum of signals from different presynaptic neurons 6. One way transmission : at the synapse, nerve signals are carried ONLY in one direction (from pre-synaptic to post-synaptic neuron/target tissue)

11. Properties of the Synapse 7. Convergence and divergence: more than one pre-synaptic neurons sending common post-synaptic neuron/target ( convergence ) or one pre-synaptic neuron sending signals to more than one post-synaptic neuron/target ( divergence )

12. Classification of sensory receptors According to nature of transmission (Functional classification) Electrical synapses Chemical synapses In electrical synapses, electrical signals from pre-synaptic neuron is transmitted directly into post-synaptic neuron/ target cells through direct cell-cell contract At the point of contact, there are gap junctions that allow direct contact of cytoplasm between the two cells Depolarization in one cell hence spreads directly into other cell

13. Electrical synapse

14. Synapse: Functional classification In Chemical synapses, electrical signals across the synapse are converted into chemical signals to be transmitted to the other neuron. No direct contact between neurons and the other neuron/target cells It is the most common type of synapses in the CNS and PNS

15. Structure of chemical synapse A chemical synapse is made up of two terminals and the space between them ( a synaptic cleft ) The two terminals are pre-synaptic terminal and post-synaptic terminal Pre-synaptic terminal Is the terminal end of an axon of pre-synaptic neuron that brings electrical signals (in form of AP) to the synapse Post-synaptic terminal Is the specialized plasma membrane of the post synaptic neuron (a neuron that receives the signals) or a target cell

16. Structure of chemical synapse Pre-synaptic terminal/knob It contain cellular vesicles that carry special chemicals knowns as Neurotransmitters (NTs) It is rich in voltage gated calcium channels that open upon arrival of AP at the pre-synaptic terminal Contains SNARE proteins whose function is transporting NT vesicles to the pre-synaptic membrane for release into the synaptic cleft (via exocytosis)

17. Chemical synapse Post-synaptic terminal It comprises of specialized membrane that contain specific receptors for specific neurotransmitter molecules The membrane contains ionophores (ion channels) that tend to open upon binding of neurotransmitter molecules to their respective receptors.

18. Neuro-transmitters Are chemical molecules that are involved in signal transmission across the synapse. They can be neuropeptides NT small-molecule NT Neuropeptide NT Synthesized in the cell body of the pre-synaptic neuron and stored in large vesicles and transported to the axonal terminal, ready for release Eg : Enkephalin, endorphins, Neuropeptide-Y

19. Neuro-transmitters Small molecule NT Synthesized either in the cell body (simple amino acids such as glutamine) or at the terminal axonal end, and then stored in small vesicles at the axonal terminal, ready for release Eg : Acetylcholine, Glutamine, serotonin, and catecholamines (epinephrine, Norepinephrine, dopamine)

20. Neurotransmitter Receptors Are the receptors for NTs found in post-synaptic membrane. They mediate post-synaptic cell signalling (depolarization/hyperpolarization) upon binding with NTs They are of two (2) types: Ion channel coupled receptors G-protein coupled receptors Ion channel coupled receptors: they are bound directly to the ion channels (cations or anions channels) Eg ; Na+, K+, Ca++, Cl- channels They tend to have fast action that last shortly.

21. Neurotransmitter Receptors 2. G-protein coupled receptors: They are bound to G-protein molecules Binding of NT to these receptors leads activation of G-protein molecules that initiate second messenger system usually via cAMP, IP3,DAG through PKA that leads to phosphorylation and opening of ion channels These ones take long time to open, and last longer (seconds to minutes lasting time)

22. Mechanism of Signal transmission Transmission across the synapse takes place in 4 steps: synthesis release Receptor activation Signal termination

23. Mechanism of Signal transmission Synthesis: NT molecules are synthesized and stored in the vesicles. Release: Upon arrival of AP to the pre-synaptic terminal, Ca channels open and more Ca enter the cytosol of presynaptic terminal. More Ca leads to activation of SNARE proteins that causes transfer and fusion of the vesicles to the presynaptic membrane, leading to the release of NT into the synaptic cleft

24. Mechanism of Signal transmission Receptor activation: NT binds to their respective post-synaptic receptors (inotropes or metalotropes ) leading to opening of ion channels (commonly Na+, but can be any eg , K, Ca, Cl-) The end result of this activation is either excitation or inhibition of the post-synaptic membrane (depending on the ion channel opened)

25. Mechanism of Signal transmission Signal termination: NT have to be removed from the receptor site otherwise it might lead to continuous activation or inhibition of the postsynaptic membrane (over activation might lead to fatigue and destruction of the nerve fibers )

26. Mechanism of Signal transmission Removal of NT cab be either of 3 ways Re-uptake by the presynaptic terminal (via endocytosis or transporter proteins found in the presynaptic membrane) back to the presynaptic terminal for recycling and storage Deactivation by specific enzymes eg MAOs, Acetylcholinesterase enzyme Diffusion out of the synaptic cleft

27. Clinical Implications Myasthenia Gravis: Is an autoimmune disease in which auto-antibodies either bind or destroy the Ach receptors at the post-synaptic membrane.(usually at Neuromuscular Junction) This causes failure of NT Ach to bind on their respective receptors, hence no transmission of signals to the muscles Lambert Eaton syndrome: Autoimmune disease in which antibodies attack calcium channels in the pre-synaptic membrane. This causes of Ca influx and release of neurotransmitters

28. Clinical Implications Drugs: Most of drugs are designed to act on the synapse to block or enhance nerve signals transmissions Eg : Paralytic drugs ( suxamethonium , pancuronium ) binds with Ach receptors and blocks them from binding with Ach NT Tricyclic antidepressants (amitriptyline, nortriptyline) Inhibits NT reuptake by serotoninergic, dopaminergic and noradrenaline presynaptic neurons leading to prolonged binding of NT to their receptors leading to enhanced Acetylcholinesterase inhibitors: blocks Ach-esterase enzyme, leading to high concentration of Ach available to bind on the post-synaptic membranes usually at the NMJ (in Myasthenia gravis) Eg : pyridostigmine, rivastigmine.

29. Clinical implication Toxins: most of neurotoxins attack the synapse Eg : botulinum, tetanus Botulinum - is a protease enzyme. It prevents release of Ach which is excitatory NT by cleavage of SNAREs resulting in inhibited NT release by pre-synaptic terminal -the patient will present with weakness or paralysis of the affected site Tetanus: inhibits release of GABA which is inhibitory NT, results in uncontrolled firing of post-synaptic neurons. -the patient presents with spasticity, locked jaws and opisthotonos

30. Introduction: Definitions A reflex is a stereotyped motor response to a specific sensory stimulus Coordinated contractions and relaxations of specific muscles in response to sensory inputs Reflexes are rapid automatic responses to stimuli

31. Classification of reflexes

32. Spinal reflexes Can be monosynaptic or polysynaptic Monosynaptic : one synapse between the sensory neuron and the alpha motor neuron The sensory neuron synapses directly on the motor neuron Polysynaptic : multiple synapses  sensory neuron + at least one interneuron + motor neuron More synapses = longer delay between stimulus and response But several muscle groups can be involved so can produce a more complex response

33. Mono- and polysynaptic reflexes

34. The reflex arc Five steps Arrival of stimulus and activation of receptor Activation of sensory neuron Information processing (SC, brain, etc) Activation of motor neuron Response by effector

35. The spinal reflex arc

36. THANK YOU FOR LISTENING

37. References Vanders Human Physiology 15 th edition Guyton and Hall Textbook of Medical Physiology 14th edition www.uptodate.com www.Medscape.com