Lecture 5: Nerve Impulse and Synapse 2024-2025 PDF

Physiology School of Medicine Founded in 1845 Course: MD137 2024-2025 Introduction to Physiology Lecturer: Lecture 5:...

Physiology School of Medicine Founded in 1845 Course: MD137 2024-2025 Introduction to Physiology Lecturer: Lecture 5: Dr. Leo Quinlan Nerve impulse and the synapse [email protected] Sensory system Nerve impulse and the synapse Collect data and pass it on Learning Outcomes 3 Describe the basic role of neurons in sensory physiology Explain with examples how physical stimuli are translated to nerve APs How are stimuli classi ed. Explain what is meant by sensory adaptation What is a synapse and how does it work EPSP versus IPSP fi General Properties of Sensory Systems 4 Stimulus Internal External Energy source Receptors Sense organs Transducer Afferent pathway CNS integration  RECEPTOR TYPES 5 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES Sensory receptors onvert stimulus into neuronal activity e either endings of afferent neurons or specialized cells adjacent to an afferent euron RECEPTOR TYPES 6 Somatic Senses 7 Enables body to feel, ache, chill Responsible for touch and pain Receptors: Broadly distributed Responds to many kinds of stimuli Somatosensory Pathway 8 The sensory pathways convey the type and location of the sensory stimulus The type: because of the type of receptor activated The location: because the brain has a map of the location of each receptor Somatic Senses – Internal Stimuli 9 Touch Temperature Pain Itch Proprioception Pathway  PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES Stimulus Type Information on stimulus type 10 Each sensory receptor is particularly sensitive to one stimulus type or modality Each sensoryTable receptor is particularly 1: Sensory receptor classes and theirsensitive totype preferred stimulus one stimulus type or modality Sensory receptor Stimulus modality Location Mechanoreceptors Stretch and pressure Skin, muscle and tendons, blood vessels Thermoreceptors Cold, warmth Skin Photoreceptors Light Retina Chemoreceptors Certain chemicals Tongue, nose Nociceptors Stimuli causing tissue Throughout body damage All receptors of a sensory unit respond to same stimulus modality All receptors of a sensory unit respond to same stimulus modality Receptive elds of sensory units responding to different modalities Receptive fields of sensory units responding to different modalities often overlap often overlap Next: How is information on stimulus intensity encoded in the sensory signal? Back to index fi Adequate Stimulus 11 Normally each stimulus has a speci c type of receptor I.e Taste buds - taste I.e. Rods and cones - light The stimulus to which a speci c receptor is speci cally sensitive is termed the ADEQUATE STIMULUS fi Stimulus Intensity - Encoding PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES Information on stimulus intensity Larger receptor potential Increased ring frequency in Increased ring frequency in higher 12 Intensity More receptors activated primary unit order neurons Neighbouring units activated fi fi  Stimulus transduction Generating a sensation 13 1. Stimulus --> opening (or closing) of ion 1. Stimulus --> opening (or closing) of ion channels in specialized receptor membrane channels in specialized receptor membrane 1 2. Current flow through ion channels --> 2. Current ow receptor through potential ion channels (= graded change in --> 2 membrane potential). receptor potential (= graded change in 3. Receptor potential spreads within afferent membrane potential). 3 neuron until it reaches region with high density 3. If receptor potential reaches of voltage-dependent threshold sodium channels. If for receptor potential reaches threshold for their 4 their activation, activation, actionaction potentials potentials are are generated. generated. 4. Action potentials propagate within afferent neuron 4. Action and cause propagate potentials release of neurotransmitter along afferent neuron and cause release of neurotransmitter Next: What are sensory unit and receptive field ? Back to inde fl  PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES Information on stimulus duration Stimulus Duration 14 Some primary sensory neurons re action Some primary sensory neurons fire action potentials as long potentials as stimulus isas long others present, as stimulus do not: is present, others do not: Adaptation Adaptation = Sensory = Sensory receptors receptors decrease decrease in to in sensitivity stimulus of constant strength sensitivity to stimulus of constant strength action action potential frequency decreases potential frequency decreases Sensory Sensory receptors receptors can can be be Rapidly Rapidly adapting: Receptors signal changes in adapting: Receptors signal changes in stimulus intensity stimulus intensity Slowly adapting: Slowly adapting: Receptors Receptors signal the continued signal presence of the continued the stimulus presence of the stimulus Next: How is information on stimulus location encoded in the sensory signal? Back to index fi  itored, such as joint and muscle receptors that participate in the nating wi The afferent n for that n Adaptation Rapidly adapting receptor Afferent neuron afferent n action potentials gle point 15 Receptor potential Slowly adapting receptor N Afferent neuron action potentials Receptor potential Re Stimulus on Stimulus off Skin Time Figure 7.3 Responses of slowly adapting and rapidly adapting Figure receptors to a prolonged, constant stimulus. Rapidly adapting receptors receptors, AP Propagation 16 AP Propagation 17 Local current spread (electrotonic conduction) depolarization of nearby part of membrane can initiate the spike AP Propagation Unmyelinated 18 AP Propagation Unmyelinated 19 AP Propagation Unmyelinated 20 AP Propagation Unmyelinated 21 Chemical Synapse 22 Chemical Synapse 23  24 Chemical Synapse  Post-Synaptic Potential (PSP) 25 If PSP causes ↑ ring called an EPSP If PSP ↓ ring - called an IPSP Chemical Synapse fi fi Excitatory transmission 26 Ca2+ Na+ Na+ Ca2+ Excitatory transmission 27 Ca2+ Na+ Na+ Ca2+ Excitatory transmission 28 Ca2+ Na+ Na+ Ca2+ Excitatory transmission 29 Ca2+ Na+ Na+ Na+ -70mV Na+ Excitatory postsynaptic Ca2+ potential (EPSP) Inhibitory transmission Na+ Ca2+ 30 Na+ Ca2+ Inhibitory transmission Na+ Ca2+ 31 Na+ Ca2+ Inhibitory transmission Na+ Ca2+ 32 Cl- -70mV Cl- Na+ Inhibitory postsynaptic potential (IPSP) Ca2+ Learning Outcomes 33 Describe the basic role of neurons in sensory physiology Explain with examples how physical stimuli are translated to nerve APs How are stimuli classi ed. Explain what is meant by sensory adaptation What is a synapse and how does it work EPSP versus IPSP fi