Health Sciences I Lecture 4: Integration of Signals (Oct 2024) PDF

Lecture 4: Integration of signals Dr Isabel Hwang Division of Education School of Biomedical Sciences, Faculty of Medicine, CUHK [email protected] E-mail: [email protected] Office no: 3943 6795 Important Notice: These slides contain copyright materia...

Lecture 4: Integration of signals Dr Isabel Hwang Division of Education School of Biomedical Sciences, Faculty of Medicine, CUHK [email protected] E-mail: [email protected] Office no: 3943 6795 Important Notice: These slides contain copyright materials. Access is limited to students of MEDF1011 unless otherwise specified. Copyright © 2022 Reading access is only allowed to CUHK students of MEDF1011 1  LECTURE OUTLINE What is a chemical synapse? Postsynaptic potential can be excitatory or inhibitory Integration of neural information Temporal and spatial summation Convergence and divergence Components of a reflex arc A patellar tendon (knee-jerk) reflex General structure and function of the nervous system Concept of homeostasis to regulate physiological variables Importance of a negative feedback to achieve homeostasis Important Notice: These slides contain copyright materials. Access is limited to students of MEDF1011, unless otherwise specified. 2  TWO TYPES OF ELECTRICAL SIGNALS Graded potential Action potential Dendrites and cell body Axon hillock and axons Signals decay with distance Self-propagating Questions 1. How graded potential (GP) is formed in dendrites or cell body? 2. How the GPs in dendrites and cell body help form action Dendrites & potential at the axon hillock? cell body A chemical GP synapse GP GP GP Axon hillock Axon hillock triggers AP by summing GP Action potential 3 The all-or-none principle (from lecture 3) The threshold potential Not all depolarization events produce action potentials (AP) For axon to "fire“ an AP, depolarization must reach threshold (-55mV) - Recall: Graded potential can be depolarisation or hyperpolarisation 4 Question: How the GPs in dendrites and cell body can be converted to action potential that is triggered only at the axon hillock? Graded potential (GP) can be summed. axon hillock Excitable membranes of dendrites and cell bodies are communicating with axon terminal (chemical synapse) of another neuron 5 Important terminology Presynaptic neuron Conducts impulses toward a A single neuron post-synaptic to one cell can be synapse. Sends the information. pre-synaptic to another cell. Postsynaptic neuron Transmits electrical signal away from a synapse. Receives the information. Most neurons function as both presynaptic or postsynaptic neurons in the nervous system 6  Summation Adding effects of graded potentials called postsynaptic potentials so that incoming signals from other neurons produce a bigger change in membrane potential than does one impulse alone. Presynaptic Presynaptic axon axon Postsynaptic Postsynaptic axon neuron 7 Comparison of excitatory and inhibitory postsynaptic potentials Excitatory postsynaptic potential is a depolarization. Inhibitory postsynaptic potential is a depolarization. 8 Spatial summation- an example when only excitatory neurons are involved Refers to summation of postsynaptic potentials that occurs when several presynaptic axon terminals produces graded potentials at the same time. Presynaptic axon terminal Step 1 1 Three excitatory neurons fire GP. Their graded 1 potentials separately are all below threshold. Graded potentials produced by excitatory neurons are depolarizations only Trigger zone Action (a) potential 9 Spatial summation (excitatory only) Presynaptic Step 2 axon terminal 1 Three excitatory neurons fire. Their graded potentials 1 separately are all below threshold. 2 Graded potentials arrive at trigger zone together and sum to create a suprathreshold signal. 2 Trigger zone Action (a) potential 10 Spatial summation (excitatory only) Presynaptic Step 3 axon terminal 1 Three excitatory neurons fire. Their graded potentials 1 separately are all below threshold. 2 Graded potentials arrive at trigger zone together and sum to create a suprathreshold signal. 3 An action potential is 2 generated. Trigger zone 3 Action (a) potential 11 Spatial summation- an example when both excitatory and inhibitory neurons are involved In spatial summation, strength of a depolarization produced by one excitatory postsynaptic neuron can be reduced or cancelled out by a hyperpolarization produced by another postsynaptic neuron Step 1 1 One inhibitory and two excitatory neurons fire. Inhibitory neuron 1 Trigger zone No (b) action potential 12 Spatial summation (excitatory and inhibitory) Step 2 1 One inhibitory and two excitatory neurons fire. 2 The summed potentials Inhibitory are below threshold, so neuron 1 no action potential is generated. 2 Trigger zone No (b) action potential 13 Spatial summation (excitatory and inhibitory) Step 3 1 One inhibitory and two excitatory neurons fire. 2 The summed potentials Inhibitory are below threshold, so neuron 1 no action potential is generated. 2 Trigger zone No (b) action potential 14 Temporal summation Refers to summation of postsynaptic potentials occurs when a pre-synaptic neuron fires repeatedly at a high rate (close in time) 15 Temporal summation will fail if the two graded potentials are too distant from each other 16 Excitatory postsynaptic potential (EPSP) increases possibility of forming an action potential in the axon hillock via summation 17 Types of neural circuits A circuit is a term that describes the complex grouping of neural pathways Divergence Convergence Divergence Is a pathway that spreads Convergence is when several neurons information from one neuron to multiple synapse with a single postsynaptic neuron. A neurons. A mechanism for spreading mechanism for providing input to a single stimulation to multiple neurons in the neuron from multiple sources central nervous system 18 Categories of chemical messengers A given chemical messenger can fit into more than one category. For example, the steroid hormone cortisol affects the very cells in which it is made, the nearby cells that produce other hormones, and many Examples of effector distant targets, including muscles and cell: muscle cell liver. 19 Post-synaptic potential is produced by a chemical synapse Synapses are interaction points or junction within the nervous system and transmit electrical signals between cells Two types (chemical and electrical synapses) Synaptic terminals Dendrites Axon Inhibitory Excitatory hillock Chemical synapses releases chemicals called neurotransmitters (NT) between two neurons, Myelin sheath Receiving namely the presynaptic and cell body postsynaptic neuron. Axon 20 Chemical synapse At chemical synapses, presynaptic neurons release neurotransmitters (NT) from their Axon of presynaptic neuron axon terminals, and the NT bind to specific receptors on post-synaptic neurons. Mitochondrion Axon terminals of the Axon terminal Postsynaptic presynaptic neuron neuron hold the synaptic Synaptic vesicles that contain vesicles neurotransmitter Synaptic cleft molecules. Neurotransmitter Receptors Postsynaptic membrane 21 Sequence of events at a chemical synapse Action potential 22 1 An action potential depolarizes the axon terminal. 2 The depolarization opens Axon voltage-gated Ca2+ terminal channels and Ca2+ Synaptic enters the cell. vesicle Neurotransmitter molecules Action potential 1 Ca2+ Synaptic cleft 2 Postsynaptic cell Voltage-gated Ca2+ channel 23 1 An action potential depolarizes the axon terminal. 2 The depolarization opens Axon voltage-gated Ca2+ terminal channels and Ca2+ Synaptic enters the cell. vesicle Neurotransmitter 3 Calcium entry triggers molecules exocytosis of synaptic Action vesicle contents. potential 1 3 Ca2+ Synaptic Docking cleft protein 2 Postsynaptic cell Voltage-gated Ca2+ channel 24 1 An action potential depolarizes the axon terminal. 2 The depolarization opens Axon voltage-gated Ca2+ terminal channels and Ca2+ Synaptic enters the cell. vesicle Neurotransmitter 3 Calcium entry triggers molecules exocytosis of synaptic Action vesicle contents. potential 4 Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell. 1 3 Ca2+ Synaptic Docking cleft protein 2 4 Receptor Postsynaptic cell Voltage-gated Ca2+ channel 25 1 An action potential depolarizes the axon terminal. 2 The depolarization opens Axon voltage-gated Ca2+ terminal channels and Ca2+ Synaptic enters the cell. vesicle Neurotransmitter 3 Calcium entry triggers molecules exocytosis of synaptic Action vesicle contents. potential 4 Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell. 1 3 5 Neurotransmitter binding Ca2+ initiates a response in Synaptic the postsynaptic cell. Docking cleft protein 2 4 Receptor Postsynaptic cell Voltage-gated Ca2+ channel Cell 5 response 26 Termination of neurotransmitter (NT) effects 27 The nervous system The central nervous system comprises the brain and the spinal cord. Sensory Input provides Integration CNS takes Motor Output all the incoming executes the central the central nervous information, interprets it, nervous system system with information then determine an commands to cause the about the internal and appropriate effector appropriate effector external environment response response 28 Types of neurons There are 3 types of neurons in the nervous system: Types of neurons Sensory (afferent) Interneurons Motor (efferent) neurons (association neurons) neurons To conduct signals To integrate To conveys signals to from sensory receptors information in the effector cells to the central nervous central nervous system To determine an system (sensory input and send it to motor appropriate effector and perception) neurons response To monitor changes To integrate, interpret To execute movement inside and outside the and to process such as of body, secretion of body emotion, learning and hormones memory 29 A spinal reflex initiates a response without input into the brain Sensory information goes to the brain Spinal reflexes are simple behaviors produced by central nervous system (CNS) pathways that lie entirely within the spinal cord. Spinal Stimulus cord Sensory information Integrating center Interneuron Command to A spinal reflex initiates muscles or a response without input glands from the brain. Response 30 Spinal reflex functions via a reflex arc A reflex arc is a neural pathway by which a stimulus reflexively induces a response It has five components: 1. receptor, 2. sensory neuron, 3. integration center, 4. motor neuron, and 5. effector A reflex arc produces a specific, rapid, involuntary and automatic response to a particular stimulus and always causes same response E.g. The patellar tendon (also called knee-jerk) reflex 31 The patellar tendon (knee jerk) reflex Quadriceps (extensor) Hamstrings (flexor) A group of four muscle at the A group of three muscles at front of the thigh the back of the thigh To begin: tapping the patellar tendon below the kneecap (Motor neuron) 32 Extension and flexion Flexion is bending a joint. Extension is straightening a joint. 33 The patellar tendon (knee jerk) reflex is a mono- synaptic reflex Afferent Step 1 neuron To begin: tapping Muscle the patellar tendon spindle below the kneecap Quadriceps Stimulus Patellar tendon Muscle spindle is the receptor in this reflex; when stretched, action potentials are triggered and travel in afferent neurons to the spinal cord which is the integration center 34  Afferent Step 2 neuron To brain Muscle spindle Quadriceps Stimulus 1 Patellar tendon Efferent neurons In the spinal cord, the afferent neuron synapses 1 with efferent neuron that innervate the quadriceps to contract, and the leg ‘kick’ forward (extend). 35  Afferent Step 3 neuron To brain Muscle spindle Quadriceps Stimulus 1 2 Hamstrings Patellar tendon Interneuron Efferent neurons Afferent neurons from muscle spindle also synapse 2 with an inhibitory interneuron innervating the efferent neurons going to the hamstrings causing it to relax. 36  Afferent neurons also ascend Step 4 to the brain, forming synapses with various Afferent interneurons neuron To brain Muscle spindle Quadriceps Stimulus 1 2 Patell Hamstrings ar tendo n Interneuron Efferent neurons Simultaneous excitation of the quadriceps and inhibition of the hamstrings causes leg extension during the knee jerk reflex. 37 Homeostasis A central principle of physiology to maintain a relatively constant internal environment (dynamic constancy) Homeostasis refers to the dynamic mechanisms that detect and respond to deviations in physiological variables from their “set points,” by initiating effector responses that restore the variables to the optimal (physiological) range. Components of the internal environment that are regulated: Temperature Volume Composition Requires organ systems integration Disruption of homeostasis (called pathophysiology) is the basis for disease and even death 38 Changes in blood glucose concentration during a typical 24-hour period Blood glucose levels increase after eating, and then levels return to their set point via homeostasis. This is an example of dynamic constancy: Levels change over short periods of time but remain relatively constant over long periods of time. 39 Homeostasis is governed mainly by ‘negative’ feedbacks A negative feedback system brings about responses that move a variable opposite to the direction of its original change Negative feedback is the most common type of control system used in the body. Example: when body temperature rises, negative feedback ensures that it decreases and returns to normal range. 40 General components of a reflex arc that functions as a negative feedback control system 41 A homeostatic control reflex system maintains body temperature when room temperature decreases 42  Learning outcomes To describe how two methods of summation help trigger an action potential at the axon hillock. Summarize the sequence of events in a chemical synapse. To summarize ways that help to terminate a neurotransmitter effect at the chemical synapse. To compare an excitatory and inhibitory postsynaptic potential. To summarize the sequence of events in a patellar tendon reflex. To recall the three major components of a homeostatic control system To define a negative feedback with an example. To recognise that most homeostasis involves integration of multiple organ functions. Provide an example. Important Notice: These slides contain copyright materials. Access is limited to students of MEDF1011, unless otherwise specified. 43 Required reading: Basic Concepts in biomedical sciences I Chapter 2, Homeostasis Page 8-13 44 44  The end of this lecture Copyright © 2024 The Chinese University of Hong Kong 45