Synapses Topic 2 PDF

Synapses Chapter Objectives (1 of 2) By the end of this chapter, you should be able to: 2-1. Describe how Charles Sherrington used behavioral observations to infer the major properties of synapses. 2-2. Explain how EPSPs and IPSPs produce temporal and spatial summation. 2-3. Discuss the importa...

Synapses Chapter Objectives (1 of 2) By the end of this chapter, you should be able to: 2-1. Describe how Charles Sherrington used behavioral observations to infer the major properties of synapses. 2-2. Explain how EPSPs and IPSPs produce temporal and spatial summation. 2-3. Discuss the importance of inhibition in the nervous system. 2-4. List and explain the sequence of events at a synapse, from synthesis of neurotransmitters, through stimulation of receptors, to the disposition of the transmitter molecules. Chapter Objectives (2 of 2) 2-5. Distinguish between ionotropic and metabotropic receptors and explain how each of them works. 2-6. Discuss how certain drugs affect behavior by their effects at synapses. 2-7. List some hormones and their effects. Concept of the Synapse Introduction Neurons communicate by transmitting chemicals at junctions, called “synapses.” The term was coined by Charles Scott Sherrington in 1906 to describe the specialized gap that existed between neurons. Sherrington’s discovery was a major feat of scientific reasoning. The Properties of Synapses Sherrington Investigated how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc Example Leg flexion reflex: a sensory neuron excites a second neuron, which excites a motor neuron, which excites a muscle Three Important Points about Reflexes Sherrington’s observations Reflexes are slower than conduction along an axon. Several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus. As one set of muscles becomes excited, another set relaxes. Speed of a Reflex and Delayed Transmission at the Synapse Sherrington found a difference in the speed of conduction in a reflex arc from previously measured action potentials. He believed the difference must be accounted for by the time it took for communication between neurons. Evidence validated the idea of the synapse. Temporal and Spatial Summation Sherrington observed that repeated stimuli over a short period of time produced a stronger response. Temporal summation: repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak. Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not. Spatial summation: Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse. Excitatory Postsynaptic Potential (EPSP) Presynaptic neuron: neuron that delivers the synaptic transmission Postsynaptic neuron: neuron that receives the message Excitatory postsynaptic potential (EPSP): graded depolarization that decays over time and space (type of graded potential) The cumulative effect of EPSPs is the basis for temporal and spatial summation. Inhibitory Synapses Sherrington noticed that during the reflex that occurred, the leg of a dog that was pinched retracted while the other three legs were extended. Suggested that an interneuron in the spinal cord sent an excitatory message to the flexor muscles of one leg and an inhibitory message was sent to the other three legs Thus, the idea of inhibitory postsynaptic potential (IPSP)—the temporary hyperpolarization of a membrane Occurs when synaptic input selectively opens the gates for positively charged potassium ions to leave the cell, or negatively charged chloride ions to enter the cells Serves as an active “brake” that suppresses excitation Recordings from a Postsynaptic Neuron During Synaptic Activation The periodic production of action potentials despite synaptic input EPSPs increase the number of action potentials above the spontaneous firing rate. IPSPs decrease the number of action potentials below the spontaneous firing rate.  1. Identify which type of synapse each are (A, B, C and D) Test yourself 2. Explain what is happening in the neuron: 1. Which ions are moving in/out? 2. What is happening to the voltage or potential of the neuron? 3. What is the likely outcome of the neuron (will it fire or not?) Terms to use: Graded potential IPSP EPSP Temporal summation Spatial summation A B C D Excite the neuron Inhibit the neuron Neuron is more likely to fire an action Neuron is less likely to fire an action Action potential Chemical Events at the Synapse James W. Kalat, Biological Psychology, 14th Edition. © 2024 Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part. 14 The Discovery of Chemical Transmission at Synapses German physiologist Otto Loewi The first to convincingly demonstrate that communication across the synapse occurs via chemical means Otto Loewi’s experiment Found that stimulating one nerve released something that inhibited heart rate, and stimulating a different nerve released something that increased heart rate Realized that he was collecting and transferring chemicals, not loose electricity The Chemical Events at the Synapse The major sequence of events allowing communication between neurons across the synapse The neuron synthesizes chemicals (neurotransmitters) Action potentials travel down the axon Released molecules diffuse across the cleft, attach to receptors, and alter the activity of the postsynaptic neuron The neurotransmitter molecules separate from their receptors and are return to the presynaptic neuron for recycling (re-uptake) or diffuse away. Some postsynaptic cells may send reverse messages to slow the release of further neurotransmitters by presynaptic cells. Transmission at a Synapse Synthesis of Transmitters Neurons synthesize neurotransmitters and other chemicals from substances provided by the diet. Acetylcholine synthesized from choline found in milk, eggs, and nuts. Tryptophan serves as a precursor for serotonin. Catecholamines contain a catechol group and an amine group (epinephrine, norepinephrine, and dopamine). Effects on the Postsynaptic Cell The effect of a neurotransmitter depends on its receptor on the postsynaptic cell. Transmitter-gated or ligand-gated channels are controlled by a neurotransmitter. Ionotropic effects: occur when a neurotransmitter attaches to receptors and immediately opens ion channels Occur very quickly (sometimes less than a millisecond after attaching) and are very short lasting Rely on glutamate or GABA Metabotropic effects: occur when neurotransmitters attach to a receptor and initiate a sequence of slower and longer lasting metabolic reactions G-Proteins G-protein activation: coupled to guanosine triphosphate (GTP), an energy storing molecule Increases the concentration of a “second-messenger” The second messenger communicates to areas within the cell. May open or close ion channels, alter production of activating proteins, or activate chromosomes Variation in Receptors Many neurotransmitters attach to more than one type of receptor. Because different receptors control different functions, drugs can have specialized effects on behavior. A given receptor can have different effects for different people, or even in different parts of one person’s brain. Drugs That Bind to Receptors Many hallucinogenic drugs distort perception. Video camera with solid fill Chemically resemble serotonin in their molecular shape (e.g., LSD) Stimulate serotonin type 2A receptors (5-HT2A) at inappropriate times or for longer duration than usual, thus causing their subjective effect MAGIC MUSHROOMS Psylocibin converts into Psylocin in the brain, an active compound that binds to receptors, activating their function Impacts the default mode network of the brain (what your brain is doing when you’re not doing anything) Video camera with solid fill Video camera with solid fill Video camera with solid fill Drug effect of Psylocibin The DMN Magic mushrooms as a treatment Inactivation and Reuptake of Neurotransmitters Neurotransmitters released into the synapse do not remain and are subject to either inactivation or reuptake. During reuptake, the presynaptic neuron takes up most of the neurotransmitter molecules intact and reuses them. Transporters are special membrane proteins that facilitate reuptake. Negative Feedback from the Postsynaptic Cell Negative feedback in the brain is accomplished in two ways: Autoreceptors: receptors that detect the amount of transmitter released and inhibit further synthesis and release Postsynaptic neurons: respond to stimulation by releasing chemicals that travel back to the presynaptic terminal where they inhibit further release Effects of Drugs on Dopamine Transmission The active chemicals in marijuana (a cannabinoid) bind to receptors on presynaptic neurons This changes the signaling to the post-synaptic neuron; both excitatory and inhibitory messages from many neurons. Video camera with solid fill Cannabinoids The active chemicals in marijuana that bind to anandamide or 2-AG receptors on presynaptic neurons known as retrograde signalling. When cannabinoids attach to these receptors, the presynaptic cell stops sending signals. In this way, the chemicals in marijuana decrease both excitatory and inhibitory Video camera with solid fill messages from many neurons. Electrical synapses Electrical Synapses A few special-purpose synapses operate electrically. Faster than all chemical transmissions Gap junction: the direct contact of the membrane of one neuron with the membrane of another Video camera with solid fill Depolarization occurs in both cells, resulting in the two neurons acting as if they were one. Endocrine system  Hormones Chemicals secreted by a gland or other cells that is transported to other organs by the blood where it alters activity Produced by endocrine glands Important for triggering long-lasting changes in multiple parts of the body The Pituitary Gland Attached to the hypothalamus and consists of two distinct glands Anterior pituitary: composed of glandular tissue Hypothalamus secretes releasing and inhibiting hormones that control anterior pituitary. Posterior pituitary: composed of neural tissue Hypothalamus produces oxytocin and vasopressin, which the posterior pituitary releases in response to neural signals. Negative Feedback in the Control of Thyroid Hormones