Understanding Brain Dissection and Histology Part 4

Neurobiology Exam 1 of serotonin and makes the person extremely happy then the brain overcompensates 7-8 hours later and re-uptakes too much of the serotonin which leaves the person grumpy. MDMA use leads to lasting brain damage via depletion of serotonin within the nervous system. Lecture Summary: • Electrical Synapses: • Fewer in total number throughout the nervous system • Mechanical and electrically conductive link = Gap Junction • Virtually instantaneous transmissions • Lack ability for gain/summation • Some are bi-directional • More reliable, less likely to fail or be altered • Chemical Synapses: • More common throughout the nervous system • Cell communication via chemical diffusion across the synaptic cleft • Depends on neurotransmitter release which causes a delay in transmission • Capable of summation • Unidirectional • More likely to be blocked or fail • A synapse is a small extracellular space that is generally between about 3-40 nm across, which depends on the mode of synaptic transmission. • Two main classes of synapses: • Electrical Synapses: “SPARK” mechanical and electrically conductive link between two cells that is formed by a gap junction. • Chemical Synapses: “SOUP” specialize synaptic cleft junctions through which cells signal to one another via chemical diffusion • Electrical and chemical transmissions can both occur at the same synapse. Neurobiology Exam 1 Synaptic Plasticity How do neurons differ from one another relative to their neurotransmitters and receptors?! III. Neurotransmitter Variability: ! Multiple different neurotransmitters utilized by neurons which has huge implications for its function. ! Glutamate: Major excitatory neurotransmitter in the brain GABA: Major inhibitory neurotransmitter in the brain IV. Neuron Receptor Variability: Receptors are more diverse than neurotransmitters. *Most neurons have both Inotropic Receptors and Metabotropic Receptors Inotropic Receptors: Resemble ion channels, with a central pore through which ions can flow. They open when activated by a specific neurotransmitter (or other ligand, which makes them ligand-gated channels); their effect may be excitatory or inhibitory but nonetheless occurs quickly and lasts only short time. *Three Types: K-Receptor: Diffusion of Potassium, K+. (Ligand-Gated Channel) AMPA-Receptor: Diffusion of Sodium, Na+. (Ligand-Gated Channel) NMDA-Receptor: Diffusion of Potassium, K+; Calcium, Ca+; Sodium, Na+. (LigandGated & Voltage-Gated Channel) Inotropic Ligand-Gated Ion Channel Receptors (K & AMPA) Diagram:! • Neurotransmitter binds to the ligand receptor of a closed inotropic ligand-gated ion channel • Ion channel opens up and ions are able to flow through their central pore Neurobiology Exam 1 Inotropic Ligand-Gated & Voltage-Gated Ion Channel Receptor (NMDA) Diagram:! ! ! • When glutamate binds to one of the receptors located on one of the four subunits in the extracellular domain, the NMDA receptor can change shape which widens its central pore and allows Ca2+, K+, and Na+ ions to flow through. • The pore will only change shape and let ions through when the neurotransmitter Glutamate binds to its receptors AND the postsynaptic membrane is already depolarized which is caused by AMPA receptors which allows the NMDA channels to function. • The depolarization due to AMPA receptors on the postsynaptic neuron, relieves the voltage-dependent channel block placed on NMDA channels by the binding of Mg2+; by kicking off Mg2+ from its receptor on the NMDA channel via depolarization, the channel is now able to change shape, open, and allow ions through. • Once depolarization is complete, the ion channel opens and allows large quantities of Ca+ in, effecting the metabolic machinery of the cell, resulting in the addition of more AMPA receptors to the postsynaptic neuron membrane which essentially strengthens the neuron by allowing it to respond more rapidly and strongly to future releases of Glutamate. Neurobiology Exam 1 Metabotropic Receptors: Have no central pore (which means no ions move through it directly), but open and close nearby channels via intracellular signaling cascades (which means they are indirectly linked to ion channels). Has an extracellular domain that contains a neurotransmitter binding site and an intracellular domain that binds to G-Proteins; their effects take longer to develop but lasts a long time. -When activated by a neurotransmitter, triggers an intracellular signaling cascade that ultimately opens or closes a nearby ion channel. G-Proteins: Molecular switches which initiate a signaling cascade intracellularly. Metabotropic Receptor Diagram: • • • • • Molecule of neurotransmitter binds to the extracellular domain receptor. Receptor in turn activates G-Protein within the intracellular domain. G-Protein activates an enzyme which produces a second messenger protein. Second messenger activates a nearby ion channel; channel opens. Ions enter cell and changes the postsynaptic potential; either depolarization or hyperpolarization (opening and closing of the channel causes changes in potential) Neurobiology Exam 1 ! How are synapses strengthened in invertebrates (e.g. Aplysia) Plasticity: The quality of being easily shaped or molded; connections between neurons can and do change with experience but it doesn’t necessarily mean that new neurons are added to the nervous system. -Brains are better than computers in that brains are capable of changing according to their outside stimulus/experience whereas computers are “stuck” and cannot adapt to their surroundings/experiences. Aplysia californica: The Sea Hare/Marine Snail; They shoot ink to escape predators due to synaptic transmission within their nervous system. They have a small number of very large neurons which made it easy to study since it isn’t too complex. Able to learn things; a variety of behaviors which can be modified through experiences to include the modification of innate/ natural reflexes by experience.! -Good model organism which allowed development of some of the most important work regarding invertebrate plasticity and answering how can the brain rewire itself as a result of “mental exercise” which is difficult to study in humans. Gill Withdrawal Reflex: Involuntary reflex to protect its gills. When the Aplysia is relaxed, the siphon and gills are visible from above; when the siphon is touched gently, structures are temporarily withdrawn and covered with a protective flap of mantle. *Reflex can be triggered when you dissect away the body and leave the neurons to the siphon and gills; called semi-intact preparation. Habituation: Decrease in the strength of a behavior induced typically by low-intensity stimuli. Sensitization: Increase in the strength of a behavior induced typically by high-intensity stimuli. Habituation of the Reflex: The response decreases as a result of repetitive, monotonous, tactile stimuli. The organism learns to stop responding to the stimulus because it isn’t harmful; will no longer withdraw the gill in response to stimuli. Short Term Habituation: Rapid presentations of a stimulus with a short interval between presentations; rapid habituation but spontaneous recovery. -When the siphon is first stimulated by a squirt of water, the Aplysia retracts its gill to protect it. This reflex is mediated by the sensory neurons synapsing directing upon the motor neurons which withdraw the gill. -If the siphon is squirted repeatedly, the animal soon habituates to the stimulus. It no longer retracts its gill. This short-term habituation results because the sensory neurons release less neurotransmitter onto the motor neuron; changes in the neural activity occur. Neurobiology Exam 1 Long Term Habituation: One stimulus presentation a day; more long-term effects with less spontaneous recovery. -If the siphon is squirted repeatedly over days, the animal habituates faster and faster each day and eventually shows almost no response. This long-term habituation is due to a retraction of some sensory synaptic terminals from the motor neurons; changes in the neural structure occur. *The intended use of habituation is to eliminate unnecessary responses to stimuli which presents no danger. Neural Basis for Habituation: ! *Sensory neuron innervates the siphon. *Motor neuron innervates the gill. • Gill Withdrawal Reflex: ! 1.) Stimulus activates receptors in the siphon. ! 2.) Sensory neuron synapses on the motoneuron ! that withdraws the gill (AKA the reflex circuit) • Habituation:! 1.) Siphon is squirted repeatedly over the course ! of an hour resulting in no retraction. ! 2.) Repeated activation leads to a progressive ! decrease in neurotransmitters released upon the ! motor neurons (more specifically, decreased ! glutamate released by the sensory neurons). ! 3.) Reduction of neurotransmitter levels ! terminates the signal Sensitization: A form of non-associative learning in which a strong (usually noxious/harmful/ extremely unpleasant) stimulus enhances/increases the response to a variety of subsequent stimuli; enhances the size of the response; a strong stimulus can bring back the gill withdrawal reflex after being habituated. *With a noxious stimulus, gill reflex is potentiated (strengthened reflex response) and becomes faster and stronger. Neurobiology Exam 1 Neural Basis for Sensitization: 1.) Input from the skin receptors activate Sensory ! Neuron #2. ! 2.) They synapse on a facilitating interneuron, ! that excites Sensory Neuron #1 via ! Serotonergic Synapses ! 3.) Activation of the Motor Neuron. ! 4.) The consequence of the sensitization process! is to increase the size of the EPSP in the Motor ! Neuron without increasing the response of Sensory! Neuron #1. ! 5.) Thus, gill withdraw is stronger and more ! forceful. ! ! *The release of serotonin causes a greater response! *Increases the duration of the action potentials ! *This change in the EPSP amplitude is mainly due to neurotransmitter release and is said to be Pre-Synaptic. *Central to the mechanism underlying sensitization is a set of neurons that are activated by noxious stimulation of the skin and then release serotonin at several locations in the Aplysia nervous system to include the presynaptic terminals of the sensory neurons in the gill withdrawal reflex circuit. Experiments have shown that blocking serotonin receptors on those sensory neurons prevents the ability of noxious stimuli, such as tail shocks, to induce sensitization. Conversely, pumping a bit of serotonin out of a micropipette onto the sensory neurons can sensitize the gill withdrawal reflex, even if no tail shocks are applied. Together, these studies indicate that serotonin release is necessary and sufficient for sensitization of Aplasia’s gill withdrawal reflex. In general, showing that a neural mechanism is necessary and sufficient for a particular behavior (or change in behavior) goes a long way to establishing a causal link between the two. (Page 73) *Serotonin release onto siphon sensory neurons ultimately leads to increased glutamate release onto the motor neurons. Neurobiology Exam 1 Mechanism of Short-Term Sensitization: 1.) Serotonin is released from the Pre-Synaptic Terminal of the Interneuron following propagation of an action potential down the interneuron, releasing the synaptic vesicles full of serotonin. ! 2.) The Serotonin binds to a metabotropic Serotonin Receptor on the Presynaptic Axon Terminal of Sensory Neuron #1. ! 3.) Activation of these Serotonin metabotropic receptors activates the enzyme Adenylate cyclase. ! ! 3a.) Adenylate Cyclase synthesizes cyclic adenosine monophosphate (cAMP). ! 3b.) Increased cAMP levels activate protein kinase A (PKA). ! 4.) PKA phosphorylates. ! 4a.) Closes K+ channels, broadening the action potential due to delaying the rate of depolarization by not letting K+ leave the cell. ! 4b.) Opens Ca2+ channels allowing an influx of Ca2+ ions into the cell. ! 4c.) Ca2+ mobilizes synaptic vesicles filled with Glutamate for exocytosis.! 5.) Activation of this Serotonin pathway = increased neurotransmitter release = larger EPSPs; more Ca+ = more synaptic vesicles releasing their Glutamate; more vesicles = larger EPSPs = probably triggering an action potential in the post-synaptic neuron (motor neuron in this case). Neurobiology Exam 1 Short-Term Sensitization Mechanism Diagram: Neurobiology Exam 1 Long-Term Sensitization: Requires the translation of new proteins from RNA (ie protein synthesis) to develop structural changes in the neurons i.e. grow new synapses. *Spaced learning is better than massed learning at creating long-lasting memories. Mechanism for Long-Term Sensitization: 1.) Repeated noxious stimulus leads to repeated release of Serotonin from the Pre-Synaptic Terminal of the Interneuron following propagation of an action potential down the interneuron, releasing the synaptic vesicles full of serotonin onto the axon terminal of the Pre-Synaptic Sensory Neuron #1. These high levels of Serotonin cause high levels of PKA. 2.) So much PKA accumulates in the Pre-Synaptic Sensory Neuron that it moves to the nucleus. 2a.) PKA Phosphorylates and activates the transcriptional activator/transcription factor cAMP Response Binding Element Protein (CREB). 3.) CREB Phosphorylates and binds to cAMP Response Element (CRE) sequences which are located upstream of many genes. 4.) When bound to the CRE sequence, phosphorylated CREB interacts with nearby RNA Polymerases, which begin their task of transcribing the downstream gene to create new mRNA. 5.) The transcribed mRNAs are translated into proteins and shipped from the nucleus to other parts of the neuron. 6a.) CREB stimulates the synthesis of an enzyme called Ubiquitin Hydrolase. 6b.) Other genes induced by CREB result in the production of unknown proteins. 7a.) Ubiquitin Hydrolase stimulates degradation of the regulatory subunit of PKA;stops the regulators of PKA and allows increase. 7b.) Unknown proteins thought to cause an increase in the size of the sensory axon terminals and the addition of synaptic terminals; they create more synaptic connections from the Sensory Neuron #1. 8a.) Some PKA is persistently active and no longer requires serotonin to be activated. 9.) Overall process thus yields a long-term increase in the number of synapses between sensory and motor neurons.