Lecture 3 - Neurons and Neuronal Transmission PDF

11/11/2024 PSYC112/132: Introduction to Neuroscience Whilst we wait to get started… Week 6: Friday 15th November 2024 Discuss with the people Dr Abigail Fiske around...

11/11/2024 PSYC112/132: Introduction to Neuroscience Whilst we wait to get started… Week 6: Friday 15th November 2024 Discuss with the people Dr Abigail Fiske around you: Can psychology exist without [email protected] neuroscience? Why / why not? 1 Questions, Comments, Concerns? Please do get in touch – I’m very happy to help [email protected] Office: Fylde C42 Book a meeting with me here Message me on Microsoft Teams Post in the Discussion Forum 2  11/11/2024 Lecture 3: Neurons and Neuronal Transmission 3 Learning Objectives Identify and describe the parts of a neuron and their function Understand the process of neural conduction Understand the process of synaptic transmission By the end of the lecture, you will have a basic understanding of the anatomy of neurons and the processes by which neurons send and receive signals. 4  11/11/2024 The “Why” Neurons are building blocks of the brain Understanding how information flows through the nervous system will give you an appreciation of how these biological processes contribute to thoughts, emotions and behaviour 5 Part I: Cells of the Nervous System 6  11/11/2024 Neurons Neurons are specialised nerve cells that receive, conduct and transmit electrochemical signals in the brain Approximately ~86 billion neurons in the human brain 7 Anatomy of Neurons Neurons are surrounded by the cell membrane The soma (cell body) is the metabolic centre of the neuron and contains the nucleus (contains chromosomes) Dendrite (tree) branches out from the cell body to receive information from other neurons 8  11/11/2024 Anatomy of Neurons The axon is the long, narrow part of the neuron that carries signals away from the cell body (white matter) The axon hillock is at the junction between the axon and the cell body The axon is surrounded by myelin (fatty, insulating layer) that supports efficient neuronal transmission Nodes of Ranvier (“rahn-vee-yay”) are gaps Axon hillock between sections of myelin that support efficient neuronal transmission 9 Anatomy of Neurons The “buttons” at the axon terminal release chemicals into the synapses Synapses are the gaps between adjacent neurons across which chemicals are transmitted More on synapses to come… Axon hillock 10  11/11/2024 Types of Neurons Bipolar neuron has both an axon and a dendrite extending from the soma Predominantly found in the retina and olfactory system Pseudounipolar neurons have no dendrites! Sensory neurons are activated by sensory input from the environment Multipolar neurons have one axon and 2 or more dendrites Motor neurons transmit signals from the spinal cord to the muscles Interneurons connect motor and sensory neurons 11 Glial Cells Glial cells are non-neuronal cells in the nervous system Surround neurons and hold them in place Serve to support neuronal functioning by: Supplying nutrients and oxygen to neurons Insulating and protecting neurons Destroy and remove dead neurons Several types of glial cells… 12  11/11/2024 Types of Glial Cells Astrocytes (astro = star) are the most common type of glial cell and form the blood-brain barrier Oligodendrocytes (cell with several branches) serve to help information move faster along the axon by forming the myelin sheath Microglia (micro = small) are the brain’s immune system! Ependymal cells make up the thin membrane lining of the spinal cord and ventricles of the brain to help circulate cerebrospinal fluid Schwann cells are like oligodendrocytes but are found in the PNS rather than the CNS. Form part of the PNS immune system! 13 Cell Membrane Surrounds every neuron The neuron cell membrane is composed of a lipid bilayer (two layers of fat molecules) Protein molecules are embedded within the lipid bilayer Channel proteins (forming “channels”) allow certain molecules to pass through the lipid bilayer Signal proteins transfer a signal to the inside of the neuron when molecules bind to them on the outside of the membrane 14  11/11/2024 Part II: Neural Conduction 15 How A Signal Travels Through A Neuron Signal is received by receptors on the dendrites Causes electrical potential changes in the neuron that are interpreted by the soma Signal is sent to the axon hillock If signal is strong enough, it is sent to the axon (action potential… more soon) Myelin supports transmission of the signal by: Preventing action potentials occurring in the region of axon surrounded by myelin sheath Increases neural transmission speed Reduces total influx of sodium ions into the axon Axon terminals / synaptic buttons release neurotransmitters into the synapse to be picked Axon hillock up by the adjacent neuron 16  11/11/2024 Neurons at rest 17 Resting Membrane Potential Na+  Sodium The difference in electrical K+  Potassium potential across the cell membrane when the cell is not stimulated or is “at rest” When a neuron is at rest, its sodium channels close The resting potential is typically between -50 to - 100mV (let’s say -70mV) mV = millivolt (a measure of electrical potential) 18  11/11/2024 Resting Membrane Potential Na+  Sodium Caused by differences in K+  Potassium concentrations of ions inside and outside of the cell If the cell were equally permeable to all ions, each type of ion would flow across the membrane But since ions cannot simply cross the membrane (they need to travel through protein channels) this leads to different concentrations inside and outside of the cell 19 Resting Membrane Potential Na+  Sodium Cell membrane has “channels” that K+  Potassium allow the ions to diffuse across the membrane Membrane is more permeable to K+ movement than Na+ K+ diffuses outside of the cell much faster than Na+ leaks into the cell High Na+ concentration OUTSIDE of cell High K+ concentration INSIDE of cell Sodium-potassium pump maintains the resting potential For every ATP unit consumed… Brings two K+ ions into the cell Removes three Na+ ions out of the cell 20  11/11/2024 Neurons in action! 21 Postsynaptic Potentials (PSPs) “Disturbances” of the resting membrane potential Occurs when signal from an adjacent neuron is detected at the cell membrane of the dendrites The neuron depolarises (decreases the resting membrane potential so that it becomes less negative) Sodium channels opening This is known as an excitatory postsynaptic potential (EPSP) Increases the likelihood that the neuron will fire The neuron hyperpolarises (increases the resting membrane potential so it becomes more negative) Caused by opening of chloride channels This is known as an inhibitory postsynaptic potential (IPSP) Decreases the likelihood that the neuron will fire 22  11/11/2024 Postsynaptic Potentials (PSPs) PSPs are graded potentials = the amplitude of the PSPs are proportional to the intensity of the signal that elicits them PSPs are rapid but decremental (decrease in amplitude as they travel through the neuron) A single PSP is unlikely to have any effect on the firing of the neuron What determines whether a neuron fires is the balance of excitatory and inhibitory signals reaching the axon = summation Threshold of excitation (~65mV) must be reached for neurons to “fire” (triggering an action potential) 23 Summation Spatial summation = how PSPs produced simultaneously on different parts of the receptive membrane summate in the same soma Temporal summation = how PSPs produced in rapid succession at the same synapse summate 24  11/11/2024 25 Action Potentials Action potentials are massive but momentary (~1ms) There is a reversal of the membrane potential from -70mV to ~+50mV All-or-none response – they will be triggered if the threshold is exceeded. Their magnitude is not related to the intensity of the stimuli that elicited the response. 26  11/11/2024 Stage 1: Depolarisation Na+  Sodium Neuron becomes less K+  Potassium negative (closer to 0mV) Caused by the entry of Na+ ions into the neuron through the Na+ channels This causes the inside of the cell to become less negative If this causes the threshold potential to be reached, an action potential is triggered 27 Stage 1: Depolarisation Depolarisation at once place in the axon triggers the opening of adjacent Na+ channels As such, the action potential propagates down the axon Action potential travels from the axon hillock down the axon of the neuron to the terminal buttons 28  11/11/2024 Stage 1: Depolarisation If the axon is myelinated, then the action potential “jumps” between the gaps in the myelin (Nodes of Ranvier) which speeds up neural transmission 29 Stage 2: Repolarisation Na+ channels inactivate  channels are CLOSED and cannot be reopened until the membrane potential returns to a more negative value Then, the K+ channels OPEN to allow for the rapid movement of K+ ions from inside to outside of the cell This causes the membrane potential to become negative again 30  11/11/2024 Stage 3: Hyperpolarisation Occurs when the K+ channels are still open and the Na+ channels are closed Since K+ channels increase the permeability of the membrane to K+, but are less permeable to Na+, the membrane potential becomes more negative compared to the resting potential Hyperpolarisation ends when the K+ channels CLOSE The neuron membrane then Sometimes called the “refractory period” returns to its resting potential 31 Action Potential 32  11/11/2024 Let’s have a brain break! 33 Part III: Synaptic Transmission 34  11/11/2024 What is a synapse? The place where neurons connect and communicate with each other A structure that permits the neuron to pass an electric or chemical signal to another neuron Synapses enable neurons to form circuits to support function Electrical signals (e.g., action potential) used to send messages from one end of the neuron to another Chemical signals (e.g., neurotransmitters) pass information to the next neuron 35 Different types of synapses Axodendritic synapse – a synapse formed between the axon terminal button and the dendrite of another neuron (this is the most common kind) Axosomatic – between the axon terminal button and the soma of the next neuron Axomyelenic – between the axon terminal button and the myelin sheath of the next neuron Synapses can form between any two parts of the neurons 36  11/11/2024 37 Pre-synaptic Neuron Action potential travels down the pre- synaptic neuron to the terminal buttons Reaches the axon terminal and causes changes in electrical properties inside the cell that impact the synaptic vesicles Synaptic vesicles are tiny, sac-like organelles that contain neurotransmitter molecules Synaptic vesicles attach to the cell membrane and release neurotransmitters into the synaptic cleft (process of exocytosis) 38  11/11/2024 Neurotransmitter Release Action potential reaches the axon terminal Change in potential causes voltage-gated calcium (CA2+) ion channels to open and allow calcium to enter the cell Calcium interacts with synapsin (protein) which supports the synaptic vesicle to mobilise the neurotransmitters for release This then facilitates a process where the synaptic vesicle fuses to the cell membrane of the neuron The synaptic vesicle then empties the neurotransmitters into the synaptic cleft The synaptic vesicle is recycled so that it can be used again 39 Post-synaptic Neuron Neurotransmitters diffuse across the synaptic cleft Neurotransmitters bind to receptors (proteins) on the membrane of the post- synaptic neuron This can cause changes in the membrane potential making a neuron more (excitatory) or less (inhibitory) likely to produce an action potential The neurotransmitters job (getting a message to the next neuron) is now complete! 40  11/11/2024 Post-synaptic Receptors Different types of receptors that enable neurotransmitters to transmit different kinds of messages throughout the brain Ionotropic receptors – associated with ion channels 41 Post-synaptic Receptors Different types of receptors that enable neurotransmitters to transmit different kinds of messages throughout the brain Ionotropic receptors – associated with ion channels Metabotropic receptors – associated with signal proteins 42  11/11/2024 Neurotransmitter Removal Neurotransmitters need to be removed from the synaptic cleft Otherwise, they disrupt neural processing as they continue to send messages to the post-synaptic neuron or stop other molecules from being able to bind to the receptor Diffusion –only a small % Enzymatic degradation – enzymes can break down the neurotransmitter molecule so that they can be taken back up into the pre-synaptic neuron Reuptake – transport protein in the pre- synaptic cell membrane can take the neurotransmitter back into the cell where they can be recycled and reused 43 Synaptic Transmission - Summary 44  11/11/2024 HOMEWORK 1. READ CHAPTER 4 OF THE TEXTBOOK 2. Check out the short YouTube videos on this topic in the Optional Reading table for Lecture 3 3. Look at the prep work for Lecture 4 ahead of next Wednesday’s lecture 45 Thank you for your attention, engagement and contributions! 46