Week 5 PDF - Neuroscience Introduction

Different “Scales of Explanation” in Psychology Social group / community Synapse Molecules Person Neuron...

Different “Scales of Explanation” in Psychology Social group / community Synapse Molecules Person Neuron Neural circuits / networks Brain Neuron Communication in the Brain  The Neuron Axon Terminals or Synapses Myelin Input Output  The Neuron Cell Body common to all cells contains nucleus and all structures necessary for cell functioning (DNA)  The Neuron Dendrites Unique to neurons Receives signals – input zone Many per neuron, receives input from many other neurons  The Neuron Axon Terminals Axon Hillock Myelin Axon Unique to neurons Sends signals – output from axon hillock at cell body to axon terminals One per neuron – only one axon for output Wrapped in myelin for efficient transmission of signals along the axon  The Neuron Axon Terminals Axon Hillock Myelin Axon Terminals Terminal boutons / buttons Form synapses with other neurons Secrete neurotransmitters to send signals across synapses to other neurons  Glial Cells Brain contains neurons and Glial Cells – Glial Cells are supporting cells for neurons – 3 types of Glial Cells Oligodendrocytes – Produce the myelin sheath that wraps around axons Astrocytes – Supply nutrients from blood to the neurons – Maintain “blood-brain barrier” Microglia – brain’s immune system – Clean up foreign or toxic substances  The myelin of axons 0.3 - 4µm Oligodendrocytes form myelin sheath by wrapping around the axon Essential for efficient communication, for propagation of signals along axon Multiple Sclerosis involves loss of myelin, disruption of efficient neural communication throughout the body Synapses Axon Terminals (neuron 1) to Dendrites (neuron 2) Synapses – Join axon terminals of one neuron to dendrites of another neuron for transmission of signals between neurons Neural signals go one-way – Pre-synaptic: (before the synapse) From cell body to axon terminal – Post-synaptic: (after the synapse) From dendrite to cell body 100 billion neurons each with 10,000 synapses 1,000 trillion connections Neurons: Electrical Signals Neuron Signals = Action Potential Electrical signal “pulse” travels along the axon Fixed size – either on or off, signal or no-signal (not large or small) Terms to know: Membrane Potential Resting Potential Action Potential Cell Membrane Wall 70% of the brain is water Water surrounds cells – Extra-cellular fluid – outside cell Water fills the cells – Intra-cellular fluid or cytoplasm – inside cell Cell Membrane forms barrier between extra-cellular and intra-cellular fluid ions and electrical potential across cell membrane Sodium (Na+) and Potassium (K+) positively charged ions Different concentrations outside and inside cell, across cell membrane Gives difference in electrical charge (potential) across cell membrane u t he re io n s o o s i ti v e o r e p M here Fewer positive ions in  Membrane Potential: Resting Potential Membrane Potential Definition: Difference in the electrical charge (voltage) between inside and outside cell, across cell membrane wall Resting Potential Definition: At rest (i.e. NOT during an action potential) more positive ions outside than inside the cell gives overall negative potential (voltage) inside compared with outside the cell – Difference in electrical charge (voltage) at rest = -70mV t her e ns o u i ti v e io re p o s Mo Difference (resting potential) -70mV here Fewer positive ions in ion channels in Cell Membrane ion channels in cell membrane wall open and close to pass or block movement of ions across cell membrane – ions move between intra- and extra-cellular fluid (in and out of cell) – movement of ions changes electrical potential 3 important types: – Sodium Potassium Pump – Voltage-dependent ion channels – Ligand-gated ion channels  Ion Channel 1 : Sodium Potassium Pump Actively pumps Na+ and K+ across cell membrane Overall pumps positive charge out of cell (3 Na+ out for every 2 K+ in) – Positive charge will naturally move towards negative area (opposites attract) Maintains negative resting membrane potential (approximately -70mV) Uses Energy – about 25% of body total energy; 70% of brain energy! Action Potential Input from other neurons (via synapses on dendrites) increases membrane potential Transmission of electrical signal along axon If voltage exceeds threshold, triggers action potential Depolarisation of cell: membrane potential goes back to zero Repolarisation: membrane potential back to -70mV resting potential Ion Channel 2 : Voltage-dependent ion channels Voltage-dependent ion channel, closed at resting potential Open when membrane potential reaches threshold voltage Allows flow of ions across cell membrane – E.g. positive ions (Na+) can flow from outside into the cell (because positive charge will naturally move towards negative area) Causes depolarisation of cell (voltage less negative = closer to zero) Na+ open Depolarisation Na+ close K+ open Repolarisation Fig. 48-6 Voltage-dependent ion channels: Na+ and K+ Action Potentials Depolarisation / repolarisation is fast – occurs in less than 0.002 seconds Repolarization “undershoots” – Refractory Period – more difficult for another action potential to occur – further to threshold to trigger another action potential Fixed Size and All-or-None principle: If threshold level is reached, action potential of a fixed sized will occur. The size of the action potential is always the same for that neuron. All-or-None: Either a full action potential is “fired” (if membrane potential reaches threshold) or there is no action potential. There are no “large” or “small” action potentials. The strength of the neuron signal is determined by the rate of repeated action potentials. Synapses Axon Terminals (neuron 1) to Dendrites (neuron 2) Synapses – Join axon terminals of one neuron to dendrites of another neuron for transmission of signals Neural signals go one-way – Pre-synaptic: Axon – Axon Terminal – Synapse – Post-synaptic: Synapse – Dendrite – Cell body The Synapse Pre-Synaptic Neuron Post-Synaptic Neuron Sending Signals: Neurotransmitter Release Action Potential Depolarisation of axon terminal (action potential) triggers release of neurotransmitter Neurotransmitter acts on receptor on post- synaptic neuron to open ion channels and pass signal Chemical signal neuron-to-neuron The Synapse Neurotransmitter Chemical “messenger” Released from pre-synaptic terminal Acts on post-synaptic receptors Common neurotransmitters? The Synapse Synaptic Vesicles Stores neurotransmitter in pre-synaptic terminal Joins cell membrane wall to release neurotransmitter into synaptic cleft Recycled: neurotransmitter taken back into pre-synaptic terminal is re-packaged into vesicles The Synapse Neurotransmitter Receptors “Gates” on post-synaptic side (neuron dendrite) Neurotransmitter in synaptic cleft joins with receptor “Activates” receptor to open ion channels on post-synaptic neuron Transmits signal by opening ion channels and changing membrane potential on post- synaptic neuron “Lock and key” – neurotransmitter receptors Each receptor only binds to a specific type of neurotransmitter Neurotransmitters only “activate” their specific type of receptor Important for drug effects – Drugs can act on specific receptors to cause specific effects (eg. L-DOPA for Parkinson’s disease replaces dopamine in the brain) Specific shape to binds to specific receptor  The Synapse Re-uptake pump “Clears” neurotransmitter from synaptic cleft back into pre-synaptic terminal Enzymes Break down neurotransmitter in synaptic cleft Both stop neurotransmitter signalling to post-synaptic Enzymes neuron Closes ion channels (when neurotransmitter is gone) and turns off the signal Dopamine – Parkinson’s disease Parkinson’s disease Loss of dopamine in the basal ganglia deep in the brain Primarily affects movement Treatment with L-DOPA replaces dopamine in the brain Anti-Depressant Drugs - Serotonin (neurotransmitter) SSRIs Selective serotonin re-uptake inhibitors (Prozac, Zoloft, Lexapro, Lovan, Cipramil) MAOIs Monoamine oxidase inhibitors (Nardil, Parnate) Act to keep serotonin in the synaptic cleft for longer. Enzymes increase serotonin signalling Example: Reflexes Whole brain is very complex, but think of neural communication in a very simple circuit Same principles of neural communication apply for reflexes What happens here? What determines if the motor neuron will “fire” and cause reflex? Sensory neuron (input) passes signal to motor neuron (output) to cause muscle contraction. Sending Signals: Neurotransmitter Release Action Potential Depolarisation of axon terminal (action potential) triggers release of neurotransmitter Neurotransmitter acts on receptor on post- synaptic neuron to open ion channels and pass signal Chemical signal neuron-to-neuron Ligand-Gated ion channels Neurotransmitter receptors open ion channels when neurotransmitter binds Different neurotransmitters bind to and open different ion channels (Na+, K+, Cl-) to change membrane potential in different ways Receptor binding: Neurotransmitter Can cause depolarisation (less negative) e.g. Na+ flows in Can cause hyperpolarisation (more negative) e.g. K+ flows out or Cl- flows in EPSPs and IPSPs – Excitatory and Inhibitory Receptor Channels – activated by neurotransmitters: Signals can be: Excitatory – Receptor opens channels that cause depolarisation – EPSP: Excitatory Post-Synaptic Potential – Closer to threshold for action potential Inhibitory – Receptor opens channels that cause hyperpolarisation – IPSP: Inhibitory Post-Synaptic Potential – Further from threshold for action potential Graded Potentials Excitatory and Inhibitory inputs (via dendrites) combine together – Change membrane potential on postsynaptic cell Graded Potential on postsynaptic cell depends on strength of synapse connection (on dendrite) – Strong connection causes large change in membrane potential – Weak connection causes small change (for “Neuroplasticity” lecture: “strength” of synapse changes with learning - LTP)  When do inputs trigger an Action Potential? Membrane potential at axon hillock depends on sum and timing of inputs through dendrites If enough excitatory inputs occur together close enough in time, membrane potential will exceed threshold level for action potential If membrane potential exceeds threshold level (at axon hillock) – Triggers action potential – Neuron “Fires” (sends signal along it’s axon) Graded Potentials Neural Integration – Sum of all inputs Higher Brain “Don’t drop the dinner !!” “Oww, that really hurts !!” Inhibits reflex Boosts reflex Sum of all inputs determines whether sensory neuron (input) passes signal to motor neuron (output) to cause muscle contraction. Integration of signals – whole brain Neuron receives many, many inputs – has only one output – What combination of inputs will cause this neuron to “fire” and pass on it’s signal? Brain is enormous “integrator” of information – adapts with learning (billions of neurons with millions of billions of connections) When sum of all inputs is high enough, triggers output (cell “fires”) Integration of information in the brain Imagine this neuron represents memory of your grandmother When this neuron “fires” you consciously recall your grandmother What information does this neuron need to receive to “fire” and give conscious recall of your grandmother?