6.5 Neurons & Synapses

Understanding: Neurons transmit electrical impulses. Two systems are used by internal communication: Nervous system (neurons) Endocrine system (glands & hormones) A nerve impulse is an electrical signal. Electricity is flow of electrons through a wire. Nerve impulse is flow of ions through a neuron.

Structure of a NEURON Draw & label on your whiteboard!

Structure of a NEURON

Motor Neuron

Schwann cell membrane = myelin (fatty substance)

Understanding: The myelination of nerve fibres allows for saltatory conduction. From the Latin saltāre meaning to jump, leap or spring! Nerve impulses “jump” from node to node, allowing for a faster transmission (100m/s) Na+ gates are only found at the nodes.

Propagation of a nerve impulse

Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential. Membrane potential = difference in electrical charges across a membrane Resting potential = difference in charges when neuron is not transmitting a signal About -70mV

The purpose of the Na/K pump is to restore RESTING potential!

Understanding: An action potential consists of depolarization & repolarization of neuron. Action potential = rapid rise and fall of membrane potential = depolarization followed by repolarization Depolarization = reversal of charges (from – to +) Repolarization = restoration of charges (from + to -)

Action Potential

Depolarization and Repolarization Depolarization = Na+ gates open, Na+ rushes into axon Repolarization = K+ gates open, K+ rushes out of axon

Understanding: Nerve impulses are action potentials propagated along axons of neurons. Propagate = to spread Movement of ions that depolarize one part of axon trigger depolarization in neighboring parts of axon http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

Why can’t nerve impulses go backwards? Impulse can only be initiated in dendrites of one neuron and can only be passed on to other neurons at axon terminals There is a refractive period after a depolarization that prevents propagation of an action potential backwards along an axon Nerve transmission skit!

Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential. Threshold potential = the critical level to which the membrane potential must be polarized in order to initiate an action potential About -50 mV

Skill: Analysis of oscilloscope traces showing resting potentials and action potentials. DBQ p 324

Understanding: Synapses are junctions between neurons and between neurons and receptor or effector cells. Synapse = fluid-filled gap between 2 neurons (or between neuron and muscle, neuron and gland, or neuron and receptor cells) Electrical impulse cannot pass across fluid Electrical signal must turn into a chemical signal in the synapse

Synapse between neuron & muscle = neuromuscular junction

Synapse between neuron & receptor

Understanding restated: Synapses are junctions between neurons and between neurons and receptor or effector cells.

Steps involved in Synaptic Transmission

Synaptic Transmission = chemical signal

Steps of Synaptic transmission Nerve impulse reaches end of pre-synaptic membrane. Depolarization of pre-synaptic membrane cause Ca2+ to diffuse thru channels in membrane Influx of Ca2+ causes vesicles containing neurotransmitter to move to pre-synaptic membrane & fuse with it. Neurotransmitter released into synapse by exocytosis. Neurotransmitter diffuses across synaptic cleft and binds to receptors on post-synaptic membrane. Binding of neurotransmitter causes adjacent sodium ion channels to open. Na+ diffuse into post-synaptic neuron, causing membrane to reach threshold potential. Action potential triggered in post-synaptic membrane and propagated along post-synaptic neuron. Neurotransmitter rapidly broken down and removed from synapse.

Description of nerve transmission Electrical signal  chemical signal  electrical signal Animation: http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation__trans mission_across_a_synapse.html

Understanding: Secretion and reabsorption of acetylcholine by neurons at synapses. Acetylcholine (Ach) = common neurotransmitter Loaded into vesicles then released into synapse Ach only binds to receptor for short time, so only 1 action potential initiated Acetylcholinesterase (enzyme) breaks down Ach Choline reabsorbed by pre-synaptic neuron and recycled/reused A receptor is ”cholinergic” if it uses Ach as its neurotransmitter. A synapse is ”cholinergic” if it uses Ach as its neurotransmitter.

Where do we get choline? Also: Wheat germ Brussel sprouts Broccoli Salmon Milk Peanut butter Milk chocolate

Nicotine mimics Ach! Nicotine shaped like Ach Ach involved in parasympathetic nervous system (muscle movement, breathing, heart rate, learning, memory) Ach causes release of other neurotransmitters and hormones that affect your mood, appetite, memory When nicotine gets into the brain, it attaches to Ach receptors and mimics Ach Nicotine raises the levels of dopamine in the parts of the brain that produce feelings of pleasure and reward Dopamine (the pleasure molecule) is the same neurotransmitter that is involved in addictions to other drugs like cocaine and heroin, which may explain why it is so hard for people to stop smoking.

Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to Ach receptors Neo-nicotin-oid = pesticide = kills insects Neo similar to nicotine: mimic Ach and bind to Ach receptors Acetylcholinesterase can’t break down neo, so it stays bound to Ach receptors so Ach can’t bind  insects become paralyzed & die. imidacloprid = most widely used neo insecticide Advantage: not toxic to mammals =) Disadvantage: harm honeybees & other beneficial insects =(

Beneficial insects

Understanding: A nerve impulse is only initiated if the threshold potential is reached. Nerve impulses follow an ALL OR NOTHING principle! POSITIVE FEEDBACK: Stimulus  some Na+ gates open  diffusion of some Na+ increases membrane potential  causing more Na+ gates to open If threshold potential reached, there will always be full depolarization Threshold potential also has to be reached at the synapse.

How your brain makes decisions! Post-synaptic neurons in brain & spinal cord synapse with many pre-synaptic neurons If your brain is processing information from different sources in the body, the SUMMATION of neurotransmitters released will determine whether the nerve impulse is EXCITED or INHIBITED

Nature of Science: cooperation between groups of scientists: Biologists are contributing to research into memory & learning. Research into memory & learning has typically been just an area of psychology, but now: Molecular biology Biochemistry Biophysics Medicine Pharmacology Computer science Engineering Optics Genetics

Nature of Science Centre for Neural Circuits & Behaviour at Oxford University: Professor Miesenbock – medicine & physiology Dr. Booth – engineering & optical microscopy Dr. Hens – chem & biochem Professor Waddell – genetics, molecular bio, neurobio Research methods: optogenetics  neurons genetically engineered to emit light when firing, making brain activity visible; also made to respond to light signals http://video.mit.edu/watch/optogenetics-controlling-the-brain-with-light-7659/ http://www.ted.com/talks/gero_miesenboeck?language=en Research involves both competition and collaboration. Success involves many different scientists in many different countries!