The Nervous System Cell Structure Cells of the Central Nervous System Receiver (AFFECTOR) cells from sense organs Sender cells (EFFERENT) to motor organs/muscles Also: Specialized “helper” cells Total Neurons: About 100 Billion – Specialized nerve cells in CNS and PNS Neurons Neurons: Specialized nerve cells in CNS and PNS – – – – Convey sensory information into the brain Carry out operations involved in thoughts, feeling, behavior Transmit commands to body to control muscles and organs Many different specializations Many neurons: – – – 26-29 billion in higher brain areas 70 billion in cerebellum 1 billion in spinal cord Parts of a Neuron – – – – – Soma: regulates life functions (metabolism) Contains nucleus (and chromosomes), organelles, etc. Dendrites: branched projections from soma receive transmissions from other neurons Axon: long projection extending from soma – transmits information to next neuron – axon hillock: beginning of axon, important in transmission Parts of a Neuron Terminals or Terminal Buttons – bulbs on end of branched portion of axon – synaptic bulbs or terminals – contain neurotransmitters Synapse or synaptic cleft – space between neurons – space between one neuron's synaptic bulb/other's dendrites Myelin sheath: – glue like structures that hold neurons in place Myelin Sheath insulates neurons made of glial cells or “glue cells” – In CNS: oligodendroglial cells – In PNS: Schwann cells allows synaptic transmission to occur by jumping down axon exposed area between sheath = Nodes of Ranvier Several kinds of neurons and helper cells Affector or Receptor neurons: – SENSORY neurons: embedded in sense organs – specialized to receive stimulation from environment and send to the brain May be UNIPOLAR or BIPOLAR – Single axon – Two axons: end of axon branches into two Axon and dendrites extend in several directions from body Several kinds of neurons and helper cells Effector or Motor neurons – specialized to contract muscles and stimulate glandular secretions – acted upon by nerves and neurons MULTIPOLAR: have multiple axons which extend from soma in several directions – Dendrites on one end; axon on other Several kinds of neurons and helper cells Interneurons – Connect one neuron to another in same part or region of brain/spinal cord – Multipolar – Seem to be missing the axon (it is very short or nonexistent) Several kinds of neurons and helper cells Glial cells: form the myelin sheath – Wrap around on outside of most human neurons: Insulate and wrap around neurons: – Speeds up neural transmission – Also help with storing of neurotransmitter – Important part of waste system for neuron as well Multiple Sclerosis = allergy to own myelin Neural Membrane and Its potential: HOW THE NEURON WORKS The Cell Membrane Cell membrane = – The cell wall or “skin” of the cell – most critical factor in neural communication – Only about 8 micrometers ( 8 millionths of a meter) thick Composed of lipid (fat) and protein – Lipid molecules arranged head to tail – Heads are water soluable (attract water) – Tails are water insoluable (repel water) Heads point towards surrounding fluid, tails turn away – This creates double layer membrane Neuron lipid layer Neuron Potentials Membrane holds cell together – More importantly: CONTROLS environment within and outside cell Semipermeable: – allows some molecules in/out – H20; O2, CO2 pass freely Selective permeability: – Keeps out some substances – Allows others in only under certain circumstances – Protein channels: open and close to let molecules in when neuron is active Neuron Potential Polarization is result of this selective membrane permeability: – means that the cell has an electrical charge Potential: difference in electrical charge between the inside and outside of a cell (or any two points) Neuron has three critical potentials: – Resting potential: cell is at rest – Action potential: cell is active and sending a signal – Refractory potential: cell is recovering Resting potential Neurons sit at base level: RESTING POTENTIAL – Resting potential = approximately -70 mV Why -70 mV? – 70 mV charge due to unequal distribution of electrical charges on two sides of cell membrane: – – inside of axon is negatively charged: more K+ ; more A- outside of axon is positively charged: More Na+; some Cl- Remember: axon has voltage of -70mV at resting potential How can neuron stabilize at resting potential? Diffusion: Molecules diffuse from area of high concentration to low concentration until “evened out”: Concentration gradient: ions move to side of membrane where less concentrated Electrical gradient: ions attracted to side that is of opposite charge Other factors upset this diffusion: – Neuron wall semi permeable: anions too large to pass through membrane – Negative charge of anions repels Cl- ions so they don’t move inside During an ACTION POTENTIAL, membrane undergoes changes: – opens gates into/out of cell – Results in changes to the t charge of the concentration Na+K+ Pump or Ion Pump K+ and Na+ are special: – K+ tends to move out of cell concentration gradient is stronger than electrical gradient – Na+ tends to move into cell: both gradients pull Na+ inside K+ and Na+ pass through membrane via special protein channels – Most Na+ and K+ remain in place during resting potential – Those that do get through are returned via NA+K+ pump Na+K+ Pump or Ion Pump • Na+ K ion pump = large protein molecules that move Na+ ions to outside/K+ to inside • Rate of exchange: 3 Na+/2 K+ • Maintains membrane as more negative inside than out • Metabolic process: uses energy (about 40% of energy expenditure of the cell!) Neuron Potential Polarization is result of this selective membrane permeability: – means that the cell has an electrical charge Potential: difference in electrical charge between the inside and outside of a cell (or any two points) Neuron has three critical potentials: – Resting potential: cell is at rest – Action potential: cell is active and sending a signal – Refractory potential: cell is recovering Action Potential Action potential = abrupt depolarization of membrane that allows neuron to communicate over long distances Neuron becomes excited and sends a signal via neurotransmitter release if incoming message is sufficient in strength: Causes an ACTION POTENTIAL Action Potential Dendrites (usually) receive incoming neurotransmitter – Chemical fits in “lock” on dendrite – Alters the shape of the cell wall PARTIAL DEPOLARIZATION at dendrites: – Allows changes in cell wall that will change voltage inside the neuron – THIS depolarization Is decremented: decreases with time/distance – Also called local potential because has only a local effect Action Potential All or None Law: Voltage change is either sufficient to stimulate action potential, or not (no wimpy action potentials!) Voltage changes from -70 to +40 mV and back again This “depolarization” begins at the axon hillock – inside of axon becomes negative due to movement of ions – NaCl goes out of axon outside of axon becomes positive: K+ goes in result is voltage change as switching of ions occurs depolarization moves down axon in wavelike form in myelinated neurons The Neuron Fires Voltage across cell membrane is stored energy; if this stored energy is released, get tremendous changes within cell During action potential: Na+ channels open – Remember: thousands of Na+ ions held on outside- now they rush in through these channels – Approximately 500x greater than normal number of Na+ ions – Small area inside membrane is depolarized, first to 0 and then to +30 to +40mV – This small area will then spread down axon: cell wall opens and ion exchange occurs at Nodes of Ranvier As reach peak of Action Potential K+ ions move out due to diffusion. K+ also moves out due to electrostatic pressure because the inside of the cell is temporarily positive. Membrane returns to near resting potential or BELOW This entire event takes approximately 1 millisecond. Because nearby sodium channels open, a new action potential is triggered at the adjacent patch of membrane. Refractory Period Absolute refractory period: Neuron repolarizes – cell environment moves back toward resting potential during refractory period – Resetting neuron back to resting potential – Cell absolutely cannot fire during this period Then Relative Refractory period: – Neuron can fire, – but only with extra stimulation Absolute Refractory Period Neuron cannot fire during this period Due to action of Ion pump: – Ion pump kicks into action at end of action potential – Pumps ions K+ in and Na+ out – Over does it a bit: cell ends up just below resting potential – Until returns to resting potential, very difficult, if not impossible, for cell to fire Important functions of Absolute Refractory Period When the Na+ channels close during the action potential, that part of the axon cannot fire again. This limits how frequently the neuron can fire. This also prevents backward spread of depolarization, so action potentials move only toward the terminals. Important functions of Relative Refractory Period Plays role in intensity coding in axon K+ channels open for just milliseconds longer following absolute refractory period Makes inside of cell slightly more negative; harder to fire Rate law: Stronger stimuli trigger new action potentials earlier in recovery, so the axon encodes intensity as rate of firing. Thus: only stronger stimulation can set off neuron Remember: Movement of action potentials down the axon is not a flow of ions but a chain of events. Because the action potential “jumps” from node to node this is called saltatory conduction. When the action potential reaches the terminals it passes the message on to the next cell “in line”…..and it begins again Nondecremental conduction Action potential different from local potential in several important ways: – Local potential = graded potential- it varies in magnitude depending on strength of stimulus that produced it; action potential is ungraded – Action potential obeys all or none law: occurs at full strength or not at all Action potential is nondecremental: does NOT lose strength at each successive point (local potentials do degrade) Neurotoxins and Ion channels Neurotoxins affect ion channels involved in the action potential. The puffer fish produces tetrodotoxin: blocks sodium channels. Scorpion venom keeps sodium channels open, prolonging the action potential. Beneficial drugs affect these ion channels as well. – Local anesthetics block sodium channels. – Some general anesthetics work by opening potassium channels. Ion channels can be modified to control neurons by light. – This allows greater precision in stimulating neurons and identifying pathways in the brain. Again: Three Steps for firing Resting potential: voltage is about -70mV Action potential – Dendrites receive incoming signals – If sufficient, cell goes into firing mode – – – – Voltage changes from -70mV to +40mV Ions exchange places Repeats itself rapidly down axon Only in places where myelin sheath doesn’t cover: Nodes of Ranvier Refractory Period: – – – – below resting or lower than -70mV Cell recovers from firing Absolute refractor period: Brief time period when cannot fire again Relative refractory period: Brief time period when difficult for it to fire again. Take home lesson: – Axon encodes stimulus intensity by controlling FIRING RATE not size of action potential Why so fast? Thank your Glial cells • Remember: Glial cells: – Non neural cells – Provide a supporting function to neurons – Account for 90% of cells in adult human brain • Function: Help hold neurons together, assist in neurotransmission – Provide supports for the nervous system – In periphery are rather rigid: • E.g., Schwann cells – In CNS: are soft and squishy: • E.g., Oligodenroglia cells Glial Cells speed conduction • Neurons conduct impulses from 1 to 120 meters/sec or 270 mph – Still fairly slow Since reaction time critical for survival, body found ways to increase conduction speed • Vary thickness of axons to provide less resistance • Motor neurons: diameter of 0.5 mm can attain conduction speed of 30m/sec • But: conduction speed not increase in direct proportion to size: is power function, thus must find alternative way Alternative way: use graded local potentials Myelination = solution to increasing speed of neural transmission • Vertebrates are myelinated: allows SALTATORY conduction – Action potential jumps from node to node – Myelin also helps increase speed via capacitance: resists movement of ions during graded potential Overall effect: 100x greater conduction speed; reduced work for Ion pump Other functions of Glial Cells Important in fetal development: scaffold that guides new neurons to destination Provide energy to cell Serve as waste system for neurons Aid in development and maintenance of neural connections – Get 7x more connections when glial cells available Help in conducting action potential Now: Why an action potential? Allows release of neurotransmitter Neurotransmitter is a chemical substance – Remember: even chemical substances contain a charge Several specific kinds- each act on certain neurons Most neurons respond to and release one kind of neurotransmitter Now: Why an action potential? Neurotransmitter stored in synaptic vesicles • Remember, these are at the END of the axon • Next to the synapse Action potential opens channels that allow Ca+ ions to enter terminals from extracellular fluid – Ca+ ions cause vesicles nearest the membrane to fuse with membrane – Membrane then opens and transmitter is dumped into synapse Neurotransmitter then diffuses across synapse to postsynaptic neuron and attaches to chemical receptor – And if enough NT moves across, it all starts again!