Neurons & Synapses Biology 12

The Resting Neuron When at rest, there is a difference in electrical charges between the outer and inner surfaces of the nerve cell membrane. The outer has a net (+) charge, while the inner surface has a net (-) charge due to less positive ions. This difference in electrical charge is known as polarization. The polarization is caused by the concentration difference of sodium ions and potassium ions on either side of the cell membrane.

The Resting Neuron The cell membrane is selectively permeable to these ions, but also has a sodium-potassium pump that works to actively pump these ions across the cell membrane. When working, the sodium-potassium pump forces Na+ out of the cell and K+ into the cell (3 Na+ out, 2 K+ in).

The Resting Neuron At rest, the cell membrane is freely permeable to K+, but not to Na+. This means that K+ diffuses through the membrane but Na+ does not. As a result, there is a build up of Na+ on the outside of the cell membrane.

Nerve Impulse An action potential is an impulse from another nerve or a stimulus from a nerve receptor. A nerve impulse causes: The permeability of the membrane to sodium ions suddenly increases. Sodium ions diffuse rapidly from the outside to the inside of the membrane. This reverses the polarity of the cell membrane (inside positive and outside negative).

Nerve Impulse Continued This reversal occurs in a small area of the membrane and results in a flow of electrical current that affects the permeability of the adjacent areas of the membrane. The reversal of polarization is the nerve impulse and it travels the length of the axon. High permeability of the membrane to sodium ions last only a fraction of a second and then returns to normal. The sodium pump and potassium diffusion allow normal distribution of ions to be restored.

Nerve Impulse Continued A brief recovery period occurs during which the nerve cell membrane cannot be stimulated to carry impulses. This refractory period lasts a few thousandths of a second. The rate at which an impulse travels depends on the size of the nerve and whether or not it is myelinated (unmyelinated = 2 m/s and myelinated = 100 m/s). In myelinated fibers the signal jumps from one node of Ranvier to the next. This is saltatory conduction and occurs because the membrane at the node is highly sensitive and this uses less energy due to polarization only at the nodes.

Nerve Impulse Continued For a nerve impulse to be transmitted, the stimulus must be at least a certain minimum strength or must reach a threshold. The impulses transmitted by a given neuron are all alike; a neuron operates on an “all-or-none” basis. The strength of the stimulus is measured by two effects: 1. A stronger stimulus causes more impulses to be transmitted each second. 2. Different neurons have different thresholds. A large number of neurons fire when a stimulus is stronger.

Transmission at the Synapse The transmission of the impulse across the synaptic cleft is a chemical process. Within the synaptic knob, the synaptic vesicles contain neurotransmitters (which are chemicals such as acetylcholine and norepinephrine). When an impulse reaches the synaptic knob, the synaptic vesicles fuse with the membrane of the synaptic knob and release their contents into the synaptic cleft. Special receptor proteins in the membrane of the neighboring dendrite attach to these neurotransmitters.

Transmission Continued When the impulses are arriving at a faster rate (representing a stronger initial stimulus), more neurotransmitter is released into the synaptic cleft and more impulses per second are sent. When the neurotransmitter has done its work, it is removed from the synaptic cleft by an enzyme that breaks down the molecules

Neurotransmitters Excitatory neurotransmitters are chemicals that initiate impulses in adjacent neurons. Examples include: acetylcholine, norepinephrine, histamine, and glutamic acid (an amino acid) Inhibitory neurotransmitters are chemicals that inhibit the firing of impulses. Examples include: serotonin, epinephrine, and glycine If the overall results are excitatory, impulses are transmitted down the axon to the next set of synapses. If the results are inhibitory, no impulses are transmitted.

Drugs and Synapses Many poisons and drugs affect the activity of chemical neurotransmitters at the synapses. Nerve gas, curare, botulin toxin, and some poisonous insecticides can interfere with the functioning of acetylcholine and cause muscle paralysis (death for respiratory paralysis). Stimulants cause a feeling of well-being, alertness, and excitement such as amphetamines (mimic norepinephrine by binding to receptors) and caffeine (aids in synaptic transmissions). Depressants slow the body activity or cause depression such as barbiturates (block the formation of norepinephrine). Hallucinogens such as LSD or mescaline interfere with the effect of the inhibitory transmitter serotonin.

Extra Video Links https://www.youtube.com/watch?v=-6t_n6kTj1A https://www.youtube.com/watch?v=h2H6POZowiU https://www.youtube.com/watch?v=XdCrZm_JAp0 https://www.youtube.com/watch?v=Tbq-KZaXiL4 https://www.youtube.com/watch?v=LT3VKAr4roo

Taken from: 1. Text book (Biology: The Study of Life) 2. John Broida, PhD. http://www.usm.maine.edu/psy/broida/101/neuron.JPG 3. How Stuff Works http://health.howstuffworks.com/brain1.htm 4. Airline Safety.Com http://www.airlinesafety.com/editorials/PilotsAndMemory.htm 5. Neruoscience Glossary http://shp.by.ru/spravka/neurosci/ 5. Muscel Physiology http://fig.cox.miami.edu/~cmallery/150/neuro/neuromuscular-sml.jpg