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Sending Messages: Where an electrical signal is received by a neuron and then transferred down the processes to the next neuron. This carries the signal from the stimulus detected by the PNS to the brain in the CNS to interpret and then back to the motor neurons in the PNS to respond. A: The resting neuron membrane At rest, the neuron plasma membrane is chemically and electrically polarized (more positive ions are outside than inside the cell) So, there is a charge/voltage across the plasma membrane. Outside: Positively charged with Sodium ions Inside: Negatively charged with potassium ions B: Arrival of a Stimulus (Depolarization) A stimulus is detected by: the environment (detected by the skin) Neurotransmitter (from skin or another axon terminal) Light (from neurons in the eye) This starts an action potential: Sodium moves in making the inside of the cell slightly positive. The process of depolarization takes only 1/1000 of a second! http://www.wisc-online.com/objects/index_tj.asp?objid=AP1201 C: Repolarization After the depolarization wave passes, the sodium gates close and potassium gates open, allowing potassium to diffuse to the outside of the cell. During this step, there is a short time period when a neuron is not able to fire because it must restore those charges. This is called the refractory period. This restores the electrical differences to the way they were at rest. The ions are switched… Outside is positive Inside is negative. http://www.blackwellpublishing.com/matthews/actionp.html D: Restoration of the resting state An active transport pump (Sodium-Potassium Pump) begins working to pump sodium ions back to the outside and potassium ions back to the inside of the fiber. ATP is used to accomplish this. This is the ONLY part of the process that requires energy. http://www.brookscole.com/chemistry_d/templates/student_resources/shared_re sources/animations/ion_pump/ionpump.html E. THE SYNAPSE • Bridging the gap between the axon terminal of a neuron and another neuron, a muscle or a gland: • Synapse- the point between the axon of one neuron and the dendrite of the next where they are almost in contact, but not quite touching. Also can be between the axon of a neuron and the organ it is stimulating (such as a muscle fiber at a neuromuscular junction). Synaptic cleft (or gap)- the actual space between the axon terminal and the dendrite of the next neuron or the organ to be stimulated. When an impulse reaches the end of an axon: a chemical called a Neurotransmitter is released and travels across the synaptic cleft to the second neuron or muscle or gland. • If the organ stimulated is a muscle or a second neuron, it will begin to depolarize. If the organ to be stimulated is a gland, the chemical stimulates the release of whatever substance the gland produces. • It only takes a few hundredths of a second for the impulse to cross a synapse. Why is this synapse arrangement important? This arrangement insures that the action potential goes in only one direction. Also, it is easier for the body to control. A cell can stop the signal by breaking down the neurotransmitter using enzymes at the cell membrane. The Intensity of the Stimulus: The “firing” of a neuron (generation of an action potential) is an “all-or-none phenomenon”. Each neuron has a minimum level of stimulus it needs to get started. Either it fires completely or not at all. A strong stimulus depolarizes the membrane quickly The Intensity of the Stimulus: Weaker stimuli must be applied for a longer period of time to achieve the crucial amount of current flow. Very weak stimuli are unable to trigger an action potential, because the local current flows they produce are so weak they just die out. The Intensity of the Stimulus: The amount of stimulus needed to cause action potential to be self-propagating is called the threshold stimulus. This is about 15-20 mV. When an action potential is large enough to keep going on its own, it is self-propagating. How the Brain Perceives the Stimulus The stimulus can be perceived as just sensation or strong enough to be actual pain. The intensity of the sensory input depends on 2 factors: 1. Number of sensory neurons fired (stimulated) 2. Frequency of the stimulation Medical Applications: In order for this system to work, a neuron must be able to change permeability. Calcium is needed to keep the Na and K gates closed between stimulations. The neuron also needs sufficient amounts of sodium and potassium. If a person is deficient in these 3 ions, their nervous system will not work. Medical Applications: Anesthetics work by decreasing the membrane permeability to sodium. The sodium can’t get into the cell so the neuron won’t “fire” and the person feels nothing. Medical Applications: Alcohol and sedatives also block nerve impulses by reducing membrane permeability to sodium ions. ▪ ▪ Since cold temperatures can interrupt blood circulation and cut off oxygen to neurons, drinking alcohol “to feel warm” while out in the cold is dangerous You feel better because you can’t feel the cold, but you also don’t know to get in a warmer area. This could result in frost bite. E. The Synapse: This step transfers the signal from: How it works: when an impulse reaches the end of the axon, it trigger the cell to release: this chemical moves across the: to: If the chemical reaches a muscle or another neuron, what will happen? If the chemical reaches a gland what will happen? A synapse functions to: 1. 2. The intensity of the nervous signal can vary as well. This is based on the intensity of the _______________________. Strong stimuli start an action potential instantly. Weak stimuli need repeated stimulus to get an action potential going. The minimum level of stimulus that is needed to start a reaction is the: If a stimulus is strong enough to keep an action potential going it is called: A weak stimulus can be perceived as a sensation; strong stimuli are perceived as pain. How the brain perceives the stimulus depends on two factors: 1. 2. Medical Applications: 1. Nutrient defiencies in the following ions can hault nerve impulses: 2. Anesthetics limit the cell’s ability to fire because: 3. Alcohol and sedatives work the same way as anesthetics. Dangers of using these in cold temperatures: