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ELECTROCHEMICAL IMPULSE 1. How does a neuron transfer a signal? 2. What causes neuron excitation? 3. How does the brain interpret different strength signals? 1. How does a neuron transfer a signal? • When the neuron is not relaying information, it is said to be at rest. In this situation, there is a charge difference across the membrane due to the unequal concentration of positive ions on either side of the membrane. This charge difference or resting membrane potential is -70 mV (millivolts). • Potassium ions are concentrated on the inside of the cell and sodium ions are concentrated outside the cell. Both ions move across the membrane but the membrane is 50 times more permeable to potassium ions, so more potassium ions diffuse out of the cell than sodium ions diffuse in. Because of this unequal movement, the exterior of the cell becomes positive relative to the interior. So, at rest, the membrane is said to be polarized. • The nerve impulse is an electrochemical message generated by the movement of ions through the membrane. • When the nerve is excited, the potential charge difference across the membrane changes to +40 mV. This is called the action potential. • When excitation of the nerve occurs, the membrane becomes more permeable to sodium ions. It is believed that the membrane has sodium ion channels embedded within it. As the sodium gates open, the potassium gates close. This allows for the rapid flow of sodium ions into the cell, causing a charge reversal which is referred to as depolarization. • Once the interior of the nerve cell becomes positive, the sodium and potassium gates reverse themselves and a sodium-potassium pump restores the ion concentrations to their resting levels. • The nerve cannot send another impulse again until the membrane has been restored or repolarized. The refractory period can last from 1 to 10 milliseconds. • The wave of depolarization sweeps down the length of the axon in sections. A similar analogy would be a crowd doing the wave in a stadium. 2. What causes neuron excitation? • When a sensory neuron detects a change in the environment known as a stimulus, it has to be strong enough to trigger the depolarization of the membrane. • The intensity of the stimulus must reach a set level called the threshold level before the signal will be sent. This threshold is important for it prevents small changes that don’t have an effect from sending a signal to the brain. Without the threshold, the sensory neurons would send signals continuously which would overwhelm the brain. • Once the threshold level is reached the neuron will fire at the same intensity and the same speed for any stimulus. • The size of the change cannot affect these factors of the nerve impulse. A greater stimulus does not cause a greater depolarization of the membrane. This is known as the all-ornone response. The neuron either sends the signal or it does not. 3. How does the brain interpret a signal of different strengths? • The brain is able to interpret intensity by the frequency of the impulses being sent. Therefore, if the stimulus is small a few signals will be sent, one after the other. • When the stimulus is intense, the signal will be sent repeatedly as soon as the membrane has repolarized. http://www.youtube.com/watch?v=jcZLtH-Uv8M&feature=related SYNAPTIC TRANSMISSION 1. How are signals transferred between neurons? 2. What is a reflex arc? 1. How are signals transferred between neurons? • Information from sensory neurons must be passed to interneurons (by synaptic transmission) which can then relay the information to the brain. • The small space between the ends of the neurons is called the synapse. • When the signal reaches the end of the neuron, chemical neurotransmitters (small molecules that are held in tiny vesicles at the end of the first neuron) move across the space or synaptic cleft. • The vesicles move to the end of the neuron, fuse with the membrane and dump their contents into the cleft. The next neuron in line, called the post-synaptic neuron, has receptors on its end where the neurotransmitter can bind. • Once they bind, the threshold level is reached and the signal moves down the length of the neuron. The diffusion of the transmitter across the cleft slows down the transmission of the signal so the greater the number of synapses, the slower the signal travels. • Acetylcholine is a common neurotransmitter that is capable of opening the sodium ion channels in the post-synaptic neuron. Once the message has been transmitted, the enzyme cholinesterase is released into the synaptic cleft and breaks the acetylcholine down. http://www.youtube.com/watch?v=9vZI_XKca88 2. What is a reflex arc? • In most situations, the nervous system works quickly enough to respond to changes in the environment. But, there are some situations where an almost instantaneous response is required to keep the organism from harm. This is the function of the reflex arc. • Each reflex arc contains five necessary components: the receptor, the sensory neuron, the interneuron in the spinal cord, the motor neuron and the effector. • The sensor is the portion of the nervous system that is able to detect the environmental change. It activates the sensory neuron which sends the signal towards the brain. • Once the signal reaches the spinal cord, specialized interneurons that are pre-programmed with a specific response, send a message back through the motor neuron to the effector which immediately makes the change. • At the same time, the signal travels along other interneurons to the brain for processing. • This system cuts down on the lag time required for the signal to reach the brain, by having the brain process the information and then send the appropriate response. http://www.youtube.com/watch?v=eZundDVPIYw