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The Nervous System Chapter 14 Plan – Field Trip • Wednesday 5/23 – to Case Western Neurology Laboratory: • Dr. Hillel Chiel: http://www.case.edu/artsci/biol/people/chiel. html • Time TBD. Outline • Different types of neurons • Structure of a neuron • How do neurons send messages? a) Electrical b) Chemical Purpose • The purpose of the Nervous System is to respond to change in the environment of an organism. • Example: An approaching predator, sunrise, a moving baseball. Definition • The Nervous System is a network of specialized cells (called “neurons”) that carry messages throughout the body. • [The study of these neurons is called Neurology ______________ ] Nervous System • The Nervous System is made up of three different types of neurons: 1. Sensory Neurons 2. Interneurons 3. Motor neurons 1- Sensory Neurons • Sensory neurons are neurons that detect a stimulus. • Stimuli can be sounds, heat, light, chemicals, pressure, etc. • Example: Sensory neurons in your ear detect sound, nerves on your neck can detect touch (pressure), in your nose can detect smell, etc. • These neurons then pass the message to… 2- Interneurons • Interneurons relay messages from one neuron to another. These can be divided into two different types: • A) Messengers- Neurons that are messengers send receive a message from the previous neuron and pass it along to the next neuron in line. • B) Interpreters- Neurons that receive a message and interpret the message and come up with a proper response. (Example: neurons in the brain). • Interneurons continue passing the message along until it reaches a… 3- Motor Neuron • Motor neurons receive a message from interneurons and cause a reaction in the body to the original stimulus. Motor neurons usually control a muscle and cause it to contract. Label Sensory Neuron Stimulus Interneuron Motor Neuron Effector- organ that actually reacts (usually a muscle) Majority of nerves in the body • The overwhelming majority of neurons in the body are…. Interneurons • This is because the chain to relax information is fairly long and involves many different neurons and only the very first one is a sensory neuron and the very last one is a motor neuron. How does a neuron differ from an “average animal cell”? Average animal cell (epithelial cell, white blood cell, gland cell, etc). Neuron Structure of the Neuron • Now we are zooming in on a single neuron. The neuron has 3 sections: 1. Dendrites 2. Cell body 3. Axon Structure of the Neuron – Similarities to other cells (Cell Body) The cell body of a neuron is similar to any other type of cell. It has mitochondria, ribosomes, a cell membrane, a nucleus, etc. This is because the cell needs to survive just like any other Similar cell. Different Dendrites • Dendrites are short, highly branched fibers that receive an impulse (message) from another neuron. • It transmits the message toward the cell body. Axon • The axon is a single long fiber that carries an impulse away from the cell body toward a different neuron. • This is the longest part of a neuron. • It generally does not branch until the very end, where it branches into many terminals. • Axons can be as long as a meter in certain areas. Axon Terminal Dendrite Cell body Nucleus Axon Axon (contd.) • The length of the axon is mostly covered by a white, fatty material known as myelin. • There are also gaps in the myelin known as the nodes of Ranvier. • We will get to the function of these different areas soon. The Nerve Impulse 1. Resting potential 2. Nerve Impulse 3. Refractory Period Resting Potential • When a neuron is at rest, the sodium-potassium pump moves sodium (Na+) out of the cell and moves potassium (K+) into the cell. This is done by active transport. • After this, some of the potassium leaks out of the cell, making the outside more positive than the inside. • This causes the inside of the cell has a net negative charge and the outside of the cell has a net positive charge. Nerve Impulse • When a nerve impulse reaches the dendrite of a neuron, the sodium channels open. This causes sodium to rush in (diffuse) to the cell and make the inside of the cell more positive. • This, in turn, causes “voltagegated sodium channels” to open in response to the change in charge. • This causes many more sodium to rush in. The transmission of the nerve impulse 1. The sodium rushes in to the cell. 2. This changes that area of the cell to become more positive. 3. This affects the voltage-gated sodium channels nearby causing them to open. 4. This allows sodium to rush in, etc. Speed of Transmission • This impulse moves rapidly down the dendrite membrane, down the cell body and down the axon to the axon terminal. • The impulse is able to move so rapidly down the long axon because it skips all of the regions that are covered by myelin. The impulse only affects the Nodes of Ranvier, which are not covered by myelin. • In neurons without myelin, the impulse moves at ~2 m/s. In myelinated cells, it moves at ~100 m/s. Refractory Period • After the sodium rush has caused a certain segment of the cell to become positive, the cell reverses this process by opening potassium channels and allowing the potassium (K+) to rush out, restoring the negative charge inside the cell. • After a while, the sodiumpotassium pump will exchange Na+ for K+ and the normal balance will be restored in the cell. • During this time, the nerve cannot respond to a new impulse. (This takes a few thousandths of a second). Review At a given spot in a neuron, the following events will happen: 1. Resting potential: _______ inside, _______outside. More _______ inside and more ________ outside. 2. Action potential: _______ flows inside the cell. This causes the inside to become more ________. This causes _____________ to open, allowing more ________ to flow inside. Now the inside is more ________ and the outside is more _________. 3. Refractory Period: The cell opens channels to let ________ flow out of the cell. This causes the inside to be more __________ and the outside to be more __________. Eventually, the sodium-potassium pump restores the balance for the cell, with more ________ outside and more ________ inside. We are now back to resting potential. Summary Movie • http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch apter14/animation__the_nerve_impulse.html Synapse • The gap between different neurons is called a synapse. • We have established how an electrical impulse can travel down a neuron. How does the impulse travel between different neurons? How does an impulse travel across a synapse?