Welcome to the Nervous System!
A world full of exciting electrical impulses, synapses, dedritic "fingers", and astonishing connections awaits you!
P.S... Did you know that some nerve impluses travel at 250 mph?! And that there are around 5,000 synapses in the width of a hair?
Overview of Nervous SystemThe nervous system is a vast collection of nerves and highly specialized cells known as neurons that transmit signals to all parts of the body. All vertebrates - that is, animals with backbones and a spinal column - have both central and peripheral nervous systems. The central nervous system (CNS) is made up of the brain, spinal cord, and retina. On the other hand, the peripheral nervous system (PNS) consists of the rest of the nerves: sensory neurons, ganglia (which are clusters of nerves), motor neurons, and nerves that connect to one another and the CNS.
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Specialized CellsAs the most complex system, the nervous system serves as the body control center and communications electrical-chemical wiring network. However, this doesn't happen by magic! The specialized networks that transmit these electrical signals throughout our bodies are called nerves, which are made up of neurons. Neurons are one of the most highly specialized and precious cells of our body that are perfectly adapted for their job. An important concept to remember throughout the AP Bio course is that structure directly affects function, and this is definitely the case with these cells!
The perikaryon (sometimes called the cell body or the soma) is the neuron central part. Dendrites, short branches, extend from the neuron. These input channels receive information from other neurons or sensory cells (cells that receive information from the environment). A long branch, the axon, extends from the neuron as its output channel. The neuron sends messages along the axon to other neurons or directly to muscles or glands. Neurons must be linked to each other in order to transmit signals. The connection between two neurons is a synapse. When a nerve impulse (electrical signal) travels across a neuron to the synapse, it causes the release of neurotransmitters. These chemicals carry the nerve signal across the synapse to another neuron. |
How are the impluses propagated?
Nerve impulses are propagated (transmitted) along the entire length of an axon in a process called continuous conduction. To transmit nerve impulses faster, some axons are partially coated with myelin sheaths. These sheaths are composed of cell membranes from Schwann cells, a type of supporting cell outside the CNS. Nodes of Ranvier (short intervals of exposed axon) occur between myelin sheaths. Impulses moving along myelinated axons jump from node to node. This method of nerve impulse transmission is saltatory conduction.
Like we said before, neurons are very highly specialized cells and their structure allows them to do their job perfectly. All neurons have an abundance of ion channels within their membranes since they are surrounded by charged ions (mainly sodium and potassium). At their normal resting state, neurons have a resting membrane potential of -70mV, meaning that the interior of the cell is more negative than the surrounding environment. This is an important concept to remember! Action potentials, or impulses, are caused by a series of electrical responses that occur within the cell. With the appropriate stimulation, the voltage in the dendrite of the neuron will become somewhat less negative (or in more simpler terms, more positive). This change in the membrane potential called depolarization, will cause the voltage-gated sodium channels to open. Sodium ions will rush in, resulting in a rapid change in the charge. At the peak of the action potential, that area of the neuron is about 40 mV positive. As the voltage becomes positive, the sodium channels close, or inactivate, and the voltage-gated potassium channels open. These potassium channels let potassium ions rush out of the cell, causing the voltage to become negative again (repolarization). The potassium channels remain open until the membrane potential becomes at least as negative as the resting potential. In many cases, the membrane potential becomes even more negative than the resting potential for a brief period; this is called hyperpolarization. An action potential typically lasts a few milliseconds.
Like we said before, neurons are very highly specialized cells and their structure allows them to do their job perfectly. All neurons have an abundance of ion channels within their membranes since they are surrounded by charged ions (mainly sodium and potassium). At their normal resting state, neurons have a resting membrane potential of -70mV, meaning that the interior of the cell is more negative than the surrounding environment. This is an important concept to remember! Action potentials, or impulses, are caused by a series of electrical responses that occur within the cell. With the appropriate stimulation, the voltage in the dendrite of the neuron will become somewhat less negative (or in more simpler terms, more positive). This change in the membrane potential called depolarization, will cause the voltage-gated sodium channels to open. Sodium ions will rush in, resulting in a rapid change in the charge. At the peak of the action potential, that area of the neuron is about 40 mV positive. As the voltage becomes positive, the sodium channels close, or inactivate, and the voltage-gated potassium channels open. These potassium channels let potassium ions rush out of the cell, causing the voltage to become negative again (repolarization). The potassium channels remain open until the membrane potential becomes at least as negative as the resting potential. In many cases, the membrane potential becomes even more negative than the resting potential for a brief period; this is called hyperpolarization. An action potential typically lasts a few milliseconds.
How can this action potential be propagated along the neuron? When the sodium channels are opened, sodium ions rush in; once inside they cause nearby regions of the neuron to become depolarized by moving laterally through the axon. This, in turn, causes the opening of more voltage-gated sodium channels in those regions. Thus, the sodium channel activation moves in a wave-like fashion: the action potential is propagated down the length of the neuron, from its input source at the dendrites, to the cell body, and then down the axon to the synaptic terminals. How does the action potential maintain this directional flow that is key to information processing? The sodium channels have a mechanism that avoids "back propagation" of the action potential, which would result in a confused signal. After opening, the sodium channels become inactivated as the potential becomes more positive, and they cannot open again until they are "reset" by hyperpolarization at the end of an action potential. This brief period of sodium channel inactivation, called a refractory period, prevents
bidirectional propagation of the action potential, constraining it to go in only one direction. |
To "reset" the neuron, the cell needs the assistance of sodium-potassium pumps, which actively transport the ions against their concentration gradient with the use of ATP. Na+ needs to move back out and K+ needs to move back in. To actively pump these ions, the cell needs sodium-potassium pumps (seriously, take the time and appreciate the simplicity of this name because let us tell you, you will come across some very long and complicated names in AP Bio that you will hate). Basically in this pump, 3 Na+ are pumped out while 2 K+ are pumped in at the cost of an ATP.
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Questions to Keep in Mind During the Video1. What is the basic structure, anatomy, and function of neurons?
2. What are the two main departments of the nervous system? and their responsibilites? 3. Types of neurons and their jobs? 4. What's the difference between the parasympathetic and the sympathetic systems? 5. Why are synapses essential? 6. Understand how impulses are transmitted and how the neuron "resets" itself. |
Reading Assignments
We know that it's summer and you probably don't want to waste your time studying the nervous system, but please visit these additional webpages for more information to make sure that you thoroughly understand this material!
Sodium-Potassium Pump animation:
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
CNS, PNS, and brain functions
http://faculty.washington.edu/chudler/nsdivide.html
Key Ideas to keep in mind while reading:
1. What are the main functions of the different structures of the brain? (i.e. brain stem, hypothalmus, etc.)
2. What is the difference between the CNS and the PNS?
3. How do the two brain hemispheres communicate with one another?
4. Main difference between afferent and efferent?
Sodium-Potassium Pump animation:
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
CNS, PNS, and brain functions
http://faculty.washington.edu/chudler/nsdivide.html
Key Ideas to keep in mind while reading:
1. What are the main functions of the different structures of the brain? (i.e. brain stem, hypothalmus, etc.)
2. What is the difference between the CNS and the PNS?
3. How do the two brain hemispheres communicate with one another?
4. Main difference between afferent and efferent?
Ready for the Quiz? Good Luck!
The Nervous System was made by Kathryn Fournier and Kathleen Silveira. We hope this is helpful for you guys! GOOD LUCK on the AP Bio exam in May! :)