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LAB 5 - Nervous System


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Diagram of a Neuron

The human body is a vast communications network of over 12 billion nerve cells or neurons in the brain that communicate with millions more in the body. The neuron is the functional unit of the nervous system. Neurons have three characteristic structure features. All neurons have a cell body that contains the nucleus, mitochondria and other organelles typical of eukaryotic cells, dendrites, which receive information from another cell and transmit the message to the cell body, and an axon that conducts messages away from the cell body. In most neurons the cell body is located centrally with multiple branching processes, and a thin single axon extending from it. The cell body is the main nutritional and metabolic region of the neuron. It receives signals from other cells and sends them toward the axon. Axons generate an action potential, an outgoing signal also called a nerve impulse and conducts it to the next cell. The axon is the transmitting or conductive region of the neuron.

A nerve signal is received by the neuron dendrites and travels along the axon, a thin tube up to three feet long. The axon maintains a chemical balance with more potassium ions inside and more sodium ions outside. When a signal is transmitted, the myelin sheath covering the axon allows the different ions to leak through. Potassium and Sodium ions change places creating an electric signal which travels along the membrane. The space between two neurons is called the synapse. When the impulse reaches the synapse, vesicles discharge chemical transmitters, which transmit the impulse to the next neuron. The first neuron returns to a resting state. Potassium and Sodium ions begin to move in the second neuron and the impulse passes on.

Some axons are wrapped in a insulating material called myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gaps in the myelin sheath are known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon.

Ion channels control the movement of ions across the neuronal membrane. Ion channels are either active or passive. Active channels have gates that can open or close the channel . Some active channels have voltage controlled gates. Voltage gated channels have gates that are controlled by voltage. Passive (leakage channels) are always open. Ions pass through them continuously. Ion channels are regionally located. Passive channels are located in the cell membrane on the dendrites, the cell body and the axon. Chemically gated channels are on the dendrites and cell body. Voltage gated channels are found on the axon hillock all along unmyelinated axons and at the nodes of Ranvier in myelinated axons.

When at rest, a neuron is polarized, meaning that it has an electric charge. The reason for this charge is that charged atoms (ions) of sodium (Na(+)), potassium (K(+)), or chloride (Cl(_)), either electrically positive or negative, are unevenly distributed on the inside and the outside of the neuron.

This differential charge is called a resting potential. Passage of ions across the neuronal cell membrane passes the electrical charge along the cell. The voltage potential is -70mv of a cell at rest(resting potential). When a neuron is at rest, voltage gated channels are closed. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump. The sodium potassium pump maintains the unequal concentration by actively transporting ions against their concentration gradients. If a neuron receives sufficient excitatory signals it becomes depolarized. This means that the resting potential difference between the inside and outside of the neuron becomes decreased, and the membrane potential becomes more positive. Once this occurs to a significant extent, the neuron reaches its firing threshold and generates an electrochemical signal called an action potential When a neuron reaches threshold a positive feedback is established. The positive feedback loop produces the rising phase of the action potential. The rising phase of the action potential ends when the positive feedback loop is interrupted . Two processes break the loop. The inactivation of the voltage gated sodium channels, and the opening of the voltage gated potassium channels.

Potassium leaves the cell as voltage gated channels open with less sodium moving into the cell and more potassium moving out, the membrane potential becomes more negative moving toward its resting value. This process is called repolarization.

In many neurons the slow voltage gated potassium channels remain open after the cell has repolarized. Potassium continues to move out of the cell, causing the membrane potential to become more negative than the resting membrane potential. This process is called hyper-polarization.

Steps in an Action Potential

yes.gif (6787 bytes) At rest the outside of the membrane is more positive than the inside.

yes.gif (6787 bytes) Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside.

yes.gif (6787 bytes) Potassium ions flow out of the cell, restoring the resting potential net charges.

yes.gif (6787 bytes) Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.