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Nervous System Structure and Function Pt 1 Nervous System Function • The nervous system controls and coordinates functions throughout the body, and responds to external and internal stimuli. Irritability • The basic unit of structure and function in the nervous system are specialized cells called neurons. Neurons • Neurons are specialized cells that can transmit electrical signals called impulses. • Impulses are the messages carried by the nervous system. • Neurons can be classified into three types –Motor neurons –Interneurons –Sensory neurons Neuron Structure Axon Terminals Node Neuron Structure • Cell body (soma): The largest part of a typical neuron which contains the nucleus and much of the cytoplasm. • Dendrites: Short branching extensions from the cell body. Carry impulses from the environment or from other neurons toward the cell body. Neuron Structure • Axon: Long fiber that carries impulses away from the cell body. • Axon Terminals: Small swellings at the ends of axons. • Myelin Sheath: Insulating membrane surrounding the axon of a neuron. Contains gaps called nodes that speed up the transmission of impulses. Types of Neurons 1. Sensory Neurons: Carry impulses from the sense organs to the spinal cord and brain. 2. Motor Neurons: Carry impulses from the brain and spinal cord to the muscles and glands. 3. Interneurons: Connect sensory and motor neurons and carry impulses between them. Sensory and Motor Neurons Label each neuron A B C The Nerve Impulse • The production of a nerve impulse can be compared to the flow of electricity through a wire. • The transmission of electricity depends on the movement of negatively charged electrons. • The production of nerve impulses depends on the movement of positively charged ions across the cell membrane. • The cell membrane is the primary structure involved in the production of an impulse. Resting Potential • The distribution of sodium (Na +) and Potassium (K+) ions inside and outside of a neuron is shown in the following diagram. • There are more potassium (K +) ions in the cytoplasm of the neuron than in the fluid outside of the cell. • There are more sodium (Na +) ions in the fluid outside of the cell than inside the neuron. Resting Potential Resting Potential • Because both sodium and potassium ions can diffuse across the cell membrane, the unequal distribution of these ions must be maintained by active transport. • Proteins in the cell membrane actively pump sodium ions out of the neuron and actively pump potassium ions into the neuron. Sodium and Potassium Pumps Actively Transport Na+ and K+ Across the Cell Membrane Resting Potential Sodium Potassium Pump Requires ATP Resting Potential • As a result of active transport (K+ in, Na+ out) and diffusion (K+ out, Na+ in), a negative charge builds up on the inside of the membrane and a positive charge builds up on the outside of the membrane. • The difference in electrical charge across the cell membrane of a resting neuron is called is resting potential. • A neuron has a resting potential of about -70 millivolts (mV) The Moving Nerve Impulse The Moving Nerve Impulse • In most animals, the axons and dendrites of neurons are clustered into bundles of fibers called nerves. • A nerve impulse is similar to the ripple caused when a pebble is dropped into a pond. • The ripple is caused by the up and down movement of water. The impulse is caused by the movement of ions across the cell membrane. The Moving Nerve Impulse • A nerve impulse begins when a neuron is stimulated by another neuron or its environment. • The nerve impulse travels along the axon, away from the cell body and toward the axon terminals. • The cell membrane contains thousands of protein channels. Generally the sodium channels are closed. The Moving Nerve Impulse • At the leading edge of an impulse, sodium channels open allowing sodium ions to flow into the cell. • This flow of positive ions causes a temporary change in the charges on the cell membrane. • The inside of the membrane gains a positive charge and the outside of the membrane gains a negative charge. • This reversal of charges across the membrane along the length of an axon is called an action potential. Action Potentials • A neuron has an action potential of about +30 mV. • As the impulse passes through the axon, potassium channels open allowing K+ ions to flow out of the cell. • The resting potential is now reestablished with the negative charge inside the membrane and the positive charge outside the membrane. Action Potential • An action potential is caused by positive ions moving in and then out of the neuron at a certain spot on the neuron membrane. • An action potential is initiated by a stimulus above a certain intensity or threshold. • Not all stimuli initiate an action potential. The stimulus could be a pin prick, light, heat, sound or an electrical disturbance in another part of the neuron. • Action potential is an all or nothing mechanism, just like a mousetrap or stack of dominoes. Action Potential: Depolarization • Depolarization • A stimulus causes a gate in the Na+ Channel to open. Since there is a high concentration of Na+ outside, Na+ diffuses into the neuron. The electrical potential changes to ~ +40 mV. Action Potential - Repolarization • Repolarization • Depolarization causes the K+ Channel gate to immediately open. K+ diffuses out of the neuron. This reestablishes the initial electrical potential of ~-60 mV. Action Potential – Refractory Period • Refractory Period • During this time (~ 1 msec), the Na+ and K+ Channels cannot be opened by a stimulus. • The Na+/K+ Pump actively pumps Na+ out of the neuron and K+ into the neuron. This reestablishes the initial ion distribution of the resting neuron. Summary of Action Potential Action Potential • This single action potential acts as a stimulus to neighboring proteins and initiates an action potential in another part of the neuron. • Ultimately a wave of action potentials travels from the dendrites all the way to the axon terminals. • At the axon terminal, the electrical impulse is converted to a chemical signal that stimulates a neighboring neuron. These chemical signals are called neurotransmitters. Propagation of Action Potential Dendrite End Axon Terminal End The Synapse • Synapse: Small space where a neuron can transfer an impulse to another cell. • It is a small gap that separates the axon terminal from the dendrites of the next neuron. • The axon terminals contain tiny sacs filled with neurotransmitters. • Neurotransmitters: Chemicals used by a neuron to transmit an impulse to another cell. The Synapse • When an action potential arrives at an axon terminal, the sacs release the neurotransmitters into the synapse. • The neurotransmitter molecules diffuse across the synapse from one neuron to the next stimulating an impulse or action potential in the neighboring cell. • Dopamine, seratonin, and acetylcholine are all neurotransmitters. The Synapse The Synapse Cool sites for animations http://www.blackwellpublishing.com/matthews/nmj.html (synapse and neurotransmitters) http://www.blackwellpublishing.com/matthews/channel.html (Action potentials)