Transcript File
The Nervous System & Neurons Chapter 7 Click pic Overview of the Nervous System • STRUCTURES: brain, spinal cord, & peripheral nerves • FUNCTION: Recognizes and coordinates the body’s response to changes in its internal and external environments Overview of the Nervous System • The ultimate control of all the organ systems is done by the nervous system. A. Neurons • Neurons are nerve cells that carry electrical impulses through the body • Impulse = message • Neurons are classified according to the direction an impulse travels. Click here • 3 Types of Neurons – 1. Sensory: carry impulses from sense organs to the brain – 2. Motor: carry impulses from the brain to muscles & glands – 3. Interneurons: connect sensory and motor neurons and process impulses Interneuron Sensory Neuron Motor Neuron Reflex Arc Sensory Neuron Interneuron Motor Neuron • Structure of a Neuron Nucleus Dendrites Axon terminals Cell body Myelin sheath Nodes Axon – 1. Dendrites: receive impulses – 2. Cell body: contains nucleus & cytoplasm, largest part of cell – 3. Axon: transmit impulses away from cell body – 4. Myelin Sheath: covering that insulates the axon, sending the impulse faster and gives axon a whitish appearance – Neurons with axons that have myelin make up “white matter” in the brain, while neurons without myelin are called “gray matter” – 5. Nodes: gaps in the myelin sheath where membrane is exposed – Impulses jump from one node to the next – 6. Axon terminals: transmits impulse (message) to next cell II. Transmission of an Impulse • A nerve impulse is an electrical message. • At rest, no impulse is being sent. • The neuron must be stimulated (receive input) to send a impulse. + A. Resting Neuron + ---- + + • STEP 1: At rest, the neuron is not transmitting an impulse – inside of cell has a net negative charge – – outside of cell has a net positive charge + • The cell membrane has Membrane Potential, the potential to carry an electrical current because there is a difference in charge – The ions sodium (Na+) and potassium (K +) cause the potential (K+) Click pic • Sodium-potassium pumps in the nerve cell membrane pumps sodium (Na+) ions out of the cell and potassium (K+) ions into the cell by means of active transport. • As a result, the inside of the cell contains more K+ ions and fewer Na+ ions than the outside. B. Stimulated Neuron • STEP 2: An impulse begins when a neuron is stimulated by another neuron or the environment – The stimulated impulse must reach threshold, the minimum impulse strength needed to create a new impulse (all or none) • STEP 3: At the leading edge of the impulse, gates in the sodium channels open allowing positively charged sodium (Na+) ions to flow into the cell membane. • This reversal of charges is called is called the nerve impulse or an ACTION POTENTIAL (AP) • As the action potential passes, gates in the potassium channels open, allowing potassium (K+) ions to flow OUT of the cell – This restores the negative potential inside the axon. • STEP 4: The AP jumps from node to node along the axon STEP 5: The AP continues along the axon in one direction to the axon terminals. 1. At Rest ------------2. Action Potential ++---------3. ----++----4. ---------++- Action Potential Click here C. Synapse • Synapse: the space where the axon terminal of one neuron can transfer an impulse to another neuron (or cell) – The small space between cells is the synaptic cleft – Axon terminals contain vesicles filled with neurotransmitter • Neurotransmitters (NTs) are chemical signals of neurons that transmit an impulse across the synapse to another cell – NTs fit like a lock and key with receptors Steps of Transmission • • • • STEP 6: Action Potential reaches axon terminal STEP 7: Neurotransmitter is released from vesicles into synaptic cleft STEP 8: Neurotransmitter binds to receptors on next neuron. STEP 9: Channels open, which creates a new action potential in next neuron http://highered.mcgrawhill.com/sites/0072437316/st udent_view0/chapter45/ani mations.html# Re-uptake of NTs • Re-uptake: After the NT has done its job, it is taken back into the axon terminal so it can be used again or broken down. Types & Effects of Neurotransmitters ►There are dozens of different NTs, but a few do most of the work. ►Each NT fits with its own specific receptor. ►It won’t cause a response without the right receptor Acetylcholine Serotonin Endorphins GABA • NTs can be excitatory or inhibitory – Excitatory NTs cause impulses in the next neurons – Inhibitory NTs stop an impulse from being sent in the next neuron Click here: Chemistry of Addition Video Major NTs • 1) GABA (Gammaaminobutyric acid) – Inhibitory – Effects: Relaxation, calming, anti-anxiety, and sleep – Depressant drugs like heroin, marijuana, and alcohol intensify effects of GABA • 2) Glutamate – Excitatory – Effects: Stimulates learning and memory formation – Some drugs can interfere with glutamate, causing short-term memories not to be formed • 3) Dopamine – Excitatory – Effects: Pleasure, Euphoria, Reward-centers, addiction, positive reinforcement, movement and posture – Most addictive drugs (cocaine, PCP, heroin, opiates, marijuana, and amphetamines) work by flooding the synapse with dopamine • 4) Serotonin – Excitatory – Effects: Sleep, mood, appetite, pain, body temperature – Low serotonin linked to depression, suicide, impulsive behavior, and aggression – Alcohol, stimulants, marijuana and hallucinogens alter serotonin levels • 5) Endorphins – Excitatory & Inhibitory – Effects: alertness, blocks pain, happy (runner’s high), sexual arousal – Often released after physical activity • 6) Norepinephrine – Excitatory – Effects: alertness, energy, stress, sexual arousal • 7) Acetylcholine – Excitatory – Effects: alertness, memory, muscle contraction, appetite, sexual arousal Click here: Crash Course-The Chemical Mind Effects of Drugs • Many prescription and illegal drugs can alter the brain’s chemistry. – Many drugs are agonists (enhancers) of certain NTs that cause many of the pleasurable sensations of drugs and lead to addiction. – Addiction: chemical dependency on a substance as a result of it altering the brain’s chemistry after repeated use. Ways Drugs interfere with Neurotransmission – 1) Increase number of impulses – 2) Release NT from vesicles with or without an impulse – 3) Block reuptake or block receptors – 4) Produce more or less NT – 5) Prevent vesicles from releasing NT Click pic