Transcript File
What Makes Us Human? The Psychological Answer Neural Transmission Key Terms • Dendrite – the bushy, branching extensions of a neuron that receive messages and conduct impulses toward the cell body Key Terms (Cont’d) • Axon – the extension of a neuron, ending in branching terminal fibers, through which messages are sent to other neurons or to muscles or glands Key Terms (Cont’d) • Myelin [MY-uh-lin] Sheath – a layer of fatty cells that encase the fibers of many neurons in segments – Speeds up the transmission of neural impulses Key Terms (Cont’d) • Synapse [SIN-aps] – junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron – tiny gap at this junction is called the synaptic gap or cleft Key Terms (Cont’d) • Neurotransmitters – chemical messengers – released by the sending neuron – travel across the synapse and bind to receptor sites on the receiving neuron (determining whether or not to generate a neural impulse) Cell Body • • • • Cell body Otherwise known as a soma Contains nucleus Produces and packages neurotransmitters Neurons • Neuron – the basic building block of the nervous system – Send messages electrochemically – chemicals cause an electric signal Action Potential • Chemicals in the body are ions (electrically charged) • Two important ions in the nervous system – sodium (Na) and potassium (K) Action Potential (Cont’d) • nerve cells are surrounded by a semipermeable membrane (allows some ions to pass through and blocks the passage of other ions) Resting Membrane Potential • Neurons that aren’t sending signals are "at rest" • At rest – (relatively) more sodium ions outside the neuron and more potassium ions inside that neuron Action Potential • An action potential occurs when a neuron sends information down an axon, away from the cell body • Action potentials are caused by an exchange of ions across the neuron membrane. • The action potential is an explosion of electrical activity that is created by a depolarizing current Action Potential (Cont’d) • When depolarization reaches a certain amount (threshold) a neuron will fire an action potential • If the neuron does not reach critical threshold level, then no action potential will fire Action Potential Action Potential (Cont’d) • All action potentials are the same size. • The neuron either does not reach the threshold or a full action potential is fired this is the "ALL OR NONE" principle. Action Potential (Cont’d) • During an action potential sodium ions rush into the neuron. • Because sodium has a positive charge, the neuron becomes more positive and becomes depolarized Action Potential (Cont’d) • Potassium rushes out of the cell, reversing the depolarization. • At about this time, sodium channels start to close - causing the action potential to go back – repolarization Part Two: The Synapse The Synapse • Information from one neuron flows to another neuron across a synapse • The synapse contains a small gap separating neurons The Synapse • The synapse consists of: 1. a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles, 2. a postsynaptic ending that contains receptor sites for neurotransmitters and, 3. a synaptic cleft or gap between the presynaptic and postsynaptic endings An Example Neurotransmitter Mobilization and Release • At the synaptic terminal (the presynaptic ending), an electrical impulse will trigger the migration of vesicles containing neurotransmitters toward the presynaptic membrane Neurotransmitter Mobilization and Release (Cont’d) • The vesicle membrane will fuse with the presynaptic membrane releasing the neurotransmitters into the synaptic cleft • Neurons can contain and release more than one kind of neurotransmitter • The neurotransmitter molecules then diffuse across the synaptic cleft where they can bind with receptor sites on the postsynaptic ending to influence the electrical response in the postsynaptic neuron Diffusion of Neurotransmitters Across the Synaptic Cleft Neurotransmitters • Communication of information between neurons is accomplished by movement of chemicals across a small gap called the synapse. • Chemicals, called neurotransmitters, are released from one neuron at the presynaptic nerve terminal. Neurotransmitters • Neurotransmitters then cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor • The action that follows activation of a receptor site may be either depolarization (an excitatory postsynaptic potential) or hyperpolarization (an inhibitory postsynaptic potential) Neurotransmitters • A depolarization makes it MORE likely that an action potential will fire; a hyperpolarization makes it LESS likely that an action potential will fire.