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Chapter 48 Nervous Systems travismulthaupt.com Nerve Systems A neuron is a nerve cell, and there are 100 billion in the brain. Except for sponges, all animals have some type of nervous system. The thing that sets them apart is their organization. travismulthaupt.com Nerve Systems Simple animals have nerve systems classified in nerve nets-very diffuse organization. Example: Cnidarian QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Increasing in their complexity, nerve nets are also associated with nerves. These assist with more complex movements. Example: Sea stars travismulthaupt.com Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Nerve systems with greater complexity involve cephalization. This included the clustering of neurons in the head and bilaterally symmetrical bodies. These are simple CNS’s. Example: Planarians travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. The more complex brains as well as ventral nerve cords and clusters of nerve cells called ganglia are seen in more complex invertebrates. These systems have a peripheral nervous system that connects with the CNS. Example: Annelids travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. The structure of nerve system organization is closely related to function. For example: molluscs are slow moving and don’t have a very highly organized nervous system. Example: Clams and Chitons Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. travismulthaupt.com Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Fast moving molluscs such as the cephalopods have more highly organized nervous systems. Example: Squids and Octupi Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. travismulthaupt.com Nerve Systems QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Vertebrates have a CNS consisting of a brain and spinal cord running along the dorsal side of the body, along with nerves and ganglia comprising the PNS. Example: Salamander Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. travismulthaupt.com Nerve Systems Information processing by the nervous system consisting of 3 stages: 1. Sensory input 2. Integration 3. Motor output travismulthaupt.com Nerve Systems These three stages are handled by specialized neurons. 1. Sensory neurons transmit information from sensors that detect external stimuli and internal conditions. 2. Interneurons integrate and analyze sensory input. 3. Motor output leaves the CNS via motor neurons which communicate with effector cells eliciting a change. travismulthaupt.com Form Fitting Function The organelles of a neuron are located in the cell body. Two extensions arise from the cell body: 1. Axons--longer, transmit signals. 2. Dendrites--highly branched, receive signals. travismulthaupt.com Form Fitting Function Near its end, an axon divides into several branches, each ending in a synaptic terminal. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this pi cture. travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Form Fitting Function QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. http://biologyclass.neurobio.arizona.edu/images/synapse2.jpg A synapse is the site of communication between one synaptic terminal and another. Neurotransmitters transmit the signal from a pre-synaptic cell to a post-synaptic cell. travismulthaupt.com Supporting Cells of the Nervous System Glia are the supporting cells of the nervous system. There are several different types, among them are: 1. 2. 3. 4. Schwaan cells Oligodendrocytes Radial glia Astrocytes. travismulthaupt.com 1. Schwaan Cells Schwaan cells are associated with the PNS as are glia, and they form myelin sheaths around the axons of many vertebrate neurons. travismulthaupt.com 2. Oligodendrocytes QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Oligodendrocytes are associated with the CNS and do the same thing as Schwaan cells. The myelin sheath generated by these cells forms an insulation blanket. This aids in nerve conduction. travismulthaupt.com 3. Radial Glia In an embryo, radial glia form tracks along which newly formed neurons migrate from the neural tube during development. Radial glia and astrocytes act as stem cells and give rise to new neurons and glia. travismulthaupt.com 4. Astrocytes These provide structural support, regulate extracellular ion concentrations and neurotransmitter concentrations. They are involved in dilating blood vessels, increasing blood flow to neurons, and they facilitate information transfer. They induce tight junction formation in the course of development of the CNS helping form the blood-brain barrier. travismulthaupt.com Potential Difference A typical cell has a potential difference across the membrane of -60 to -80mV. This is the resting membrane potential. The membrane voltage at equilibrium is calculated using the Nernst equation. It is called the equilibrium potential, (Eion). Eion = 62mV(log([ion]outside/[ion]inside)) travismulthaupt.com The Nernst Equation Eion = 62mV(log([ion]outside/[ion]inside)) This equation applies to any membrane that is permeable to a single type of ion. All you need to know is the ion concentration inside and outside of the membrane. A minus sign indicates the inside is more negative than the outside. travismulthaupt.com Membrane Potential This is the basis of nearly all electrical signals in the nervous system. The membrane potential can change from its resting value when the membrane’s permeability to a particular ion changes. Na+, K+, Ca2+, and Cl- all play major roles in nerve signal transmission. travismulthaupt.com Ion Channels When ion channels are always open, they are said to be ungated. Gated ion channels switch open and closed to one of three kinds of stimuli: Stretch gated ion channels sense stretch. Ligand gated ion channels open and close in response to specific signals. Voltage gated ion channels open and close due to changes in membrane potential. travismulthaupt.com Ion Channel Stimulation Stimulating gated ion channels can trigger hyperpolarization or depolarization. travismulthaupt.com Ion Channel Stimulation Hyperpolarization results in an increased magnitude of membrane potential--The inside of the membrane becomes more negative. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this pi cture. travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Ion Channel Stimulation Depolarization reduces the magnitude of the membrane potential--the inside becomes less negative. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this pi cture. travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Ion Channel Stimulation In most neurons, depolarizations are graded up to a certain threshold. Once a stimulus has reached a threshold, an action potential is triggered. QuickTime™ and a TIFF (U ncompressed) decompressor are needed to see this picture. travismulthaupt.com Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Action Potentials Action potentials are all or none. They carry signals over a long distance along axons. They are very brief, and can thus be generated at a high frequency. Both Na+ and K+ voltage-gated ion channels are involved in the production of an action potential. Both open by depolarization of the membrane. Na+ opens 1st, K+ 2nd. travismulthaupt.com Action Potentials • Na+ channels have 2 gates--an activation gate and an inactivation gate. Both must open for Na+ to get through. 1. At resting potential, the activation gate is closed, inactivation gate is open. (For Na+). Depolarization rapidly opens the activation gate and slowly closes the inactivation gate. For K+, the activation gate is closed at resting potential. Depolarization slowly opens the gate. travismulthaupt.com Action Potentials 2. When a stimulus depolarizes the membrane, the activation gates open on some channels allowing some Na+ in. Na+ influx causes depolarization opening more activation gates and so on (positive feedback). travismulthaupt.com Action Potentials 3. When the threshold is crossed, this positive feedback cycle brings the membrane potential close to ENa (equilibrium potential) during the rising phase. travismulthaupt.com Action Potentials 4. ENa is not reached: -Activation gates close most Na+ channels halting Na+ influx. -K+ activation gates open causing efflux of K+ decreasing the membrane potential. travismulthaupt.com Action Potentials 5. Undershoot occurs as too much K+ leaves the cell. Eventually, K+ activation gates close and the membrane returns to its membrane resting potential. travismulthaupt.com Action Potentials The refractory period occurs when the Na+ channels remain closed and prevent the triggering of another action potential. This is what prevents the backflow of a stimulus. travismulthaupt.com Action Potentials Myelinated axons help to increase the diameter of the nerve and thereby increase the speed at which the impulse is propagated. It also contributes to saltatory conduction which is where the action potential appears to jump from node to node along the axon. travismulthaupt.com Action Potentials--Synapses When action potentials reach the ends of axons, they contribute one of 2 general mechanisms of information transfer. 1. Electrical synapse. 2. Chemical synapse. travismulthaupt.com Synapses--Electrical 1. Electrical synapses contain gap junctions which allow electric current to flow from cell to cell. travismulthaupt.com Synapses--Chemical 2. Chemical synapses make up the vast majority of synapses. They involve the release of chemical neurotransmitters from the pre-synaptic neurons via synaptic vesicles. The synaptic vesicles interact with the dendrites of a postsynaptic neuron. travismulthaupt.com Action Potentials The diffusion of neurotransmitter through the synaptic cleft has a change on the post-synaptic neuron, either direct or indirect. travismulthaupt.com Action Potentials When the neurotransmitter binds directly to the post-synaptic membrane and opens a channel, ions can diffuse across the membrane in a process called direct synaptic transmission. travismulthaupt.com Action Potentials In indirect synaptic transmission, a neurotransmitter binds to a receptor that is not part of an ion channel. travismulthaupt.com Action Potentials This involves activation of a signal transduction pathway involving a second messenger in the post-synaptic cell. These have an overall slower effect than direct transmission, but they last longer. travismulthaupt.com