travismulthaupt.com 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 Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Nerve Systems  Increasing in their complexity, nerve nets are also associated with nerves.  These assist with more complex movements.  Example: Sea stars Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Nerve Systems  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 Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Nerve Systems  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 Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Nerve Systems  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  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  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. Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Form Fitting Function  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. Schwaan cells  2. Oligodendrocytes  3. Radial glia  4. 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  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, (E ion ).  E ion = 62mV(log([ion] outside /[ion] inside ))

travismulthaupt.com The Nernst Equation  E ion = 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 +, Ca 2+, 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. Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com Ion Channel Stimulation  Depolarization reduces the magnitude of the membrane potential--the inside becomes less negative. Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com 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. Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.

travismulthaupt.com 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 1 st, K + 2 nd.

travismulthaupt.com Action Potentials  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. Na + channels have 2 gates--an activation gate and an inactivation gate. Both must open for Na + to get through.

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 E Na (equilibrium potential) during the rising phase.

travismulthaupt.com Action Potentials  4. E Na 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 post- synaptic 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.