Chapter 46 .pptx

UNIT 9 Chapter 46: Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Four lobes of cerebral cortex Frontal lobe: (higher mental functions) •Anterior cerebral art. •Middle...

UNIT 9 Chapter 46: Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters Slides by Thomas H. Adair, PhD Copyright © 2021 by Saunders, an imprint of Elsevier Inc. Four lobes of cerebral cortex Frontal lobe: (higher mental functions) •Anterior cerebral art. •Middle cerebral art. Parietal lobe: (integrates sensory information from different modalities) •Anterior cerebral art. •Middle cerebral art. Temporal lobe: (auditory perception, semantics, memory) •Middle cerebral art. •Posterior cerebral art. Occipital lobe: (visual processing center – visual cortex) •Middle cerebral art. Surface anatomy of brain Primary motor cortex Somatosensory cortex Cranial nerves Cranial nerves Organization of the nervous system Anatomical stuff Spinal cord Organization of Nervous System • Sensory Division – tactile, visual, auditory, olfactory • Integrative Division – process information, creation of memory • Motor Division – respond to and move about in our environment Somatosensory Axis of Nervous System Figure 46-2 Skeletal Motor Nerve Axis of Nervous System Figure 46-3 3 Major Levels of CNS Function Spinal cord level Lower brain level Higher brain or cortical level Spinal Cord Level • The spinal cord level: – more than just a conduit for signals from periphery of body to brain and vice versa – cord contains: • walking circuits • withdrawal circuits • support against gravity circuits • circuits for reflex control of organ function Lower Brain Level • Contains: – medulla, pons, mesencephalon, hypothalamus, thalamus, cerebellum and basal ganglia • Controls subconscious body activities: – arterial pressure, respiration, equilibrium, feeding reflexes, emotional patterns Higher Brain or Cortical Level • Cortex never functions alone, always in association with lower centers. • Large memory storehouse. • Essential for thought processes. • Each portion of the nervous system performs specific functions, but it is the cortex that opens the world up for one’s mind. Neuron Structure 3 major components: • Soma • Axon terminal. - main body of neuron. - extends from soma to synaptic - the effector part of neuron. • Dendrite - projections from soma. - the sensory portion of neuron. Figure 46-1 Anterior motor neuron Posterior horn receives sensory information. anterior root anterior horn Also called (anterior column or ventral horn) - contains motor neurons that affect axial muscles. Figure 45-6 Review of membrane potential Vm -65 0 mV EK -86 ENa +61 ECl -70 Fig 46-8 Electrotonic potentials Subthreshold potential change vs. action potential Recap: how membrane potential and excitability relate Synaptic responses - EPSP Note the following: • no action potentials occurred in postsynaptic neuron because a threshold potential was not achieved • the excitatory post synaptic potential (EPSP) is an electrotonic response that decays with an exponential time course • the last EPSP is larger because it ?Which channels and ions can create occurs before the previous EPSP an EPSP? has decayed fully. Glutamate • opens cation channels • chief excitatory transmitter in CNS ?What type of summation is shown? Presynaptic neuron + Postsynaptic neuron Presynaptic neuron 0 mV -70 threshold -60 mV -70 epsp 10 ms Postsynaptic neuron With spatial summation, EPSP’s created by distant synapses overlap. A single neuron can have more than one synaptic terminal Spatial summation can occur with a single presynaptic neuron. Copyright © 2011 by Saunders, an imprint of Elsevier Inc. Synaptic responses - IPSP Note the following: • The post-synaptic cell is hyperpolarized. – Remember that hyperpolarization depresses excitability - inhibitory – This is an inhibitory post synaptic potential (IPSP) • IPSPs can summate too!! • IPSPs result from increases in ?Which ions are involved in membrane permeability creating an IPSP? Presynaptic neuron - Postsynaptic neuron 0 mV Presynaptic neuron -70 -60 GABA (γ-Aminobutyric acid), opens Clchannels. • chief inhibitory transmitter in adult CNS • could be an excitatory transmitter during development (because of Postsynaptic neuron mV -70 ipsp -80 10 ms Summation of Postsynaptic Potentials Spatial Summation excitation of a single presynaptic neuron on a dendrite will almost never induce an AP in the neuron. each terminal on the dendrite accounts for about a 0.5 - 1.0 mV EPSP. when multiple terminals are excited simultaneously the EPSP generated may exceed the threshold for firing and induce an AP. Summation of Postsynaptic Potentials Temporal Summation A neurotransmitter opens a membrane channel for about 1 msec, but a postsynaptic potential (EPSP or IPSP) lasts for about 15 msec A second opening of the same membrane channel can increase the postsynaptic potential to a greater level. Therefore, the more rapid the rate of terminal stimulation, the greater the postsynaptic potential. Rapidly repeating firings of a small number of terminals can summate to reach the threshold for an AP. Electrotonic potentials EPSP = IPSP = Figure 45-9 Simultaneous firing of many synapses is required to reach threshold • Often the summated postsynaptic potential is excitatory in nature but has not reached threshold levels. • This neuron is said to be facilitated because the potential is nearer the threshold for firing compared to the resting level, but not yet to the firing level. • It is easier to stimulate this neuron with subsequent input. threshold Figure 46-10 Facilitation at the squid giant synapse A Presynaptic Membrane Potential (mV) Postsynaptic Membrane Potential (mV) 0 0.5 Ca++ concentration in synaptic end of neuron 0.4 Amount of facilitation Action potentials 0.3 Facilitation B 0.2 0.1 EPSPs 0.0 10 20 Time (ms) 30 0 10 20 30 40 50 Interval between stimuli (ms) Redrawn after Purves, Neuroscience. 5th ed., Sinauer, 2012. Facilitation: • Definition – The increased transmitter release produced by an action potential that follows closely upon a preceding action potential. Note: This is not temporal summation. • Mechanism – prolonged elevation of presynaptic calcium levels following synaptic activity. Function of Dendrites in Stimulating Neurons  Dendrites allow signal reception from a large spatial area providing opportunity for summation of signals from many presynaptic neurons.  Dendrites do not transmit action potentials. They have few voltage gated Na+ channels. Figure 46-11  Dendrites transmit signals by electrotonic conduction. Transmission of current by conduction in the fluids of the dendrites. Special Characteristics of Synaptic Transmission Synaptic facilitation – enhanced responsiveness following repetitive stimulation. – mechanism is build-up of calcium ions in presynaptic terminals. – build-up of calcium causes more vesicular release of Synaptic fatigue (or short-term synaptic depression) transmitter. – excitatory synapses are repetitively stimulated at a rapid rate until rate of postsynaptic discharge becomes progressively less. – It’s a protective mechanism for excessive neuronal activity – Possible mechanism for causing epileptic seizure to end – Mechanism: See Reverberatory Circuit below. Synaptic delay – the process of neurotransmission takes time, from the time delay one can calculate the number of chemically connected series neurons in a circuit. Actions of transmitter substances on postsynaptic membrane Ion channels: o Cation channels / Anion channels o Rapid response – short lived o Small molecule transmitters (Ach, NE, etc) 2nd messenger system o Multiple responses o Prolonged responses o Neuropeptides Synaptic transmission: ion channels Synaptic transmission: 2nd messenger Figure 46-7 Neurotransmitters: 2 main types 1. Small molecule, rapidly acting transmitters 2. Neuropeptides, slowing acting transmitters Small molecule, rapidly acting transmitters Function: small molecule Usually excitatory in CNS transmitters mediate most acute responses of nervous system. Usually inhibitory GABA: Chief inhibitory transmitter in CNS Glycine: inhibitory transmitter, mainly in cord Glutamate: Chief excitatory transmitter in CNS. Accounts for >90% of the synaptic connections in CNS. Synthesized on demand; does not use vesicles – diffuses through membrane Table 45-1 What does this have to do with chicken eggs? 113 mg choline Neuropeptides, slowing acting transmitters, GFs Act on Metabotropic receptors Cause long-term changes  Changes in number of neuron receptors • Often co-released with small molecule  Long-term opening or closure of ion channels  Changes in number and sizes of synapses transmitters • Some neurons make several different peptides Table 45-1 Environmental Changes and Synaptic Transmission • Acidosis. – depresses neuronal activity. – diabetic coma – pH change from 7.4 to 7.0 usually will induce coma. • Alkalosis. – increases neuronal excitability. – Can initiate petit mal seizure – pH change from 7.4 to 8.0 usually will induce seizures. • Hypoxia. – brain highly dependent on oxygen – interruption of brain blood flow for 3 to 7 sec can lead to unconsciousness.