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Nervous System Nervous System 1. 2. 3. 4. Cells of the Nervous System • Two major cell types 1. Neurons • The Neuron Receive & Integrate Inputs Electrical and chemical communication 2. Glia • • Parts of the neuron Organization of the nervous system Neural communication Autonomic Nervous system Important for development and support Myelin Neurons are the fundamental unit of the nervous system Impulse conduction Neurotransmitter Secretion 1 Neurons – diversity of types Organization of nervous system Other sensory input dendrites cell body Motor Output Behaviour Processing Sensory Input axon Patterned activity Filters Purkinje Cell Cortical Pyramidal Neuron Experience Simple Nerve net Complex Neural ganglia Interconnected Ganglia Segmentation Anterior specialization ‘brain’ • Even ‘simple’ nervous systems can display very complex behaviour • eg nematodes, leeches, and flies can all display learning! Hydra Leech # of neurons nematode 300 Human 100 x 109 2 Central Nervous System Brain Segmented Nerve Cord Fruit Fly Embryo Peripheral Nervous System Human Division of the Motor Nervous System Autonomic Sympathetic Parasympathetic Heart rate Cardiovascular function Pupil dilation Lung function Voluntary Things you control Organization of the Vertebrate Nervous System Brain Spinal Cord Sensory Neurons (afferent) Sensory Receptors Motor Neurons (efferent) Autonomic Voluntary (somatic) Autonomic Nervous System • Parallel Systems that innervate the same target • Opposing effects on the target •Usually in balance Sympathetic Parasympathetic Dilate pupil Constrict pupil Increase heart rate Decrease Heart Rate 3 Figure 6-4 The Reflex Arc Sensory receptor Monosynaptic Sensory receptor Interneuron Polysynaptic Summary & Key Points 1. The fundamental cell of the nervous system is the neuron. 2. The neuron has functionally specialized regions 3. Evolution of the nervous system includes increasing cell number and interconnection 4. Subdivisions of the vertebrate nervous system Neural Communication Basic Concepts 1. Neurons use electrical and chemical mechanisms 4 Neural Signalling Within neurons Between neurons Bioelectric Potentials This connection is called the synapse • Neurons have an electrical potential (voltage) across the cell membrane • The inside of the cell is more negative than the outside – called the Resting Membrane Potential electrical chemical & electrical Measuring Membrane Potential amplifier microelectrode Reference electrode Membrane potential 0 mV Bioelectric Potentials • Action Potentials are rapid changes in resting membrane potential that travel down the axon • Action Potentials initiate synaptic transmission at the nerve terminal cell -80 mV Bathing solution time 5 Action Potentials Action Potential • What are they? – Rapid reversal of the resting membrane potential Overshoot Rising phase or depolarization Falling phase or Repolarization 0 mV cell 0 mV Threshold Potential -80 mV -80 mV Resting membrane potential Undershoot or afterhyperpolarization 3 ms Na+ Voltage-gated channels Na+ K+ K+ K+ K+ Na+ Membrane Potential Section of Axon Membrane Potential Na+ time time 6 Action Potential Conduction Action Potential Conduction Stimulate Action Potential axon Axon hillock = initial segment Region of neuron where AP usually starts Action Potential Conduction Record voltage Na+ Stimulate Action Potential 1. APs constant amplitude at all points along the axon 7 Sequence of Events leading to AP propagation Myelinated nerve 1. Stimulus opens Na+ channels & cause AP 2. Depolarizing current flows down the axon 3. Local depolarization opens Na+ channels downstream & initiate a new AP 4. Na+ channels close (inactivate) & K+ channels open 5. Local depolarization opens Na+ channels downstream and initiate a new AP Myelin Node of Ranvier Myelin Formed by: Schwann cells (periphery) Oligodendrocytes (central) Na+ Myelin Saltatory conduction 8 Myelin Mulitple Sclerosis Myelin increases speed of conduction by: 1. Increasing membrane resistance • Reduces ‘leakiness’→ ↑ length constant 2. Voltage-gated channels only at Node of Ranvier • • Demyelination of axons – Impaired AP conduction – Symptom depends on nerves affected • Optic nerve → blindness • Motor nerves → weakness or paralysis APs generated only at the Node Neural Signalling Within neurons Between neurons This connection is called the synapse Definitions 1. Synapse : the functional contacts between neurons and other cells 2. Synaptic Transmission: the process used by neurons to relay information from one cell to the next electrical chemical & electrical 9 Types of Synapses Direction of information flow 1. Electrical ¾ Direct flow of electrical current from one cell to the next through gap junctions 2. Chemical ¾ Secrete neurotransmitter molecules that activate receptors Chemical synapse Presynaptic nerve terminal Synaptic vesicles Synaptic cleft Neurotransmitter receptors Chemical Synapses Features: 1. Use chemical neurotransmitters 2. There is a space between the pre- and postsynaptic neuron, called the synaptic cleft 3. Neurotransmitter is stored in synaptic vesicles Postsynaptic membrane 10 Real synapse Chemical Synapses Drosophila neuromuscular junction Features: 4. Can be excitatory or inhibitory • Depends on neurotransmitter and receptor 5. Can activate ion-channels or signaling pathways 6. The amount of transmitter released is variable and can be modulated Synaptic vesicles Synaptic contacts Active Zone Sequence of Events at a Chemical Synapse 1. 2. 3. Na+ Ca++ 4. 5. 6. 7. 8. Action potential arrives & depolarizes nerve terminal Voltage-gated Ca++ channels open & Ca++ flows into the nerve terminal Ca++ causes synaptic vesicles to fuse with the plasma membrane Neurotransmitters are released into synaptic cleft Neurotransmitter binds to receptors Opens ion channels and positive current flows into postsynaptic cell Current flow gives postsynaptic potential If postsynaptic potential = threshold → Action Potential Depolarization 11 Na+ Ca++ Depolarization 12