The Neuron. Cells of the Nervous System

Transcription

1 Nervous System Nervous System 1. Parts of the neuron 2. Organization of the nervous system 3. Neural communication 4. Autonomic Nervous system Cells of the Nervous System Two major cell types 1. Neurons Electrical and chemical communication 2. Glia Important for development and support Myelin Neurons are the fundamental unit of the nervous system The Neuron Receive & Integrate Inputs Impulse conduction Neurotransmitter Secretion 1

2 Neurons diversity of types Organization of nervous system Other sensory input dendrites cell body axon Sensory Input Filters Processing Patterned activity Motor Output Behaviour Cortical Pyramidal Neuron Purkinje Cell Experience Simple Nerve net Neural ganglia Interconnected Ganglia Segmentation Complex Anterior specialization brain Even simple nervous systems can display very complex behaviour eg nematodes, leeches, and flies can all display learning! Hydra Leech nematode 300 # of neurons Human 100 x

3 Brain Central Nervous System Organization of the Vertebrate Nervous System Brain Spinal Cord Segmented Nerve Cord Peripheral Nervous System Sensory Neurons (afferent) Motor Neurons (efferent) Fruit Fly Embryo Human Sensory Receptors Autonomic Voluntary (somatic) Division of the Motor Nervous System Autonomic Voluntary Autonomic Nervous System Parallel Systems that innervate the same target Opposing effects on the target Usually in balance Sympathetic Parasympathetic Things you control Sympathetic Parasympathetic Heart rate Cardiovascular function Pupil dilation Lung function Dilate pupil Increase heart rate Constrict pupil Decrease Heart Rate 3

4 The Reflex Arc Figure 6-4 Monosynaptic Sensory receptor Polysynaptic Interneuron Sensory receptor 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

5 Neural Signalling Bioelectric Potentials Within neurons Between neurons 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 microelectrode Reference electrode amplifier Membrane potential Bioelectric Potentials Action Potentials are rapid changes in resting membrane potential that travel down the axon cell 0 mv Action Potentials initiate synaptic transmission at the nerve terminal Bathing solution -80 mv time 5

6 Action Potentials Action Potential What are they? Rapid reversal of the resting membrane potential 0 mv Rising phase or depolarization 0 mv Threshold Potential Overshoot Falling phase or Repolarization cell -80 mv 3 ms -80 mv Resting membrane potential Undershoot or afterhyperpolarization Voltage-gated channels K+ K+ K+ K+ Section of Axon Membrane Potential Membrane Potential time time 6

7 Action Potential Conduction Action Potential Conduction Stimulate Action Potential axon Axon hillock = initial segment Region of neuron where AP usually starts Record voltage Action Potential Conduction Stimulate Action Potential 1. APs constant amplitude at all points along the axon 7

8 Sequence of Events leading to AP propagation 1. Stimulus opens channels & cause AP 2. Depolarizing current flows down the axon 3. Local depolarization opens channels downstream & initiate a new AP 4. channels close (inactivate) & K+ channels open 5. Local depolarization opens channels downstream and initiate a new AP Myelinated nerve Node of Ranvier Myelin Formed by: Schwann cells (periphery) Oligodendrocytes (central) Myelin Myelin Saltatory conduction 8

9 Myelin Myelin increases speed of conduction by: 1. Increasing membrane resistance Reduces leakiness length constant 2. Voltage-gated channels only at Node of Ranvier APs generated only at the Node Mulitple Sclerosis Demyelination of axons Impaired AP conduction Symptom depends on nerves affected Optic nerve blindness Motor nerves weakness or paralysis Neural Signalling Definitions Within neurons Between neurons This connection is called the synapse 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

10 Types of Synapses Direction of information flow 1. Electrical Direct flow of electrical current from one cell to the next through gap junctions 2. Chemical 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

11 Chemical Synapses 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 Real synapse Drosophila neuromuscular junction Synaptic vesicles Synaptic contacts Active Zone Depolarization Ca++ Sequence of Events at a Chemical Synapse 1. Action potential arrives & depolarizes nerve terminal 2. Voltage-gated Ca++ channels open & Ca++ flows into the nerve terminal 3. Ca++ causes synaptic vesicles to fuse with the plasma membrane 4. Neurotransmitters are released into synaptic cleft 5. Neurotransmitter binds to receptors 6. Opens ion channels and positive current flows into postsynaptic cell 7. Current flow gives postsynaptic potential 8. If postsynaptic potential = threshold Action Potential 11

12 Ca++ Depolarization 12