neurons and synapses
DESCRIPTION
Neurons and Synapses. Types of Neurons. Sensory. Motor. Interneurons. Sensory Neurons. INPUT From sensory organs to the brain and spinal cord. Brain. Drawing shows a somatosensory neuron Vision, hearing, taste and smell nerves are cranial, not spinal. Sensory Neuron. Spinal Cord. - PowerPoint PPT PresentationTRANSCRIPT
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Neurons and Synapses
Types of Neurons
Sensory Motor Interneurons
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SpinalCord
BrainSensoryNeuron
Sensory Neurons
INPUT From sensory organs to the brain and spinal cord.
Drawing shows a somatosensory neuron
Vision, hearing, taste and smell nerves are cranial, not spinal
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SpinalCord
BrainSensoryNeuron
MotorNeuron
Motor Neurons
OUTPUT From the brain and spinal cord To the muscles and glands.
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SpinalCord
BrainSensoryNeuron
MotorNeuron
Interneurons
Interneurons carry information between other neurons only found in the brain and spinal cord.
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Structures of a neuron
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The cell body
Round, centrally located structure
Contains DNA Controls protein
manufacturing Directs metabolism No role in neural
signaling
Contains the cell’s Nucleus
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Dendrites
Information collectors
Receive inputs from neighboring neurons
Inputs may number in thousands
If enough inputs the cell’s AXON may generate an output
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Dendritic Growth
Mature neurons generally can’t divide
But new dendrites can grow
Provides room for more connections to other neurons
New connections are basis for learning
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Axon
The cell’s output structure
One axon per cell, 2 distinct parts tubelike structure
branches at end that connect to dendrites of other cells
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Myelin sheath
White fatty casing on axon
Acts as an electrical insulator
Not present on all cells
When present increases the speed of neural signals down the axon.
Myelin Sheath
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How neurons communicate
Neurons communicate by means of an electrical signal called the Action Potential
Action Potentials are based on movements of ions between the outside and inside of the cell
When an Action Potential occurs a molecular message is sent to neighboring neurons
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Ion concentrations
Cell Membrane in resting state
K+
Na+ Cl-K+A-
Outside of Cell
Inside of Cell
Na+ Cl-
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The Cell Membrane is Semi-Permeable
Cell Membrane at rest
Na+ Cl-K+
Na+
Cl-K+ A-
Outside of Cell
Inside of Cell
Potassium (K+) can pass through to equalize its concentration
Sodium and Chlorine cannot pass through
Result - inside is negative relative to outside
- 70 mv
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Resting Potential
At rest the inside of the cell is at -70 microvolts With inputs to dendrites inside becomes more positive if resting potential rises above threshold an action potential
starts to travel from cell body down the axon Figure shows resting axon being approached by an AP
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Depolarization ahead of AP
AP opens cell membrane to allow sodium (NA+) in inside of cell rapidly becomes more positive than outside this depolarization travels down the axon as leading edge
of the AP
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Repolarization follows
After depolarization potassium (K+) moves out restoring the inside to a negative voltage
This is called repolarization The rapid depolarization and repolarization produce a
pattern called a spike discharge
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Finally, Hyperpolarization
Repolarization leads to a voltage below the resting potential, called hyperpolarization
Now neuron cannot produce a new action potential This is the refractory period
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Neuron to Neuron
Axons branch out and end near dendrites of neighboring cells
Axon terminals are the tips of the axon’s branches
A gap separates the axon terminals from dendrites
Gap is the Synapse CellBody
Dendrite
Axon
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Synapse
axon terminals contain small storage sacs called synaptic vesicles
vesicles contain neurotransmitter molecules
SendingNeuron
SynapseAxonTerminal
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Neurotransmitter Release
Action Potential causes vesicle to open
Neurotransmitter released into synapse
Locks onto receptor molecule in postsynaptic membrane
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Locks and Keys
Neurotransmitter molecules have specific shapes
positive ions (NA+ ) depolarize the neuron negative ions (CL-) hyperpolarize
When NT binds to receptor, ions enter
Receptor molecules have binding sites
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Some Drugs work on receptors
Some drugs are shaped like neurotransmitters
Antagonists : fit the receptor but poorly and block the NT e.g. beta blockers
Agonists : fit receptor well and act like the NT e.g. nicotine.
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Summary
3 types of neurons The cell membrane Ion movements Action potentials Synapse Neurotransmitters Receptors and ions Agonists and
antagonists