The Nervous System. Types Central Nervous System (CNS)Peripheral Nervous System (PNS)
The Nervous System AP Biology Unit 6 Branches of the Nervous System There are 2 main branches of the...
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Transcript of The Nervous System AP Biology Unit 6 Branches of the Nervous System There are 2 main branches of the...
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The Nervous SystemThe Nervous System
AP BiologyUnit 6
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Branches of the Nervous System
• There are 2 main branches of the nervous system
• Central Nervous System– Brain– Spinal Cord
• Peripheral Nervous System– All nerves leading to rest of
body
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Anatomy of a Neuron• Dendrites = where a signal is received by the
neuron
• Cell body = contains the organelles, nucleus of the cell
• Axon = signal travels down this to get to the other end of the neuron
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Anatomy of a Neuron• Myelin = surrounds the axon to speed up
the signal
• Synaptic Terminal = end of the neuron
• Synapse = gap/space between neurons
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Question…
• What is the general pathway of a signal through a neuron?– Dendrites cell body Axon Synaptic
Terminals (then into the synapse to get to the next neuron or other cell)
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Sending Signals
• The “signal” sent through a neuron is an electrical signal
• Based on the movement of ions into and out of the cell– Causes changes in the + and – charges inside
the cell
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A Neuron at Rest• A neuron at rest (unstimulated) has a
difference in charge (voltage) across the plasma membrane = -70 mV = resting potential – This means that it is more negative inside than
outside
• The resting potential is caused by the distribution of ions on either side of the membrane
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A Neuron at Rest
• Resting potential (-70 mV) is maintained by the Sodium-Potassium Pump – Pumps Na+ out of cell – Pumps K+ into the cell– Active transport
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Ion Concentrations at Rest
• At rest, there are also open K+ channels in the membrane Allows some K+ to escape
• Leaves negatively charged molecules behind (Cl- ions, etc.) more negative on the inside than on the outside.
Ion Inside neuron Outside neuron
Na+ Lower Higher
Cl- Lower Higher
K+ Higher Lower
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Sending a Signal: Action Potential
• Na+ channels are embedded in the membrane of the neuron
• Usually, these Na+ channels are closed, but can be triggered to open when the correct stimulus is received– Voltage gated channels = open in response to a
particular change in voltage (charge)– Chemical gated channels = open in response to a
chemical binding to them
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Action Potential• STEP 1: To start an
action potential, some kind of stimulus (light, pressure, chemical, etc.) causes Na+ channels in the dendrite to open.
• This causes Na+ to flood into the neuron from outside DEPOLARIZATION
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Questions…
• Why does Na+ diffuse in from the outside? – Higher concentration on the outside
• When depolarization occurs, how is the charge inside the neuron affected? – Becomes more positively charged inside
• What would happen if Cl- channels are also opened? – Cl- would also flow in– makes the inside more
negative (cancels out the charge from Na+ coming in) -- HYPERPOLARIZED
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Action Potential• STEP 2: The change in
voltage triggers the next Na+ channel (voltage gated channel) to open.
• STEP 3: As Na+ diffuses down the neuron, it continues to trigger voltage gated Na+ channels to open. – This is what sends a signal
down the neuron towards the axon terminal.
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Action Potential
• STEP 4: Na+ voltage gated channels only open temporarily. After a short period of time, they close and an inactivation gate opens to prevent them from opening again for a little while REFRACTORY PERIOD
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Action Potential• STEP 5: The neuron is “reset”
(REPOLARIZED) by the opening of voltage gated K+ channels.
• K+ flows OUT of the neuron, making the inside more negative again. – Why does K+ flow out?
– Higher K+ concentrations inside neuron
• The Na+/K+ pump also helps reestablish resting potential.
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Saltatory Conduction
• Depolarization & Repolarization happens over and over down the axon, so the nerve impulse travels.
• Myelin sheaths insulate the axon, keeping ions from flowing out except at Nodes of Ranvier.
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Saltatory Conduction
• Wider axons yield faster conduction because there is less resistance.
• Action potentials jump from one Node of Ranvier (space between myelin sheaths) to the next, speeding up the signal.
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Communication between Neurons
• When the signal reaches the axon terminal, it triggers voltage gated Ca2+ channels to open.
• This causes vesicles that contain neurotransmitter molecules to fuse with the plasma membrane and expel the neurotransmitters into the synaptic cleft (space between neurons)
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Communication between Neurons• The neurotransmitters will diffuse across the
cleft and bind to receptors on the next neuron (postsynaptic neuron).
• This triggers a Na+ chemical gated channel to open on the postsynaptic neuron, triggering an action potential in that neuron.
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Communication between Neurons
• After the signal has been sent, neurotransmitters are eliminated from the synaptic cleft by – Diffusion = diffuse away– Reuptake – Enzyme degradation
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Communication between Neurons
• Reuptake– Neurotransmitters are actively transported back
into the presynaptic neuron repackaged into vesicles to be released again
– Recycling neurotransmitters
• Enzyme degradation– Enzymes in the synaptic cleft break down the
neurotransmitter
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Question…
• Why is it important that our bodies / medications control nerve communication? – So that signals are only sent to neurons when
needed.
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Control of Communication
• How can nerve communication be controlled?– Change conduction of impulse – Change synaptic cleft size – Change volume of neurotransmitters or vesicles – Change number of ligand-gated ion channels on
post synaptic neuron– Add a chemical that binds to ligand-gated ion
channels to block them or always keep them open– Add a chemical that binds to neurotransmitters, so
they cannot bind to the ligand-gated ion channels
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Brain Regions• Brain = organ where nervous
processes are centralized• Cephalization = evolutionary
trend in which most nervous processes are at head (anterior) region
• Different regions of the vertebrate brain have different functions – Ex. Vision, Movement,
Memory
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Overview of the Nervous System• The neuron is the basic structure/cell of the nervous
system.
• Action potentials are what send messages down a neuron.
• Pathway: Dendrites Cell body Axon Synapse
• In order for a signal to be tranmitted across the synapse, neurotransmitters are used.
• Transmission along neurons and synapses results in response.– Responses can be stimulatory or inhibitory.