Download Topic 6.5 Neuron and Synapses

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Transcript
Neurons and Synapses
6.5
The Nervous System
• Composed of cells called neurons.
• These are typically elongated cells that can carry electrical
impulses at very high speeds.
• The nervous system can be broken down into two major
parts:
– Central Nervous System (CNS)
• Brain and Spinal cord
– Peripheral Nervous System (PNS)
• Peripheral nerves that connect all parts of the body to the central nervous
system.
The Nervous System
Nerve Impulse
• Neurons transmit messages in the form of nerve
impulses.
• The impulses are electrical but involve the
movement of charged ions, not electrons.
• Nerve impulse can travel along a neuron as high as
100 meters per second.
Neurons
Neurons
• Dendrites receive chemical signals and
produce an electrical signal
• Axon transmits message
• Axon covered with a layer of fat called myelin
• Myelin does not completely cover the axon.
• Gaps are called nodes of ranvier
• Speeds up rate of impulse transmission
• Saltatory conduction
Resting Potential
• Neurons pump ions across their plasma membranes by active
transport.
• Sodium is pumped out of the neuron and potassium is
pumped in.
• Concentration gradients of both sodium(higher outside) and
potassium(lower outside) are established across the
membrane.
• The inside of the neuron develops a net negative charge
compared with the outside.
• This is the resting potential across the plasma membrane of a
cell that is not conducting an impulse.
Action Potentials
• An action potential is the reversal and then
the restoration of the electrical potential
across the plasma membrane of a cell as an
electrical impulse passes along it.
• An impulse passing along the neuron allows
sodium and potassium ions to diffuse across
the membrane (through voltage gated ion
channels).
Action Potential / Net Charges
Stage 1 in the Passage of a Nerve
Impulse
• An action potential in one part of a neuron causes an action
potential to develop in the next section of the neuron.
• This is from the diffusion of sodium ions between the region
with an action potential and the region at the resting
potential.
• If the potential rises above the threshold level, voltage gated
channels open.
Stage 2 in the Passage of a Nerve
Impulse : Depolarization
• Sodium channels open and sodium diffuse
down the concentration gradient
• This reduces membrane potential and causes
more sodium channels to open.
• Entry of positively charged sodium ions into
the neuron develops a net positive charge.
• Depolarization of the membrane occurs
reversing the membrane potential
Stage 3 in the Passage of a Nerve
Impulse : Repolarization
• Potassium channels open after a short delay.
• Potassium ions diffuse out of the neuron down the
concentration gradient through open channels.
• Exit of positively charged potassium ions develops a
net negative charge inside the cell compared to
outside.
• Repolarization is the restoration of the membrane
potential to its original resting state.
Stage 4 in the Passage of a Nerve
Impulse (refractory)
• Concentration gradients of sodium and potassium are
restored.
• This is done through the removal of sodium from the neuron
by active transport and the addition of potassium through
active transport.
• This restores the resting potential of that part of the neuron.
now again ready to conduct another nerve impulse.
• Sodium ions diffuse along inside the neuron from an adjacent
region that has already depolarized and initiate
depolarization.
Action Potential / Net Charges
Nerve Synapse
• A synapse is a junction between two nerves.
• The plasma membranes of either neuron is
separated by a narrow fluid gap called the synaptic
cleft.
• Messages are passed across the synapse in the form
of chemicals called neurotransmitters.
• Neurotransmitters always pass in the same direction
from the pre-synaptic neuron to the post-synaptic
neuron.
Synaptic Transmission
Synaptic Transmission
• Stage 1: action potential reaches the axon
terminal of the pre-synaptic neuron
• Stage 2: calcium channels open and Ca++ ions
diffuse into axon terminal
• Stage 3: Ca++ influx causes vesicles with
neurotransmitters to fuse with the membrane,
releasing their contents by exocytosis
Synaptic Transmission cont.
• Stage 4: neurotransmitters diffuse across the
gap (synaptic cleft) and bind to receptors of
the post-synaptic neuron
• Stage 5: binding of neurotransmitters on
receptors cause sodium ion channels to open
triggers an action potential
• Stage 6: neurotransmitters are either broken
down or taken back up by the pre-synaptic
neuron
Neural Transmission
• Both nerve impulses and synaptic
transmissions have a threshold potential
• If this threshold is not reached, the impulse
stops
• A typical post-synaptic neuron will be in
communication with many pre-synaptic
neurons
Acetylcholine
• Involved with contraction of muscles
• If acetylcholine triggers an impulse, the
muscle contracts (black widow)
• If not enough acetylcholine, muscle does not
contract (botox)
• Acetylcholinesterase is an enzyme that breaks
down the neurotransmitters
Insecticide use
• Neonicotinoids are synthetic compounds that
bind to acetylcholine receptors
• End up blocking the action of acetylcholine
• Works on insects more so than mammals
• Used as an insecticide
• Some controversy as to whether it is now
disrupting some ecosystems (honeybees)