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Biology
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35-2 The Nervous System
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35-2 The Nervous System
The nervous system
•controls/coordinates functions in thebody
•responds to internal & external stimuli.
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Neurons
Neurons
Messages carried by the nervous system areelectrical signals called impulses.
The cells that transmit these impulses are calledneurons.
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Neurons
Neurons are classified according to where the animpulse travels.
•Sensory neurons carry impulses from thesense organs to the spinal cord and brain.
•Motor neurons carry impulses from the brainand spinal cord to muscles and glands.
•Interneurons connect sensory and motorneurons and carry impulses between them.
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Neurons
Structures of a Neuron
Axonterminals
Myelin sheath
Cell body
Nodes
Axon
Dendrites
Nucleus
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Neurons
The largest part of a typical neuron is the cell body.
It contains the nucleus and much of the cytoplasm.
Cell body
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Neurons
Dendrites carry impulses from the environmenttoward the cell body.
Dendrites
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Neurons
Axon - long fiber that carries impulses away from thecell body.
The axon ends in axon terminals.
Axonterminals
Axon
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Neurons
Axon is surrounded by an insulating membrane -myelin sheath.
Nodes - gaps in the myelin sheath.
Impulses jump from one node to the next.
Myelin sheath
Nodes
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The Nerve Impulse
How is a nerve impulse transmitted?
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The Nerve Impulse
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The Nerve Impulse
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The Nerve Impulse
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The Nerve Impulse
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The Nerve Impulse
Threshold
A stimulus must be of adequate strength to causea neuron to transmit an impulse.
The minimum level of a stimulus that is required toactivate a neuron is called the threshold.
A stimulus that is weaker than the thresholdproduces no impulse.
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The Synapse
The Synapse
At the end of the neuron, the impulse reaches anaxon terminal.
The neuron makes contact with another cell at thissite.
The neuron may pass the impulse along to thesecond cell.
The location at which a neuron can transfer animpulse to another cell is called a synapse.
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The Synapse
The synaptic cleftseparates the axonterminal from thedendrites of the nextcell.
Synaptic cleft
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The Synapse
Terminals containvesicles filled withneurotransmitters.
Neurotransmittersare chemicals used bya neuron to transmitan impulse across asynapse to anothercell.
Vesicle
Neurotransmitter
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The Synapse
As an impulsereaches a terminal,vesicles sendneurotransmittersinto the synapticcleft.
These diffuse acrossthe cleft and attachto membranereceptors on thenext cell.
Receptor
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The Synapse
The next cell is simulated.
If the stimulation exceeds the cell’s threshold, a newimpulse begins.
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The Synapse
Moments after binding to receptors,neurotransmitters are released from the cell surface.
The neurotransmitters may then be broken down byenzymes, or taken up and recycled by the axonterminal.
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DIVISIONS OF THENERVOUS SYSTEM
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The Central Nervous System
The Central Nervous System
•The central nervous system relays messages,processes information, and analyzesinformation.
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The Peripheral Nervous System
The Peripheral Nervous System
•The peripheral nervous system is all of thenerves and associated cells that are not part ofthe brain and the spinal cord.
•The peripheral nervous system includes cranialnerves, spinal nerves, and ganglia.
•Ganglia are collections of nerve cell bodies.
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35-2
Neurons that carry impulses from the brain andspinal cord to the muscles are
a.interneurons.
b.sensory neurons.
c.resting neurons.
d.motor neurons.
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The part of the neuron that carries impulsestoward the cell body is the
a.axon.
b.myelin sheath.
c.dendrite.
d.nodes.
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35-2
The minimum level of a stimulus that is requiredto activate a neuron is called its
a.action potential.
b.resting potential.
c.threshold.
d.synapse.
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35-2
Chemicals that are used by a neuron to transmitimpulses are called
a.neurotransmitters.
b.synapses.
c.axons.
d.inhibitors.
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An action potential begins when
a.sodium ions flow into the neuron.
b.potassium ions flow into the neuron.
c.sodium and potassium ions flow into theneuron.
d.sodium and potassium ions flow out of theneuron.
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The Nerve Impulse
The Nerve Impulse
The Resting Neuron
When resting, the outside of the neuron has a netpositive charge.
The inside of the neuron has a net negativecharge.
The cell membrane is electrically charged becausethere is a difference in electrical charge betweenits outer and inner surfaces.
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The Nerve Impulse
The sodium-potassium pump in the nerve cellmembrane pumps sodium (Na+) ions out of the celland potassium (K+) ions into the cell by means ofactive transport.
As a result, the inside of the cell contains more K+ions and fewer Na+ ions than the outside.
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The Nerve Impulse
Sodium-Potassium Pump
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The Nerve Impulse
More K+ ions leak across the membrane than Na+ions. This produces a negative charge on the insideand a positive charge on the outside.
The electrical charge across the cell membrane of aneuron at rest is known as the resting potential.
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The Nerve Impulse
The inside of the membrane temporarily becomesmore positive than the outside.
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The Nerve Impulse
This is called a nerve impulse, or an actionpotential.
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The Nerve Impulse
As the action potential passes, K+ ions flow outrestoring the negative potential inside the axon.
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The Nerve Impulse
The impulse continues to move along the axon.
An impulse at any point of the membrane causes animpulse at the next point along the membrane.