Transcript Peripheral Nervous System

Part I - The Nervous System
Function:
• To coordinate the actions of your body
• To ensure effective behavior
• To maintain the internal environment
within safe limits (homeostasis)
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Workings of the Nervous System
Messages are relayed throughout the body via
electrochemical messages from the brain or
through chemical messengers – hormones
There are more nerve cells in the body than there
are visible stars in the Milky Way!
1 cm3 of brain tissue houses several million
neurons with each connecting with several
thousand others
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Nervous Tissue
The nervous system is divided into a Central nervous system
=brain & spinal cord and a
Peripheral nervous system
= nerves carrying sensory & motor
information between the CNS &
muscles & glands.
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Central Nervous System
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Peripheral Nervous System
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Neuron Structure
-A nerve cell is called a neuron
-Neurons are composed of dendrites
that receive signals, a cell body with a
nucleus, and an axon that conducts a
nerve impulse away.
Sensory neurons take information to the
CNS.
Motor neurons take information from the
CNS to muscles
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Neurons
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Parts of a Neuron
dendrites – receive information conducting
towards the cell body (~200 dendrites/cell
body)
cell body – location of the nucleus, high
metabolic rate (so contains mitochondria)
axon– may be 1m long, very thin, conducts the
impulse towards other neurons
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Sensory and Motor Neurons
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Myelin Sheath
nodes of Ranvier– the unmyelinated
sections of a myelinated neuron,
impulses “jump” between the nodes of
Ranvier
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The Nerve Impulse- Action
Potential
The nervous system uses the nerve
impulse to convey information.
Voltage (in millivolts, mV) measures the
electrical potential difference between
the inside and outside of the axon.
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The Action Potential
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Membrane Polarization (Resting Potential)
When an axon is not conducting a nerve
impulse, the inside of an axon is negative
(-70mV) compared to the outside (+40mV);
-This is the resting potential.
To establish the –70mV potential in the cell:
• Na+ is actively pumped out of the cell
• K+ is actively pumped into the cell
Sodium pump
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The Action Potential
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Membrane Depolarization
When the nerve cell is excited, the
membrane DEPOLARIZES (Action
Potential)
The membrane’s polarity changes:
– Na+ channels open, Na+ rushes in, K+
gates close
The positive ions flowing in causes a
charge reversal to +40 mV inside the
neuron
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Depolarization- Action
Potential
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Membrane Repolarization
Once the charge becomes positive, the
Na+ gates close, K+ gates open,
eventually restoring the charge inside
the neuron to –70 mV (but the Na+
excess is inside and K+ excess is
outside!)
The Na/K Pump restores the ion
concentrations inside and outside the
cell
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Membrane Repolarization
During the repolarization, the nerve
cannot be reactivated – this is called
the refractory period (1 to 10 ms) and is
a recovery time for the neuron
The pump requires ATP in order to
operate
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Refractory Period
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Fig. 48-13
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
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Transmission Across a Synapse
• The junction between neurons or
neurons & effectors is called the
synapse.
• Transmission of a nerve impulse takes
place when a neurotransmitter molecule
stored in synaptic vesicles in the axon
bulb is released into a synaptic cleft
between the axon and the receiving
neuron.
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Synapse structure and function
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Neurotransmitter Molecules
Acetylcholine (ACh)
Norepinephrine (NE)
Neurotransmitters are removed from the
synapse by the enzyme
acetylcholinesterase (AChE) that breaks
down acetylcholine.
This prevents continuous stimulation or
inhibition
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Other Neurotransmitters
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Serotonin
Dopamine
GABA
Glutamate
• *see table of neurotransmitters and
their functions
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Reflex Arc Diagram
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