Transcript Chapter 35-2

Chapter 35-2
Nervous System
A Neuron
Structure of the Neuron
 Neurons come in all shapes & sizes
 Have common features
Nucleus
Axon
terminals
Cell body
Myelin sheath
Dendrites
Axon
Nodes
Nerve Impulses
 Resting Neuron – when at rest:
- outside of cell = + charge
- inside of cell = - charge
- cell membrane = electrically charged
- due to the opposite charges on both sides
- caused by the - & + elements crossing the membrane
by active transport
Sodium Potassium Pump
 Na+/K +pump
 How it works:
1. nerve cell membrane pump Na +out of cell & K+ into the
cell by active transport
2. as a result: inside – ↑ K+ and ↓ Na+
outside - ↑ Na+ and ↓ K+
3. the nerve cell membrane allows more K+ to cross the
membrane
result - K+ goes outside of cell –
produces – charge inside cell
+ charge outside cell
Figure 35-6 Resting Potential
Resting Potential
Section 35-2
 Electrical charge across the cell
Outside of cell (+ charge)
membrane of a neuron in its
resting state
Cell membrane
Inside of cell
(- charge)
Moving Impulse
 A neuron remains resting until it receives a stimulus large enough
to start a nerve impulse
 An impulse begins when a neuron is stimulated by:
1. another neuron
2. by the environment
Process of Impulses
1.
2.
When it begins, the impulse travels rapidly down the axon, away
from cell body towards the axon terminals
This causes the charges to reverse the membrane potential (- to +)
- results from the leading edge of impulse opening Na channels
which allows Na to flow into the cell
- Action Potential – the reversal of charges
from – to +
- also called “nerve impulse”
Section 35-2
Process of an Impulse
Action Potential
At rest.
Action Potential
At the leading edge of the impulse, the
sodium gates open. The membrane becomes
more permeable to Na+ ions and an action
potential occurs.
As the action potential passes, potassium
gates open, allowing K+ ions to flow out.
Action Potential
The action potential continues to move along
the axon in the direction of the nerve impulse.
Process of Impulses cont.
3. As the action potential passes, gates in the K+ channels open, allowing
K+ to flow out
- this restores the resting potential
- reversing the charge back to normal
4. The axon potential continues to move along the axon until it reaches the
organ or gland
Nerve impulses are “self-propagating”:
- an impulse at any point on the membrane
cause an impulse at the next point on the
membrane ( row of dominoes)
Threshold
 The strength of an impulse is always the same – either there is an
impulse or there is not
 A stimulus must have enough strength to cause a neuron to start an
impulse
 The minimum level of a stimulus that is required to activate a
neuron is called the THRESHOLD
All or None Principle
 Either the stimulus will produce an impulse or it will not produce
an impulse.
 No in between
 If you stack a row of dominoes and push the first one:
- either they all fall down or they don’t move at all
Synapse
Figure 35-8 The Synapse
Section 35-2
 The location at which a neuron
can transfer an impulse to
another cell
 Motor neurons pass impulses to
muscle cells
Direction of Impulse
Dendrite of
adjacent neuron
Axon
Receptor
Vesicle
Axon
terminal
Synaptic cleft
Neurotransmitter
Figure 35-8 The Synapse
Section 35-2
Synaptic Cleft
• A space that
separates the axon
terminal from the
dendrites of the next
neuron
• Contains tiny
vesicles filled with
neurotransmitters
Direction of Impulse
Dendrite of
adjacent neuron
Axon
Receptor
Vesicle
Axon
terminal
Synaptic cleft
Neurotransmitter
Figure 35-8 The Synapse
Section 35-2
Neurotransmitters
• Chemicals used by
a neuron to
transmit an
impulse across the
a synapse to
another cell
Direction of Impulse
Dendrite of
adjacent neuron
Axon
Receptor
Vesicle
Axon
terminal
Synaptic cleft
Neurotransmitter
How Neurotransmitters Work
1.
2.
3.
4.
When an impulse arrives at the axon terminal, the vesicles release the
neurotransmitters into the synaptic cleft
Neurotransmitters diffuse across the cleft and attachment themselves
to receptors on the membrane of the next cell
This stimulus cause positive Na to rush across the membrane,
stimulating the next cell
If stimulation exceeds the cell’s threshold, a new impulse begins
Life of a Neurotransmitter
 Only a fraction of a second after binding to its receptors, the
neurotransmitter is released from the membrane.
 Neurotransmitters are either:
1. broken down by enzymes
2. taken up & recycled by the axon terminal