1. Notes
The Nervous System
Chapter 35
Section 2

2. Notes
The Nervous System
Chapter 35
Section 2

3. Nervous System = Communication
Function:
Receives Stimuli
Interprets information
Causes a response
-Controls and coordinates bodily functions
-Responds to stimuli (inside or outside the body)

4. What is the basic functional unit of the nervous system?
Neuron!
What does the neuron do?
Carries messages throughout the body
How does it carry the messages?
By conducting electrical signals
What are these signals called?
Nerve impulses!

5. What does a neuron look like?

6. Neuron Anatomy Three parts to a NEURON: Cell body:
Large, central portion of the neuron where all organelle are located
What is its job?
Interpret incoming signals
Cell Body

7. Neuron Anatomy 2. Dendrites: Short, highly branched fibers
what is its job?
Carries impulses toward the cell body
Referred to as the afferent process

8. Neuron Anatomy 3. Axon: Long, slightly branched fiber What is its job?
Carry impulses away from the cell body
Referred to as the efferent process

9. The Axon
Axon Hillock
fiber
Axon Terminals

10. Additional Parts of a Neuron
Neuron Anatomy
Additional Parts of a Neuron
Dendrite
Cell body
Axon terminal
Myelin sheath
Nodes of
Ranvier
Axon
Nucleus
Structure of a Typical Neuron

11. Neuron anatomy What do you remember? dendrites 3 1 2 4 5 6 7
Myelin sheath
nucleus
Direction
of impulse
Cell body
axon
Nodes of Ranvier
Axon terminals

12. Three Types of Neurons II. Motor Neurons:
Carry impulses to muscles and glands
Cause a response to some stimuli
III. Interneurons:
Connect sensory and motor neurons
Allow for quick response (reflex action)

13. Three types of neurons

14. Neurons What is grey matter? What is white matter?
Collective cell bodies and dendrites of all neurons
What is white matter?
Myelinated nerve fibers
Axons of all neurons
Can be approximately one meter in length
White matter
Gray matter

15. What is a Nerve? Bundle of axons held together by connective tissue.
What color is a nerve?
White!
Why?
Because axons are white matter and they compose nerves

16. How are nerves held together?
Connective tissue
What is this connective
tissue called?
Neuroglial cells (nerve glue)
Approximately half of the volume of the brain is composed of neuroglial cells
Most brain tumors develop in mesoglial cells – NOT neurons

17. What do neuroglial cells do?
Support the axons
Insulate the electrical impulses
Like electrical tape insulates electric wires this prevents “leaking” of electric signals

18. An example of a neuroglial cell
Schwann cell
This wraps around the axon in multiple layers
It is composed of a fatty material called…
Myelin
Regular breaks in the myelin sheath are called…
Node of Ranvier

19. Impulse speed Some neurons are fast, good conductors of impulses
Other neurons are slow, poor conductors of impulses
What distinguishes these two types?
Diameter of the axon
Myelination
Fastest axons have a large diameter and are myelinated. How much faster?
Up to 100 times faster! (Reflexes are this type)

20. The Nerve Impulse
How are messages (impulses) carried by the nervous system?
As electrical and chemical signals.
How does the impulse develop?
Charged particles (ions) move across the cell membrane
A neuron is ready to transmit an impulse when it is in the resting state.
Resting membrane potential is -70 mV

21. Resting Membrane Potential
The inside of the cell has a negative charge as compared to the outside of the cell membrane.
How does this charge difference develop?
There are more Na+ (sodium) ions outside and fewer K+ (potassium) ions inside.

22. The Nerve Impulse How does the impulse begin?
The neuron is stimulated by another neuron or by stimuli from the environment.

23. The Nerve Impulse
If the stimulus is strong enough, it reaches threshold level.
This stimulates an impulse.
If the stimulus is not strong enough, no impulse occurs (all-or-none principal)

24. The Nerve Impulse What happens if threshold is reached?
An action potential (nerve impulse) begins
What is an action potential?
Rapid reversal of membrane potential in response to a stimulus
How does this happen?
Sodium channels open allowing Na+ to flood into the cell.
The membrane potential rises to +30 mV (rising phase) as inside of cell becomes more positive

25. The Nerve Impulse Please, please, tell us what happens next…
When the membrane potential reaches +30 mV, the sodium channels close.
Potassium channels open and K+ flows out of the cell.
This causes the membrane potential to become more negative again (falling phase).

26. The Nerve Impulse +50 Membrane potential (mV) -70 Na+ channels close 2
K+ channels
open, K+
begins to
leave cell
K+ leaves
cell
Membrane potential (mV) 1
Na+
channels
open, Na+
enters cell
Threshold of
excitation
K+ channels
close 3
-70
Excess K+ outside
diffuses away

28. The Nerve Impulse

29. The Nerve Impulse Action potential +40 Depolarization Repolarization
Depolarization
Repolarization
Voltage (mV)
Failed
initiations
Threshold
-55
Resting state
-70
Stimulus
Refractory
period
Time (ms)

30. The Nerve Impulse
What happens when the action potential passes down the axon?
The resting potential is restored via the sodium/potassium pump.
Now the membrane is ready to transmit another impulse.
extracellular fluid
cytoplasm
Na+
ATP
ADP K+

31. The Nerve Impulse
An action potential only moves in one direction down the axon.
From axon hillock, thru axon fiber to the axon terminal.
The action potential is regenerated at each Node of Ranvier down the length of the axon.

32. The Nerve Impulse
What happens when the impulse reaches the axon terminal?
It must pass thru the synapse.
What is a synapse?
It is the gap between two neurons, or between a neuron and organ (effector).
Why is there a gap?
So the neurons don’t short each other out!!

33. The Synapse How do neurons pass the impulse across the synapse?
Using chemicals called neurotransmitters
What do neurotransmitters do?
Stimulate the dendrites of other neurons or membranes of other cells.
If a dendrite is stimulated, it sends its message to the cell body and the message is passed on
If a muscle or gland is stimulated, a reaction occurs in that organ.

34. The Synapse

35. The Synapse Model of the Synapse Axon of presynaptic cell
synaptic vesicles
Axon of
presynaptic cell
neurotransmitters
receptors for
glycoprotein
synaptic cleft
Glycoprotein
(ligands)
Model of the
Synapse

36. The Synapse

37. Neuron Smear:
Identification: Note distinctive shape of neuron, with long processes (dendrites and/or axons, 5) extending out from main cell body.
Features to Know:
The large, irregularly shaped cell body (3) contains a darker nucleus (2), which contains an even darker-staining nucleolus (1).
There are also numerous supporting glial cells, though only their small dark nuclei (4) are easily seen.

40. Myelin sheath
Nerve
Neuron