1. Chapter 35-2
Nervous System

2. Structure of the Neuron
A Neuron
Structure of the Neuron
Neurons come in all shapes & sizes
Have common features
Axon terminals
Myelin sheath
Nodes
Cell body
Axon
Nucleus
Dendrites

3. 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

4. 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

5. Figure 35-6 Resting Potential
Section 35-2
Electrical charge across the cell membrane of a neuron in its resting state
Outside of cell (+ charge)
Cell membrane
Inside of cell (- charge)

6. 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

7. Process of Impulses
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”

8. Process of an Impulse Section 35-2 Action Potential At rest.
As the action potential passes, potassium gates open, allowing K+ ions to flow out.
Action Potential
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.
The action potential continues to move along the axon in the direction of the nerve impulse.

9. 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)

10. 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

11. 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

12. Dendrite of adjacent neuron
Figure 35-8 The Synapse
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

13. Dendrite of adjacent neuron
Figure 35-8 The Synapse
Synaptic Cleft
Section 35-2
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

14. Dendrite of adjacent neuron
Figure 35-8 The Synapse
Neurotransmitters
Section 35-2
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

15. How Neurotransmitters Work
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

16. 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