1. Nervous System What you need to know: Three Basic Functions
Functional Organization
Neuron Structure
Reflex Arcs
Nerve Impulse physiology
Brain Structure/Function

2. Basic Functions of the Nervous System

3. Three Basic Functions of N.S.
Sensory input —gathering information
To monitor changes (stimuli) occurring inside and outside the body
Integration
To process and interpret sensory input and decide if action is needed
Motor output
A response to integrated stimuli
Muscles and/or glands are activated

4. Three Basic Functions of N.S.
Muscles,
Glands
(1)
(2)
(3)
Figure 7.1

5. Functional Organization of the Nervous System

6. Functional Organization of the Nervous System
(maintains homeostasis)
(“fight or flight”
response)
Functional
Organization
of the
Nervous
System
IMPORTANT: Direction of arrows indicates direction of nerve impulse.

7. Functional Organization of NS
Central nervous system (CNS)
brain
spinal cord
Integrate sensory information, then send “response” signals to muscles and glands
Peripheral nervous system (PNS)
nerves outside the brain and spinal cord
transmits nerve impulses to/from CNS

8. Functional Classification of the Peripheral Nervous System
1. Sensory (afferent) division
Nerve fibers that carry information to the central nervous system
2. Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system

9. Peripheral Nervous System
Muscles,
Glands
Figure 7.1
Slide 7.3b
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10. Functional Classification of the Peripheral Nervous System
Motor (efferent) division
Two subdivisions
Somatic nervous system = voluntary
Autonomic nervous system = involuntary
Figure 7.1
Slide 7.3c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

11. Organization of the Nervous System
„ Sympathetic nervous system
„allows body to function under stress
„ “fight or flight” response
„ Parasympathetic nervous system
„ controls vegetative functions
„ feed or breed or rest and repose
„ constant opposition to sympathetic system
Slide 7.4
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

12. Organization of the Nervous System
Slide 7.4
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

13. Functional Organization of the Nervous System
IMPORTANT: Direction of arrows indicates direction of nerve impulse.
(maintains homeostasis)
(“fight or flight”
response)

14. Functional Organization of the N.S.
Central Nervous System
(brain and spinal cord)
Peripheral Nervous System
(cranial and spinal nerves)
Brain
Cranial
nerves
Sensory division
Sensory receptors
Spinal
cord
Spinal
nerves
Motor division
Somatic
Nervous
System
Skeletal muscle
Autonomic
Nervous
System
Smooth muscle
Cardiac muscle
Glands 14
(a)
(b)

15. Neuron Structure

16. Sensory Neuron

17. Relay Neuron

18. Motor Neuron

19. Neuron Structure

20. Neuron Anatomy Extensions outside the cell body (processes):
Dendrites – conduct impulses toward the cell body
Axons – conduct impulses away from the cell body
Figure 7.4a
Slide 7.10
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21. Structural Classification of Neurons
Multipolar neurons – many extensions from the cell body
Figure 7.8a
Slide 7.16a
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22. Structural Classification of Neurons
Bipolar neurons – one axon and one dendrite
Figure 7.8b
Slide 7.16b
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23. Structural Classification of Neurons
Unipolar neurons – have a short single process leaving the cell body
Figure 7.8c
Slide 7.16c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

24. Reflex Arcs

25. Central process (axon)
Sensory
neuron
Spinal cord
(central nervous system)
Cell
body
Ganglion
Dendrites
Peripheral
process (axon)
Afferent
transmission
Interneuron
(association
neuron)
Peripheral
nervous system
Receptors
Efferent transmission
Motor neuron
To effectors
(muscles and glands)
Figure 7.6

27. Physiology of a Nerve Impulse

28. Physiology of a Nerve Impulse: Anatomy of Human Neuron
Dendrite
Axon
terminals
Cell body
Myelin sheath
Node of Ranvier
Image References: LifeART. (1998). Super Anatomy Collection 1-9. Baltimore, MD: Williams & Wilkins, All rights reserved. 28

29. Physiology of a Nerve Impulse
In a typical nerve cell, the largest portion is the cell body, which contains the nucleus. Dendrites extend from the cell body and receive impulses that are transmitted to the cell body and out through a long extension called an axon. Some neurons have a fatty outer layer of insulation called the myelin sheath. The myelin sheath is interrupted at intervals, causing impulses to “jump” the gaps (called Nodes of Ranvier) and speed the transmission rate. Impulses travel in one direction only: in through the dendrites and out through the axon. Bundles of neurons are called nerves.
The connecting space between neurons is called a synapse. Neurotransmitters at the synapse must be activated for the impulse to continue to the next neuron.
Electrical impulses in neurons depend on relative concentrations of ions inside and outside of the cell’s membrane. Voltage-gated channels affect the membrane potential of the neuron. At resting potential (polarized state), the inside of the neuron is more negatively charged than the outside of the neuron. The reversal of polarity (or depolarization) across the membrane - the action potential - causes voltage-gated sodium channels to open and sodium ions to flow into the axon. This begins a chain reaction that causes sodium channels to open along the axon. The action potential conducts rapidly down the axon. Voltage-gated potassium channels open, allowing potassium ions to flow out of the axon, returning the membrane potential to negative. The resting potential is regained as sodium-potassium “pumps” restore the original concentrations of sodium and potassium inside and outside the membrane.
References: Campbell, N.E. & Reece, J.B. (2002). Biology,(6th ed.). San Francisco: Benjamin Cummings.
Raven, P.H. & Johnson, G.B. (2002). Biology, (6th ed.). McGraw-Hill.

30. Nerve Impulses This is simplified model of how nerves work
Not all biochemical mechanisms influencing transmission of nerve impulses are included in this model
This model focuses on how electrochemical gradients and potential energy are involved in the transmission of impulses
Figure 7.9a–c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

31. Nerve Impulses
Electrical charges exist as ions (particles with + or – charges) inside and outside the nerve’s cell membrane.
An electrochemical gradient is a difference in the overall electrical charges on opposite sides of a membrane.
Figure 7.9a–c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

32. Nerve Impulses
The electrochemical gradient across a neuron’s membranes result in potential (stored) energy.
The potential energy used by nerve impulses is stored as the natural attraction between particles with opposite (+ and -) charges.
Figure 7.9a–c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

33. Nerve Impulses
When a stimulus is perceived by sensory receptors, potential (stored) energy gets converted to kinetic energy (energy of movement). This change begins the action potential (nerve impulse).
Figure 7.9a–c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

34. Nerve Impulse – electrochemical gradient
Resting (polarized) membrane
Energy is stored as a strong electrochemical gradient.
Na+ (sodium ions) outside the membrane can’t move through the membrane in the resting state
membrane is impermeable to Na+ in the resting state
Figure 7.9a–c
Slide 7.18
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35. Nerve Impulse – electrochemical gradient
Resting (polarized) membrane
The strength or weakness of the electrochemical gradient across the membrane can be measured and quantified.
Figure 7.9a–c
Slide 7.18
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36. Nerve Impulse - stimulus
Stimulus makes membrane permeable to Na+
If stimulus is strong enough, the membrane becomes permeable to Na+
Na+ diffuses rapidly into the nerve cell (this is a passive process and occurs simply due to existence of much higher Na+ concentrations outside the cell)
Figure 7.9a–c
Slide 7.18
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37. Nerve Impulse - depolarization
Diffusion of Na+ into cell
As Na+ diffuses into the cell, the solution outside the cell becomes less positive (more negative) and the fluid inside the cell becomes more positive (less negative)
The electrochemical gradient has been weakened.
Figure 7.9a–c
Slide 7.18
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38. Nerve Impulse – action potential
Initiation of Action Potential (nerve impulse)
A slightly weakened electrochemical gradient will not result in an action potential
If the electrochemical gradient has been weakened enough, then an action potential will be generated
Figure 7.9a–c
Slide 7.18
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39. The Action Potential
If the action potential (nerve impulse) starts, it is propagated over the entire axon
In other words, a nerve impulse is all or nothing
There is no such thing as a “weak” or “strong” action potential (nerve impulse) or one which only goes part way along a neuron.

40. The Action Potential
After Na+ rapidly diffuses into the cell, sodium-potassium pumps rapidly pump K+ (potassium ions) out of the neuron to repolarize the membrane.
This action requires ATP, a molecule which carries a lot of potential energy which can be used for many cellular functions.
Use of the Na+-K+ pumps is “expensive” in that it requires energy.

41. Nerve Impulse Propagation
The impulse continues to move toward the cell body
Impulses travel faster when fibers have a myelin sheath
Figure 7.9c–e
Slide 7.20
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42. Continuation of the Nerve Impulse between Neurons
Impulses are able to cross the synapse to another nerve by using neurotransmitter molecules
Neurotransmitters are released from a nerve’s axon terminal
The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter
An action potential is started in the dendrite
Slide 7.21
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43. How Neurons Communicate at Synapses
Figure 7.10
Slide 7.22
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44. An impulse moves in only one direction across a synapse - from an axon to the dendrites or cell body of another neuron.

46. Why is it important that a neurotransmitter is broken down after it initiates an action potential (nerve impulse)?
Why is it important that a neurotransmitter is released from the receptor site after initiating the action potential?

47. Why is it important that a neurotransmitter is broken down after it initiates an action potential (nerve impulse)?
Answer: If neurotransmitters were not broken down, they could continue to stimulate impulses (action potentials) in the “receiving” neuron.
Why is it important that a neurotransmitter is released from the receptor site after initiating the action potential?
Answer: Release of neurotransmitters from receptor sites makes those sites available to receive new signals in the future.