1. Human Biology: Nervous System
Lesson 1: Neurons to Reflex Arc
(Inquiry into Life pg )

2. Today’s Objectives
Analyse the transmission of nerve impulses, including:
Identify and give functions for: dendrite, cell body, axon, axoplasm, axomembrane
Differentiate among sensory, motor, and inter-neurons with respect to structure and function
Explain the transmission of a nerve impulse through a neuron, using applicable terminology (see handout)
Relate the structure of a myelinated nerve fibre to the speed of impulse conduction, with reference to applicable structures (see handout)
Identify the major components of a synapse (see handout)
Explain the process by which impulses travel across a synapse
Describe how neurotransmitters are broken down in the synaptic cleft
Describe the structure of a reflex arc (see handout), and relate its structure to how it functions

3. Neurons

4. Learn Some Stuff
With a partner, prepare a large diagram which includes the following terms:
Motor neuron, interneuron, sensory neuron, axon, dendrite, cell body, node of Ranvier, myelin sheath, Schwann cell
Below each term write one or two sentences to describe the function of each.
Be sure to explain the difference between motor neuron, interneuron and sensory neuron.
Reading pages 320 – 321 will help!

5. Structures and Functions
1) Dendrites
Conduct a nerve impulse (message) towards a cell body
Many dendrites enter a cell body
2) Cell Body
Contains the nucleus and cell organelles needed to keep the cell alive
Only a single axon leaves a cell body
Relays impulse from dendrite to axon
3) Axons
Conduct a nerve impulse away from the cell body

6. Structures and Functions
4) Myelin Sheath
Protective coating of Schwann Cells around larger axons
5) Nodes of Ranvier
Interrupted areas on the Myelin Sheath
Speeds up transmission of impulse
Animation
6) Motor End Plates
In close proximity to muscles and organs
From here the impulse is chemically transported to the organs
7) Ganglia
A collection of cell bodies outside of the CNS

7. Types of Neurons (see animation)
1) Motor Neuron -Efferent Neuron: Moving toward a central organ or point. -Relays messages from the brain or spinal cord to the muscles and organs.

8. Types of Neurons
2. Sensory Neuron (see animation) -Afferent Neuron: Moving away from a central organ or point. -Relays messages from receptors to the brain or spinal cord

9. Types of Neurons
3. Interneuron (associated neuron or Connector Neuron) (see animation) -Relays message from sensory neuron to motor neuron. -Make up the brain and spinal cord.

10. Bases of Comparison
Sensory Neuron
Interneuron
Motor Neuron
Length of Fibers
Location
 Function
Long Dendrites and short Axon
Cell body and Dendrite are outside of the spinal cord; the cell body is located in a dorsal root ganglion.
Conduct impulse to the spinal cord.
Short Dendrites and short or long Axons
Entirely within the spinal cord or CNS.
 Interconnect the Sensory neuron with an appropriate Motor Neuron.
Short Dendrites and long Axons
Dendrites and the cell body are located in the spinal cord; the Axon is outside of the spinal cord.
 Conduct impulse to an effector (muscle or gland)

11. ***A nerve is composed of long fibers of a number of Neurons***

12. Impulse Generation (Action Potential)
Nerve impulses are electrical messages
If we measure the voltage of a resting neuron using a voltmeter, we will see a reading of -60~-70 millivolts
Voltage is a comparison of electrical charge between two points
When the neuron is stimulated, the charge changes briefly to +40 millivolts (mv), then back to -60 mv
(-60 mv means that the inside is 60 mv more negative than outside)
If we hook up our voltmeter to a machine called an oscilloscope, we can see the change in voltage over a period of time

13. There is a difference in ion distribution on either side of the membrane of a neuron.
At Rest: Na+ outside the Neuron
K+ and large negatively
charged organic molecules inside the neuron.

14. The concentration of sodium ions Na+ is greater outside the axon than inside.
The concentration of potassium ions K+ greater inside the axon than outside.
This unequal distribution is due to the sodium- potassium pump which actively transports Na+ out and K+ into the axon

15. Action Potential
The membrane is more permeable to K+ ions, and some K+ diffuses back while Na+ does not.
This unequal charge distribution, along with the large negative molecules, causes the inside to be more negative than the outside. (Polarity)
An action potential is a rapid change in polarity across an axonal membrane as the nerve impulse occurs.
An action potential occurs when a stimulus causes the axonal membrane to depolarize to a certain level, called threshhold.

16. Na+ K+ Na+ Na+
__________________________ K+ K K+
  Na+ K+ Na+ Na+
This situation is called Resting Potential. -60mv
When the axon or dendrite is stimulated, sodium gates open which allows some Na+ to enter the Axoplasm (interior). Now, the inside becomes more positive than the outside by 40 mv. (see video)

17. Na Na+ Na+
  K+ -- K+ K K+
  Na+ K+ Na+ Na+
This is called the Upswing Phase of the action potential. The charge changes from –60 mv to +40 mv. The change is called Depolarization.
  After the sodium gates have opened, then potassium gates open. K+ exit the axoplasm.

18. K K+
-- K+ Na+ -- Na+
Na K
K Na+ K+ K+
K+ K+
This is called the Downswing Phase of the action potential. The charge returns to –60mv. The change is called Repolarization.
**Note: Charge is back to normal, but ions are reversed

19. Finally, there is a Recovery Phase in which the sodium/potassium pump (ACTIVE TRANSPORT) returns Na+ to the outside and K+ to the inside.
This period of pumping is called the refractory period.
During the refractory period, another action potential cannot be created.

20. Action Potential
So far we have only been looking at one point on the Axon or Dendrite
The opening of the sodium gates in one area causes the sodium gates in the next area to open
We get a wave motion (chain reaction) moving down the nerve fiber

22. 1. RESTING POTENTIAL
-Charge is –60mv
-Na+ outside
-K+ inside
2. UPSWING OF ACTION POTENTIAL
-Depolarization
-Charge from –60mv back to +40mv
-Na+ moves inside
-(Sodium gates opened)

23. 3. DOWNSWING OF ACTION POTENTIAL
-Repolarization
-Charge from +40mv back to –60mv
-K+ moves outside
-(Potassium gates opened)
4. RECOVERY PHASE
-Charge is –60mv
-Sodium/Potassium Pump
-Moves Na+ out and K+ inside
****NOW NEURON CAN BE RE-STIMULATED ****
**REMEMBER THIS IS A WAVE MOTION DOWN THE NEURON**

24. + + + + + + + + + + Na+ K+ + + + + - - + + + + + K+
Na+ K+
Na+ K+

26. Myelinated Vs Unmyelinated Fibers
Myelinated Neuron
Schwann cells
Nodes of Ranvier

27. Myelinated Neuron
In vertebrates, where most long nerve fibres have myelin sheath around them, the Schwann cells restrict ion movement across the axomembrane and the impulse “jumps” between successive nodes of Ranvier, thus speeding up the impulse.
This type of “jumping” transmission is called saltatory conduction.

28. It is Schwann cells that wrap around the nerve fiber
It is Schwann cells that wrap around the nerve fiber. When it is myelinated, it is covered by Schwann cells. Impulse skips from node to node. (see video)

29. UnMyelinated Neuron
Each action potential starts the next action potential, causing a wave along the entire neuron.

30. In myelinated axons and dendrites, the impulse can travel up to 200m/s
In myelinated axons and dendrites, the impulse can travel up to 200m/s. In unmyelinated fibers, the impulse can be as slow as 0.5 m/s.
This difference in speed is because the action potential is able to jump over the myelin sheath. Depolarization only occurs at the nodes of Ranvier.

31. Synapse (see video)
Each axon branches off and ends with a swelled tip or terminal knob that lies close to but not touching the dendrite of another neuron. (or an organ). The entire region is called a synapse.

32. Transmission of nerve impulses across a Synaptic cleft is carried out by chemicals called Neurotransmitters.
These substances are stored in vesicles at the end of the axon. Noradrenalin (speeds up activity or excitatory) and acetylcholine (slows down activity or inhibitory) are examples of neurotransmitters.

33. When an impulse reaches the end of the axon like it usually would, not only does Na+ come into the axon, but Ca+2 as well since voltage gated calcium channels are opened.

34. This calcium binds with contractile proteins that pull the Neurotransmitter vesicles to the membrane surface. The vesicles join with the cell membrane, forcing the neurotransmitter into the cleft (exocytosis)
**Ca+2 causes the microfilaments to contract and pull the synaptic vesicles to the presynaptic membrane*

35. Neurotransmitter’s job is to increase the permeability of the sodium ions on the postsynaptic membrane.
The Neurotransmitter binds to specific receptor sites on the dendrite of the next neuron. If enough transmitter substance is received, the neuron will “fire” and continue the impulse.

36. A neurotransmitter only has a short period to work once it has been released into the synaptic cleft.
Enzymes rapidly break down the transmitter substance to clear the synapse so the next impulse can be transmitted.
Monoamine oxidase breaks down noradrenaline and Acetylcholinesterase breaks down acetylcholine.(see video)

38. Pain killers such as Tylenol act as an enzyme to break down the neurotransmitter to decrease the pain impulse. A natural painkiller in the body is Prostaglandin.
An impulse can only travel across a synapse in one direction. Only the axon contains neurotransmitter vesicles, so the impulse can only travel one way

39. AXON  DENDRITE across a synapse.
**** ALL OR NONE LAW (threshold): If enough neurotransmitter is received by the postsynaptic fiber, it will fire 100%. (all). If not enough substance is received, it will not fire at all. (none)

40. There are excitatory and inhibitory neurotransmitters in the body
There are excitatory and inhibitory neurotransmitters in the body. When two excitatory neurotransmitters work together to cause an action potential, it is called summation.

41. Why does the brain not have to be involved?
Reflex Arc 
Reflexes are automatic, involuntary responses to changes occurring inside or outside the body. Some involve the brain (such as blinking the eye), while others do not (such as moving your hand away from a hot object).
Why does the brain not have to be involved?
If it were, by the time the impulse traveled to the brain, the brain figured out what was happening, and sent a response to the body, serious damage might occur.

42. So the body evolved a method of by passing the brain.

43. Stages of Reflex Arc
1. Receptor is stimulated and formulate message. ie. nerve impulse
2. Sensory neuron takes the message to the Central Nervous System. (spinal cord)
3. Interneuron passes the message to a motor neuron.

44. 4. Motor Neuron takes the message away from the C. N. S
4. Motor Neuron takes the message away from the C.N.S. to the effector (muscle/organ)
5. The muscle receives the message and contracts.
***The brain finds out later what had happened***