1. Nerve Impulses and Reflex Arcs
1-2 December 2014

2. Neuron Functions What are the functions of the nervous system?
Receive sensory input, interpret & make a decision, and effect a response
Individual neurons have two major functional properties: irritability and conductivity.
Irritability = ability to respond to a stimulus and convert it to a nerve impulse
Conductivity = ability to transmit the impulse to other neurons, muscles, or glands.
How do irritability and conductivity relate to the functions of the nervous system?

3. Irritability
Irritability = ability to respond to a stimulus and convert it to a nerve impulse
What type of stimulus?
Receptor neurons respond to specific stimuli (e.g. touch, temperature, pressure, tension, sight, hearing, chemical)
All other neurons (interneurons, motor neurons) respond to neurotransmitters released by other neurons
What is a nerve impulse?
A quick switch in voltage potential (charge difference) across the membrane that travels all the way along the axon of the neuron
Occurs due to flow of ions across the membrane
All – or - nothing
Now to understand the process of a nerve impulse / action potential in much more detail …

4. Resting Potential
Neurons at rest have an electrochemical gradient across the cell membrane, known as the resting potential. The resting potential is approximately -70mV.
Extracellular fluid
Cell membrane
cytoplasm

5. Resting potential The resting potential is maintained by:
Sodium-potassium pump
(3 Na+ pumped out for every 2 K+ pumped in)
Greater membrane permeability of K+
(K+ can diffuse back out to some degree)
Organic anions (- ions) within cell
This ‘rest state’ takes energyto create
Extracellular fluid
Na+
Cell membrane K+
Organic anion
cytoplasm

6. Action Potential
An action potential involves the rapid depolarization and repolarization of the membrane.

7. Action potential
When a stimulus is applied to a nerve, some Na+ gates open, allowing Na+ to diffuse in.
Extracellular fluid
Na+
Na gate
Cell membrane K+
K gate
Na / K pump
Organic anion
cytoplasm

8. Action potential
Once a threshold is reached, all Na+ gates open, causing depolarization of the membrane.
When the membrane is depolarized, the inside of the membrane is more positively charged than the outside.
Extracellular fluid
Na+
Na gate
Cell membrane K+
K gate
Na / K pump
Organic anion
cytoplasm

9. Action potential
Membrane repolarization occurs when Na+ gates close and K+ gates open, allowing net diffusion of K+ outside.
Repolarization returns the membrane to resting potential (more negatively charged inside)
Extracellular fluid
Na+
Na gate
Cell membrane K+
K gate
Na / K pump
Organic anion
cytoplasm

10. Action potential
The K+ gates close and the resting potential is maintained by the Na+ / K+ pump
Watch me!
Extracellular fluid
Na+
Na gate
Cell membrane K+
K gate
Na / K pump
Organic anion
cytoplasm

11. Action Potential – Turn & Talk
Scholar with more siblings …
At rest, what ions are most abundant outside the cell, and which are most abundant inside the cell?
Which side of the membrane is more negative at rest?
Scholar with less siblings …
Describe how the movement of ions causes
Depolarization
Repolarization

12. Action Potential – Turn & Talk
Scholar with more siblings …
At rest, what ions are most abundant outside the cell, and which are most abundant inside the cell?
Which side of the membrane is more negative at rest? At rest, Na+ ions are mostly outside the membrane, while K+ and anions are mostly inside. The inside of the cell membrane is more negatively charged.
Scholar with less siblings …
Describe how the movement of ions causes
Depolarization – Na+ ions rush into the cell
Repolarization – K+ ions rush out of the cell

13. Nerve Impulse Propagation
The nerve impulse moves along the axon. The change in voltage of one area triggers the depolarization of the next area. Repolarization follows immediately.

14. Nerve Impulses
In myelinated neurons the impulse “jumps” from node to node, rather than traveling the whole length of the axon – makes the impulse transmission much more efficient.
Watch me!
..and me!
Saltatory conduction is 10-60X faster than smooth conduction

15. Irritability vs. Conductivity
Generation and propagation of nerve impulse along one neuron= irritability Conductivity is the ability of one neuron to signal another. This occurs in an entirely different fashion at the synapse, or gap, between neurons.

16. Synapses The fluid-filled space between neurons is called a synapse
Chemicals
called
neurotransmitters
carry the nerve
impulse across
the synapse.

17. Synapses The nerve impulse reaches the axon terminal.
Ca+ gates open, allowing Ca+ into the axon.
The Ca+ causes
vesicles containing
neurotransmitters
to empty into the
synapse

18. Synapses
The neurotransmitters diffuse across the synaptic cleft and binds with receptors of the next neuron.
Na+ channels open
in the dendrites of
the post-synaptic
neuron
Post-synaptic
neuron depolarizes
7. Remaining
neurotransmitter is
broken down.

19. Synapses
Pre-synaptic neuron Action potential  calcium ions release  neurotransmitter release Post-synaptic neuron Neutrotransmitter uptake  sodium gates open  action potential Note: information travels as electrical signal within neurons and as chemical signal between them
Watch me!

20. Reflex Arcs
Watch me!
Reflexes are rapid, predictable, involuntary responses to stimuli.
May be somatic or autonomic
Contain 5 elements

21. Closure
What were our objectives today, and what did we learn about them?
What was our learner profile trait and how did we use it?
How does what we did today relate to our unit question?
HOMEWORK: Quiz on Thursday / Friday!

22. Exit Ticket Count off by three.
Identify the steps that take place during an action potential. Make a diagram of the cell membrane demonstrating the changes that occur during each step.
Draw a diagram of the voltage changes that occur during an action potential. Label the diagram with the changes that occur within the cell during each step.
Identify the steps that take place during synaptic transmission. Make a diagram of the events that within the pre- and post-synaptic neurons during each step.