1. Warm Up
What is actually occurring inside the cell when a neuron has an “impulse” traveling through it?

2. Action Potential
Chapter 7 (pg )

3. Objective Checklist
Describe the events that lead to the generation of a nerve impulse and it conduction from one neuron to another.
Explain how the sodium-potassium pump restores the resting membrane potential.
Define reflex arc, and list its elements.

4. Functional Properties of Neurons
Irritability – ability to respond to stimuli
Conductivity – ability to transmit an impulse

5. Neurons at Rest
The plasma membrane at rest is polarized (negatively charged)
Fewer positive ions are inside the cell than outside
Inside = potassium (K+)
Outside = sodium (Na+)

6. Initiating a Nerve Impulse
Many different types of stimuli can excite neurons to become active and generate an impulse
Examples:
Light excites eye receptors
Sound excites ear receptors
Pressure excites cutaneous receptors

7. Initiating a Nerve Impulse
A stimulus causes sodium (Na+) channels to open.
Sodium ions diffuse into the cell, making the inside of the cell more positive.
This area of the membrane is said to be depolarized.

8. Conducting a Nerve Impulse
Voltage-gated sodium channels respond to a change in charge
Local depolarization causes adjacent sodium channels to open.
The depolarization travels down the nerve axon and is known as an action potential.

9. Aside: All or Nothing The nerve impulse is an all-or-none response.
It does not occur unless the local depolarization is strong enough.
It is either propagated over the entire axon, or it doesn’t happen at all.

10. Restoring Resting Potential
Potassium ions (K+) rush out of the neuron after sodium ions rush in, which repolarizes the membrane.
Repolarization restores the negative charge inside the cell.
Until repolarization occurs, a neuron cannot conduct another impulse.

11. Restoring Ion concentrations
After repolarization occurs, the initial concentrations of the sodium and potassium ions are restored using sodium-potassium pumps.
ATP is used to pump excess sodium ions out of the cell and to bring potassium ions back in to the cell.
Three sodium ions are pumped out; two potassium ions are pumped in.
Animation: Nerve Impulse
Animation: Na/K Pump

12. Transmitting Impulses to other Neurons
Neurotransmitters are released from the axon terminal
They cross the synapse and stimulate dendrites on an neighboring nerve
A nerve impulse is an electrical and a chemical event. Why?

13. Saltatory Conduction
Impulses travel faster when fibers have a myelin sheath.
That is because electrical current cannot flow across the myelin.
The impulse jumps from node to node, a process known as saltatory conduction.

14. Introduction to Reflexes
Reflex – rapid, predictable, and involuntary responses to stimuli
Reflex arc – the neural pathway that a reflex travels; involves the PNS and CNS

15. Types of Reflex Arcs Autonomic reflexes Somatic reflexes
Regulate activity of smooth muscle, the heart, and glands
Examples: salivary reflex, pupillary reflex*
Somatic reflexes
Activation of skeletal muscles
Examples: patellar reflex, withdrawal reflex
The * denotes a reflex that requires brain involvement, which is uncommon

16. Examples of Somatic Reflex Arcs
Which one of these do you think takes longer? Why?
Most reflex arcs have five elements and require interneurons

17. Importance of Reflex Arcs
Most reflex arcs do not require communication with the brain.
Reflex transmission is important in evaluating the condition of the nervous system.
Changes in reflex arcs often occur before other symptoms of disease have become obvious.

18. Objective Checklist
Describe the events that lead to the generation of a nerve impulse and it conduction from one neuron to another.
Explain how the sodium-potassium pump restores the resting membrane potential.
Define reflex arc, and list its elements.