1. Neural Pathways and Transmission

2. Neural Circuits
There are two basic responses to stimuli coming in from the environment
The Reflex Arc
The Neural Circuit

3. 5 basic steps to the reflex arc
The reflex arc is an automatic response which cuts out the pathway to the brain
The body responds without thinking about it
5 basic steps to the reflex arc

4. Reflex Arc
Stimulus: The receptors receive information from the environment big enough to illicit a response from the nervous system
Sensory Neurons: Carry information from the receptors to the interneurons in the spinal cord
Interneurons: transmit the message through the spinal cord immediately to the motor neurons
Motor Neurons: carry the message away from the CNS to the peripheral nervous system to illicit a muscle response
Muscle Response: When the effectors (muscles) respond to the stimuli received from the motor neurons
Ex. Hand pulls away in response to a hot burner on a stove

5. REFLEX ARC

6. The Neural Circuit
The neural circuit is very similar to the reflex arc, however, the information is carried all the way to the brain to allow for decision making and analysis of the stimuli
Once the information is relayed to the CNS via the interneurons, the information continues onto the brain
Brain decides response and carries the message back to the spinal cord via interneurons to be carried to the motor neurons and illicit response
Ex. After hand has been pulled away, respond “ouch”

7. Electrochemical Impulses
But how does information travel within the neuron to reach the CNS?
Each neuron is polarized
This means that charged particles, called ions, line the membrane of the neuron, causing a difference in charge across the membrane
Potassium and sodium ions line the membrane in unequal distribution to cause a positive exterior and a negatively charged interior
Potassium (K+) is concentrated on the interior of the cell, sodium (Na+) on the outside
When the neuron is stimulated, the charges reverse, causing the message to be carried down the neuron in one direction only

8. Neuron Ion Movement
Neuron membrane has more K+ channels than Na+ channels
Channels – facilitated diffusion
More potassium out than sodium in
Na/K Pump
Active transport
Moves K+ back into the neuron
Moves Na+ out of neuron

9. Resting State of the Neuron
At resting, sodium lines the exterior of the cell membrane, potassium lines the interior of the cell, although not in equal proportions
This creates an overall charge that is negative within the cell at rest
The internal environment is generally at –70 mV

10. Depolarization
When the stimuli elicits a response big enough to break the neuron’s threshold, the neuron will transmit the message down the axon to other neurons
This is an “all or nothing” response, firing regardless of the strength in stimuli, or not firing at all
If threshold is reached, the neuron will have a response
During this period, the membrane becomes more permeable to sodium than potassium

11.
Depolarization
Ion channels specific for sodium open within the cell membrane, allowing sodium to move into the neuron
This causes a very temporary reversal in charges, in which the interior is now positively charged, and the exterior is negatively charged
Internal environment is, on average, 30 mV at this state
This depolarization can also be referred to as the action potential, which shows the differences in charges from resting to depolarization
Once the maximum amount of sodium has diffused in, the cell begins to repolarize

13. Repolarization
During repolarization, the cell must remove the sodium from the internal environment within the cell
This is accomplished using sodium/potassium pumps, a form of active transport, requiring ATP
Repolarization only takes about 0.001s, in which the cell returns to it’s resting state
During this process, the cell cannot be stimulated again, until the resting state has been re-established

14. Hyperpolarization
For an instant after repolarization, the cell over-compensates by transporting more sodium than necessary across the membrane, called hyperpolerization
The cell becomes slightly more negative than normal
Sodium will move back across the membrane until resting state is returned (-70 mV)
Again, no new impulses can be fired at this point
The refractory period is the time after depolarization, through repolarization, and to hyperpolarization

16. Action Potential of a Myelinated Neuron (saltatory conduction)
In myelinated axons, depolarization happens only at the nodes (A) and an action potential jumps to the next node (B).
The red arrows show the direction of the nerve impulse and the
black arrows show the flow of ions.

17. How do we determine intensity of a response???
Neuron threshold and the All-or-None Response
The threshold level is the minimum level of a stimulus required to produce a response – each neuron has its own threshold level - once threshold has been met the neuron will fire
All-or-none response:
The intensity of the nerve impulse and speed of transmission remain the same
How do we determine intensity of a response???