1. P.
Ch 48 – Nervous System pt 1

2. Peripheral nervous system (PNS) Central nervous system (CNS)
Overview
Sensor
Effector
Sensory input
Motor output
Integration
Peripheral nervous system (PNS)
Central nervous system (CNS)

3. Types of neurons
Sensors detect external stimuli and internal conditions and transmit information along sensory neurons
Sensory information is sent to the brain or ganglia, where interneurons integrate the information
Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

4. Parts of a neuron Dendrites Stimulus Axon hillock Nucleus Cell body
Presynaptic cell
Signal direction
Axon
Synapse
Neurotransmitter
Synaptic terminals
Postsynaptic cell
Parts of a neuron

5. Evolutionary Adaptation of Axon Structure
The speed of an action potential increases with the axon’s diameter
In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase
Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

6. Axon
Myelin sheath
Schwann cell
Nodes of Ranvier
Node of Ranvier
Layers of myelin
Nucleus of Schwann cell

7. Nerve Signals
Membrane potential - the electrical charge difference across a membrane
Due to different concentrations of ions in & out of cell
Anions more concentrated inside, Cations more concentrated outside
Resting potential – the membrane potential of an unstimulated neuron
About -70 mV (more negative inside)

8. Changes in membrane potential act as signals
Concentration of Na+ highest outside, concentration of K+ highest inside cell.
Sodium –potassium pumps use ATP to maintain concentration gradients
Energy is potential chemical energy

9. A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell
The resulting buildup of negative charge within the neuron is the major source of membrane potential

10. What determines the membrane potential?

11. Key
Na K
Sodium- potassium pump
Potassium channel
Sodium channel
OUTSIDE OF CELL

12. Maintenance of Resting Potential
- Note gradients for sodium and potassium
- To maintain resting potential, you need to keep concentration gradients constant
- ions have a charge, so need facilitated diffusion through membrane (is this with or against the gradient?)
- there are selective ion channels that allow “leakage”

13. Maintenance of Resting Potential
To keep sodium & potassium in right gradients, sodium-potassium pump uses ATP to maintain gradients
The sodium-potassium pump pumps 2K+ in and 3Na+ out each time.

14. What is the result of changes in membrane potential?
Graded potentials – based on magnitude of stimulus
Action potentials – all or nothing depolarization

15. Types of ion channels: Ungated ion channels – always open
Gated ion channels – open or close in response to stimuli
Ligand gated ion channels (chemically gated) ion channels–in response to binding of chemical messenger (i.e. neurotransmitter)
Voltage gated ion channels – in response to change in membrane potential
Stretch gated ion channels – in response to mechanical deformation of plasma membrane

16. Graded potentials
The magnitude of the change (in polarization) varies with the strength of the stimulus
These are not the nerve signals that travel along axons, but they do have an effect on the generation of nerve signals

17. Hyperpolarization
Stimulus
Threshold
Resting potential
Hyperpolarizations
50
50
Membrane potential (mV)
When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative
This is hyperpolarization, an increase in magnitude of the membrane potential

18. Depolarization
Stimulus
Threshold
Resting potential
Depolarizations
50
50
100 1 2 3 4 5
Membrane potential (mV)
Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential
For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell

19. Graded potentials occur up to a particular voltage, the threshold voltage
If depolarization reaches the threshold voltage, then an action potential is triggered.

20. Action Potentials
Signals conducted by axons, transmitted over long distances
Occur as the result of gated ion channels that open or close in response to stimuli
A specific dynamic change in the charge across the membrane of a cell, one that occurs either totally or not at all (“all or nothing”

21. Action Potential Steps: 1) resting state 2) threshold
3) depolarization phase
4) repolarization phase
5) undershoot
Threshold
Resting potential
50
50
100 1 2 3 4 5
Membrane potential (mV) 6
Action potential

22. Resting state
Most voltage gated sodium(Na+) and potassium gated (K+) ion channels are closed, membrane potential is at -70 mV

23. Threshold
An electric stimulus causes the voltage-gated Na+ channels open first and Na+ flows into the cell
The flow of Na+ causes more voltage – gated Na+ channels to open, so more depolarization
(what kind of feedback is this)
If this depolarization reaches threshold potential, then more Na+ gates open

24. Depolarization phase
Membrane potential rises rapidly as sodium ions rush into cell
Also known as “rising” phase

25. Repolarization phase
The Na+ voltage gates close shortly after opening, so Na+ inflow stops
The K+ voltage gates open, so K+ rushes out
There is a loss of positive charge, so cell returns to resting state
“Falling” phase

26. Undershoot
The membranes permeability to K+ is higher than at rest, so membrane potential dips down lower than resting potential
K+ gates close, so membrane potential returns to normal

27. Refractory period
After an action potential occurs, there is a period of time when another action potential cannot be triggered
During undershoot – sodium channel inactivation gates are closed – haven’t had enough time to reopen yet

28. Membrane potential (mV) 50
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
Sodium channel
Potassium channel
Action potential
Threshold
Resting potential
Time
Membrane potential (mV)
50
100
50
Na K
Key 2 1 3 4 5
Resting state
Undershoot
Depolarization
Rising phase of the action potential
Falling phase of the action potential

30. How do action potentials “travel” along a neuron?
Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membrane
Action potentials travel in one direction – towards synaptic terminals

31. Action potentials are self-propagating across the axon.
Plasma membrane
Action potential 1
Cytosol
Na
Action potentials are self-propagating across the axon.

32. Axon Plasma membrane Action potential 1 Cytosol Action potential 2 NaK 2
Na K

33. Axon Plasma membrane Action potential 1 Cytosol Action potential 2
Na
Action potential K 2
Na K
Action potential K 3
Na

34. Why doesn’t it travel backwards?
The refractory period is due to inactivated
Na+ channels, so the the depolarization can only occur in the forward direction.

35. Speed of action potentials
Speed is proportional to diameter of axon, the larger the diameter, the faster the speed
Several cm/sec – thin axons
100 m/sec in giant axons of invertebrates such as squid and lobsters

36. http://www.youtube.com/watch?v=omXS1bjYLMI Nerves with giant axons
Ganglia
Brain
Arm
Nerve
Eye
Mantle
Nerves with giant axons

37. Speeding up Action potential in vertebrates
Myelination (insulating layers of membranes) around axon
Myelin is deposited by Schwann cells or oligodendrocytes.
Cell body
Schwann cell
Depolarized region (node of Ranvier)
Axon

38. Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found
Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction