1. Neurons The two principal cell types of the nervous system are:
Neurons – excitable cells that transmit electrical signals.
There are 150 different types.
Vary in function and size.
Supporting cells (neural glia)– cells that surround and wrap neurons.
Oligodendrocytes, Schwann cell, Astrocytes

2. Supporting Cells: Neuroglia
The supporting cells
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth

3. Cells of the CNS

4. Fundamental Types of Neurons
Sensory (afferent) neurons
detect changes in body and external environment
information transmitted to brain or spinal cord
Interneurons (association neurons)
lie between sensory and motor pathways in CNS
90% of our neurons are interneurons
Integration and retrieving information
Motor (efferent) neuron
send signals out to muscles and gland cells
organs that carry out responses called effectors

6. Structure of a Neuron Cell body (soma) Axon single, central nucleus
contains many multi-branched dendrites
Which receive signals from other neurons.
Axon
(nerve fiber) arising from axon hillock for rapid conduction
Axon terminals release will neurotransmitters that communicate a chemical message to another nerve or muscle

7. Neuron Physiology Living nerve cells are polarized
The inside of the cell (Intracellular fluid) ICF is negatively charged compared to the outside of the cell extracellular fluid (ECF)
The cell is able to maintain a resting membrane potential of -70 mV (negative charge on the inside of membrane by active transport and specific voltage gated channels.

8. Resting Membrane Potential

9. Resting Membrane Potential
The cell membrane is considered a semi-permeable membrane which selectively allows things in and out of the cell.
Large negatively charged molecules found in the ICF such as proteins and phosphates are confined to the inside of the cell. The membrane is impermeable to these molecules which contributes to the ICF maintaining RMP of -70mV

10. Ionic Basis of Resting Membrane Potential
Na+ concentrated outside of cell (ECF)
K+ concentrated inside cell (ICF)

11. Basis of the Resting Membrane Potential
Since Na+ ion are more concentrated in the ECF when a specific voltage gated Na+ channel opens Na+ will always rush into the cell by diffusion.
Since K+ ion channels are more concentrated in the ICF when a specific voltage gated K+ channel opens K+ will always rush out of the cell by diffusion
In order to keep the resting membrane potential at –70 mV the cell is constantly hydrolyzing ATP with the Na+,K+-ATPase pump.

12. Resting Membrane Potential

13. Chemical Excitation

15. Depolarization 22

16. Repolarization 24

18. Action Potentials (APs)
There are 3 phases to an AP:
Depolarization
a reduction in the polarity of the membrane potential by allowing Na+ to enter the cell.
Repolarization
membrane potential returns towards the resting value closing Na channels and opening K+ channels. K+ travels along its concentration gradient out of the cell returning the inside of the cell to a negative value.
Hyperpolarization
Slow closing K+ channels cause the inside of the cell to be more negative than the resting value
All APs have the same magnitude regardless of the size of the stimulus

19. Action Potentials

20. The Refractory Period Absolute refractory period
as long as Na+ gates are open
no stimulus will trigger AP
Relative refractory period
as long as K+ gates are open
only especially strong stimulus will trigger new AP
Refractory period is occurring only to a small patch of membrane at one time (quickly recovers)

21. Impulse Conduction in Unmyelinated Fibers
The action potential in trigger zone begins impulse
Nerve signal (impulse) - a chain reaction of sequential opening of voltage-gated Na+ channels down entire length of axon
This is a very slow process. (2 m/sec)
We need something to speed the process up.

22. Impulse Conduction - Unmyelinated Fibers

23. Saltatory Conduction - Myelinated Fibers
The velocity of an action potential propagates along the length of the axon depends on:
axon diameter
The larger the diameter of the axon the greater the velocity of the action potential travels along the axon to the axon terminal.
Myelin sheath increases the diameter of sections of the axon which dramatically increases impulse speed. (120 m/sec)

24. Myelin Sheath
Myelin is a white, fatty insulating covering around most of the long axons. It plays an important role in both conduction velocity and protection of the axon.
In the CNS Oligodendrocytes can myelinate many different neurons.
In the PNS Schwann cells are can only myelinate a portion of one axon.

25. Diseases of the Nervous System
What would be worse?
A disease that attacks neurons of the CNS
A disease that attacks neurons of the PNS
What are the deficits one might expect to see if the neurons loose their Myelination?

27. Saltatory Conduction
Notice how the action potentials jump from node of Ranvier to node of Ranvier.

28. Presynaptic Neurons Presynaptic neurons
Nerve signal(AP) opens voltage-gated calcium channels allowing it to diffuse into the synaptic knob.
Calcium triggers the release of a neurotransmitter such as acetylcholine (Ach) from the (synaptic vesicles).
The neurotransmitters are released into the synaptic cleft.

29. Postsynaptic Neuron
Neurotransmitters diffuse across the synaptic cleft binding to ligand-gated channels on the postsynaptic neuron.
Graded Potential
Specific neurotransmitters can influence the permeability of the voltage gated channels.
This influences the post synaptic neuron to become more likely to generate an AP (depolarization) or less likely (hyperpolarization).

30. Postsynaptic Potentials- EPSP
Excitatory postsynaptic potentials (EPSP)
a positive voltage change causing postsynaptic cell to be more likely to fire
result from Na+ flowing into the cell
glutamate and aspartate are excitatory neurotransmitters

31. Postsynaptic Potentials- IPSP
Inhibitory postsynaptic potentials (IPSP)
a negative voltage change causing postsynaptic cell to be less likely to fire (hyperpolarize)
result of Cl- flowing into the cell or K+ leaving the cell
glycine and GABA are inhibitory neurotransmitters

32. The membrane potential of a real neuron typically undergoes many EPSPs (A) and IPSPs (B), since it constantly receives excitatory and inhibitory input from the axons terminals that reach it.

33. Summation - Postsynaptic Potentials
Net postsynaptic potentials in trigger zone
firing depends on net input of other cells
The trigger zone takes in both EPSP and IPSPs. If there are more EPSPs threshold will be reached.
temporal summation
single synapse receives many EPSPs in short time
spatial summation
single synapse receives many EPSPs from many cells

34. Summation of EPSP’s
Does this represent spatial or temporal summation?

35. Action Potentials vs. Graded Potentials
Action Potentials (All-or-none phenomenon)
action potentials either completely, or not at all
Graded Potentials ( sub threshold)
EPSP and IPSPs are
are graded (vary in magnitude with stimulus strength)
are decremental (get weaker the farther they spread)
are reversible as K+ diffuses out of cell

36. Clinical Applications
Many drugs work by altering neuronal functions.
Block the receptor site
Beta blockers: prevent sympathetic input to the heart and various organs
Block the reabsorption of the neurotransmitter
SSRI: Prevent the reuptake of serotonin so it stays in the synaptic cleft longer and continue to stimulate nerve
Bind to the receptor site
Curare blocks the (ACh) acetylcholine receptors by binding to the same position on the receptor