Outline-2. BIO 3360, Neural Function

I. Neurons and Glial Cells comprise the Nervous System

A. Overall function –Communication:receive information, process it and transmit it via electrical and chemical signals. Also, function in stimulus-response mechanisms (reflexes)

B. Typical Neuron– dendrite for signal reception, cell body for signal integration, axon for signal conduction, & axon terminal for signal transmission

1. Cell body

a. Nuclei Groups of cell bodies in CNS

b. Ganglia Groups of cell bodies in PNS

2. Cytoplasmic Processes

a. Dendrite Receptive end

b. Axon (nerve fibers)Conducting end

3. Nerve Bundle of axons in PNS

C. Neuroglia“Nerve glue”, supporting and protecting tissue

1. Schwann Cells PNS

a. Myelin

1. White matter

2. Function – Insulate and greatly speed up rate of nerve impulse transmission

b. Regeneration of axons [1-5 mm/day]Possible due to Schwann cells providing structural scaffold. Slow process.

2. Oligodendrocytes Myelin production in CNS

3. Astrocytes Maintenance of blood brain barrier

II Electrical signals in neurons

A. Polarized; Depolarized; Hyperpolarized –resting membrane potential exists; inside of cell becomes less negative; inside of cell becomes more negative. All cells have a resting membrane potential but only nerve cells and muscle cells can change this RMP by depolarization and hyperpolarization.

B. Resting membrane potential can change –changes in the membrane potential are the result of changes in the permeability of the membrane to ions

1. Graded potentials –for short distances

2. Action potentials

C. Na+ and K+ membrane channels

1. K+ channels are nearly always open

2. Gated channels –these are channels that open and close

a. Chemically regulated channels –ligand regulated and associated with local potentials

b. Mechanically regulated channels –also associated with local potentials

c. Voltage regulated channels –associated with action potentials and only present in muscle and nerve cells

D. Review of Resting Membrane Potential and the Na+/K+ pump

E. Graded Potentials– change in resting membrane potential resulting in either depolarization or hyperpolarization

1. Localized

2. Magnitude varies/graded

3. AKA Receptor potentials, Synaptic potentials, Pacemaker potentials

4. Stimulus results in change in membrane permeability

a. Light stimulating vertebrate photoreceptors result in hyperpolarization because membrane becomes LESS permeable to Na.

b. Light stimulating invertebrate photoreceptors results in hypopolarization or depolarization due to the membrane becoming MORE permeable to Na.

5. Graded potentials decrease with distance –therefore graded potentials are important signaling over short distances

F. Action potentials

1. Very quick, very large and can propagate signals over long distances

2. Only neurons and muscle cells

3. Depolarization to threshold– positive feedback loop

4. Neurons & muscle cells have voltage-gated ion channels

5. Action potential results from transient changes in the membrane permeability to Na+ and K+ due to opening and closing specific ion channels

6. At rest,  K+ leak channels are open but few Na+ channels are open

7. Depolarization: The stimulus causes Na+ channels to open and since they are voltage regulated gates, this stimulates more to open and at about -55mV, threshold, this becomes a run away cycle with numerous Na+ channels opening and Na+ flooding into the cell so that it actually becomes positive inside the cell.

8. Repolarization occurs because the Na+ channels are only open for a short time but the K+ channels open and positive potassium ions leave the cell – in fact it hyperpolarizes the cell for a short time until all of the channels close.

9. Na/K pump returns ion concentrations to the resting levels

10. Action potentials are “All or None

Question: If AP are all or none, how can you determine a weak stimulus from a strong stimulus?

11. Refractory period

a. Absolute refractory period– no stimulus can trigger another AP. This is due to Na+ channel inactivation

b. Relative refractory period– A stimulus that is very strong may trigger another AP. This is due to K+ channels still being open

G. Propagation of Action Potentials =Nerve Impulse

1. Unmyelinated axons (gray)

a. Current flow

b. Unidirectional based on origin of AP & the fact that AP’s cannot go backwards due to refractory periods

c. Slow ~ 2mph

2. Myelinated axons (white) –a fatty electrical insulator

a. Myelin made by oligodendrocytes in CNS and Schwann cells in PNS

b. Not continuous, gaps are called Nodes of Ravier

c. Saltatory conduction– leaping conduction from one node to the next and can be up to 250 mph

d. Newborns have incomplete myelination

e. Largest myelinated axons have fasted conduction because it is easier for ions to move through large axons