Lecture 8-9 — Introduction to cells of the nervous system, graded potentials and action potentials

Neurons

• Specialized to carry electrical signals and to communicate with other cells

• Can communicate with distant targets

• High density of ion channels

• Special transport mechanisms to move material from one end to the other

• Secrete signalling molecules

Cell body

• Part of neuron that holds on to nucleus and genetic material — synthesizes proteins

Axon

• Long and thin part that may be myelinated or unmyelinated

• Stretches towards other neurons

Pre-synaptic terminals

• Filled with synaptic vesicles and neurotransmitters

Synaptic cleft

• Space between neuron of interest and post synaptic cell

• Takes in the neurotransmitter

Myelin sheath

• Covers most of the axon

• Nodes of Ranvier

• Insulating layer

Node of Ranvier

• Spaces in between myelin sheath

Axon Hillock

• Initial segment, Trigger zone

• Has a high density of ion channels

• Action potentials get generated

• Site where neuron decides whether it should send an AP down the axon

Dendrites

• Emanate from the cell body

• Specialized for gathering information

• Other neurons will make synaptic contact

Efferent

• Information coming out

Afferent

• Information coming in

Oligodendrocytes

• Myelinates axons in brain and spinal cord (CNS)

• Has a number of process and forms myelin sheaths that encircle various portions of an axon

Ependymal cells

• Line ventricles, make neural stem cells (CNS)

Microglia

• Immune cells of the CNS

Astrocytes

• Blood brain barrier, trophic factors, take up excess water and K+, neural stem cells, pass lactate to neurons

• Wraps blood vessels and protects them — prevents leaks

• Trophic factors

• Release a lot of K+

• May dedifferentiate

• Pass lactate to neurons

Satellite cells

• Trophic factors (PNS)

Schwann cells

• Myelinate axons in PNS

• Includes lower motor neurons, sensory neurons

Bipolar neurons

• Has two processes

• Receives information

Pseudounipolar

• Unipolar means one projection

• In early development, looked like a bipolar neuron

• Bipolar, pretends to be unipolar

• Like a wire

Voltage gated ion channel

• Changes in MP open the channel

• Threshold

Receptor Channels

• Ligand gated ion channels

• Gate when they bind a ligand

  • Neurotransmitter, cGMP, Ca++

Phosphorylation gated

• Open or close when phosphorylated (addition of P)

Stretch gated

• Often found in sensory systems

• Pulling gates open

Temperature gated

• Changes in temperature open or close channel

Graded potentials

• Signals often communicated from one neuron to the next

  • Site of generation can be almost anywhere

• Caused by the flow of ions through a few ion channels

• Small “sub-threshold” changes MP

• Can be depolarizing or hyperpolarizing

• Proportional to the size of the stimulus

• Can be summed

• Can be short-lasting or long-lasting

Graded Potential example

• Travels like ripples on a pond

  • Moves outward from the source and degrades as it moves farther away

  • Takes time to get from synapse to the axon hillock

• Eventually degrades to nothing

  • Electrical resistance in the cytoplasm

  • Cell membrane is leaky to ions

Action Potentials

• Wave of depolarization that actively propagates across neuronal membrane

  • regenerative not passive

  • Requires the membrane to be depolarized past a threshold

  • Requires voltage-gated ion channels

• All or none

• Fast

• Large amplitude about 100mV (RMP→peak)

• ALWAYS depolarizing

• Refractory period

• CANNOT be summed

• AP generation is the axon hillock (neurons)

Ionic Basis of the Action Potential

1. RMP

2. Depolarized by graded potential

3. Membrane depolarizes towards threshold

• V-G Na+ channels open, Na+ enters cell

• V-G K+ channels open slowly

4. Rapid Na+ entry depolarizes cell

5. Na+ channels inactive and K+ channels fully open

6. K+ leaves cell, membrane potential repolarizing

7. K+ channels remain open and additional K+ leaves cell, hyperpolarizing

8. V-G K+ channels close, less K+ leaks out, Na+ channels being to recover

9. Cell returns to resting ion permeability and RMP — Na+ channels recovered

Afterhyperpolarization

• Caused by K+ leaving the cell

• Pulls inactivation gate out of the pore and closes ion channels

Refractory period

• Period of time which you cannot simulate another AP

• Absolute

• Relative

Absolute Refractor Period

• Occurs during the very beginning of the rising phase

• V-G ion channels are ALREADY opening as fast as they can

• You cannot simulate another AP during this phase

Relative Refractory Period

• Some of the channels have recovered from activation to allow another AP

• Some still haven’t

• Requires more stimulus in order to perform AP

Influence of extracellular K+

• Resting permeability is HIGH ~ 50 leak ion channels per 1 Na+

• Increase causes depolarization moving away from the threshold

• +ve charge moves out

Local Current Flow

• When a section of axon depolarizes, +ve move into adjacent sections of the cytoplasm

• +ve flows towards the depolarized region

Refractory Region

• Large amounts of K+ leave causing hyperpolarization

• Needs large amounts of +ve to override K+

Myelination characteristics

• Formed from concentric layers of glial cell membrane

• Excellent insulators

• Very few ion channels

• Increases electrical efficiency

• Thick

Saltatory Conduction

• Instead of travelling as a wave, the action jumps from node to node

• Local conduction between the nodes, ion channels activation at the nodes

• Na+ channels high concentration on the sides of nodes

AP velocity — Myelination

• Increases velocity because insulated areas have less leakage of Na+ and K+

• Myelinated conducts faster than non-myelinated axon

AP velocity — Increase Axon diameter

• Increases velocity because as the axon radius becomes larger, internal resistance decreases (inverse square realtionship)

• A large axon conducts much faster than a smaller axon

• Think of sewage pipes