Chapter 2

Structure and functions of cells of the nervous system

Cells of the Nervous System

Supporting Cells Glia (glial cells) - Supporting cells that “glue” the

nervous system together; 3 most important types are: Astrocytes Oligodendrocytes Microglia

Summary: Things to think aboutSummary: Things to think about

Membrane potentialsMembrane potentials Lipid bilayer Ion types (cations and anions contributing) Distribution of ions across the membrane Membrane proteins

Channels Pumps/transporters:

Passive vs active movement of ions

Action potentialsAction potentials Threshold Temporal explanation of ion movement across the

membrane.

An Action PotentialAn Action Potential

Temporal and sequential importance of ion transfer across the membrane.

Dependent on voltage-gated (dependent) channels

Figure 2.21

Factors Influencing Conduction Factors Influencing Conduction VelocityVelocity

Saltatory conduction High density of Na+ V-D at

Nodes of Ranvier

2 advantages of Saltatory Conduction

Economical Much less Na+ enters cell

(only at nodes of Ranvier) mush less has to be pumped out.

Speed Conduction of APs is faster

in myelinated axons because the transmission between the nodes is very fast.

Communication Between Communication Between NeuronsNeurons

Some Simple VocabSome Simple Vocab

Details of a SynapseDetails of a Synapse

Figure 2.28 Figure 2.28

The SynapseThe Synapse

Synaptic Synaptic transmissiontransmission- transmission of signal from one cell to another

Neurotransmitter Postsynaptic

potentials Excitatory Inhibitory

Scanning electron micrograph (real) shows the synapses between nerve fibres (purple) and a nerve cell (yellow). Magnified 10,000 times. NOVA

Release of Release of NeurotransmittersNeurotransmitters

VesiclesVesicles

After synthesis, NTs are stored in vesicles (lipids).

Varying numbers of vesicles at the button

Terminal button could contain both large and small sized vesicles

Small, clear

Large, dense core

False colour electron micrograph

Scanning electron micrograph- nerve ending (broken) with vesicles

Small vesicles (neurotransmistters)Small vesicles (neurotransmistters) Synthesized in the terminal button and

packaged in synaptic vesicles

Large dense core (typically Large dense core (typically neuropeptides)neuropeptides) Assembled in the cell body, packaged in

vesicles, and then transported to the axon terminal.

Vesicle and Release ProteinsVesicle and Release Proteins

Vesicle Transporters: Get substances into vesicles Each vesicle: 1000s

NT molecules

Trafficking Proteins: Docking Release Recycle

Vesicle PoolsVesicle Pools

Very few vesicles are docked (<1%) Most in the reserve pool (85-90%) Recycling pool (10-15%)

Neurotransmitter ReleaseNeurotransmitter Release

The arrival of an AP at the terminal opens dependent Ca2+ channels

The entry of Ca2+ causes vesicles to fuse with the terminal membrane and release their contents

Exocytosis Exocytosis

Release of NeurotransmittersRelease of Neurotransmitters

Figure 2.31

Release of NeurotransmittersRelease of Neurotransmitters

Figure 2.31

1.Synaptic vesicle migrates to presynaptic membrane.

Docked

Release of NeurotransmittersRelease of Neurotransmitters

Figure 2.31

2. Vesicle fuses with presynaptice membrane.

Release of NeurotransmittersRelease of Neurotransmitters

Figure 2.31

3. Neurotransmitter is released into the synaptic cleft.

Recycling of vesicle material (<1sec)

1.Kiss and Run (leave) Release most NT,

reseals and moves into cytoplasm to be refilled

2.Merge and Recycle Vesicle fuses completely

with the membrane

3.Bulk Endocytosis Large pieces of the

membrane fold in to reform vesicles

Figure 2.33

Vesicles After ReleaseVesicles After Release

Pos

Activation of Activation of ReceptorsReceptors

I. Postsynaptic Receptors

I. Postsynaptic receptorsI. Postsynaptic receptors

• Released NT molecules produce signals in postsynaptic neurons by binding to receptors• Receptors are specific for a given NT

Ligand Ligand – a molecule that binds to another

A NT is a ligand of its

receptor

1) 1) Ionotropic Ionotropic ReceptorsReceptors

Figure 2.34

Receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site.

Ionotropic Ionotropic ReceptorsReceptors

NT binds and an associated ion channel opens or closes, causing a PSP

If Na+ channels are opened, for example, an EPSP occurs

If K+ or Cl- channels are opened, for example, an IPSP occurs

Inhibitory e.g.Inhibitory e.g.BZP receptors BZP receptors (hyperpolarize(hyperpolarizes)s)

Excitatory Excitatory e.g.e.g.Nicotinic Nicotinic (N1) (N1) receptors receptors (depolarizes(depolarizes))

2) Metabotropic Receptors2) Metabotropic Receptors• Slower variety (short cut faster than second messenger system)• Actions are reliant on activation of G-proteins located in the internal membrane of the postsynaptic cell• 2 basic varieties: 1) short cut 2) second messenger

1) Short cut 2) Second messenger

Figure 2.35

Ionic Movement During Postsynaptic Potentials

Figure 2.36

1) REUPTAKEREUPTAKE Mediated by transporter molecules on

neurons and glia After it is taken up it may be degraded or

recycled in vesicles

Figure 2.37

2) ENZYMATIC 2) ENZYMATIC DEGRADATIONDEGRADATION

Removal at the cleft E.g. Cholinergic synapses

(ACh)

Neuromuscular junction

ED can occur in the synapse or in the cytoplasm

Used to recycle: ACh -> choline by ACh-

esterase (AChE)

3) DIFFUSION3) DIFFUSION

Away from the synapse Glia cells

Transporters for uptake

II. II. AutoreceptorAutoreceptorss

Sensitive to neurotransmitter released by presynaptic terminal

Act as safety valve to reduce release when levels are high in synaptic cleft (autoregulation)

Excitatory Post-Synaptic PotentialExcitatory Post-Synaptic Potential

• Transmitter causes the receptor sites to open gated ion channels that permit Na+ into the cell (depolarizing event)

• Known as an EPSP

Inhibitory Post-Synaptic PotentialInhibitory Post-Synaptic Potential

Transmitter causes the receptor sites to open gated ion channels that permit K+ out of the cell or Cl- into the cell (hyperpolarizing event)

Known as an IPSP

INTEGRATION of Input SignalsINTEGRATION of Input Signals

1. Spatial Summation2. Temporal Summation

SPATIAL SUMMATIONSPATIAL SUMMATION 1. Summation of EPSPs1. Summation of EPSPs

Two distinct synaptic inputs onto postsynaptic cell• Same time• EPSP + EPSP = larger EPSP• Cell is depolarized

+

+

SPATIAL SUMMATIONSPATIAL SUMMATION 2. Summation of IPSPS2. Summation of IPSPS

• Two independent inhibitory inputs• Postsynaptic cell hyperpolarized

-

-

SPATIAL SUMMATIONSPATIAL SUMMATION 3. Summation of EPSP 3. Summation of EPSP and IPSP and IPSP

EPSP (depolarizing) and IPSP (hyperpolarizing) inputNot net change in membrane potential

+

-

TEMPORAL TEMPORAL SummationSummation

Single synapse initiating a sequence of membrane events

Presynaptic InhibitionPresynaptic Inhibition• Axoaxonic- decreases NT released• Presynaptic facilitation can occur also (increasing

NT released)