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 about  Membrane 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 potentials  Threshold  Temporal explanation of ion movement across the membrane.

An Action Potential  Temporal and sequential importance of ion transfer across the membrane.  Dependent on voltage-gated (dependent) channels Figure 2.21

Factors Influencing Conduction Velocity  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 Neurons

Some Simple Vocab

Details of a Synapse Figure 2.28

The Synapse  Synaptic transmission  Synaptic transmission- 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 Neurotransmitters

Vesicles  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)  Synthesized in the terminal button and packaged in synaptic vesicles  Large dense core (typically neuropeptides)  Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal.

Vesicle and Release Proteins  Vesicle Transporters: Get substances into vesicles  Each vesicle: 1000s NT molecules  Trafficking Proteins:  Docking  Release  Recycle

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

Neurotransmitter Release  The arrival of an AP at the terminal opens dependent Ca 2+ channels  The entry of Ca 2+ causes vesicles to fuse with the terminal membrane and release their contents Exocytosis

Release of Neurotransmitters Figure 2.31

Release of Neurotransmitters Figure Synaptic vesicle migrates to presynaptic membrane. Docked

Release of Neurotransmitters Figure Vesicle fuses with presynaptice membrane.

Release of Neurotransmitters Figure 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 Release

Pos Activation of Receptors I. Postsynaptic Receptors

I. 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) Ionotropic Receptors 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 Receptors 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. BZP receptors (hyperpolarizes) Excitatorye.g. Nicotinic (N1) receptors (depolarizes)

2) 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

REUPTAKE 1) REUPTAKE  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 DEGRADATION  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) DIFFUSION  Away from the synapse  Glia cells  Transporters for uptake

II. Autoreceptors  Sensitive to neurotransmitterreleased by presynapticterminal  Act as safety valve to reducerelease when levels are highin synaptic cleft(autoregulation)

Excitatory 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 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 Signals 1. Spatial Summation 2. Temporal Summation

SPATIAL SUMMATION 1. Summation of EPSPs Two distinct synaptic inputs onto postsynaptic cell Same time EPSP + EPSP = larger EPSP Cell is depolarized + +

SPATIAL SUMMATION 2. Summation of IPSPS Two independent inhibitory inputs Postsynaptic cell hyperpolarized - -

SPATIAL SUMMATION 3. Summation of EPSP and IPSP EPSP (depolarizing) and IPSP (hyperpolarizing) input Not net change in membrane potential + -

TEMPORAL Summation  Single synapse initiating a sequence of membrane events

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