Membrane Proteins

1. Constitute more than 20% of the membrane.

2. Confers on neuron many characteristics that distinguish it from other cells.

3. Different proteins have different properties and this gives a neuron its specific features.

4. Neurons can change the type of proteins and thus their functional characteristics.

5. Proteins can be grouped into several categories.

Transport Proteins

Ion Channel - allows the passive flow of ions.
Cylindrically shaped hollow core proteins (.3-.6 nm).

Sensitivity of neuron depends upon the types and numbers of transport proteins

Modification by neuron is essential component in the adaptability of neurons.

Ion channel consists of several chemically similar sub-units, each composed of helical strands that cross the membrane.
These are connected by chains of amino acids which help confer the special properties.

Leakage channels - open to ion flow all of the time.

Presence ensures a continual movement of ions.

Conformational change - Open or close through a structural change.

Ligand gated channel - responds to signaling molecule.

Voltage gated channel - regulated by difference in electrical potential across membrane.

Stretch sensitive channel - responds to a mechanical force.

Connexons - gap junctions between neurons (electrical synapses).

Ion Pumps

Expends energy (ATP) to move one or more ions across membrane against the chemical gradient.
1. Sodium pump or sodium-potassium exchange pump (3K+ vs 2Na+)

2. One molecule of ATP is hydrolyzed to adenosine diphosphate (ADP) for power.

3. 25-40% of brain metabolic activity is to power the transfer.

Signaling Proteins

1. Some imput in release of transmitter substance.

2. Others for message reception

Receptors

1. Highly concentrated in post-synaptic membrane.

2. Trans. Substance is signaling molecule and cause pore to open.

3. Classified by response they cause in the post-synaptic membrane.

a. Ionotropic receptors - open for ion flow.

b. Metabotropic receptors - a biochemical cascade - takes longer to develop but lasts longer.

Proteins and Other Enzymes

First protein activated by the metabotropic receptor is guanine nucleotide binding protein or G protein.
Initiate a biochemical sequence and thus a response in the neuron.

Binding Proteins

Help hold cells together

Subcellular Elements

1. Microfilaments - 5 nm diameter - important in development.
   
   Form loose network just under the membrane.

2. Neurofilaments - 10nm
   
   Twisted coils of protein with opening in center.
   Found in dendrites and axons but most in axons.
   Help provide structural support
   Form dense tangles in Alzheimer's patients.

3. Microtubules - 23-25nm - hollow
   Composed of protein - tubulin
   Used for long distance transport.
   In state of dynamic equilibrium.
   Polarized - end near cell body negative and closed.
   Other end positive and open.

Axonal Transport

Anterograde transport -- .5 to 400 mm/day.
Retrograde transport - 200 mm/day.
All synthesis of proteins takes place in the cell so receptors, signaling proteins, and binding proteins, as well as enzyme for transmitter synthesis must all be moved from some to site of use.

Mechanisms of Transport

Needs energy (Hydrolysis of ATP) and microtubules for transport.
Kinesin - a motor protein powers anterograde transport.
7 families of proteins
Different proteins move different materials
Kinesin molecule has head and 2 tails.
Head to molecule and tails to microtubule
Moves 8 nm per step.

TERMS you should know