Download Transport in dendrites can also occur. The mechanisms are similar

Transport in dendrites can also occur. The mechanisms are
similar to axonal transport but involves different kinesin
motor proteins.
Energy production in the nervous system – Neurons in the
vertebrate brain make use of the oxidation of glucose to
generate ATP as a source of energy that can be used to
promote energetically unfavourable reactions. Most
glucose is completely oxidised to CO2 and H2O which
means oxygen is also required. Glycolysis occurs in the
cytosol while the TCA cycle and the ETC occurs in the
mitochondria. This is where the main ATP production takes
place. A widespread distribution of mitochondria and glycolytic enzymes in neurons ensures that
ATP production can occur locally near the sites of energy use. The major use of ATP in neurons is to
provide energy to establish and maintain ionic gradients across nerve cell membranes. The main
protein involved in maintaining ionic gradients is the Sodium potassium ATPase. This moves sodium
out of the cell and potassium in to the cell against their concentration gradient. ATP provides the
energy for this. Other uses of ATP include the synthesis and transport of proteins and membranes,
the synthesis of neurotransmitters and for intracellular signalling.
Electrical signalling – All electrical signalling involves the movement of ions in aqueous solution. The
ions involved are sodium, potassium, calcium and chloride. Electrical signals occurs across or near
the cell membranes of neurons. Non-random movement of these ions occurs because of
concentration gradients, electrical gradients and membrane transporters and pumps.
Water has a polarised structure, one side of the molecules is delta negative and the other is delta
positive. The means that water molecules interact weakly by hydrogen bond. Charged ions and polar
molecules dissolve in the wat but non-polar molecules do not because they disrupt the hydrogen
bonds and don’t replace them with other bonds.
Cell membranes – All animal cells are surrounded by a cell membrane which consists of a
phospholipid bilayer, together with other constituents. There are 4 types of constituents that are
common in the cell membrane. They are phospholipids, cholesterol, glycolipids and membrane
proteins.
1. Phospholipids – They consist of a glycerol
backbone (a 3 carbon chain), bonded to three
groups which are 2 long chain fatty acids and a
phosphate group with an attached small polar
molecule (usually choline). Long chain fatty
acids are non-polar which means that electrical
charge is distributed over the entire molecule.
They are insoluble in water and will tend to form globules when added to a watery solution.
However, the phosphate group and the attached polar molecule are highly soluble in water.
This means that phospholipids in an aqueous environment tend to align themselves that
their non-polar tails cluster together where they exclude water and the polar ends face
outwards into the water. In this way, the phospholipids form bilayers. This forms the basis of
the cell membrane.
2. Cholesterol – This molecule consists of a 4 ring structure with an attached short carbon
chain. It is very non-polar and mixes well with the non-polar fatty acid chains inside of the
phospholipid bilayer. It further increases the fluidity of the membrane and reduces it
permeability. In some part of the membrane, cholesterol may be concentrated and these
lipid rifts often contain high concentrations of membrane proteins.
3. Glycolipids – They are found only in the outer leaflet of the membrane and consist of two
hydrophobic tails with a polar sugar group making the third component. The hydrophobic
tails insert into the inner parts of the lipid bilayer and the charged sugar groups protrude out
the top into the extracellular space where they interact with water. Glycolipids are
important in cell recognition functions during growth and development of contacts between
neighbouring cells.
4. Membrane proteins – Cell membranes perform an essential function in all cells which is
separating potentially toxic chemicals in the external environment from the rather delicate
machinery on the inside. The cell needs to have supplies such as amino acids and glucose
entering the cell and waste products leaving the cell. This is achieved by specific membrane
channels, pumps and exchangers as well as other processes such as endo and exocytosis and
phago and pinocytosis. Therefore, membrane proteins are required to move molecules
between the intra and extracellular space. The proteins associated with this can be divided
up into 4 families; Receptors, ion channels, transporter proteins and structural proteins.
Receptors – have NT binding sites on the outside face of the cell.
Ion channels – make pores through the membrane to allow particular charged ions to pass
through.
Transporter proteins – help polar molecules pass in and out of the cell.
Structural proteins – Act as an anchor for intra or extracellular skeletal elements give the
cell shape despite its fluidity.
Membrane permeability – The basic structure of the membrane, based on the bilayer of
phospholipids is semi-fluid at normal body temperature. That is, the membrane has no stiffness and
is readily deformed but it does not tear easily. The membrane is very impermeable to all charged
and polar molecules and to all charged ions. Some non-polar molecules can cross very easily, CO2
and O2. Most water movement across the membrane is through ion channels or aquaporins.
Lipophilic molecule such as steroids and drugs can cross the membrane easily.
The fatty acids in the phospholipids can be either saturated meaning that there are no double bonds
or unsaturated which means there are one or more double bonds. Typically unsaturated fatty acids
do not from a straight chain which means they do not pack as tightly and make it more fluid. At least
one of the fatty acids is usually unsaturated which makes it particularly fluid.
Protein structure – The conformation of the proteins is determined by their amino acid sequence.
Some AA are polar while some are not. When several NP AA are located in a protein sequence, the
protein can span the NP part of the membrane and the proteins can cross backwards and forwards
across the membrane. The 3D part of the proteins is affected by the electrical charge across the
membrane, the pH of the medium and the NT interacting with binding sites. Phosphorylation
controls the ion channels and receptors.