Hersenen en gedrag
Deel 1 – Cellen en informatieoverdracht (college 1, 2 en 3)
Literatuur Hoorcollege 1
Kalat 1.1 – The cells of the nervous system
Neurons and Glia
The nervous system consists of two kinds of cells:
a) Neurons – receive information and transmit it to other cells.
b) Glia – cells that enhance and modify the activity of neurons in many ways.
The brain, like the rest of the body, consists of individual cells.
Santiago Ramón y Cajal, a pioneer of neuroscience
Cajal’s research demonstrated that nerve cells remain separate instead of merging
into one another. How the separate cells combine their influences is a complex and
still mysterious process.
The structures of an animal cell
Neurons have much in common with the rest of the body’s cells.
- The surface of a cell is its membrane – a structure that separates the inside of
the cell from the outside environment.
- All animal cells have a nucleus – the structure that contains the chromosomes.
- A mitochondrion – the structure that performs metabolic activities, providing
the energy that the cell uses for all activities.
Mitochondria have genes separate from those in the nucleus of cell, and
mitochondria differ from one another genetically. People with overactive mitochondria
tend to burn their fuel rapidly and overheat. People whose mitochondria are less
active than normal are predisposed to depression and pains.
- Ribosomes – the sites within a cell that synthesize new protein molecules.
Proteins provide building materials for a cell and facilitate chemical reactions.
Some ribosomes float freely within a cell, but others are attached to the endoplasmic
reticulum, a network of thin tubes that transport newly synthesized proteins to other
locations.
The structure of a neuron
The most distinctive feature of neurons is their shape. All neurons include:
- A soma (cell body) – contains the nucleus, ribosomes, and mitochondria. Most
of a neuron’s metabolic work occurs here.
- Dendrites – branching fibers that get narrower near their ends. The surface is
lined with specialized synaptic receptors, at which the dendrite receives
information from other neurons. The greater the surface area, the more
information it can receive.
, - Axon – thin fiber, which conveys an impulse toward other neurons, an organ,
or a muscle.
Although a neuron can have many dendrites, it can have only one axon, but the axon
may have many branches. The end of each branch has a swelling, called a
presynaptic terminal. At this point, the axon releases chemicals that cross through the
junction between that neuron and another cell.
a) Afferent axon – brings information into a structure (a from admit).
b) Efferent axon – carries information away form a structure (e from exit).
Within the nervous system, a given neuron is an efferent from one structure and an
afferent to another.
c) Interneuron – if a cell’s dendrites and axon are entirely contained within a
single structure.
The shape of a neuron determines its connections with other cells and thereby
determines its function.
Glia
The brain has several types of glia:
a) Astrocytes – wrap around the synapses of functionally related axons. By
surrounding a connection between neurons, an astrocyte shields it from
chemicals circulating in the surround. It also helps synchronize closely related
neurons, enabling their axons to send messages in waves. They are therefore
important for generating rhythms, such as your rhythm of breathing.
b) Microglia – act as part of the immune system, removing viruses and fungi from
the brain. They also contribute to learning by removing the weakest synapses.
c) Radial glia – guide the migration of neurons and their axons and dendrites
during embryonic development.
The blood-brain barrier
Because of the blood-brain barrier, many molecules cannot enter the brain. The
barrier protects the nervous system from viruses and many dangerous chemicals.
How the blood-brain barrier works
The blood-brain barrier consists of an unbroken wall of cells that surround the blood
vessels of the brain and spinal cord. A few small, uncharged molecules such as
water, oxygen, and carbon dioxide cross the barrier freely. So do molecules that
dissolve in fats (e.g. drugs, vitamins A and D).
For certain other chemicals, the brain uses active transport, a protein-mediated
process that expends energy to pump chemicals from the blood into the brain. these
chemicals are glucose (the brain’s main fuel), amino acids (building blocks of
proteins), and a few other chemicals.
Certain hormones, including insulin, also cross the blood-brain barrier.
,The blood-brain barrier is essential to health. However, the barrier poses a difficulty
for treating brain cancers, because nearly all the drugs used for chemo fail to cross
the blood-brain barrier.
Nourishment of vertebrate neurons
Neurons rely heavily on glucose (a sugar), the only nutrient that cross the blood-brain
barrier in large quantities. Because metabolizing glucose requires oxygen, neurons
need a steady supply of oxygen.
Although neurons require glucose, glucose shortage is rarely a problem, except
during starvation. A more likely problem is an inability to use glucose. To use glucose,
the body needs vitamin B1 (thiamine).
, Literatuur Hoorcollege 2
Kalat 1.2 – The nerve impulse
The axon regenerates an impulse at each point. Although the axon’s method of
transmitting an impulse prevents a touch on your shoulder form feeling stronger than
one on your toes, a touch on your shoulder reaches your brain sooner than will a
touch at your toes. However, your brain is not set up to register small differences in
the time of arrival of touch messages (unnecessary.)
In vision, however, your brain does need to know whether one stimulus began slightly
before or after another one. This is solved by the fact that axons from more distant
parts of your retina transmit impulses slightly faster than those closer to the brain.
➔ The properties of impulse conduction in an axon are amazingly well adapted to
your needs for information transfer.
The resting potential of the neuron
Messages in a neuron develop from disturbances of the resting potential.
When at rest, the membrane maintains an electrical gradient, also known as
polarization – a difference in electrical charge between the inside and outside of the
cell. The electrical potential inside the membrane is slightly negative with respect to
the outside, mainly because of negatively charged proteins inside the cell. This
difference in voltage is called the resting potential.
Forces acting on sodium and potassium ions
The membrane has selective permeability – some chemicals pass through it more
freely than others do. When the membrane is at rest, the sodium and potassium
(ions) channels are closed. Certain types of stimulation can open these channels,
permitting freer flow of either or both ions.
The sodium-potassium pump is an active transport that requires energy. This pump is
effective only because of the selective permeability of the membrane, which prevents
the sodium ions that were pumped out of the neuron from leaking right back in again.
When the neuron is at rest, two forces act on sodium, tending to push it into the cell.
a) Electrical gradient – sodium is positively charged, and the inside of the cell is
negatively charged. Opposite electrical charges attract, so the electrical
gradient tends to pull sodium into the cell.
b) Concentration gradient – sodium is more concentrated outside than inside, so
sodium is more likely to enter the cell than to leave it.
Because the sodium channels are closed when the membrane is at rest, almost no
sodium flows except for what the pump forces out of the cell.
Potassium is subject to competing forces:
a) Electrical gradient – tends to pull it into the cell (same as sodium).
b) Concentration gradient – potassium is more concentrated inside the cell than
outside, so this gradient tends to drive it out.
