Neurons- Lecture 4 Flashcards
Why do complex Organisms need a Nervous system?
-cells on the inside of the body are not in direct contact with the outside world
-cells live in different environments
-cells have become specialised
For the organisms to function, the activities of all these cells need to be co-ordinated.
Which Two systems to co-ordinate cell activities?
Endocrine system:
▪ specialised to secrete chemicals (‘hormones’) into the bloodstream
▪ provide slow, overall co-ordination of cell activities.
Nervous system:
▪ specialised to transmit electrical impulses between two or more cells
▪ provide fast and precise co-ordination
Define Neural Impulses
Neural impulses = basis for constant and rapid communication between cells (constant
and rapid control and adjustment of ongoing cell activities)
Discuss the Function of Neurons
Function:
-Generation & transmission of electrical impulses
-Electrical impulses reach specific targets
▪ Modifies activity of he target cells
▪ Allows selective control of specific target structures
Electrical activity modulated by integrated input from other cells
▪ ‘Input’ used to adjust ‘output’
▪ Combination and integration of signals from different sources
▪ Structured communication
Describe the Form and Size of Neurons
Special requirements: Virtually no possibility to store energy
-Therefore, glucose (sugar) and oxygen must be constantly supplied
-Without supply, neurons stop working within seconds, and die within minutes!
Life span: Neurons do not divide (develop from ‘neural stem cells’)
-Neurogenesis almost complete ~5 months after conception: after this, dead neurons usually not replaced!
-Neuron death part of normal brain development: 20-80% of all neurons die during maturation!
Describe Glia Cells
Provide ‘protected environment’ for neurons to survive
• Develop – like neurons! – from neural stem cells
• About as many glia as neurons in the brain
Describe Astrocytes
star-shaped
physical & nutritional support for neurons (‘Blood-Brain-Barrier’):
▪ transport nutrients from blood vessels to neurons
▪ waste products away from neurons
▪ hold neurons in place
take part in neural signalling (!!)
Describe microglia
small
-mobile for defensive function
▪ produce chemicals that aid repair of damaged neurons
▪ digest dead neurons (‘phago-cytosis’)
Describe Oligodendroglia
large, flat branches, wrapping themselves around axons
consist of fatty substance, insulating the axon (‘myelin sheath’)
Describe Resting Potential
Resting potential
• Ion gradients and protein channels: Ion concentrations differ between inside and outside of the cell
-If membrane were non-permeable, electrical potential would remain static (no electrical activity)
But: Protein channels in the cell membrane allow ions to enter / leave the cell
-If membrane channels were just passive ‘holes’, membrane would depolarise (electrical potential would disappear) = > again, no electrical activity
Sodium/potassium pump and membrane potential:
o Active channels work against the equilibrium
o Neurons need energy just to maintain their
=resting potential
Describe signal transmission in cells
All based on movement of electrically charged particles (ions):
o Ion-specific channels in cell membrane are ‘gates’ that can open (either by chance or in response to stimulation)
If positive ions enter (or negative ions leave): membrane depolarises (inside less negative than usually)
If negative ions enter (or positive ions leave): membrane hyperpolarises (inside more negative than usually)
Explain Electrotonic transmission: [Soma and Dendrites]
Electrical & concentration gradients sweep ions along the membrane
o Passive & Graded:
▪ Initial magnitude depends on how many ions entered
▪ Some ions will get lost on their way => signal decays
Define Action Potential
Action Potential
o Electrical & concentration gradients push/pull ions across the membrane
o Active & Not Graded: self-replicating with constant magnitude (no decay)
Explain Action Potential
Voltage gated membrane channels:
o Na+ channels open or close in response to
electrical changes at the membrane
o Sequence of events:
Start: Membrane depolarised
→ Na+ channels open
→ Na+ ions enter the cell
→ Membrane depolarises further
Describe threshold potential and the Hodgkin-Huxley cycle
o IF membrane potential at axon hillock remains below ~ -50mV
=> resting potential returns
o IF membrane at axon hillock depolarises beyond ~ -50mV
=> all Na+ channels (at axon hillock) open
=> action potential generated (‘triggered’)
Describe the Electrochemical processes during an action potential
Enough positive ions have arrived that threshold has been reached:
▪ so many Na+ ions enter the cell that inside becomes
more positive than outside (complete depolarisation)
o complete depolarisation causes
▪ Closing of Na+ channels: No more Na+ ions enter the
cell
▪ Opening of K+ channels => K+ ions rush out =>
membrane repolarises
o K+ channels close when resting potential is restored
▪ briefly, fewer K+ ions inside than outside the cell =>
membrane hyperpolarized (inside more negative
than usual)
Conduction of the action potential
o Originates at axon hillock and travels down the axon
o Each burst of depolarisation = trigger that opens Na+ channels in adjacent sections of the
membrane
o Why does the action potential not travel backwards??
▪ During hyperpolarisation (after AP has just passed through), membrane more difficult to
depolarise
▪ whereas membrane ‘in front of’ AP is still at resting potential and hence relatively more
easy to depolarise
Describe the Properties of the action potential:
o Does not decay during transmission
o Always strong enough to depolarise next area of membrane ahead of it
o ‘All-or-nothing’ phenomenon: either generated or not - can not be generated with different
intensities!
o Can not be produced continuously (minimal time bet-ween subsequent APs 2 - 5 ms)
o Fast
▪ However, for some purposes they might not be fast enough =>
Describe Saltatory conduction in Mammals
o In mammals, the axons of sensory and motor neurons are myelinated, resulting in much fast-
er transmission
▪ Myelin (fat) prevents inflow and outflow of ions
▪ Electrical charges are transported inside the axon, without the need to produce an AP (for
a rough analogy, think Newton’s cradle)
▪ Every 1 or 2 mm, myelin sheath interrupted by small gaps (Nodes of Ranvier)
▪ Only at these nodes, a new AP is generated - the action potential ‘jumps’ from node to
node
Describe Signal transmission and information:
o Electrical impulses can not be modified!
o How are different types of information ‘coded’?
▪ Qualitative: Determined by the place in the brain where the signal is received
▪ Quantitative (how strong a stimulus is): represented in a neuron’s ‘firing rate’ - a strong
input will cause the neuron to send out signals more quickly
