Signalling in the nervous system L9 Flashcards
what is the nervous system
A complex network of interconnecting neurones
Neurones –electrically excitable cells which release neurotransmitters
what are the two types of signalling
- electrical: action potentials
- Site for drug action - chemical: neurotransmitters
what are the different types of neurones
sensory
- pseudo unipolar or bipolar
intermediate
- multipolar
motor
- multipolar
what features do all neurones have
- cell body
- dendrites
- axon hillock
- axon
- terminal
describe the permeability of the major ions
Na+: impermeable
K+: membrane is permeable (leak)
Cl-: membrane is permeable
A- (large anions): fixed inside
why may positive ions that are permeable not distribute equally
the concentration gradient (chemical force) is in competition with the electrical (ionic) force
the positive ions will be attracted to negative ions that are impermeable
Ions diffuse in accordance with the combined forces of concentration gradient and ionic gradient- electrochemical gradient
what are the two forces acting on ion flux
- Chemical gradient: ions move from areas of high concentration to areas of low concentration.
- Electrical gradient: ions move to areas of opposite charge.
what do the sum of chemical and electrical gradient determine
the net electrochemical gradient acting on an ion.
what does Na+/K+ ATPase do
pumps 3 Na+ out for 2 K+ in so Na is concentrated outside cell whilst K+ is concentrated inside the cell
describe movement of K+
The membrane is partly permeable to K+ ions and they tend to move down their concentration gradient
but they are forced to the inside by the ionic gradient and also transported by the Na+/K+ ATPase
describe the movement of Cl-
The membrane is partly permeable to Cl- ions and they tend to move down their concentration gradient into the cell
but they are forced to the outside by the ionic gradient
what is the composition of ions during resting membrane potential
Na and Cl outside cell
K and A- are inside cell
- inside is negative and outside is positive
how do neurones alter their membrane potentials
they have voltage gated channels that open when membrane reaches certain voltage
describe the action potential
- voltage gated Na+ channels open at -55mv (threshold)
- this allow more Na+ to enter causing rapid depolarisation
- membrane potential rises
- depolarisation - voltage gated Na channels become inactive after 1ms
- Na ions pumped out by Na/K ATPase
- membrane potential falls
- repolarisation - when membrane is slightly depolarised, voltage gated K+ channels open
- K goes out cell
- reduces membrane potential
- Movement of K+ is mainly responsible for speeding up repolarization - K+ channels close slowly
- Permeability to K+ is greater than at rest so K+ continues to leave the cell
- Membrane potential falls below RMP
- This is the ‘after hyperpolarization’
how does action potential propagate
saltatory conduction
