Nerves: How do they Work? Flashcards
How does signalling occur in the body?
Chemical:
- short distance (e.g. synapse)
- long distance e.g. hormones
- relatively slow
Electrical:
- graded potentials - short distance
- action potentials - long distance (fast upto 120m/s)
Electrical nerve signal key points:
- membrane is a poor insulator in aqueous environment
- cytoplasm is relatively poor condictor
- signal decays over 2-3mm
- nerves can be long > 1m so need a propagating signal –> action potential
What is resting membrane potential?
- separation of opposite charges across a membrane
Due to:
- permeability to key ions
- concentration gradient for key ions
- gradients maintained by active transporters
All cells have a resting membrane potential
Excitable cells: nerve and muscle
- can produce rapid changes in membrane potential
Where do Na ions have a higher concentration?
Where do K ions have a higher concentration?
Which ions are more permeable?
What is the purpose of anions?
Na ions present outside the cell
K ions present inside the cell
K is most permeable, leaving sodium virtually impermeable.
Anions are inside the cell, and impermeable, so remain inside, creating a negatie charge/pool inside
What does a sodium potassium pump do?
- transports Na out
- transports K in
- ratio: 3Na out - 2K in
- helps maintain concentration gradient
- uses ATP
What ion is the membrane permeable to?
Explain the chemical and electrical gradient of this ion:
What is the equilibrium potential for Na and K?
Membrane is permeable to K ions
Chemical gradient: concentration gradient K+ flows out
Electrical gradient: protein anions cannot flow out –> K+ attracted in
Balance at equilibrium potential: chemical and electrical gradients balance out
For K = -90mV
For Na = +60mV
What is resting membrane potential?
- mainly due to effects of K and Na
- the ion with the greater membrane permeability will drive the membrane potential
- RMP will be nearer to K equilibrium potential
- Goldmans Equation
- Em = -70mV
At what voltage do sodium channels open?
What happens next?
What happens in response to rapid depolarisation overshoot?
- when graded potential or current from APs reaches -50mV, this trggers voltage gated Na channels to open rapidly
- Na flows into axon
Membrane potential rises to +30mV (depolarisation) due to rapid depolarisation and almost reaches equilibrium potential of Na
In response to depolarisation:
- inactivation of Na channels
- K channels start to open after a slight delay, leading to a rapid repolarisation to -90mV
What do LA do?
- block sodium channels and cannot get depolarisation (reversibly) so action potentials cannot pass
How are action potentials propagated?
- local current flow to adjacent area
- raises potential to threshold
- voltage gated Na channels open
- +ve feedback
- propagation along axon
Why do action potentials only travel in one direction?
Refractory period:
- Absolute refractory period
- Na channels not in resting configuration
- an action potential cannot occur
- Relative refractory period
- the K channels are open
- membrane is hyperpolarised
- an AP can occur but requires much larger stimulus
What affects nerve conduction velocity?
Width of axon:
- increase axon diameter (wide diameter = faster conduction)
- lower diameter
- faster conduction
Myelination:
- myelin sheath = insulator
- node of ranvier - exposed axon
- local current flows from node to node
- much faster conduction velocity
How are nerves classified?
- c fibres (narrow 0.2-1.5microns) 0.2-2m/s - walking
- A-delta (1.5microns) 5-30m/s - cycling
- A-beta (6-12microns) 35-75m/s - race car
- A-alpha (13-20microns) 80-120m/s - aircraft
What do each nerve fibre supply?
- A-alpha: supply muscles, take sugnals from CNS (efferent), but also from periphery to CNS (afferent)
- A-beta: mechanoreceptors - touch
- A-delta and C fibres - nociceptive pathway, afferent from periphery to CNS - sensation of pain
What is a barrier?
Selective permeability?
Passive transport/diffusion?
- a barrier defines what is inside/outside a cell
Selective permeability: property of living cell membrane that allows the cell to control which molecules pass through membrane into, or out of cell
Passive transport: process in which an ion or molecule passes through cell membrane by a concentration gradient
