3.2 nerve impulses

what is a nerve impulse

an electrochemical change (message) that travels along a nerve fibre

why is a nerve impulse considered electrochemical

involves a change in electrical voltage, brought about by changes in chemicals

define a potential difference

created by the separation of positive and negative charges. due to ions creating a negative charge inside the cell, and a positive charge outside the cell

why are the charges negative inside the cell and positive outside the cell?

innercellular fluid: high concentration of positive potassium ions AND negative organic ions

extracellular fluid: high concentration of positive sodium ions

define membrane potential

difference in concentration of ions inside and outside of the cell membrane

define resting membrane potential

membrane potential of unstimulated nerve cells, -70mV due to fluid inside cell = more negative than the outside

what are the names of the protein channels that ions move through to get through the phospholipid bilayer

leakage channels: open all the time

voltage-gated channels: only open when the nerve is stimulated

how do sodium and potassium ions move across the cell membrane

through a carrier protein called the sodium-potassium pump

explain the movement of sodium and potassium ions across the cell membrane

pump moves 2 potassium ions into the cell for every 3 sodium ions that are removed- net negative charge of inside the cell

is the movement of sodium-potassium ions in the cell passive or active?

active bc its movement is against the concentration gradient, uses ATP

define action potential

the rapid depolarisation and depolarisation of the cell membrane

what are the 3 steps in action potential

1. depolarisation

2. repolarisation

3. refractory period

describe depolarisation

-increase in membrane potential

-occurs usually at -55mV

ligand gated sodium channels open;

more sodium ions move into the cell, making the intracellular fluid less negative, increasing the potential difference

-if stimulus is strong enough, sodium ion voltage gated channels open allowing for higher influx of Na ions

describe repolarisation

closing of sodium channels (ligand gated & voltage gated), and opening of potassium ion (voltage gated) channels.

flow of sodium ions into cell decreases and flow of potassium ions out of cell increases. inside of the cell = more negative, reducing the membrane potential

what is the refractory period

when the sodium ion-voltage gated channels become inactivated after they open, they are unresponsive to stimulus. for a brief period of time the membrane does not undergo action potential

differentiate between the difference in transmission of nerve impulses along unmyelinated vs myelinated fibres

myelin sheath acts as insulator on the cell membrane, preventing the formation of an action potential. therefore the action potentials jump between the nodes of ranvier. myelinated fibres= faster travel of nerve impulses

what is it called when nerve impulses jump between the nodes of ranvier

saltatory conduction

how can you differentiate the strength of a stimulus

strong stimulus = depolarisation of more nerve fibres than a weak stimulus

strong stimulus = produces more nerve impulses in a given time than weak stimulus

explain how nerve impulses are transmitted across a synapse

-when nerve impulse reaches synapse, calcium ion voltage-gated channels open

-calcium ions flow INTO cell

-stimulates the release of neurotransmitters by exocytosis

-neurotransmittors diffuse across a synapse and bind to the receptors of the next neuron, stimulating action potential.