3.6.2 Nervous Coordination

structure of neurone

-dendrons = collect electrical signals/impulses

-cell body = contains organelles + lots of rER

-axon = passes electrical impulses from cell body to dendrites of other cell

-axon terminals = forms synapse with other neurone

resting potential

potential difference across membrane of neurone when not stimulated

-around -70mV

how resting potential is established

-membrane is more permeable to K+ ions more than Na+ ions due to more potassium channels so K+ ions can diffuse out of cell

-however in sodium-potassium pump, 3 Na+ ions pumped out while 2 K+ ions pumped in through active transport

-also few Na+ ions move in through channels by facilitated diffusion

-so establishes electrochemical gradient as there are more positive sodium ions outside

threshold potential

-triggers action potential → increases membrane potential

-need to have enough Na+ ions inside cells to have positive potential

action potential

-is positive potential

-around +40mV

stages in generating action potential
1) stimulus + 2) depolarisation

-causes sodium channels to open so membrane becomes more permeable to Na+ so they move in more so potential becomes more positive

-if membrane reaches threshold, the voltage-gated Na+ channels open so more Na+ ions move in

stages in generating action potential 3) repolarisation

voltage-gated Na+ channels close and voltage-gated K+ channels open so K+ ions diffuse out down gradient so potential becomes more negative

stages in generating action potential
4) hyperpolarisation + 5) resting potential returned

-lots of K+ ions moving through channels quickly causing an ‘overshoot’ so potential becomes more negative than resting potential

-ions channels reset and conc gradients maintained

refractory period + importance of it

the ion channels are recovering and cannot be forced open - no stimulus is large enough to reach action potential

-ensures unidirectional action potential
-ensures discrete impulses
-limits frequency of impulse transmission

‘all-or-nothing’ principle

any stimulus that causes the membrane to reach threshold potential will generate an action potential

-all action potentials have the same magnitude → but a larger stimulus reaches threshold more quickly so greater frequency of impulses

FACTORS that affect speed of conductance -myelination

• myelin sheath = electrical insulator, made from myelin-rich membranes
  -made of Schwann cells = wrap around axon, carry out phagocytosis + nerve regeneration
  -between each, there are nodes of Ranvier which are short gaps

saltatory conduction

-in myelinated neurones = does happen, impulse jumps from node to node so does not travel the whole axon length - depolarisation only occurs at nodes

-in non-myelinated neurones = does not happen, impulse travels as a wave of depolarisation along whole length of membrane

FACTORS that affect speed of conductance -axon diameter

greater diameter = faster

-less resistance to flow of ions = quicker depolarisation

-less ‘leakage’ of ions = maintains membrane potential

FACTORS that affect speed of conductance -temperature

higher temperature = faster

-faster rate of diffusion of ions

-faster rate of respiration = more ATP for active transport for pump

-temp too high = denatured membrane proteins