The Nerve Impulse

The Resting Potential

resting potential =
Potential Difference

The Action Potential

Why occurs?
Neurones...

are highly
specialised cells

are polarised

or

  • have potential difference
  • have electrochemical gradient
  • across cell surface membrane

caused by excess of +ve ions (Na+)
outside membrane compared with inside

  • outside membrane +ve relative to inside
  • of about 70mV

maintained as when neurone NOT conducting an impulse

  • cell surface membrane = IMPERMEABLE (to flow of Na+)

if permeable, Na+ diffuse IN,
down conc gradient

  1. when neurone is stimulated
  • membrane PERMEABLE to ions
  1. due to positive ions outside relative to inside
  • ions DIFFUSE INTO NEURONE
  • down conc gradient
  1. potential difference decreases to
  • THRESHOLD POTENTIAL
  • inside -55mV relative to outside
  • ∴ gated ion channels open
  1. opening of gated ion channels
  • rapidly INCREASES rate of diffusion
  • of ions INTO neurone

5.causes DEPOLARISATION

  • inside positive to outside
  • of +40mV

reversal of potential difference

  • called ACTION POTENTIAL
  1. peak of AP (inside +40mv to outside)
  • RECOVERY PHASE (refractory period) starts
  • +ve ions diffuse/pumped OUT of neurone
  1. REPOLARISATION
  • resting potential restored
  • +ve ions move OUT of neurone

repolarisation must occur before a further AP can develop
why?

  • gated ion channels are closed
  • resting potential not fully restored

entire electrochemical
sequence = 4 milliseconds

  1. HYPERPOLARISATION
  • slight overshoot
  • inside more NEGATIVE than normal resting potential
  • (more -ve than -70mV)
  1. resting potential restored
  • now axon can conduct another nerve impulse

Propagation of
Nerve Impusle

Important
Features

REFRACTORY
PERIOD

THRESHOLD
STIMULUS

ALL OR
NOTHING LAW

during repolarisation

  • membrane not depolarised again
  • until resting potential restored

FUNTIONS

ensures AP propagated in
1 DIRECTION ONLY

limits no. of AP fired
& ensures each AP discrete entity

  • axons physiologically capable transmit impulse either direction
  • uniformly excitable
  • region just generated AP = unresponsive
  • only excitable region = AHEAD
  • AP only occur
  • following completion repolarisation
  • max frequency = limited duration refractory period
  • stops AP merging
  • enables time delay before membrane depolarised again

D: level of stimulus neurone requires
before AP is produced

only at a critical point will an AP occur

  • small degree of depolarisation can occur
  • without resulting in AP

when stimulus BELOW threshold
= NO RESPONSE

when stimulus AT or ABOVE threshold
= RESPONSE to SAME LEVEL

D: once threshold stimulus
reached, AP occurs

AP occurs or
it does not

AP all have

  • SAME INTENSITY
  • regardless of INTENSITY of STIMULUS

Factors Affecting Speed
of Nerve Impulse

AP = wave of
depolarisation

region behind
immediately repolarised

as 1 part of membrane depolarised

  • sets up local (electrical) circuit with areas adjacent on either side
  • positive ions pass on inside moving forwards
  • negative ions pass on outside moving backwards
  • occurring continuously = creates wave of depolarisation

Action potentials propagate along an axon by a flow of current in a series of localised circuits

Myelin Sheath

Diameter of
Axon

  • acts as electrical insulator
  • prevents depolarisation

at Nodes of Ranvier

  • break myelin sheath
  • depolarisation can take place

local circuits form BETWEEN NODES ONLY

  • sections surrounded by myelin sheath = BYPASSED

AP jump from 1 node to next = SALTATORY CONDUCTION

thicker axon,
faster nerve impulse

larger diameter

  • less leakage of ions
  • smaller SA:V ratio

Temperature

higher temp, faster
rate of diffusion of ions

small diameters

  • large SA:V ratio
  • means too much leakage occurs
  • difficult to maintain potential gradient

myelinated neurones

  • faster speeds
  • relatively few channel ions
  • underneath myelin sheath
  • (ion channels conc @ nodes of Ranvier)

myelination overcomes effect of
small diameter