Neuronal communication

• Mammalian sensory receptors

  • Each type of receptor detects a specific stimulus

  • Act as transducers → convert a stimulus to an electrical impulse

  → a generator potential is produced
• Pancinian corpuscles

  • Receptors in the skin which detect pressure (mechanical stimulus)

  • A sensory neurone ending surrounded with layers of connective

  tissue called lamellae

  • When pressure is applied

  → pressure deforms the lamellae

  → stretch-mediated Na+ channels in the sensory neurone membrane open

  → Na+ diffuse into the sensory neurone

  → if the threshold is reached, a generator potential is established (the start of an action potential)

  • Greater pressure opens more Na+ channels

  → threshold more likely to be reached
• Neurones

  • Sensory neurones → transmit electrical impulses from receptors to the central nervous system (CNS)

  • Relay neurones → found in the CNS

  → integrate input from sensory neurones and output via motor neurones

  • Motor neurones → transmit electrical impulses from the CNS to effectors
• Myelination

  • Both the dendrons and axons of sensory neurones can be myelinated
  • The axons of motor neurones and relay neurones can be myelinated
  • Schwann cells wrapped around the axon or dendron make up the myelin sheath
• Myelin sheath


  Provides electrical insulation
• Saltatory conduction

  1. Depolarisation only occurs at the nodes of Ranvier (gaps between Schwann cells)

  2. Depolarisation jumps from node to node
• Saltatory conduction

  Increases the rate of propagation of nerve impulses
• Non-myelinated neurones

  Depolarisation must travel along the whole length of the axon
• Non-myelinated neurones


  Slower rate of propagation of nerve impulses
• Resting potential

  • Sodium ions (Na+) are actively transported out of the neurone and potassium ions (K+) are actively transported into the neurone by the sodium-potassium pump

  → requires ATP

  → sets up electrochemical gradients of Na+ and K+

  • Voltage-gated Na+ channels are closed → Na+ cannot diffuse into the neurone down the electrochemical gradient

  • K+ ion channels are open → K+ diffuse out of the neurone down the electrochemical gradient

  • The neurone cell membrane is more permeable to K+ than Na+

  • The membrane is polarised

  → has a potential difference of -70mV
• Action potential

  1. Sodium-potassium pump still working as in resting potential

  2. Depolarisation (from -70mV to +30mV)

  3. Repolarisation (from +30mV to -70mV)

  4. Hyperpolarisation (from -70mV to -90mV)

  5. Return to resting potential (from -90mV to -70mV)
• Depolarisation

  • A stimulus causes a few voltage-gated Na+ channels to open
  • Na+ diffuse into neurone down the electrochemical gradient
  • If the threshold (-55mV) is reached, more voltage-gated Na+ channels open, permeability to Na+ increases, and the membrane depolarises
• Repolarisation

  • Voltage-gated K+ ion channels open
  • Membrane permeability to K+ increases further and many K+ diffuse out of the neurone
  • Voltage-gated Na+ channels close so no diffusion of Na+
• Hyperpolarisation


  • Voltage-gated K+ channels are slow to close so too many K+ diffuse out of the neurone (causes the overshoot to -90mV)
• Return to resting potential

  • Voltage-gated K+ channels close
  • The sodium-potassium pump redistributes ions
• Action potentials are all-or-nothing.

  They will only happen if the threshold is reached. A weak stimulus may not

  reach the threshold.
• Positive feedback in nerve impulses

  • Opening of a few voltage-gated Na+ channels increases the permeability to Na+

  • Na+ diffuse into the neurone and the cytoplasm becomes more positively charged

  • This causes opening of more voltage-gated Na+ channels and further increase in permeability to Na+

  • Positive feedback continues until the membrane depolarises (if the threshold is reached
• The refractory period


  1. Starts during repolarisation

  2. Ends when resting potential is re-established

  3. Voltage-gated Na+ channels are inactivated and cannot open again to begin another action potential

  4. Limits the frequency of impulse transmission

  5. Produces discrete impulses

  6. Ensures action potentials are unidirectional
• When Na+ in the axon diffuse sideways from a depolarised section of the membrane


  They can only trigger the depolarisation to continue on the side not in the refractory period
• Stronger stimulus


  Results in more frequent action potentials travelling along the neurone
• Excitatory cholinergic synapses

  • Excitatory → binding of the neurotransmitter to receptors depolarises the postsynaptic membrane

  • Cholinergic synapses use acetylcholine as the neurotransmitter
• Excitatory cholinergic synapses


  1. Depolarisation of presynaptic neurone membrane

  2. Voltage-gated Ca2+ channels open

  3. Calcium ions enter presynaptic knob

  4. Synaptic vesicles move and fuse

  5. Acetylcholine released into synaptic cleft

  6. Acetylcholine binds to receptors on postsynaptic membrane

  7. Na+ ion channels open

  8. Na+ diffuse into postsynaptic neurone

  9. Acetylcholine broken down by acetylcholinesterase

  10. Products reabsorbed, acetylcholine resynthesised
• Synapses ensure unidirectionality. The process only works one way

  due the receptors only being on the postsynaptic membrane.
• Inhibitory synapses

  • Inhibitory → binding of the neurotransmitter to receptors hyperpolarises the postsynaptic membrane

  • Neurotransmitter binds to receptors on the postsynaptic membrane

  → causes Cl- channels on the postsynaptic membrane to open

  • Cl- diffuses into the postsynaptic neurone and the membrane hyperpolarises

  → potential difference becomes more negative than -70mV

  • More Na+ have to diffuse into the neurone to reach the threshold so depolarisation is less likely
• Acetylcholine is excitatory in skeletal muscle but inhibitory in cardiac muscle.
• Summation

  • Enables fine tuning of responses

  • Temporal summation

  → action potentials arrive in quick succession at a synapse

  → more neurotransmitter is released into the synaptic cleft

  → the postsynaptic membrane is more likely to reach the threshold level and depolarise

  • Spatial summation

  → many neurones connect onto one neurone

  → a weak stimulation of each neurone means a small amount of neurotransmitter is released into the synaptic cleft from each neurone

  → the combined neurotransmitter means that threshold is reached and the postsynaptic membrane depolarises